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Green Architects' Lounge

Ventilation for Your Tight House — Part 2

Our guest, Sonia Barrantes, continues the conversation about the best options for a healthy, comfortable ventilation system

Image 1 of 4
Adam Cohen and Wolfgang Feist check out the inner workings of the Air Pohoda ERV. According to the manufacturer, this ERV can operate continuously at cold temperatures without the need of an electric resistance heater.
Image Credit: Photos #1 and #3: Air Pohoda
Adam Cohen and Wolfgang Feist check out the inner workings of the Air Pohoda ERV. According to the manufacturer, this ERV can operate continuously at cold temperatures without the need of an electric resistance heater.
Image Credit: Photos #1 and #3: Air Pohoda
An installed Zehnder ComfoAir 350.
Image Credit: Photo #2: Alex Willson
The Air Pohoda ERV requires a bit more space than your typical ERV. Working in tandem, a pair of Lunos through-wall ERVs are excellent for discrete, small, or hard-to-reach spaces.
Image Credit: Photo #4: 475 High Performance Building Products

Our conversation with Sonia Barrantes continues. (If you missed it, here is a link to Ventilation for Your Tight House — Part 1.)

We’ve come to realize that we all want simple rules of thumb to guide our design process. Unfortunately, there isn’t a rule of thumb for everything and we’re going to have to rely on some common sense, good advice, and good old-fashioned engineering to get this balanced ventilation system right.

Our cocktails are refreshed and we’re ready to go.

The highlights:

Hot products on the market. We talk about some examples of the latest ERV equipment out there, our experience with them and our excitement about the ones we haven’t tried yet (Lunos, CERV, Air Pohoda, Zehnder).

Demand control ventilation. A feature that is new to the ERV market. These guys know when you need more fresh air and when you don’t.

How many cfm? Sonia enlightens us on her rules of thumb for providing quantitative ventilation.

Continuous commissioning. Is the system you installed really performing as it should?

Maintaining balance. What do you do when the Viking range hood kicks on? How do we handle the imbalance? Sonia enlightens us.

A six-digit idea. Can’t we have residential dryers that exhaust and supply directly with the outside air?

When should you hire a mechanical engineer? Sonia shares the idea that perhaps there are levels of service that an engineer can provide that are tailored to your needs.

Multifamily buildings. When the scale increases, how do we adapt these same ideas? Autonomous units or a consolidated system?

As always, Phil leaves us with a song that you should be listening to in studio. For this episode he chose one that everyone on your block will listen to when you play it: “Dimed Out” by Titus Andronicus.

Thanks for tuning in. Cheers!


Chris: Hey, welcome back everybody! This is the Green Architects’ Lounge. This is probably Part Two of the Ventilation podcast.

Phil: It definitely is Part Two.

Chris: All right. It’s definitely Part Two. We’ve re-freshened our drinks, we’re back at it, we’ve got Sonia Barrantes… (I’m sorry!)

Phil: Barrantes!

Chris: Barrantes.

Phil: Is that too much of a roll?

Sonia: No, that’s great.

Chris: Barrantes. Barrantes. Barrantes. Right.

So, Sonia — now, let’s see — we’ve covered HRVs, ERVs, whatever… Oh, we were going to talk about different kinds of ERVs and HRVs. Do you want to do that before we get into anything else?

Phil: Yeah, let’s talk a little bit about that.

Chris: Off-mike, we started talking about all the different products we know. Phil, you started with the Lunos.

Phil: But, I mean, just to generalize: some of the products that we typically spec…

Chris: Yeah.

Phil: Sonia mentioned a product called Zehnder – which is a favorite. [See Image #2, below.] It’s a favorite of the Passive House crowd, because it’s high efficiency and low energy. Energy efficiency is a big deal.

Chris: And they’ll come out and commission it themselves. You know, Zehnder will come out, and you can get a high level of confidence with that.

Sonia: They also do designs. So, if you send them your floor plans…

Chris: Right!

Sonia: …they will locate supply and exhaust…

Chris: So you don’t need an engineer!

Sonia: …with their rainbow vision, they will place everything for you.

Chris: Exactly.

Phil: They’re not the cheapest system.

Chris: Nooooo.

Sonia: Nope.

Chris: But you know, you get…

Phil: Well, put it this way: they’re not the cheapest system, but they are the most expensive.


Chris: Well said. Well said. And there’s a slew of others, from Venmar to…

Phil: Venmar is a really good system – we use that a lot. And it’s got pretty low energy.

Sonia: RenewAire is good.

Chris: RenewAire. Is UltimateAir – is that one? There, that was one.

Sonia: There’s also Fantech.

Phil: Fantech is more affordable, higher loads.

Chris: American Standard has some.

Sonia: Nah.

Chris: [He laughs.] Enough said!

Sonia: Yes.

Chris: They’re not the most expensive.

Sonia: No.

Chris: There you go.

Phil: There are a few, fairly new players on the market. Air Pohoda is one. [See Image #1 at the top of the page.] Chris, you’ve just specified that recently.

Chris: Yeah. I’m kind of excited about this, Phil. So, Air Pohoda is a pretty cool device. It’s bigger than your other ones. [See Image #3, below]. So, hey architects: you’d better plan for that.

Phil: Bigger is better.

Chris: Bigger is better, that’s what I’ve…

Sonia: That’s what she said.


Chris: I’m so glad you said that, Sonia.

But, like a Zehnder, the main trunks of supply and return come off the unit and they go to a manifold and then are dispersed in little Zehnder-like tubes, which are — my rule-of-thumb — (we’ll talk about rule-of-thumbs later but) — I always have 12 cfm per tube. That’s just stuck in my head – tell me if I’m wrong later.

Sonia: That is correct.

Chris: All right, yeah. Sweet! But, the cool part about this unit is that it’s got these baffles in it.

You’re going to talk about the Lunos probably in a little bit, where it reverses the flow of the air. So, one of the problems that you have (let’s just pick a Venmar, you know, your typical Venmar unit): if it’s zero degrees outside and it’s been running for a while, well that core is going to get cold, very cold – in fact it’s going to freeze up on you. And the unit detects that and so it runs in recirculation mode. It shuts off its connection to the outside and just recirculates interior air through the core until it thaws out. That’s a little bit of an energy loss in having that.

The Air Pohoda has this very complex series of baffles within the core so it will actually reverse the flow of air through the core without reversing the supply and return in the house. So, it will – without you knowing it – it will just switch direction through the core to keep it the right temperature. So it can operate continuously down to super, super cold temperatures. It also has different filters in it. If you imagine an HRV filter as being something impermeable, like metal – a bunch of metal straws put together so close that they are imparting their heat on one another. And in an ERV, that filter is like Tyvek, you know, where that moisture…

Sonia: It’s a heat exchange, although not a filter.

Chris: A heat exchange! Thank you, Professor. Doctor.


And so the Air Pohoda has both of these. It knows the relative humidity of the air going in, going out, and coming in from the house and being supplied to the house and it will switch to an HRV filter or an ERV filter. It will switch direction as it sees it and it will link to your sleek new iPad Touch device that you can go in and check on it whenever you want.

So, yeah. It’s really neat.

Phil: It’s not cheap.

Chris: It’s not cheap. And it’s from the Czech Republic.

Phil: Efficiency-wise, isn’t it like a 96% efficiency?

Chris: Well, yeah. It’s one of those classic sliding scales. It depends on which test you look at. Basically, we’re seeing it – it’s definitely 90+. But, you know, in the last house that we just did – and it’s the one and only install I’ve ever done of it – the low end is low. A little too low. We need to bump it up. But the cool part is that the software – you can just tweak it (I could tweak it if I wanted, which is scary!). You know, it’s not that simple. I shouldn’t say it’s that simple. You need to know what you’re doing. But, there you go. Air Pohoda — boom!

Phil: So there’s this other new product – it’s called the CERV – Conditioning Energy Recovery Ventilator?

Chris: Right, which is exciting!

Phil: It’s super-exciting; it’s the magic box that they’ve had overseas for years. It’s kind of like a first chance that us Americans have had to use something like this. And honestly, I don’t know that much about it. I’ve talked about using it with another builder. They’ve only sold about 50 units so far.

It essentially uses an air-source heat pump. And it can provide conditioning up to about 8,000 BTUs – which is cooling to about 5,000.

Chris: That’s sizable for your Passive House – if you’re in a tight house – especially in our cold climate. Geez! That’s all you would need.

Sonia: So that’s like the American version of the Nilan.

Phil: Right. Right. Right.

Chris: Nice. And we’ll put a link to that, because there’s an article in GBA… right? We’ll put that on the side. Check out the side over there.

Phil: It’s very interesting, because it basically modulates itself. It can turn on and off when it decides that you need fresh air. It can also detect contaminants in the air.

Chris: No way!

Phil: Yeah.

Sonia: From high-pollution events?

Phil: Yeah.

Chris: From high-pollution events. Is that what you just said?

Phil: Yeah. So, stay tuned for this. It’s pretty interesting. Right now they’re not super-cheap – I think it’s about eight to ten grand or so for an install.

Chris: But, that’s like a Zehnder, though.

Sonia: A little more.

Chris: A little more than a Zehnder.

Phil: A little more, but this one has conditioning.

Chris: Right, exactly.

Phil: So, it’s just another one that’s coming on. People are thinking about this stuff and there are some better answers out there on their way.

Sonia: Is that demand-controlled ventilation (because you said it turns on and off as you need it)? So, demand-controlled ventilation (I forgot) in my ideal…

Chris: Yeah?

Sonia: …it would have that. So, basically you’d have a CO2 monitor somewhere in the house. And what it could do is – and Zehnder will do this (actually anything that takes an input signal) – as the carbon dioxide goes up — you have a party; Phil’s making drinks…

Phil: That happens.

Sonia: It ramps up your ventilation. And then, everybody leaves and the carbon dioxide levels come down, and it ramps the ventilation down. So, demand-control ventilation is really…

Chris: Cool.

Sonia: …really cool.

Phil: From Green Building Advisor: “By all accounts, the CERV solves a lot of problems without using much energy. It provides balanced ventilation. It is smart enough to stop ventilating when the indoor air is clean. It increases the ventilation rate when the CO2 level rises — for example, when there are 20 people in the house for a party” — Thank you, Sonia. You’re reading Martin Holladay’s mind, here — “and decreases the ventilation rate when the CO2 level falls — for example, when the house is unoccupied.” Smart.

Chris: Smart. Man, the future’s bright, Phil and Sonia! It’s going to be great.

Phil: We’re going to talk about the Lunos. [See Image #4, below.] Sonia, what can you tell us about the Lunos?

Sonia: The Lunos is… basically, they’re paired ceramic core fans – so they’re great for retrofit because you can just put them right through the wall (there’s no ductwork) – and what happens is they take turns. So, one sucks, the other blows. And as you’re exhausting air, it heats up the ceramic core and then it reverses direction and the air comes in and the ceramic core…

Chris: Yes?

Sonia: …discharges its heat to that air. So, they tend to be fairly expensive for the cfm…

Phil: Mm-hmm.

Sonia: …but in the right application where you don’t have room to run ductwork, or it’s a retrofit like we’ve talked about.

Chris: Right.

Sonia: So, 15 (I think we said, Phil: 20 cfm?)

Phil: 15-20, yup.

Sonia: So, in an average-sized house (let’s say 2,500 square feet) with three bedrooms, you need about 105 cfm. And you said it’s about $1,000 per pair.

Phil: Right.

Sonia: So, it’s starting to get up to the level of a Zehnder.

Phil: Right. But then, you talk about no ductwork.

Sonia: No ductwork.

Phil: A grand, and you poke it through a few holes in the wall.

Chris: There could be some times where you really need to ventilate a space that you can’t get ductwork to, and that might be a way to do it.

Phil: I know.

Sonia: And it could be great for a room-by-room (you know, a bonus room or something like that) where you don’t want to run ductwork.

Chris: And that was a great segue to talking about: How much ventilation do you supply to your people or spaces, because you just rattled off a real quick (dare I say) rule of thumb?

Sonia: I had some cheat sheets.


Chris: All right. Sonia, what’s your rule of thumb for deciding “how much cfm do I supply?” How do I know what to do?

Sonia: Well, I start with the code. So, for ASHRAE it’s the number of bedrooms plus one is the number of people, and a certain cfm per person, and then cfm per square foot. So I just rattled off that rule of thumb, but I’ve got some numbers. So, a 2,500 square foot house with three bedrooms is 105 cfm. A 1,000 square foot house with two bedrooms is 53 cfm. So you can kind of see the levels. And this is for fresh air coming into the house.

Chris: Right. And this is continuous?

Sonia: Continuous fresh air.

Chris: Right. Right.

Sonia: And the thing to remember though, is: there’s the spirit, again, and then there’s the letter.

Chris: Right.

Sonia: So, if I wanted, I could put a closet in my house and bring in 105 cfm and exhaust that out of that closet and technically I’m bringing in the right amount of supply air into my house.

Chris: Wow. That is awful.


Sonia: It’s awful. It’s not getting to my occupants. So, when we’re talking about the mixing and Rob Aldrich, these are the things that we as architects and engineers need to make that we’re considering — that it’s not just “Oh, I’ve brought 105 cfm into the house.” Rather, it’s “where I brought it in and did it mix and is it going to be cold on my occupants.”

Chris: Right. “Is it the right thing to do?”

Sonia: Right. Because if it’s uncomfortable, they’re going to turn it off.

Chris: Yeah. And they’re going to call us and complain.

Sonia: Right.

Chris: And we’ll have to explain ourselves.

Phil: Yeah, it’s a good question: how many times do people install HRVs or ERVs and they just get turned off because they make a lot of noise or, if it’s the summertime, they turn it off and they forget to turn it back on when they close the windows.

Chris: Right. Or, I don’t know if you’ve ever (I’m sure you have, Phil) gone back and done some form of commissioning one way or another on a house, and… Like this last house with Air Pohoda I just mentioned, we learned, “Oh, we need to turn this thing up. You know? It’s not supplying what we thought it would. It just needs to be tweaked a little” – which is great if the unit can handle that.

How many times – I think about in the past – I have several houses where we installed an ERV, patted ourselves on the back (“Good job!”) and, you know, we followed the plan. But did we go back and do a Duct-Blaster thing; did we figure out exactly what the flow is out of each one of these ports going in and out of the unit? No. So… yeah? Is it okay? Probably.

Phil: Yeah, we’ve done that. Do you know how many times we’ve done that? Once.

Chris: Once?

Phil: Once. That’s the honest truth. We rely on the folks doing the right thing.

Chris: That’s awesome!

Phil: But we don’t really know if they are doing that.

Sonia: How did it turn out when you went back and checked?

Phil: Pretty good. We did pretty good.

Chris: Pretty good — but you made an adjustment maybe?

Phil: We made a couple of adjustments.

Chris: Yeah. So, I mean — I’m just throwing it out there. It’s a good thing to do. It’s a learning experience, especially when the numbers are lower than you thought — significantly, in a couple of places. You say, “Woah! What happened here?”

Sonia: Well, it’s really… I mean, it’s going to be a cultural change… right? So right now, you get your furnace checked every year.

Chris: Yeah, right.

Sonia: Well, eventually it might be that once a year, your HVAC specialist comes and checks the balance on your system. It’s called “continuous commissioning.”

Chris: Wow.

Sonia: You never stop checking on it.

Phil: Mm-hmm. But there’s got to be a fear factor involved to inspire people to do it. It’s either going to cost them money or health. Otherwise — “Eh, it’s fine.”

Chris: Or comfort. But that’s probably not going to happen.

Phil: No, I think they’ll be less attuned to comfort.

Chris: Yeah.

Sonia: Well interestingly — again, Lawrence Berkeley National Labs, in their study, they found that the health-related deaths due to air quality rivals traffic accidents and infectious diseases.

Chris: No!

Phil: Are you serious?

Sonia: Totally.

Chris: Wow!

Sonia: Those guys don’t lie.

Phil: Wow!

Sonia: They’re serious nerds, so…

Chris: Well, I think that was the fact of the podcast right there.

Phil: Yeah. Scared straight!

Chris: Scared straight.

Phil: Commission your HRV.

Chris: Yeah. And install them.

Sonia: Install it.


Phil: Install it in the first place. How about that?

Chris: Great. Let’s talk about this balanced system. Sonia, I’ve got this great house, and it’s airtight, and then I’ve got this awesome Zehnder / Air Pohoda system. And then I walk over to my 1600 cfm Viking range hood and I turn that sucker on and my eardrums pop. What do we do? How do we handle makeup air? How do we handle the imbalance that we, as users, put on this?

Sonia: Well the first thing I would do, as an architect, is try to convince my client that they do not actually need 1600 cfm.

Chris: Amen.

Sonia: Then if you lose that battle, by code, if you’re exhausting more than 400 cfm on your range hood, you have to provide a dedicated makeup air system that is actually interlocked with your range hood. So, that means when your range hood turns on, the makeup air system comes on. That’s from the International Mechanical Code 2009, so the International Energy Conservation Code 2009 also says that the mandatory [maximum] airtightness at 50 pascals is… anyone want to guess?

Chris: Three!

Sonia: Seven.

Phil: Seven!

Sonia: So, at 400 cfm at a 7 ach50 house is where you have to put in makeup air…

Chris: Dang!

Sonia: Well, now we have a 0.6 or a 1.0 ach50 house, I think, logically, you can understand that it’s going to be much less than 400 cfm where you have to worry about makeup air. And I think, later we’re going to talk about all the bad things that can happen when you negatively pressurize your house.

Chris: Right. Well, I think we’re there now.

Sonia: Really?

Chris: Yeah, let’s do it. Yeah, so let’s say that happens. I mean, it is very easy to find a range hood that goes 300, 500, 900, 600 cfm. Easy as pie.

Phil: Right. Especially if you’re putting a range over an island…

Chris: Oh yeah.

Phil: It’s going to suck from all sides.

Chris: Oh yeah. So let’s talk about it now.

Sonia: Range hoods kind of get amped up about it. Right?

Chris: Well, yeah.

Sonia: So here’s the thing: if you’re cooking with a gas range top, you need to exhaust to the outside.

Chris: Right.

Sonia: So this whole “Passive House recirc’ing range hoods exhausting from the kitchen” – if you have a gas range, you need to exhaust to the outside. (That’s because of all the nasty stuff that comes from combustion products of gas.)

Chris: Right. So, boom!

Phil: Don’t screw around.

Chris: Right. Even if you’re not even on this blog (energy-efficient in mind) and you just have a gas…

Sonia: Right. You need to direct-vent to the outside. So now if you have a tight house – whether or not you have a balanced ventilation system – you need a makeup air system that is separate from that. Right?

Chris: Gotcha.

Sonia: You can get away with, typically – so you need a minimum of 100 cfm – even in a tight house you can probably get away without using a direct vent. Passive House, you’re on the line.

Chris: Right.

Sonia: So 100, 150 — you’re probably okay. Once you start talking the 300 cfm fans need to really look at it. I actually had a friend, Andrew Calise — you might know him — Strategic Energy Group?

Chris: Oh yeah, yeah, yeah, sure.

Sonia: So he’s blower-door testing a development in the greater Portland area and he came to me and said, “These houses are so tight.” And they not Passive Houses, they’re just…

Chris: Tight.

Sonia: They’re just tight. He’s concerned that they’re not getting the natural ventilation. So we go in there and we’re doing a walk-around and I see that they have gas fireplaces, and I was like…

Chris: Woah!

Sonia: So they’re going with exhaust-only.

Phil: Mm-hmm.

Sonia: So, I helped him calculate – based on the tightness of the house – what they should set the exhaust-only bathroom fan to make sure they’re getting their required ventilation. Then I see this gas fireplace and I say, “Wait a second. We have this continuous, exhaust-only fan and let’s say they put in a 600 cfm range hood (or a 300 cfm range hood)…

Phil: Mm-hmm.

Chris: Oh man!

Sonia: So you have to think about that. So I asked him to run the blower-door and negatively pressurize the house equivalent to what the possible cfm could be…

Chris: Right.

Sonia: And then read the negative pressurization. So, put 350 cfm out the blower door and then see what your pressure in the house is. And so, there are different ranges for if you have a closed combustion — so, if you have a natural gas water heater that gets its air from the outside and it’s piped to the water heater…

Chris: It’s closed. Right.

Sonia: It’s closed, so you can go fairly negative.

Chris: Hmmm.

Sonia: If you have an open-combustion appliance (which could be a fireplace or a wood stove, anything like that), you can’t go much more than 5 pascals before you’re looking at potentially backdrafting very nasty stuff into the house.

Chris: And 5 pascals is, I mean, I’m just winging it…

Sonia: It’s not hard.

Chris: That’s like… you can get 5 pascals at like, maybe 70 cfm?

Sonia: It depends how tight your house is.

Chris: Yeah. I mean, that’s easy. In that scenario, what would be a solution? This kitchen hood exhaust, let’s say it cranks at 150 cfm (which is a respectable cfm). But if you’ve got a gas fireplace over there, then you’re going to say, maybe you should put in a vent with a damper that’s actuated when this thing goes on.

Phil: My sense is you’re not going to like the “just crack a window” idea.

Sonia: Right. Well, I mean, you can do that. But the one thing you can’t control is occupant behavior — right?

Chris: Right.

Sonia: So, the first owner might be like, “Don’t worry about it. I’ll crack a window every time.” But then, the next owner may not. This is actually – if I were forcing the situation – I might put in a trickle vent. Right? Do you agree?

Chris: Mm-hmm. Yeah. Yeah.

Phil: Mm-hmm.

Sonia: And once you get above certain cfm’s you actually have to power the air in… right?

Chris: Mm-hmm.

Sonia: So sometimes people bring the air in the basements…

Chris: Yup, that’s right.

Sonia: So it mixes before it comes up. All I can say is the way you solve the problem — you have lots of ways to solve the problem, but you have to be aware that is a problem.

Chris: Yeah.

Sonia: That could be a problem. So, you’re building super-tight houses; you know, there’s the Viking range hood; 1200 cfm; 300 cfm. You need to think about it.

Chris: Right.

Phil: There’s a very simple answer to this.

Chris: Don’t put them in.

Phil. Yeah. Just eat out every night.


Phil: Or order in.

Chris: Is that what you do?

Phil: Order in is okay, too.

Chris: Yeah. Stop cooking!

Phil: That’s what the Germans do.

Sonia: Well, now you can go to the electric cooktop.

Chris: Yeah, exactly. I was going to say…

Sonia: There’s still some products, but you put a recirc’ing range hood, you get the grease and the smoke out, and then you let your balanced ventilation system…

Chris: Handle the rest.

Sonia: …handle the rest.

Chris: And that’s a great Passive House strategy.

Phil: Right. I mean, do Passive Houses use gas to cook?

Chris: No! No. Absolutely not.

Phil: Right. So you just don’t see it.

Chris: Does not happen.

Sonia: So, if someone has their heart set on a gas cooktop, you need to direct-vent and you just need to behave appropriately.

Chris: So, I was at a conference – I think it was the Passive House North America conference – and I forget who was talking, but they referenced a guy from Zehnder who was in the room who had this idea. Maybe you were there?

Sonia: Probably Aubrey Gewehr, maybe?

Chris: May have been, may have been.

Sonia: I don’t know if I’m butchering that phonetically.

Chris: Maybe. But whoever this genius was, he came up with the idea of taking one of those downdraft ranges (you know, the Jenn Air whatevers) and just hacking that. Using that same ductwork, he put an actuator on it and just made it so when the hood goes on, the actuator opens. So, the air comes out – whoom! – right there. The makeup air is coming into the house at the Jenn Air location right next to the burners, so it just goes right under the cook surface and…

Sonia: So it just cools your food off as you’re cooking.

Chris: Right!

Phil: I’m pretty sure that there’s a product…

Chris: There should be!

Phil: …that Zehnder makes that’s not available here.

Chris: Oh yeah?

Phil: Yeah. I know a little bit about it.

Chris: Oh, you’re saying this like you’re…

Sonia: …being so coy!


He’s being coy about a fan.

Chris: I think he is! Well, Phil is “in the know” with the Zehnder higher-ups.

Phil: That’s all I can say at the moment.

Chris: He’s in the inner circle. Well, you don’t have to share with us. I’ll turn off the mike and you can tell me all about this…

Phil: Okay, I will.

Chris: …super-secret product.

Phil: Perhaps. Perhaps.

Chris: That’s fantastic. Yeah, and while I’m on it, I have a “hot zig” or a six-digit idea – which we have every once in a while.

Phil: Zig away!

Chris: It’s probably a hot zig, because I don’t think you could make six digits off something like this. No! You know what? It’s a six-digit idea for the first company like GE or Whirlpool or one of those guys. I mean, how simple would it be – you’ve got a dryer that exhausts – why not have a dryer that also supplies and exhausts to the outside? I mean, it’s just its own contained air unit, so you turn it on and it doesn’t affect your envelope — your air volume. You can seal the ductwork and just, it’s its own thing. Wouldn’t that be genius?

Phil: That doesn’t exist?

Chris: No!

Phil: It feels like it should.

Sonia: It exists on the commercial.

Chris: Does it?

Sonia: Yeah.

Chris: Oh, all right.

Phil: And is somebody making six digits from it?

Sonia: I’m sure they’re making six digits on it.

Chris: All of my six-digit ideas are already…

Phil: So you’re talking about, essentially like, a sealed wood stove or a sealed fireplace…

Chris: Yeah, why not a sealed dryer?

Phil: …that has an outside air intake.

Chris: Yeah. I’ve had a number of residential projects where it’s time to choose the dryer. “What dryer should I choose?” — and, you know, there’s always a certain number of cfm that that thing is going to eject from your house.

Sonia: I’m actually super-excited because Jake found a heat-pump dryer. Have you heard of these?

Chris: Oh yes!

Phil: Yeah.

Sonia: It won a technology award last year.

Chris: Nice!

Phil: I heard they’re pretty cool, but they take a really long time to dry.

Sonia: Well, the condensing dryers definitely take a long time to dry.

Chris: This is different.

Sonia: I think the heat pump is different.

Phil: Oh, okay!

Chris: I’m very excited to hear about that.

Sonia: I listened to your Allison Bailes podcast…

Chris: Yes, yes.

Sonia: …and I almost drove off the road, I was so excited because he mentioned the second law of thermodynamics which happens to be a favorite of mine as well.


Sonia: So, as soon as I get some free time, I’m going to look at that from a second-law point of view…

Chris: Nice!

Sonia: …because, if you’re stealing heat from your Passive House to dry your clothes, then it’s not necessarily a good thing. But the potential is really there.

Chris: Right. We must have been talking about my cold-climate refrigerator idea at that point.

Phil: Yeah, right.

Sonia: I don’t know. I’m not sure.

Chris: That’s all right. We won’t go into that. Later, you guys will have to plow through the archives to find that idea. But anyway…

Phil: You have a question right here, Chris: “Wood stoves and fireplaces – are we still doing those?”

Sonia: We are.

Chris: Yeah, we’ve got clients who want them, and I’m in that position where we are heavily persuading them not to when it’s a super-tight house.

Sonia: Have you been successful?

Phil: Well, we’re going to fight here.

Sonia: Because, I’ve lost that fight.

Chris: Well, I’ve lost that fight.

Phil: Why say no? They’re wonderful things.

Chris: I feel like we’ve had this conversation on the podcast before.

Phil: It’s such a great elemental thing to have a fire in your house.

Chris: Yeah, I am a caveman inside and I want a fire.

Sonia: A caveman with gray hair.

Chris: Yeah.

Phil: Yeah.

Sonia: Yeah.

Chris: Yeah, thank you.

Phil: Cavemen never got gray hair because they never lived that long – is that what you’re trying to say? Back in the day?

Sonia: I just don’t think they had gel.

Phil: Oh, okay.

Chris: I think she said “great hair,” not “gray hair.”

Sonia: Yeah — great hair.

Chris: Thank you, Sonia.

Sonia: You’re welcome.

Chris: Up yours, Phil.


Sonia: The thing to think about is, what are they building the house for?

Chris: Right.

Sonia: If they are telling me they are building an energy-efficient house and they put a fireplace in, I’d be like, “Well, it’s not energy-efficient, but if you want a fireplace, okay.”

Chris: Right.

Sonia: So, Zehnder – once again, I’m partial to Zehnder – they have what’s called a “fireplace protection mode.”

Chris: Wow!

Sonia: So, what happens is: if your fireplace starts to negatively pressurize your house (because it is pulling combustion air)…

Chris: Right, it’s drafting.

Sonia: …the Zehnder will stop sucking air out of the house.

Phil: Really?

Sonia: It does some magic thing and basically you tell the Zehnder on the control panel that you, in fact, have a fireplace or a wood stove and it will monitor the pressurization of the house and it will stop sucking. [Editor’s note: This description by Sonia Barrantes isn’t quite technically accurate. For further information on this issue, see Comment #1 by John Rockwell on this GBA page.]

Phil: I think that’s pretty brilliant!

Chris: I think that’s the second biggest fact of the podcast right there.

Phil: That’s great.

Chris: That’s nice.

Phil: That’s a five-digit idea, at least.

Chris: Oh yeah, at least! So, that’s wonderful to know, and I wonder if other ERVs will pick up on that.

Sonia: And I don’t know that other ones don’t. I just know that Zehnder does.

Chris: Right. So what do you think the cfm output of a fireplace is? I bet somewhere in the archives at GBA, someone brilliant has said that. Martin probably knows. If he were here… [Editor’s note: an open wood-burning fireplace consumes between 200 and 600 cfm of room air.]

Phil: He knows everything.

Chris: Yeah. But, again, there’s another point where we have to provide makeup air usually. But you’re thinking you can use the Zehnder?

Sonia: Oh I’m not saying you can use the Zehnder to provide the makeup air, because it’s balanced ventilation — so, by definition, it doesn’t provide makeup air. Right?.

Chris: Interesting. Well see, I thought – okay, this is a great conversation! – ERVs can go out of balance to a certain degree (which, I thought the rule of thumb was half of their rated cfm). So, if I am running the dryer, if the dryer is ejecting at 50 cfm, then my ERV can probably handle that (it’s just supplying more than it itself is exhausting.). It is using the dryer to do that exhaust so, its efficiency is going to go down. The air is going to be closer to the outdoor temperature as that happens. So, a dryer is not as crucial if your ERV can handle that bump. But, if I am running the dryer and I turn on the kitchen exhaust or and I have a fire in the fireplace, then I might be 150, 200, 300 cfm and that may be beyond the entire capacity of the ERV. So, you need makeup air for those things.

Sonia: I don’t know off the top of my head what the limits are for the ERV and, if you think about it, it is a hole through your wall.

Chris: Yeah.

Sonia: So, even if none of the fans are running, you have a direct connection to the outside.

Chris: Right.

Sonia: So, if you pull hard enough, you’ll probably be able to pull makeup air through the ERV.

Chris: Gotcha.

Sonia: But, we shouldn’t design to that.

Chris: Right. Right. So, if we have a client – I have that client who says, “I want a wood stove; I’m going to have a wood stove. I have this wood lot. I want to (just whatever) — I want that caveman feeling of fire on these days.” Even though I say, “Well, you’re going to cook yourself out of your room, but that’s fine if you want to do that.”

Sonia: In that case, I would probably consider some sort of makeup air damper that opens. You know, you could put a temperature sensor in the chimney…

Chris: Oh!

Sonia: …and when it gets above a certain temperature, it opens a damper. But, keep in mind that the more complex…

Chris: Right.

Sonia: …the more you have to maintain, and the more things break.

Phil: Mm-hmm.

Sonia: I might do something like that but, bring it into the house somewhere where it’s not going to be a direct draft on somebody.

Chris: Right.

Sonia: Somewhere where it can mix before it gets into the house.

Phil: So, Sonia, at what point should someone hire a mechanical engineer?

Chris: Ooh, that’s a good question!

Phil: Do you need a mechanical engineer in the house?

Sonia: That is a great question.

Chris: Well, yeah. Because houses… I mean, if you and I were doing a commercial building, we’re hiring a mechanical engineer.

Phil: Yeah, we’ve got to have one.

Chris: It’s just way too much to handle. But tons of houses get built without…

Phil: Yeah, most of them. And you know, Chris and I, we consider ourselves like we know what we’re doing for the most part, and we rarely hire mechanical engineers for houses.

Chris: For houses.

Sonia: We’re hiring Sonia for this bigger house that we’re doing right now. It’s very complicated, so we’re going to need her help. But it’s an interesting question: What’s that threshold? How often do you get hired? When do you recommend people hire you?

Sonia: That is a great question. I think the first thing is it really depends on your team. So, if you as architects are really sharp – Kaplan Thompson and Briburn – and you guys are in this world — so you recognize when it gets beyond your comfort level. Right?

Chris: Right.

Sonia: If you have a really great contractor who can do the energy modeling and do the equipment sizing and do the distribution systems, and you have a good relationship, and they do good work (their ducts are sealed, things are installed correctly), then I would feel more comfortable going without a mechanical engineer. But, depending on the level of your team, and also we’re going through this paradigm change in the building industry…

Chris: Right.

Sonia: …where now, all of a sudden, mechanical ventilation is being put into residential homes (which we never did before) – especially up here. We don’t do forced air. Right?

Chris: Yeah, right.

Sonia: We have 4% openable windows and we’re good.

Chris: Right.

Sonia: So I think, as we’re going through this change, it would be more likely – and it could be a mechanical engineer who just helps you at the beginning: does some initial energy modeling, helps you with the equipment sizing, gives you some design tips to consider.

Phil: Mm-hmm.

Sonia: And then maybe checks in again later in the design to make sure everything looks pretty good. And then it goes out to bid and maybe the engineer even reviews the bids from the contractors.

Chris: Nice.

Sonia: So, I think it’s not necessarily like you get a full design – you don’t need a mechanical engineer to run all your ducts or anything like that – but they should just kind of look over your shoulder and raise red flags and help with some basic calculations.

Chris: Nice! Good advice. All right, what else am I missing? Anything? What do you want to talk about, Phil?

Phil: I know we talked mostly about houses, but what about multifamily?

Chris: Right.

Phil: How does the equation change here? It certainly gets more complicated. And for a lot of architects doing houses, the next step is starting to do a couple of…

Chris: Larger, multifamily.

Phil: Do the systems change dramatically?

Sonia: They’d get bigger, potentially. Right? So, I mean, we can compartmentalize and we could put a single-family ventilation system in every apartment. But then Chris goes, “Holy [bleep], Sonia, look at all those building penetrations! That looks awful! And my amazing siding design is now compromised.”

Chris: Right! Riddled.

Sonia: Riddled. It looks awful.

Chris: “Can we consolidate this and save some…?”

Sonia: So, the things you get when you have a single-family system is you have boost mode. Right? So, when you have a high-pollution event in the bathroom…

Chris: Yes?

Sonia: …you can boost it. But, if I’m in a multifamily with a single unit that’s serving multiple homes, I don’t get that option, okay?

Chris: Right.

Sonia: Similarly, if I’m turning it down – so let’s say I have one apartment that’s full of kids and they’re cooking a lot of pasta.

Chris: Yes.

Sonia: And in the next apartment I have a single, slightly older person. Right?

Chris: Yes.

Sonia: Well, they all get the same ventilation rate. I mean, you could rebalance the system so that they get less. So either you turn down or turn up as a building and back down. And so those are things that you have to consider. I would say, on the balance, it makes sense to go with a consolidated system mostly for cost reasons. And then you just have to have a smart balancer and then – we talked about continuous commissioning – it really makes sense to go make and check it seasonally.

Chris: Gotcha.

Sonia: Or as tenants change.

Chris: But if I had a developer or a client (or whatever, a combination of that) and they were adamant that, “No, everyone’s going to have their own autonomy. Each unit.” It’s a condo or something like that. Then that would behoove the architect to design the exterior.

Sonia: Absolutely.

Chris: So that these features, these penetrations are thought about.

Sonia: Absolutely.

Chris: And are not missing from the elevations.

Sonia: And that’s a great point. If you have a homeowners’ association, if it’s a condo…

Chris: True.

Sonia: …who’s going to pay the electricity for the…?

Chris: Right. It can get complicated.

Sonia: It can get very complicated.

Phil: Mm-hmm.

Chris: That’s interesting. Do you think there’s greater efficiency overall – from a macro level, from an alien looking down at the planet, and looking at this one building saying, “What’s the better thing to do?”

Sonia: Well, ironically, in almost any other case, with larger systems you get economies of scale. But I have found (in my relatively limited experience) that with the larger boxes, you get lower efficiency. So, I’m not going to get 92% on 1200 cfm anymore.

Chris: Interesting. So, it might be more hardware, more product installed, but then it will each be at 90% as opposed to…

Sonia: And so now, when you’re getting those lower efficiencies, you’re probably talking about some sort of duct-mounted coil…

Chris: Mm-hmm.

Sonia: …to condition the air (to heat it up or cool it down when it’s really cold outside) because you’re getting that “too cool” air.

Chris: Gotcha. All right.

Sonia: That would definitely be a time to bring in a mechanical engineer – if you’re going multifamily.

Chris: Oh yeah. Oh yeah.

Sonia: That would definitely be the right time.

Chris: Oh yeah. If you’re an architect doing a multifamily without a mechanical engineer, “Phssssst!”

Phil: Scared!

Chris: Bush league!


You need to get your act together. We’re talking to you.

All right. I don’t know. Phil, you feel good? Sonia, do you feel good?

Sonia: I feel great! With another cocktail, I’ll feel better.

Chris: Hey! We can help you out.


Phil: I’ve got an idea: so let’s play a rocking song to close out the podcast.

[The episode closes with a song by Titus Andronicus: “Dimed Out.”]


  1. user-1123197 | | #1

    Kitchen ventilation comments, future prospects
    Chris, Phil, and Sonia, thanks for a great conversation. Really interesting and informative. I do feel compelled to comment on and take issue with some of the things you said about kitchen ventilation.

    A vented range hood should be installed for homes with every type of cooktop -- electric resistance, electric induction, or gas. It's not just the combustion pollutants that need to be removed, but also the pollutants generated from cooking itself. These latter pollutants are generated by electric cooktops just as much as by gas cooktops. Also, ovens should be vented.

    Non-combustion cooking pollutants can include fine and ultrafine particles, acrolein, and polycyclic aromatic hydrocarbons; chronic exposure to all of which carries non-trivial long-term health risks. Recirculating range hoods do not remove these pollutants. (there is one expensive model from vent-a-hood that claims to, but in general, they don't do much if anything). Here's a report that discusses this issue in not-quite-excruciating detail, if you're interested:

    We still have a way to go to calibrate our priorities for indoor air pollutants. In most houses, cooking is perhaps the most significant pollutant source from a morbidity standpoint, and it is not widely recognized as such.

    And finally one caveat about the range hood cfm numbers you quote in relation to depressurization. It's certainly a real issue that needs to be considered for each home. However, we've found that there are often (nearly always) significant discrepancies between the advertised cfm numbers that manufacturers quote for their products and the measured cfm in installed conditions. It's not unusual for the measured cfm to be 50% or even 40% of the advertised flow. Sometimes even less. Range hoods that are HVI rated have HVI (i.e. third-party tested) flows listed in the HVI directory or on the product itself. These numbers tend to be much closer to the ones that we measure (i.e., the numbers we measure are typically 80-100% of the HVI numbers).

    We're currently working on a capture efficiency test for range hoods, and have found that a well-designed range hood can remove 80% of cooking pollutants at 200 cfm for a typical 60,000 Btu/hr cooktop. If things go well, soon architects, engineers, contractors, and homeowners will have a new performance metric -- called capture efficiency -- to consider when choosing a range hood. Thanks for reading and thanks again for the conversation.

  2. Christopher Briley | | #2

    Cooked food for thought
    Thanks Chris! I always love comments that improve our blog and take the learning to the next level. Ever the student, I shall review your link and see what I can retain and incorporate into my ongoing work. I feel like this is an area that no one has completely figured out yet. It's a good reason to always cook on the grill outdoors and eat out at fancy restaurants.

  3. jhrockwell | | #3

    Our ERV/HRV Comfosystem DOES NOT monitor house pressure to detect if a wood stove is drafting or a clothes dryer is in use.

    In winter, to prevent any condensate from freezing in the exchanger core, some HRV’s intentionally go into a temporary imbalanced mode, ramping down the cold air intake while maintain exhaust fan speed. This depressurizes the house, and if a wood stove is in use, can result in back-drafting of combustion byproducts like carbon monoxide, a possibly fatal situation.

    Therefore, in houses with wood stoves and fireplaces, the frost protection mode described above MUST NEVER BE USED. Our ComfoAir units must be set to “open fire program” to override the default of imbalancing to prevent depressurization. This is done during the system commissioning phase (a service we provide).

    Instead, Zehnder provides an electric resistance preheater and/or a geothermal ground loop preheater for effective frost protection.

    John Rockwell
    Northeast Technical Sales Engineer
    Zehnder America, Inc.
    (603) 422-6700

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