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Green Architects' Lounge

The Multi-Zone Heat Pump Issue

Why do multi-zone air-source heat pumps usually perform worse than single-zone ductless minisplits?

In this episode, Dana Fischer (center) shares his minisplit heat pump expertise with GBA's green architects, Phil Kaplan (left) and Chris Briley (right).

In cold climates, multi-zone heat pump systems (or multi-split systems) are developing a reputation for under-performing  in terms of energy efficiency. In a nutshell, this happens when you ask a large capacity heat pump to eke out small amounts of heating or cooling. Your efficiency will come nowhere close to what was advertised or what you used in your energy model.

Don’t freak out! Chris and Phil invited a special guest, Dana Fischer of Mitsubishi, to share a beverage and get to the bottom of this problem and how to properly approach the design of multi-zone heat pumps for your energy-efficient house or building.

The Cocktail

Hot Buttered Rum (serves 4)
2 cups water
1/2 stick (1/4 cup) unsalted butter
1/4 cup packed dark brown sugar
1 teaspoon cinnamon
1/2 teaspoon freshly grated nutmeg
1/4 teaspoon ground cloves (fresh is yummier)
1/8 teaspoon salt
2/3 cup dark rum (we used Meyer’s)

Bring water, butter, brown sugar, cinnamon, nutmeg, cloves, and salt to a boil in a 1 1/2- to 2-quart saucepan over moderately high heat. Reduce heat and simmer, whisking occasionally, for 10 minutes. Remove from heat and stir in the rum. Serve hot. Thanks, Epicurious!

The Mocktail

The Fischer Mocktail
1/2 oz. Twisted Shrub’s Blueberry Lemon Shrub
2 oz. unsweetened cranberry juice
1/4 oz. maple syrup
3 dashes Fee Brother’s Whiskey-Barrel-Aged Bitters

Serve with crushed ice, and pop a few fresh cranberries on top if you got ’em, just for a little pretty.

The Highlights

  1. How does a heat pump work? It’s all about the physics of refrigeration (and phonons!)
  2. Single zone vs. multi-zone, what’s the difference?
  3. What’s the problem? Why are multi-zones having efficiency issues? It’s not always an appropriate solution.
  4. How bad is the problem? You could see that advertised COP of 3 actually be 2 or even 1!
  5. Solutions: Proper sizing; proper zoning; supplemental or integrated systems. Understand what you’re asking the heat pump to do.
  6. How about ducted systems? Good solution, but they require greater thought and expense.

Pet Product

I share a pet product I’m considering adopting:  Glavel. It’s air-entrained recycled glass nuggets. At roughly R-2 per inch, it can be used to replace the EPS or XPS rigid insulation under your slab (as well as the crushed stone).

Closing Song

Phil shares a song that we should all be playing while planning out our next awesome projects. In honor of the topic, Phil chose “Two-headed Boy” by Neutral Milk Hotel. (See what he did there?)


Never miss an episode and take the podcast with you! Subscribe to the Green Architects’ Lounge on iTunes or from wherever you download your podcasts. The show’s Theme Music is Zelda’s Theme by Perez Prado. 

Special thanks to our sponsor Pinnacle Window Solutions

The Transcript

Chris: Hey everybody! Welcome to the Green Architects’ Lounge podcast. I’m your host, Chris Briley.

Phil: And I’m your host, Phil Kaplan. Hey, Chris!

Chris: Hey, Phil! How’s it going?

Phil: It’s cold today. Chilly. You know what I saw driving in?

Chris: No.

Phil: On the time-and temperature building?

Chris: I do know what you saw.

Phil: I saw zero.

Chris: A big old zero.

Phil: Goose egg.

Chris: Donut.

Phil: That’s right.

Chris: Nothing. It’s cold.

Phil: It’s cold. So, it’s a perfect day to talk about what we’re going to talk about today.

Chris: That’s right. And this is — we should warn everybody — this is a cold climate episode. This is an issue that is affecting mostly people in the colder climates. We’re going to talk about the multi-zone heat pump “issue.”

Phil: Issue. Challenge!

Chris: Challenge. It’s not a problem. We’re not going to say that, because that’ll make everyone panic and move away from heat pumps. We’re not saying that. We’re saying that there’s an issue. Everyone needs to be aware of it and they need to start designing their things better (their heat pump systems).

Phil: That’s right. So, here today, to illuminate us on the issue and how to solve it, is…

Chris: Dana Fischer.

Phil: Welcome, Dana.

Dana: Hey, glad to be here. It’s great to see you guys, as always.

Phil: Yeah.

Chris: Good to see you.

Phil: Great to have you. It really is.

Chris: Yeah.

Phil: We’ve known Dana for years and years. It turns out that, Chris and Dana, you guys go way back.

Chris: Way back.

Dana: Yeah, we were chatting about this early today. Going back more than a decade on solar stuff, and those heady days of solar thermal and Chewy Wonkers.

Chris: Yeah, that’s right. You were at the Chewonki Foundation. (What did you call it? It’s not camp).

Dana: Well, they have a camp too, but it’s a campus. It’s a different…

Chris: They have these huge energy summits and, there you were up there, presenting this Maine company doing solar thermal. It was really great and inspiring, and I thought, “Man, that guy’s got his act together.”

Dana: [Laughs]

Chris: And look at you now! You’ve spent tons of time at Efficiency Maine. How many years?

Dana: Yeah, I was at Efficiency Maine for seven years working on all manner of heating systems and weatherization, and working on the loan program and getting that running. So, it was a great time.

Chris: Yep.

Phil: And now you represent Mitsubishi in Maine and New Hampshire.

Dana: Yeah. I cover these two states for Mitsubishi Electric heat pumps. I support the network of installers and distributors in the two states. I’m trying to make sure that everybody’s armed with all the technical information they need, and that they’re out there doing best practices and doing well by their customers. So, it’s a great transition for me.

Phil: So, Dana’s here because he’s been a great resource. He’s our go-to guy when things happen or things go wrong with our Mitsubishi heat pumps.

Chris: Right.

Phil: And just to be clear, this is an issue with all heat pumps…

Chris: All heat pumps. Right.

Phil: …not just Mitsubishi’s. Dana has a real good beat on it, so he’s going to be able to talk broadly about some of these issues.

Dana: Yeah. And I think that this is something — we were chatting — that has come up just in the last couple of years with the arrival of more cold-climate versions of multi-zone systems, and it really comes down to sizing and selection of units, and it has to do with the limitations of how the heat pump multi-zone systems operate, and working around that. So, we’re going to get a great chance to talk through all those different parts.

Chris: Right after we talk about this cocktail.

[The guys jaw about this episode’s cocktail.]

Chris: So, here we go.

Phil: And just for a little back up, we’re not really going to go into detail about heat pumps and how they work. We’re going to have a brief intro from Dana, who’s going to tell us a little bit about it. But if you want to find out more, in our Episode 31, we talked about how to choose your mechanical systems. So, go back and listen to that if you want a little bit of a refresher.

Chris: Right. And we are going to talk about how they work, but not in the details of how we’re sizing them and why we’re choosing them and all that.

Phil: Thanks, Chris. Yes.

Chris: Right? Yes. So, maybe we ought to start there.

Phil: Yeah. How does a heat pump work, Dana? Give us the… You’re going to sound a lot smarter than we are! It’s not hard to do.

Chris: Yeah, it’s not hard to do. We have our own gimmicks of talking about refrigerators — moving heat, not creating heat, all that jazz — but you’ve probably got a better…

Dana: Well, you know, I think (like you and probably some segment of the listeners), I totally blacked out every time the concept of refrigeration came up in any science class in high school or anywhere else. It was just not very interesting. Who could understand how that whole thing worked?

And then when heat pumps started to come along, we really (at the state level) had to look at what’s going on here. How are these things operating? I really had to dive back into the textbooks and look at some of this. And then going around and talking to people about it, I’ve tried this spiel many, many times, and really the thing that seems to get people’s attention is to really just come right out and say that the refrigerant that you use in these systems (called 410a, which is relatively newer refrigerant technology) has a boiling point of -50 degrees Fahrenheit.

So, with a boiling point that low, when it gets squirted into the outdoor unit and into a low-pressure state, it’s heading for its boiling point at -50 degrees (which is substantially colder than it is even tonight), so all the heat energy that’s in the air can get absorbed by that 410a (I don’t think it reaches its lowest boiling point, but it’s approaching that level). And so, it warms it to whatever outdoor ambient temperature it is.

And so, on a night like tonight, it’s warming it up by going into that radiator, and the cold, zero-degree wind blowing past it with a big fan heats up the refrigerant from what it would be to approximately zero degrees, and then that refrigerant goes into the compressor and the compressor on the outdoor unit (which runs on an electric motor) compresses that refrigerant, reducing its volume. And when it reduces its volume, it’s also increasing the density of the heat energy.

Chris: Yeah. So it releases energy by compressing it. Right?

Dana: Well, this gets into one of those other things that you tend to blackout on during high school, which is the gas laws. The heat energy remains the same — anything maintains its heat energy throughout. So, when you compress that gas, you also compress the heat energy. And that manifests itself as being at a higher temperature. So, when that refrigerant comes out of the compressor, it’s not zero degrees, it’s 140 or 150 degrees Fahrenheit (which is plenty hot).

So, then the compressor pushes it along through copper tubes to the indoor unit, where there’s another fin-tube radiator and a fan, and it blows indoor air across that hot coil with that hot refrigerant, and transfers that heat into the house. The refrigerant cools down to roughly room temperature and then gets circulated back out to the outdoor unit. And then the cycle continues.

Chris: Right. So, Dana, I’m sure there are people out there thinking, “But Dana, it’s zero degrees outside! How can there possibly any energy for this thing to grab? It’s not hot. How is it grabbing heat from zero degrees?”

Phil: And that’s the biggest thing that freaks people out. We’re with clients and they say, “Oh yeah, I heard it doesn’t work so well when it gets cold.”

Chris: Yeah. Well, its efficiency does go down. Right?

Phil: Right. But we still hear it.

Chris: They think it’s like the old ’80s one, where it’s below freezing and they stop working, basically. But these are so different.

Dana: Yeah. You know, a lot of times people think of heat energy as like waves, or they think about a wood stove, or feeling the heat coming from the sun or something. But you can also think of heat energy as a particle, as a packet of energy. The equivalent of light that we always talk about is a photon coming from the sun. (It’s light and it’s light energy and it can generate electricity across the solar panel).

In the thermal world, the equivalent (which nobody ever hears about so much) is called a phonon, and that’s a packet of heat energy. And that particle of heat energy is in everything. Everything that has any kind of temperature whatsoever has a density of heat particles in it (these phonons). So, we’re constantly generating heat in these particles and emitting them as we walk around and everything has a certain density and these heat particles move from higher density (whatever’s hotter) to a lower density of heat particles (whatever’s colder).

And so, the absence of heat particles would be absolute zero — like minus 270-something Celsius, or zero Kelvin. Oh yeah, that’s like outer space cold! So, when it’s minus 10 degrees Fahrenheit outside, or zero, or 10 degrees above, or 20 degrees above, there’s still a fair amount of this heat energy drifting around, floating through space.

And so, when the refrigerant goes into the outdoor coil at -50, it just has an absence of heat particles, and so those heat particles that are loose in the air cling to that coil on the outside, get absorbed, and get compressed and transferred into the house.

So, it’s just a different way of thinking about heat energy. You’re capturing all of this heat energy that’s drifting around outside and moving it into your house, and then the efficiency of your envelope of your house is really like how well it retains all of those heat particles, how well it traps that heat energy and prevents it from escaping (from air flow or low levels of insulation, that sort of thing).

Chris: Perfect. I think we covered that. We’ve lost half of our listeners, and we’re going to bring them back around.

Phil: [Snores]

Chris: Bunch of nerds! Oh my God! [Laughter]

Actually, our listeners… we kept most of them. The new listeners are probably like, “Oh my God, I just wanted to hear about hot buttered rum.” But, no!

Phil: No! some of our listeners are totally like me. I’m going to use “phonon” in a sentence tomorrow.

Chris: I am too!

Phil: I am totally psyched to do that! I’ve never done that before.

Chris: You know, the density of phonons in this cocktail is much higher than the cocktail we proposed earlier.

Phil: Well done, well done!

Chris: All right.

Phil: So, give us a basic understanding, Dana: single-zone versus multi-zone. I think there are people out there that think, “Oh yeah. I’ve got a heat pump, I understand that. I see that cartridge on my wall. How do I know if I have a single-zone or a multi-zone?” What’s the difference?

Dana: Yeah, so we’re talking about what are generally called minisplit or multi-split systems. A minisplit heat pump —  it’s called a split unit because there’s one unit indoors and one unit outdoors, and it’s called mini because it’s much smaller than the traditional heating and refrigeration systems that were out there that would have split design. So, a one-to-one system is where you have one outdoor unit that has two copper refrigerant lines that go to one indoor unit, and it circulates refrigerant just between those two units.

A multi-split or a multi-zone heat pump will have one outdoor unit that can go to multiple indoor units (two, three, five, eight indoor units). It’s the largest residential version that’s available right now. And so, people would be like, “Well, geez, that’s awesome! Because if I want to have four or five or six indoor units, I only have to have one outdoor unit.” And that’s totally true.

The piece that we’re going to get to is, really, that you should design or select the outdoor unit and the capacity of the outdoor unit based on the load of the building as opposed to the number of heads that are in the house. So, it’s two separate questions.

Phil: Right. And that outdoor unit is — people have seen them all over the place — it looks kind of like a big box fan.

Dana: Yeah.

Phil: The inside units, we call them heads. Sometimes, those units…

Chris: Or cassettes. Wall cassettes.

Phil: Cassettes, right. They’re about 4 feet wide, about 18 inches tall.

Chris: And you see them near the ceiling usually. They’re usually mounted up high. They come in all kinds of models. You can actually get them to go into the ceiling. We’ll talk about that in our solutions section — ducted systems and that sort of thing. And there are even floor units. I was asked the other day, “Have you ever spec’ed a floor unit?” I don’t think I have.

Phil: No, architects don’t do that.

Chris: [Laughs]

Phil: We want them the hell out of the way!

Chris: We want them out of the way.

Phil: But they are more efficient, I understand.

Chris: That’s what I hear. For heating.

Dana: Well, they can be a good choice, particularly if you have a kneewall or a hallway that really warrants it, but you can’t put furniture in front of them. So, it’s got to be in a place where it’s not going to be obstructed by the occupant or any future occupant.

Chris: So, we talked about the single one-to-one ratio, and we’ve got the multi-zone. So maybe actually now we talk about the problem, which is: Now you have this big outdoor unit and then you’ve got a bunch of little indoor units, and it’s in a super-tight house. Wouldn’t you say, this is why we’re seeing this resurgence of heat pumps? It’s because people like you and I, Phil, we’re designing these Passive Houses, these super-tight houses?

Dana: Yeah. So, we should get right to the point on this. The issue that we’re really trying to work on up to and discuss is that, in certain applications, a multi-zone heat pump is inappropriate. If you put too large of a multi-zone system into a circumstance with too low of a load, we’ve seen cases where people will have poor performance, poor temperature control, and high electricity costs — higher than they were anticipating. Especially in low-performance homes where they’re building a super-tight enclosure, and it’s very well insulated, they’re expecting to have very low energy costs. But because the heat pumps that are invariably selected and oversized for the system, that’s not what they’re experiencing.

Chris: So, let me paint a scenario (and then we’ll take it from there), where there’s a dude, and he wants a nice, big, beautiful house with many rooms, and he’s got many plans for these many rooms, and so he wants all these different zones. He hires a green architect (we’ll say, a sustainable-type of architect), and he says, “Now, make it super-duper efficient.” And so, we reduce the energy demands. “So now, dude, you could heat this whole thing with just one multi-zone heat pump. One 42,000 BTUH multi-zone thing.” And he says, “Yeah, great. But I want every room to have perfect-zoned temperature control, so I’ve got a cassette in every room.”

So now we have one big outdoor unit and we have a bunch of indoor units. And then here’s where the problem happens. It’s fall. It’s a little chilly. One of the cassettes on the north side says, “Hey, I want a little bit of heat. Just a little bit.” And it’s asking this big heat pump that’s outside to start supplying. Right?

Dana: Yeah. Yup. That’s exactly right. Especially a lot of the high-performance homes that you guys are designing are a much lower load. So, at design temperature (which in this neck of the woods is about -6 degrees Fahrenheit), it’s not unheard of to have a high-performance home that would have a peak heating load demand of 20,000 or 25,000 BTUH. And so, you would expect that that would be the amount of heat that you would need at that point in time. And then, likewise, you want to think, “Okay, well, more than half of the season is spent between 20 and 30 degrees Fahrenheit.” So, that’s not the same level of load (it’s about half of the peak load), so in these high-performance homes, your heating load might be 10,000 or 12,000 BTUH — half of what you would anticipate.

So, when the homeowner says, “Well, I want to have five or six or seven different cassettes throughout the house in order to control all these different zones,” and you key in on these larger multi-zones that do produce 40,000 to 50,000 BTUH of heat — we kind of skipped over this — but  one of the consequences of having the multi-zone systems is, because of all of the line sets going all over the place with all these different tubes going up through everywhere, in order for the system to ensure its longevity, it needs to be able to have great confidence that it’s circulating refrigerant and refrigerant oil through every single orifice all the way through and coming back and lubricating all the different components in the compressor.

And so, the lowest that multi-zone systems can scroll down is roughly 35% of their maximum capacity. So, if you have a 45,000 BTUH system that’s rated to put out 45,000 BTUH of heat at zero degrees or 5 degrees Fahrenheit, the minimum that it’s going to be able to scroll down to is in the ballpark of 15,000 BTUH — whether you need it or not.

The graph with the bell curve (upper left) generically shows the amount of time that heat pump systems experience the  design load (purple and blue) and the amount of time spent at median load (gold). The bar graph (below) shows the heating capacity range in kBTU/h of certain Mitsubishi units. The graph gives an indication of when these units experience optimum efficiency.  E.g., if a split system (5c42NAHZ) is asked to provide only 10 kBtu/h, it’s below its optimal efficiency and will work overtime to shed the excess energy of its minimum output.

Phil: Mm-hmm.

Dana: So, if you have a house that’s really super-tight, you’re like, “Oh well, pffft! The load is 25,000 BTUH. Okay, we’ve got from 50,000 to 15,000. Okay, well that’s in the right range.” Well, when you have half of that load (which is actually the majority of the heating season), it’s bouncing underneath the minimum output of the multi-zone outdoor unit. And so, it’s turning on and turning off (which is really, one of the key efficiency problems of days of old with your dad’s heat pump that really wasn’t designed for this climate and would just turn on and off and use all kinds of electricity). So, you really (by oversizing the system), you’re reverting back to some of the failings of the prior generation of heat pumps.

Phil: So, when this happens, Dana, a typical coefficient of performance — the number of units of energy in versus the number of units of energy out — in an ideal situation… COP of about 3?

Dana: Yeah, you can get pretty close to a COP of 3 (meaning one unit of energy in for three units of energy moved) in the highest performance, well-matched systems in Maine through our fun winters. With multi-zone systems, I think you could get a seasonal average of around 2.6 or 2.7 for your COP. It’s going to be a little bit less efficient, just because of the size of the system and the motors that are involved. But if you’re dramatically oversized, it might be lower – like 2! Which is still…

Phil: Or less.

Dana: Yeah.

Phil: I’ve been talking to Rob Aldrich. He does a lot of testing with Steven Winter out of Connecticut.

Dana: Yeah. No, it’s true.

Phil: And he’s seeing 1.6 on a regular basis, and some that are even below one. I mean, we’re talking below electric resistance.

Dana: Well, I haven’t seen anything that low, but I have to say that what everybody is trying to get to, is a place where the heat pump is running all the time and idling along, providing just the right amount of heat but rarely going through any shutdown cycle and going through a defrost mode maybe every three or four hours unless there’s a rainstorm or something going on. The objective is to make these things run smoothly and quietly all the time.

And so, there’s this problem with running heat pumps — it’s mostly humans. So, they’ll change the temperatures and run them up high and then run them down low. And they’re mis-sized or inappropriate, and that leads to these issues and complaints around a lack of efficiency or high electric costs.

And so, the plea is to look at the heating load. When you’re out there designing it for people, look at the heating load, and consider your options around it. So, if you have a client who says, “I want five or six indoor zones,” that’s great! Give the people what they want. It’s just that you may want to work on including multiple outdoor units, and have smaller outdoor units that will match the load over the entire season, and not just meet what their needs are at the peak load.

Phil: Does that mean additional cost to the homeowner?

Dana: Not necessarily. Because, it turns out that there’s some matching, actually, in terms of the outdoor unit cost for a larger one. You can get, let’s say, a three-zone or two-zone system. The outdoor unit is not really much different in cost than two single one-to-one smaller units. So, you might see more in cost if you were like, “I’m going to put six outdoor units” as opposed to one, but you’re going to get redundancy and lower operational costs and greater temperature control in the individual zones too.

Phil: Yeah. And actually, that’s what we’ve been doing recently since we’ve heard about some of these issues.

Dana: Yeah. I mean, multi-zone — if you have a building or a home that truly has a load calling for the equivalent of 700, 800, 900, 1,000 gallons of oil per year or more, they’re a beautiful piece of machinery. When a multi-zone is appropriately matched to a building, it just can provide a very high level of efficiency and comfort throughout an entire (and pretty sizable) space. But, there really are limitations. If you put a Mack truck on the outside of the building — we were talking about analogies earlier — if you just put too much system on the outside, it’s just not going to suit you well.

Phil: Right. We talked about this bad-ass Corvette (a racing vehicle that can go fast with power), and when you tell it that it can only go two miles per hour, it’s not very happy with that. It’s meant to go. And if you don’t let it go, it sputters out. It conks out. It’s not happy. It’s not doing what it’s meant to do. So, it sounds very similar to what you’re talking about.

Dana: Yeah. So I guess that’s where people think about it as an issue or a problem or something like that. It’s really that there are inherent limitations on any piece of equipment, and heat pumps are no different.

So, when we think about the buildings and designs of the future (which, we’re on a pathway similar to what other northern countries have seen) in the decade ahead, we’re likely to see the majority of Maine and New Hampshire and Massachusetts homes install heat pumps. Norway went from some small percentage of heat pumps to more than 50% of homes with heat pumps in a twelve-year period between 2003 and 2015. They installed, in Norway, close to a million heat pumps in that period of time.

And they have about four times the population of Maine, so if you were to scale it to Maine, that’s like 250,000 heat pumps here and another 250,000 heat pumps in New Hampshire. So far, Maine has somewhere between 30,000 and 35,000 homes, so we’ve got another eight times what we’ve done so far in the last six years before we’re done. And maybe even beyond that. With all of the policy work going on around renewable energy and electrification, it’s really an exciting time in heat pumps. And we can really anticipate a future where this is going to be everywhere and more common.

So, part of our duty as super energy-efficiency geeks is to make sure that people understand these constraints and limitations, to maximize their performance and make sure that we’re doing our most — that when we get to ten years from now, we turn around and look back and we’re really pleased with the fleet of high-performance systems that have been installed. And that they’ve been installed well.

Chris: Nice. I think that was a great way to end this first segment. We’ll take a quick break and then we’ll come back and we’ll talk about solutions and the way to approach designing these multi-zone systems.

Phil: Outstanding. I want solutions, Chris.

Chris: All right. Stay with us.

[Advertising break]

Chris: Yeah, we’re back. Hey, Phil!

Phil: Hey, Chris!

Chris: Hey, Dana!

Dana: Yo!

Chris: Yo! So, yeah — maybe we should take this moment to first thank Pinnacle Window Solutions for being… We’ve got some nice sponsors now. That’s great!

Phil: Yeah, absolutely!

Chris: But we should also say that we don’t just let anybody sponsor us because, I mean, everyone wants to sponsor us. But we’re like, “Hey…” In all seriousness, we wouldn’t let people sponsor if it’s not products we would use.

Phil: That we would specify.

Chris: Right. Specify, use, endorse even.

Phil: Or endeavor to specify.

Chris: Right. Right. And Chris Brill, who’s the owner — you and I and actually all of us in the room, we know him. Great guy!

Phil: Fantastic guy. And we have been specifying his windows in our high-performance homes for years. We are very happy to have Pinnacle as a sponsor.

Chris: Absolutely! Can’t say enough good things about him, and the service they provide. And they’re local. That’s one of the things: you can get a European window stateside (their Logic brand is made in Pennsylvania). So, kudos there.

Phil: So, here’s a little reminder Chris: We’re going to redo our episode on windows at some point.

Chris: I know. We keep threatening each other. “We’re going to redo windows!” Our windows episode, which you can’t find now, because it’s stinkeroo, man…

Phil: Dana, this was our very first, this was number one.

Chris: Right. I think it was number one. Yeah. We were just getting used to this podcast stuff.

Phil: Just getting going.

Chris: But more than that — it’s dated. And maybe this is a good opportunity to say: This problem we’re talking about? It might be over and solved in a year or so. Look at the dateline on this, guys, because heat pumps are rapidly changing technology.

Phil: Yeah. And the multi-zones are fairly new.

Chris: Right.

Dana: Yeah. The Hyper-Heat multi-zones have really only been around for a short period of time. And it’s really only been identified in the course of people studying their electric bills and looking at what’s going on. And most of this is really occurring in cold climates where we do have this substantial portion of our heating season and this “half of load” period. So, this kind of thing doesn’t really happen once you get south of New York.

Chris: Right. Where their peak load is not as — They’re not designed to -6 degrees.

Dana: Yeah. Once you’re like, “Well, half of our heating season is 50 degrees,” it’s just not really a big deal.

Chris: Right. So maybe it’s a good moment — before we get into solutions and the approaches that we would take. We’re not necessarily picking on Mitsubishi. You’re here because of your affiliation with Mitsubishi. So, are they out there running tests and trying to figure out how to grapple with this? How to — instead of putting in a one-number COP, and then, ta-da! That’s the efficiency!

Dana: I think this is actually one of those things that really differentiates Mitsubishi in the U.S. marketplace, is really the level of engagement with the marketplace and trying to adopt solutions for buildings in all of our different climates. This limitation around multi-zones is really not limited to Mitsubishi. In fact, Mitsubishi products are able to scroll down lower than competitive products. But they’re all going to see this. So whether you’re designing with Fujitsu or Daikin or LG, these are the same kind of things that you’re going to want to take into consideration. And be mindful that it might not be as easy to find information about the limitations on some of the other ones. I want everybody, regardless of what your favorite flavor is, to be very cautious about this.

Chris: As designers — you’ve got this nice rule of thumb of 35%. Maybe that’s where we start. We as designers can start feeling or sensing that we’re creating a multi-zone system where we know it’s going to catch demand below that turndown.

Dana: Yeah. It comes back to your sizing. So, when installers are out there, they’re not all sizing. But presumably when we’re talking about new construction or a significant retrofit, we’re talking about doing energy models and Manual J. I think one of the key tactics to avoiding this, in any circumstance, is to not be overly conservative with your energy model. I’ve seen plenty of people who have a lot of experience in HVAC who are like, “Well, it sometimes gets down to -20, so I’m going to use -20 in my energy model.”

Phil: Right, right. Worst-case scenario.

Dana: That’s wrong!

Chris: That is wrong.

Dana: It’s just wrong! And so, for most of Maine and New Hampshire (and probably across through Vermont), the design heating load is between 0 and -6 or -8. You can get into some extremes where it’s -10 or -13 but, for the most part, it’s in that ballpark. If you dial the model down to -20, you’re definitely going to be oversizing your system. So that’s to be avoided.

The other part of it is, you definitely have to check out the client and what they want for distribution and what their desires are for how the system will interact. But you really should not have a single core compressor that exceeds the calculated heating load by more than 20%. I think if you were further south, you could say maybe don’t exceed by 50%.

Chris: Mm-hmm.

Dana: But I think that when you’re trying to cover all the seasonality in these far northern states, you really want to be very close to exactly the same amount of heating load, or up to 120%. Along the same line, one of the things that I’ve bumped into in some of my reading from the Heat Pump Association in Norway (that has a lot of experience in cold climates and has been doing this for a long time), is that you can find documentation where they say the ideal efficiency for the front cost of the system and the overall operational performance of the system comes when you size it to around 80% of the heating load of the house…

Chris: Wow.

Dana: …which seems kind of strange to us, because we’re like, “Well, what happens when it’s 100%?”

Phil: Right.

Chris: So, 20% of the time when I need it most, I don’t have it!

Dana: And so, their recommendation is that’s where you have a second heat pump or you have an alternate form of heat, like a pellet stove or a wood stove or electric [resistance] heat in the perimeters of the house. But it gets back to what we were including in the prior segment — talking about having the system coast all the time and running all the time.

And so, if you were to size to 80%, there really is no circumstance (very, very limited heating circumstances) where you would be below the low output level of different heat pumps (if you were to size that level), and then you have a booster heat…

Chris: Mm-hmm.

Dana: …a second stage of heat in order to accommodate those extreme events, like today.

Chris: Right.

Phil: Yeah, that’s fascinating, Dana. I think that’s just a really cool idea, a cool rule of thumb. I mean, because, Chris, I know you guys do very similar things that we do…

Chris: Mm-hmm.

Phil: …and we’ll have backup electric sometimes, and we’ll have pellet stoves or wood stoves.

Chris: Yeah. Like your bathroom has a tile floor that’s got the electric radiant or maybe even an entryway that has electric radiant or you have a little radiant panel.

Phil: Mm-hmm.

Chris: That’s sometimes how we handle that zone issue, where you’re nervous about it — “Oh, is the bathroom going to be warm enough with this system in this house?”

Phil: That’s right. It’s about distribution.

Chris: It’s about distribution. But perhaps, you have that electric-resistance heat just as a booster. It’s not your backup — it’s actually a secondary part of the design. Maybe it can…

Dana: Integrated, too.

Chris: Integrated! That’s a great word.

Dana: You have the controls integrated so that it’s seamless. You set the temperature at 70 degrees and the house overall meets those conditions to maintain comfort throughout. And so, you can accomplish it with all heat pumps, but you need to take into consideration the size of the different systems that are included. Or you could have your supplementary backup.

We haven’t really gotten to ducting, but…

Chris: Right.

Dana: …to lead us down that road, Mitsubishi has ducted systems that have — you can add a module that has what they call a toaster kit on it.

Chris: Mm-hmm. Yeah.

Dana: It looks a lot like a toaster. You could probably put a very big slice of bread in there.

Chris: Huh. Very funny! [Laughter]

Dana: It basically comes on when the heat pump is experiencing a droop of the set point temperature. So, you set the room temperature in the living room at 70 degrees, the heat pump’s operating, and for whatever reason, it’s so freaky cold and windy outside that it isn’t able to maintain the set point temperature. So the electric [resistance element] comes on to supplement that. And it’s totally seamless. The homeowner doesn’t know any different. And the period of time that it’s on is so limited — just a few hours per year…

Chris: Mm-hmm.

Dana: …that they don’t really notice the difference. But if they were to try to accommodate that much more heat energy at those coldest temperatures, it would require a substantially larger system that would be more expensive up front. It would be more expensive to operate throughout the entire season.

Chris: So, a ducted system is another way to circumvent this problem. And that is, by using the same — Instead of having that one cassette on the wall, that one cassette is now in a different configuration. It’s in your ceiling. Or let’s say — and it is ducting that air to your bedroom wing. It’s handling this zoning component.

Dana: Yeah, right. One of the limitations — people will put the smallest heads in a few different bedrooms, and so that drives them towards having a large multi-zone system. But all the manufacturers have ducted systems that are slim ducts or full air handlers and they can be parked and ducted into a few adjacent rooms on the second floor.

And so you could say, “Well, I’m going to put a single-zone system that may be larger (like a 15,000 or 18,000 BTUH one-to-one) that’s just totally dedicated to the primary living space and the core of the house. And then I’m going to put either a multi-zone or a one-to-one with a ducted unit up in the attic or in a chase that can duct into a couple of smaller bedrooms.” And either they’ll all be at the same temperature or you can use automated dampers to control and zone them.

But by having just that one smaller-sized coil, you’re able to maintain an even temperature throughout and provide pretty even cooling. We’ve seen plenty of examples of that. And that can be a great solution to provide that distribution.

Chris: Mm-hmm.

Dana: So that all the rooms get heating or cooling when they need them, without having to add multiple zones and multiple outdoor units.

Chris: Right. But again, it’s also knowing what you’re doing when you’re doing a ducted system. Now you have to — you’re bringing in a whole…

Dana: There’s a whole bunch of math and magic to doing ducted systems.

Chris: Right!

Dana: And that has to be done properly. And of course, in Maine if you do anything [with ducts] in the attic, it sure as heck better be rigid.

Chris: Yup!

Dana: And insulated and sealed.

Chris: Yup. Right. And so, you’re inviting in some more costs — because when there’s more complexity, there’s more cost.

Phil: And my rule of thumb is, it’s usually two times as much for a ducted system as opposed to a one-to-one.

Chris: Hmmm. Is that what you’ve…?

Phil: That’s what I’ve heard.

Chris: I don’t know if I’d… That’s interesting.

Dana: I don’t…

Chris: It’s more.

Dana: It’s definitely more. You know, duct work is not cheap. It’s labor-intensive, especially when it’s done correctly, and so it definitely adds cost. Maybe double is an appropriate match, but I wouldn’t expect it to be more than that.

Phil: Right.

Chris: And your whole house — it’s probably not a binary decision — either this whole house is ducted (or not), or it’s a split system.

Phil: Right.

Chris: They are split. I shouldn’t confuse that.

Phil: A one-to-one…

Chris: Right.

Phil: …versus a ducted.

Chris: Right. So, you might have a portion of your building that’s ducted, and then another portion that’s…

Dan: Yeah. And it’s totally true that homeowners continue to — when they are not familiar with heat pumps, there are people who would be like, “Ah, I don’t want to see that on my wall.” Honestly, once heat pumps are on your wall… We’ve talked to people who thought that they would be a real problem and, some weeks later, they’re like, “I love it so much, I can forgive it a hundred sins!”

Chris: Mm-hmm.

Dana: But, using a ducted system is one way of making the heating system invisible.

Chris: Right.

Dana: You don’t really see it.

Phil: So, Dana, what is the ideal application for a multi-zone in this climate? Is there still one?

Dana: Oh, there definitely is. I think that it really comes down to that sizing component.  A typical Maine home — I mean, you guys are really focused on high-performance homes, but — it is extremely common for a home in Maine to be in need of weatherization…

Phil: Mm-hmm.

Dana: …but currently using 800 or 900 gallons of oil.

Chris: Right.

Dana: There are hundreds of thousands of houses in that category across the state of Maine. And so, if somebody puts a three- or four-zone system into one of those houses, and parks a decent-sized head in their main living space, a moderate-sized one either in their basement or up in master bedrooms, they’re leaning on that core system. So, the core unit that’s maybe a 18,000 or 24,000 BTUH unit that’s in their main living space (and that’s typically on and drawing and running), that can be incredibly efficient and really run well.

It really comes down to, when you start creeping down to lower loads and tighter buildings that you need to be more cognizant of it.

Chris: Right.

Dana: And sometimes people that are doing new construction — even if they’re not like, “Oh, I’m going to adhere to net-zero standards”, they’re just sort of like, “Spray foam everything!”

Chris: Yeah.

Dana: As soon as you walk down that road (or a contractor sees someone walking down that road), it’s a good time to really think about one-to-ones just to make sure that you’re not going to oversize.

Chris: So, we’ve got an integrated approach, and we’ve got using a ducted system as an approach, and… Is there anything else that we’re missing? Like are there — I don’t know — is there an ace up someone’s sleeve or some other magic bullet to keep efficiency under control?

Dana: Well, one thing we haven’t really mentioned in this is, part of this task, and part of the benefit of multiple single zones where it’s really warranted in high-performance homes, is really understanding how to sell it.

And it does have advantages. You have redundancy. So that you’re able to, if something happens with one unit, you’re not totally without heat, and you do have control throughout. But people are generally concerned about having multiple units outside. And it’s been shown that you can set up multiple units outside and have them aesthetically pleasing. So, architects are known for being creative and finding solutions. And so part of this challenge is a challenge to architects and engineers to find innovative ways of positioning and locating these outdoor units so that they enhance the use of the building efficiently and aesthetically.

Chris: Mm-hmm. Great!

Phil: Or just hide the [blank].


Dana: Yeah, there’s ways of doing that, too. I’ve seen some crazy hiding places. I saw one that was on a well or something. It was crazy.

Chris: Oh yeah. You told me about that! They built a swimming-pool — a hiding place vault for their mechanical systems. We don’t need to go into that, because that’s going to get really crazy.

Dana: Yeah.

Chris: So, I think that kind of wraps it up, unless there’s anything else that you have, Dana, on that?

Dana: Well, Chris, Phil, I’m just really thankful that you guys had me come in today. I’m really excited about heat pumps every day and I’m just thrilled to be able to share it in this venue. So I really appreciate you having me on.

Chris: Well, we’re thankful to have you.

Phil: Yeah, we appreciate you illuminating us on some of the ins and outs in that great detail. I think I learned a lot. I thought that was really cool.

Chris: Yeah, I did too! I’m learning, so I think this will be a valuable podcast to others.

Phil: Yeah. This is one of the things that Chris and I always talk about. What do we want to find out more about?

Chris: Right. And that’s why we do this podcast.

Phil: Yeah.

Chris: If we find ourselves doing the research on something, then we’re like, “This is a podcast topic.”

Phil: Yeah. Have you got a pet product for us today?

Chris: I have a pet product!

Phil: I want to hear (even though I kind of already know it).

Chris: Yeah, you do. But…

So, it’s called “Glavel.” Not gravel, but Glavel with an “L,” as in…

Phil: It sounds gleat.

Chris: It’s gleat.

Phil: It sounds like a gleat ploduct, Chris.

Chris: Oh, gross. [Laughter]

Chris: So basically, it’s glass. The reason why it’s pronounced that way is because it’s glass. If you have crappy recycled glass that is just of a quality that no one wants anymore — even the recyclers are like, “Ugh! That stuff!” — it’s all contaminated and brown and blue and green, or it’s been recycled eight times already, or it’s got whatever contamination…

Right now, in Europe is where this comes from. You can take that glass and you can entrain air into it, so it’s like lava rock, like pumice. If I put it into your hands… I don’t have any. Actually, I had a sample somewhere in here. There was a bag of it. It’s very light lava rocks or something like that.

Phil: Yeah, it’s weird stuff.

Chris: It’s weird and it’s like holding light-weight pumice in your hands.

Phil: Yeah.

Chris: But the idea is: let’s get rid of the foam under your building. EPS or XPS foam under the slab. What other options do you have when you’re bearing on it? Not many. Almost none. So, this is something that you can actually… It has about an R-2 per inch. And you can bear on it like crushed stone. So, you can prep a slab. Instead of your crushed stone, you have Glavel and maybe sand or whatever, and your vapor barrier and your slab and boom! And that’s actually what we’re doing with the school that we’re working on together — KTA and Briburn.

Phil: That’s right. Yeah.

Chris: We’re going to try — it’s a Living Building Challenge building — so we’re trying to not have petroleum products all over the place. So, can we do this without foam? Can we? We’re trying.

Phil: And how much are we going to need of it, Chris? If it’s R-2…

Chris: We’re going to need many cubic yards of this stuff.

Phil: Well, just about the depth, I mean.

Chris: At least a foot.

Phil: A foot is going to get us R-20.

Chris: Yeah.

Phil: That’s not bad.

Chris: Yes.

Phil: More than that.

Chris: Uh?

Phil: There’s twelve inches in a foot. [Laughter]

Chris: Yes, thanks!

Phil: And there’s R-2 per inch. That’s R-24.

Chris: Math is hard after a buttered rum. [Laughter]

Yeah, you were doing great with math and I’m doing terribly. Anyway, there’s a company out of Vermont — oh God, I hope I have this right — they want to produce it stateside. And they’re ramping up and gearing up. And we’re hoping to have their plants online for our project. But if they don’t make it, then we’ll be boating it over. But if this is successful, I’m going to be using it all the time. It’s supposed to be cost-effective, and (yeah!) we cannot be dependent on that petroleum foam stuff.

Phil: Yes. Outstanding! Me likey.

Chris: Yeah, great! All right.

And so, that just leaves the song, Phil.

Phil: Oh! The song!

Chris: Yeah, what do we have for today? I know you’re really proud of this one.

Phil: [Laughs.] It’s one of my favorite bands of all time. Neutral Milk Hotel.

Chris: I’ve heard of them, yes.

Phil: Yeah. It’s “In the Aeroplane Over the Sea.”

Dana: You guys are way cooler than me.

Chris: Oh yeah?

Phil: Only in a couple of things. Most things, probably not.

Chris: Yeah. I was going to say, based on this podcast, I don’t think anyone’s going to say…

Phil: “Those guys are cool!” [Laughter]

Chris: Bunch of nerds! So, yeah. Okay. Neutral Milk Hotel.

Phil: We’re going to play their song. (Wait for it)…

Chris: Yeah?

Phil: “Two-Headed Boy.”

Chris: Ah.

Phil: See? Multi-zones. Multiple heads.

Chris: He’s a multi-zone boy. Oh, that’s very good. Nice, Phil.

Well Phil, once again, a real pleasure doing a podcast with you. Dana…

Phil: Cheers!

Dana: Thanks for the mocktail. It was really great!

Phil: Thanks for joining us. Great fun. Everyone, stay warm.

Chris:  Yup. And we’ll see you next month for the next podcast on a great topic that we will decide soon.

Phil: All right.

Chris: Take care, everybody.

Phil: Bye.

[The episode closes with a song by Neutral Milk Hotel: “Two-Headed Boy.”]


  1. rhl_ | | #1

    Hi, can the editors/authors correct the words “sine wave” to normal distribution? There is no sine wave there.

    1. GBA Editor
      Martin Holladay | | #3

      Good point. How about "bell curve"?

      1. rhl_ | | #4

        Sure, but normal distribution is a bit better.

        1. Expert Member
          Dana Dorsett | | #5

          "Gaussian distribution" would be even better, giving credit where credit is due, leaving very limited alternative interpretations.

          1. GBA Editor
            Martin Holladay | | #6

            C'mon! The idea is to direct the readers' attention to a certain part of the image -- not to confuse the ordinary builder. Time to move on to other issues.

          2. Expert Member
            Peter Engle | | #8

            Neither "Gaussian" nor "normal" distribution applies. They only apply if the mean is right in the middle and if the rest of the curve follows certain mathematical rules, one of which is that the curve is symmetric about the vertical axis drawn through the mean value. Heating and cooling curves won't even be close. "Bell Curve" is probably better because it suggests the shape without any specific mathematical function implied.

            And Martin is just stuffy (below).

  2. rhl_ | | #2

    This is an interesting article. I assume all of it applies to the cooling situation in the same way. Any corresponding data (similar to that one plot) for cooling?

  3. joshdurston | | #7

    This was a great listen.

  4. Patrick_OSullivan | | #9

    I listen to quite a lot of podcasts. How in the world did I not realize this existed? Great episode and now I'm going back and listening to past episodes.

  5. AndrisSkulte | | #10

    Just found out about the podcast, and subscribed on the DownCast app.

    Is it possible to take a single zone Mitsubishi mini split and add a 2nd wall unit? Our fairly open downstairs needs heads at each end of the house, but not two zones, and the efficiency & 'turndown' of the single units are appealing over a multi-zone compressor. Seems silly to have 4 outdoor units for a 1600 sq ft cape.

    Time to catch up on past episodes. Keep up the good work!

  6. FluxCapacitor | | #11

    Very informative. Seems Dana is the go-to name for heat pump experts.

  7. charlie_sullivan | | #12

    An air to water heat pump like Chiltrix seems to address most of the issues discussed here very nicely, both accomplishing the objectives that drive people towards multi splits better than multi splits do, and avoiding the problems with multi splits. They are subject to the challenge that it's hard to find an installer with the right skills, but if enough people start using them, that problem can easily be solved.

    They aren't subject to the problems of multi splits needing to ensure that coolant is circulated throughout a huge system, because the coolant is circulated only in a very short factory sealed loop in the outdoor unit. That has the additional advantage that the quantity of high global warming potential coolant in the system is greatly reduced, and because all of the connections in that system are made in the factory, the chance of releasing that potent greenhouse gas is greatly reduced.

    And in terms of the desire to provide a heat source in every little room, whether that desire is rational or not, small panel radiators go down to as low heat output as you might want and are inexpensive to add to the system. And their slim fan coil wall units are lower visual impact than wall mount minisplit heads.

    I know from experience that it is hard to find someone to install a residential chilled water system correctly. In mine (GSHP, not Chiltrix), I have needed to install the pipe insulation myself--plumbers I've hired have been more inclined to rip it off then to put it on. But it's not really a difficult skill to learn. The system design is also a skill that needs to be learned, but that doesn't necessarily need to be somebody local, and if we are serious about efficient electrification of heating it's worth getting people wrapped up on doing that correctly.

    Maybe that could be a topic for a future podcast--have someone from chiltrix as a guest, and/or someone who has designes one of those into a project.

    1. Jon_R | | #13

      Conceptually, a Chiltrix using fan coils and a multi-split are very similar - water vs refrigerant should be about the same in terms of design/install complexity. But there is a familiarity/support issue.

    2. jameshowison | | #14

      Is there a defrost cycle on the Chiltrix? The Mitsu unit's defrost cycle is much noisier than they advertise, we had to pay to move the units away from outside bedrooms.

      I wondered if an air-to-water based system can use the previously created hot water to do defrost?

  8. jameshowison | | #15

    Is there any effort to change the "heating/cooling" load to a line-graph visualization of expected load through the year (vertical axis load, horizontal axis day-of-year). Then the operating range of each unit could be drawn as a shaded zone across that curve (changing with outdoor temperature). The parts of the load curve that poke above the shaded areas would be the periods when the system had insufficient capacity, the parts when it was below the shaded section would be when it would be short cycling.

    If one could do that room by room then one could identify rooms likely subject to over-heating due to bleed from refrigerant flow when one unit calling for heat but others not.

    The mitu software could/should do this, but it can't even identify minimum capacity for head units.

  9. jameshowison | | #16

    Could someone lay out the process needed to understand whether a system has these issues? Can the data be gathered from the units themselves, or do you need to install something on the electrical lines? I guess I'm asking what's needed to calculate actual in-service CoP?

    1. charlie_sullivan | | #17

      Directly measuring in-service CoP is hard with an air-source system. The manufacturer could develop a model that would allow estimating it from variables the control system has access to, but it would only be an estimate.

      If you log electric power consumption you can certainly see when short cycling is happening. So you get a qualitative idea of how well it's doing. But you don't know COP without a thermal measurement too.

      Thermal measurements on the air coming out of a minisplit are hard. You can get a temperature, but not all the air is at exactly the same temperature, and measuring the air flow is even harder.

      If you have water carrying the heat to or from the heat pump, it's much easier to measure temperature and flow rate. If you have those either on the source side (in a ground-source heat pump) or in the output side (in a hydronic heat delivery system) you can get reasonably precise measurements of COP reasonably easily.

    2. Jon_R | | #18

      > what's needed to calculate actual in-service CoP?

      IMO, an accurate way to determine this for heating is to run electric resistance heat on alternate days and meter both separately. Over a month or more, there should be little error.

      1. jameshowison | | #19

        Sadly, neither way will work, since I don't have either resistance heat nor ground-source or hydronic delivery. Does it work to know the heating/cooling degree days and electric power consumption for some period (e.g., a month). That assumes some consumption sensor on the relevant circuits.

        Frustrating to me, really, that they are allowed to advertise particular CoP numbers but they are not verifiable in installations.

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