How to Choose the Right Mechanical System
There's no one system that works for all projects, so how do you choose?
With any house, there are so many variables that influence the decision to choose one particular mechanical system over another: climate, house size, cost, local availability and cost of fuels and materials, and the lifestyle and preferences of the occupants. There is no “one-size-fits-all” system that we can reliably prescribe for all projects. Phil and I sat down over a good winter cocktail to share our views, anecdotes, battle scars, and wisdom on this important subject.
The Bobby Burns Cocktail: “We'll tak a cup o' kindness yet for auld lang syne.”
Demand: Let demand be your guide. After climate, the peak load demand of your home is probably the most important thing to understand and define. (This means you'll have to perform some energy modeling or at least a heat-loss calculation.) The chart developed by the Portland Building Science Discussion Group should help you identify the systems to consider.
Climate impact: Let your conscience be your guide. For some people, it's not all about cost efficiency. It's also about limiting emissions and the ecological footprint of the system itself. You may rule out oil and propane altogether.
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Cost: Then, of course, there's the cost of the system to consider. In the podcast, I mention the statistic that for most buildings (not just houses), the initial construction cost represents only 11% of the building's total cost. I had a chance to chase down where the statistic is from: Solutions for Energy Security & Facility Management Challenges by the Association of Energy Engineers. (By the way, the calculation only considers operating costs for the first 40 years.)
That makes the efficiency of your system highly important. But the consideration of the initial cost is an inescapable reality. To help with this, we've provided a chart that shares some rule-of-thumb numbers for the systems mentioned. Of course, costs vary widely among different regions and installers. With time, this chart will become meaningless as the market changes.
Cost offsets: Remember, the goal here is to shrink the mechanical system in order to save money to offset the added cost of building a robust envelope.
GSHPs and radiant floors: Do they still make sense? Sometimes, maybe.
Six-digit idea: Let's start importing (or better yet, manufacturing) the Magic Box here in the States.
Hot Zigg!: The Lunos e2 looks like a cool product that is approaching active ventilation in a whole new way. This is not an endorsement, as Phil and I haven't used them yet, but something we're keen to explore.
Phil's pick: The song of the episode is “I'm Not Talking” by A.C. Newman. It's a great album for the studio.
Thanks for listening. Cheers.
Chris: Hey, everybody, welcome to the Green Architects’ Lounge podcast. I’m your host, Chris Briley.
Phil: And I’m your host, Phil Kaplan.
Chris: We should start with an apology. Hey, everybody, sorry — we haven’t been recording much.
Phil: We’re going to pretend that people have been missing us?
Chris: Yeah. Sorry, folks. Today we’re doing how to choose a mechanical system. This is a conversation we have a lot with our clients. They want to know what the best one is for their house.
Phil: And there is a logical way to choose one. Chris and I have our systems — they’re not foolproof — but we’ve talked to some smart people and we’ll talk you through it.
[The guys jaw about Chris’s new partnership with Harry Hepburn, the possibility of having Bill McKibben on a future podcast, the NESEA conference, and this episode’s cocktail, the Bobby Burns.]
Phil: So how do we start choosing a mechanical system?
Chris: Let’s start with a chart. Imagine a building science discussion group filled with architects, raters, and builders. When asked to choose a mechanical system, each person chimes in with the best one. If you get anything from this podcast, it’s that there is no one solution.
Phil: Logic is only a small part of choosing a mechanical system.
Chris: “If that, then this” just doesn’t happen. So the building science discussion group decides to make this chart that shows when we consider the different systems based on the peak heating loadRate at which heat must be added to a space to maintain a desired temperature. See cooling load. — we’ve divided it up into 0 to 25,000 BtuBritish thermal unit, the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit in temperature—about the heat content of one wooden kitchen match. One Btu is equivalent to 0.293 watt-hours or 1,055 joules. , 25 to 50,000 Btu, 50 to 75,000 Btu, 75 to 100,000 Btu, and 100,000 and over — there are certain systems you look at, depending on peak heating load. Ductless minisplits for 0 to 25,000 Btu are a great choice. If it’s 75,000 or more, possibly you’d consider ground-source heat pumps. The chart has solar thermal, a woodstove for point-source heating — so you’d have to design for this one source of heat.
Phil: An open plan, compact.
Chris: And the downstairs communicates with the upstairs thermally. You’ve also got air-source heat pumps, which are fantastic for the lighter loads, and electric in general — electric boilers, baseboards — and of course, oil and gas boilers and furnaces.
I was talking to Marc Rosenbaum today, a mechanical engineer at South Mountain, and asked him how he chooses a mechanical system. Now that climate change has entered the conversation and has become a priority, let’s get away from fossil fuels. So, for Marc, it’s either electric or biomass. It’s a matter of heat load. If the heat load is rather high, 50,000 or more Btu, you’re looking at biomass. But some people may not want to schlep logs of wood to their wood boiler of woodstove, or they may not want to deal with pellets, which may not be available in their area. With lighter heating loads, you’re looking at electrical heat pumps — ground-source heat pumps or air-source heat pumps.
Phil: So, that’s it, we’re done! Cheers!
Chris: Well, with the ductless minisplits, it’s easy to take care of a lighter load house. Martin Holladay had a fantastic article in Fine Homebuilding.
Chris: We recommend that everyone read that. It also covers the system in a passive houseA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. — UltimateAir has a combination unit.
Phil: A hot water coil off the ERVEnergy-recovery ventilator. The part of a balanced ventilation system that captures water vapor and heat from one airstream to condition another. In cold climates, water vapor captured from the outgoing airstream by ERVs can humidify incoming air. In hot-humid climates, ERVs can help maintain (but not reduce) the interior relative humidity as outside air is conditioned by the ERV..
Chris: We’ve talked about a cold water coil for cooling. The thing is to trick the ERV into activating when there’s a demand for heating or cooling, not just fresh air.
Phil: We’re talking about a really low heating load: 10,000 Btu or even lower.
Marc has his ideas of what’s important, and so do we. Marc does not believe in sacrificing energy for looks. Aesthetics are important to architects in a different way than to engineers.
Chris: Clients are the same way. If they walk by a ductless minisplit cassette and think, “I hate that thing, the way it sits there on the wall and mocks me,” then it’s not worth it; it’s not going to work.
Phil: We all decide what’s most important for us.
Chris: Clients might just be looking at operation costs. They’re not trying to save the planet; they’re trying to save money. They’re trying to be the smartest kids on the block and have a really efficient house.
OK, Phil, let’s say a client comes to you and says, “I want a ground-source heat pump.”
Phil: Let’s go through the logical reasons about why we don’t use them. And the client says, “Yeah yeah yeah, but I still want one.”
Chris: And it’s not out of the question, but the conditions have to be right for that to be a smart choice. That does happen, but increasingly less.
Phil: We had another client who wanted electric baseboard. And it wasn’t a passive house. Close to it, but…
Chris: Just electric baseboard?
Phil: Just electric baseboard because they wanted a silent system. They were going to buy enough PVPhotovoltaics. Generation of electricity directly from sunlight. A photovoltaic (PV) cell has no moving parts; electrons are energized by sunlight and result in current flow. to offset it. But it wasn’t a great idea in terms of energy use.
Chris: I’ll share a battle scar. In the Redfern house — a little LEEDLeadership in Energy and Environmental Design. LEED for Homes is the residential green building program from the United States Green Building Council (USGBC). While this program is primarily designed for and applicable to new home projects, major gut rehabs can qualify. Platinum house with 40 grand of hardware on the roof in PVs and solar thermal, and demand is really, really low — we cheaped out on an electric boiler. It’s a hot water heater, essentially, for the boiler. It really bothered me. Why at that last moment did we cheap out? I understand the economics. It’s really not going to be on that often. It’ll be offset by solar. But we could have put three more panels on to get to net-zero.
Phil: Remember, panels were a lot more expensive then — 8 or 9 grand per kilowatt. It’s less than half that now.
Chris: Well, we could have gone with one ductless minisplit and spent only a couple thousand more than with this electric boiler. And yeah, if you compared the two, would the minisplit ultimately have paid for itself based on how little it would be used? But the efficiency of it would have been worth it for the planet. To go that far and then cheap out… But the happy ending is there’s now a woodstove in the house and the boiler never comes on.
Phil: So one of the things that bothers you about a house is the climate damage, the CO2.
Chris: That’s a big part of the metric.
Phil: But for some people, number 1 is cost. That’s always a conversation.
Chris: That’s right. You did this passive house for $135 per square foot, and still people said, “My God, that’s expensive for a little house.” We’re competing with houses already on the market.
Phil: If we can reduce the cost of mechanical systems, we can put the extra dollars into the envelope. At 15,000 Btu, you can get yourself an HRV(HRV). Balanced ventilation system in which most of the heat from outgoing exhaust air is transferred to incoming fresh air via an air-to-air heat exchanger; a similar device, an energy-recovery ventilator, also transfers water vapor. HRVs recover 50% to 80% of the heat in exhausted air. In hot climates, the function is reversed so that the cooler inside air reduces the temperature of the incoming hot air. and a pre-heater. Eight to ten grand for that system; that’s pretty awesome. If the Btu go up to 25 to 30,000, can we stay below 10 grand?
Chris: That’s really hard. You need something in there to handle the load. A ductless minisplit.
Phil: Minisplits work even when it’s 0 degrees outside. If you’re real disciplined and have a tight, compact house, you could do it. It would be ductless and cost 9 to 12 grand. That’s assuming you have an electric resistance water heater.
Chris: What if I’ve got natural gas right out there on the street?
Phil: A combo hot water tank and air handler — a ducted system, but the gas is right there. Ten to 12 grand. When would you use a hot water heat pump? Probably around loads of 50,000 or more.
At fairly low loads, if aesthetics are important, you go to ducted minisplits. You’re adding a good 10 grand, maybe not that much. People spend a lot of money on pretty. Minisplits still stay off of fossil fuels, which is nice.
Chris: What if my house is way bigger?
Phil: Then you go to the gas boiler. It’s still not a bad system when you have a big house that’s an energy guzzler.
Chris: The gas boiler can be 94 to 98 percent efficient. And still cleaner than oil.
Phil: In the higher loads, wood pellet boilers make sense. Ground-source heat pumps.
Chris: They’re still there.
Phil: We didn’t talk about radiant.
Chris: The more robust the envelope gets, the more costly the radiant system — the hundreds of feet of tubing, the circulator pumps in every zone, the computer components to monitor things…
Phil: A complex system, locked in concrete.
Chris: You’re talking about a $25,000 system, depending on the load and the size of the house. You’re talking about buying a lot of hardware for radiant floors. And if your house is really well insulated and the demand is so low — let’s say 15,000 Btu — then you have a boiler pushing fluids through a slab…
Phil: They break all the time. And it’s rarely going to get the call.
Chris: If the computer component is smart enough, it’s tweaked the temperature to be in the comfort zone, which is below your body temperature.
Phil: And then it’s going to be cold underfoot.
Chris: Or not warm. You won’t even notice it’s on. But if you have a really leaky house, radiant might be a great system.
Phil: It comes down to what’s important to you. We advocate staying off fossil fuels, keeping loads down real low, keeping it simple.
Chris: We’re trying to get rid of this component.
Phil: The best mechanical contractors try really hard not to take your money.
OK, time for a 6-digit idea: the Magic Box. It’s a combination appliance with an HRV, a heat pump and a water heater. Zehnder makes them. But we can’t get them here yet.
Chris: What about the Lunos? They look like dryer vent type penetrations. It’s a way to handle the ERV without an ERV. It’s two holes in the building with ceramic filters in them. The filter is the temperature of the air. One opens up and breathes in. Then the other hole in the wall lets the breath out.
The better our buildings get, the more we need better and better technologies to run those better buildings.
[The episode closes with A.C. Newman’s song “I’m Not Talking.”]
- Maine Green Building Supply, Portland, ME
- Kaplan Thompson Architects
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