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Green Architects' Lounge

Ventilation for Your Tight House — Part 1

Now that your house is tight, getting your ventilation system correct is vital

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This episode's cocktail: The Sweet Tart.
• 1.5 oz. vodka
• 0.5 oz. Aperol
• Juice from half a lemon
• A couple of dashes of citrus bitters

Shake with ice. Serve in a chilled glass. Garnish with a cherry.

This episode's cocktail: The Sweet Tart.
• 1.5 oz. vodka
• 0.5 oz. Aperol
• Juice from half a lemon
• A couple of dashes of citrus bitters

Shake with ice. Serve in a chilled glass. Garnish with a cherry.

Phil's homemade bitters collection.

In this episode we are assuming that you are preparing to design or build a super-tight house and you’re interested in the best way to provide fresh air for its occupants.

In the old days, you’d just “let the house breathe” [shudder]. But those days are long gone. A healthy house leaks, while an energy-efficient house controls how it leaks — and this episode is all about the latter.

With all of the advances in air-sealing methods, ventilation equipment, and the myriad of system choices available, what once was easy is now far more difficult. Luckily for us, we have Sonia Barrantes (a mechanical engineer, Naval flight officer, and fiddler) as our guest to help guide us through the decision-making process.

I should mention that we suffered some technical difficulties just prior to recording. As a result the sound quality is slightly off and there’s some audible background noise. But the good news is that we were able to record a decent episode and had a great time with Sonia (and Phil’s “Sweet Tart” cocktails).

The highlights:

Why ventilate? So your house is tight. That’s awesome from an energy-efficiency standpoint, but without proper ventilation, that can lead to some serious problems: comfort problems, humidity, the buildup of toxins, or particulate contamination.

Ventilation options. There’s a difference between doing right by code and doing right by your occupants. Sonia helps us navigate the difference and walks us through some the best options. Spoiler alert: We’re quite fond of a balanced ventilation system using efficient ERVs.

ERV vs. HRV. What’s the difference and which should you use? And when?

Placement of the diffusers and grilles. We talk about the best places to supply and exhaust. We also talk about the Coanda effect.

How efficient are these ERVs? A straightforward question with a wiggly answer.

Be sure to check out Part Two, where we cover the different equipment options and how to handle the imbalance that your exhaust appliances place on your system.

Thanks for tuning in. 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. Hey Chris, good to see you… again!

Chris: Great to see you after a long hiatus! We apologize, listeners, but we have day jobs and families and it’s summer now…

Phil: And it’s summer in Maine. So, come on. You can’t expect us to sit in and record these things… unless we have a drink in our hands.

Chris: Which we do. Cheers!

Phil: Cheers!

Sonia: Cheers!

Chris: Oh, and that other voice: We have a special guest with us, and that is Sonia Barrantes.

Sonia: Barrantes.

Chris and Phil: Barrantes!

Phil: Welcome, Sonia.

Sonia: Thank you! I’m pretty excited to be here with the local “starchitects.”

Chris: Starchitects?! Thank you. Tell us about your background. We know you as a mechanical engineer who actually has built her own Passive House.

Sonia: Well, actually, my partner Jake kicked me off the job site about a year ago. He said I could come back when it was time to sand drywall. But he’s been watching some YouTube videos and he’s decided that is actually a critical job. So I don’t know when I’ll be allowed back in.

Phil: Oh wow!

Sonia: So, he’s actually built the house almost entirely himself.

Chris: But you’ve been integral?

Sonia: I would like to think so. Yes.

Chris: I would like to think so, because we have you on the podcast and we’d like to think that you were all over that. And you also are, or were, a fighter pilot?

Sonia: Well, actually… Jake was a naval aviator, a Navy test pilot. I was a naval flight officer. In Top Gun, he was Maverick and I was Goose.

Chris: Goose, I’m so sorry!

Sonia: No. Everyone needs a Goose.

Chris: Yeah, exactly. I just feel bad for Goose. (He dies.) Spoiler alert!

Sonia: We didn’t fly Tomcats. I mean, I have flown Tomcats, but I didn’t do it for my actual jet.

Chris: That’s cool, though! So you’re not your typical mechanical engineer — which is why you’re on this podcast.

Phil: Yeah, the reason Sonia’s here is that Chris and I have both been thrilled to work with Sonia over the last couple of years.

Sonia: Thanks, guys.

Phil: Because she’s a mechanical engineer who gets it — which is really rare. It really is. I mean, architects — there are too many of us around already. I’m sure you can agree; we’re a dime a dozen.

Chris: Oh yeah, this room is already crowded.


Phil: There’s already one too many.

Chris: One too many. Two is one too many.

Phil: One of us has got to go, Chris.

Chris: Exactly, and we’re going to decide it by who drinks the most.

Phil: Don’t dare me.

Chris: It’s on.

Phil: It’s on. Let’s wait till after we finish the podcast.

Chris: Okay, so you guys probably already know — because we have a mechanical engineer on — we’re going to talk about ventilation. This podcast is going to be titled something like, “Ventilation for Your Super-Tight House.”

We are assuming that you’ve come to us and have already listened to our air-sealing podcast and are already building that really tight house and you’ve probably gotten really far in your design. And then somebody says something about putting in a wood stove — “so how do you want to provide makeup air for your stove?” — and then you realize, “Oh my gosh! I really have to learn a lot about ventilation.”

It’s not just handing it over to your typical engineer who’s going to say — you know how they do that, Sonia — they just do some neat calculations, slap some mechanical equipment in there, and…

Phil: Oversize it just to cover their asses…

Chris: Exactly.

Phil: …and move on. Right.

Chris: Right.

Phil: But, we’ve mentioned “ventilation” in just about every podcast (I would imagine) and we’ve never dedicated an entire podcast to it. So we’ve decided to kick it up a notch. And Sonia’s here to help us sound smart because Chris and I struggle with that.

Chris: Yeah, especially after a drink or two.

Phil: Yeah, can we talk about the drink?

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

Chris: So, let’s start with the fact that you have a tight house. Right? How tight is your house, Phil, in this scenario?

Phil: That’s a really good question. We talk about 1.0 ach50 (one air change per hour) and that’s what we have in our specs, typically, at 50 Pascals. When you’re going for Passivhaus, you go to 0.60, often we’ll do that as well. We don’t always do Passivhaus, but those numbers are kind of what sit in our specs typically.

Chris: Yeah. You’re basically shooting for as low as you can go, pretty much. I mean, there’s not a “too tight” if we’re going to ventilate this thing.

Phil: Right.

Chris: We’ve said this before: 25% of your heat loss, usually, is through your leaks in your house – that’s with your windows and doors shut. Not just through conduction through your envelope, but through convection leaks and all that stuff.

So, let’s say you’ve got your tight house at 1.0 ach50. And so now you have issues, maybe, if you’re not ventilating – because now, every little odor — What do we call those, Sonia?

Sonia: High pollution events.

Chris: If you invite your flatulent friends over and they have a high-pollution event, that sucker could linger for a really long time. You get your general stuffiness, which…

Phil: I’m glad I didn’t make my bean cocktail tonight.

Chris: I’m impressed you have a bean cocktail!

Phil: Nah, I don’t have a bean cocktail. I have a bean bitters, but no bean cocktail.

Chris: So, you get high humidity. If you’re not ventilating, you’ve got high humidity (which is also stuffiness). And with humidity, you can get damage to your walls (internal to the assemblies). You can get mold and mildew. And, any toxins in your house that you’ve brought in – formaldehyde in your cabinetry, or VOCs from your paint sealants, or radon…

Phil: What else did we forget, Sonia?

Sonia: You’re forgetting particulate matter, like from cooking. Cooking gives off all kinds of chemicals. LBNL (Lawrence Berkeley National Laboratory) did a study on that, and a gas cooktop gives off some really nasty stuff.

Chris: Just sitting there.

Sonia: Just sitting there.

Chris: Natural gas.

Sonia: Yup. And the particulate matter that is generated by cooking, if it gets down below – this is totally nerdy! – 2.5 microns, you can actually inhale that and cause damage to your lungs. There’s a lot of…

Chris: Grease-laden vapors.

Sonia: Yeah, absolutely.

Chris: So, we have to ventilate, right? We have your tight house. We have to do something – not only for comfort, but for health.

Phil: So what are our options?

Chris: Sonia, what are our options? I come to you as an idiot architect. I’ve got this super-tight house and people are telling me that I really ought to ventilate. What do I do?

Sonia: Well, you have to separate it between your options by code and your options by what really is effective.

Phil: Right. So, what does code say? What’s the baseline?

Sonia: So, code says – it depends on your state, obviously. Most states have what’s called an equivalency for ASHRAE 62.2. So you can either comply with the International Mechanical Code, the residential code, or ASHRAE 62.2. So, in Maine we have the option to comply with ASHRAE 62.2.

ASHRAE 62.2 requires a certain amount [of ventilation] based on the number of bedrooms and the square footage of the house. You have to get that much ventilation air – fresh air – into the house. You can either do it by exhaust-only – which, I think, we’ll talk about.

Chris: Right.

Sonia: Or you could do it by providing supply air and exhaust.

Chris: Right. A balanced system.

Sonia: It could be. It could be balanced.

Chris: Oh. I see. Or you could do exhaust-only with makeup air.

Sonia: Right. And so, you could kind of get into the weeds. I don’t know if I’ve ever heard of a code inspector coming into a residential home and running those calculations for you. But technically, if you’re going to comply with ASHRAE, you can either do continuous ventilation or intermittent ventilation. What you need is, over a certain period of time, the same amount of air. So, if you do intermittent ventilation, you have to ventilate at higher rates than if you do continuous.

Chris: Are you supposed to do — I thought (maybe you can correct me) that kitchens had to have, like, 5 cfm continuous exhaust.

Sonia: 5 ACH.

Chris: Oh!

Sonia: Yeah. You have two options in ASHRAE 62.2. You can either do 100 cfm intermittent – which is basically a “when you use” — when you’re cooking — or you can do 5 ACH continuous. (And that is ACH of your kitchen.)

Chris: Gotcha.

Sonia: And I don’t really know, in these open floor plans…

Chris: Right. How do you even… I’m sure the code enforcement officers…

Sonia: I’m sure they never check. But, I mean, the thing is, for you guys as architects – you’re obviously trying to comply with code, but – you’re trying to provide your occupants a healthy environment.

Chris: Exactly right.

Sonia: So, it’s really the spirit of the law that we’re going after tonight, I think.

Chris: Absolutely. Well said.

Phil: That’s right. So, when we talk about exhaust-only – that was the first example you gave: exhaust-only ventilation – so we have fans that exhaust directly from the house. And if we do that in a tight house, we somehow have to allow for air to come in. Because it’s so tight, where is it going to come in?

Chris: Right.

Phil: And we’ve used trickle vents in the past. We don’t do that anymore. Are there people still doing that? Sonia, do you specify that?

Sonia: I don’t. I haven’t.

Chris: That’s fair.

Phil: A trickle vent is basically a hole in the wall for fresh air.

Chris: Maybe it has a flapper on it, you know. And Bob’s your uncle. And usually it’s in the basement. The last time I even talked about doing a trickle vent, it was in the basement. It’s been a while since I’ve done a basement on a new house. So there you go.

Phil: You and I, Chris, we talk a lot about cost and budget and we say, “Listen, if we’re going to change the world, we’ve got to keep our numbers down and our overall costs down.” So maybe exhaust-only, in some cases, is better than nothing.

Chris: Right.

Phil: And we put this hole in the wall and we deliver some cold air at certain places, but it’s not much.

Chris: Right. And here’s what happens – and it happens on the cheap: People are trying to save money and they put in one of those nice Panasonic continuous exhaust-only fans and then they walk away and say, “I did it. Done. Great!” But, what they’re really doing is forcing the building to fail – unless they put in trickle vents, which they probably didn’t do. So then, they’re using their insulation as an air filter and it’s going to collect all that stuff, and they’re making all the windows and doors leak under that pressure.

So that’s why I’ve philosophically despised the exhaust-only systems. Though, I recognize when it’s being used, if it’s just like a one-room kind of apartment place. I don’t know…

Sonia: You’re also heating that – I know we’re going to talk later about energy recovery, but – you’re also heating that area.

Phil: So, one general rule of thumb – if we’re having consensus here: Don’t do that anymore. Don’t worry about exhaust-only ventilation. Spend a few extra bucks and…

Chris: Do a balanced system… right? So, Sonia, tell us about… What is your perfect balanced system?

Sonia: My perfect balanced system is obviously balanced… right? It has the same amount coming in as going out. We’re controlling where and when and how much comes in. Typically we supply in the places where people need the air the most. Did you guys go to the Indoor Air Quality Conference?

Chris: Uh, I missed that one.

Phil: No, I did not. I feel guilty I didn’t go.

Chris: I wanted to go.

Sonia: It was excellent. They actually flew a guy out from Washington State who works for the Washington State University extension and he did a presentation. Apparently in Washington in 1991, they started requiring homes to ventilate. And so, they were one of the first states to adopt it, so they have all these years of data. So they actually went and instrumented several of these houses and they actually measured the carbon dioxide concentration…

Chris: Oh wow!

Sonia: …in master bedrooms at night, during the day, through all these forced-air systems, balanced ventilation. And I was shocked at how high the carbon dioxide gets in the bedroom at night, especially with two people in it.

Chris: Really? Maybe that’s why you get sleepy…


Sonia: So, my perfect system is supplying in the places where people are most likely to spend most of their time. So I personally would supply in the bedrooms and, of course, we have to exhaust out of kitchens and bathrooms anyway.

Chris: Right.

Sonia: You try and set it up to kind of think in 3D rainbow-vision and you want to mix the air through the house, so you get good, consistent air-mixing – good, consistent air quality – throughout the house. And that would be my goal.

Chris: Nice.

Sonia: From an energy perspective? I want it to have energy recovery so I’m using the outgoing air to heat or cool the incoming air. That’s my ideal system.

Chris: Beautiful! I like how you use rainbow vision.

Phil: Right! Sonia, you had me at rainbow.


Chris: Alright. One thing I always like to say (and maybe this is my quote – either that or I’ve been saying it for so long I don’t know if I attribute this quote to somebody else, and that poor person is listening to this podcast and…)

Phil: It’s probably me, Chris.

Chris: Nope! I’m pretty sure it wasn’t you.

Phil: If it was intelligent-sounding, it probably was someone else.

Chris: Well alright, it goes like this: “A healthy house leaks and an energy-efficient house controls how it leaks,” which is very akin to what you just said. (I’m paraphrasing that).

That brings us to ERVs and HRVs – which of course, anyone who’s on this site, they already know these things exist and what they basically are, but for the sprout who’s just tuning in for the first time: Basically we’re talking about tempering the incoming air with the exhaust air. And then we get into the eternal debate of which is better for your house or your situation: ERV (which is the energy-recovery ventilator) versus the HRV (which is the heat-recovery ventilator, which came first). And they started transferring not just heat but latent moisture as well, and then they said, “Well, let’s call it an energy-recovery ventilator.” So that’s the difference between an HRV and ERV.

Phil: Or “enthalpy recovery.”

Chris: Enthalpy recovery!

Sonia: Nice!

Phil: Oh, I remember it actually – to tell the difference in my head. I’ve got to say it again a couple of times…

Chris: Twice a year.

Phil: …and I still get confused. That’s right.

Chris: Well, I’ve never really understood the technology behind an enthalpy wheel. You know those were some of the first ERVs we’d ever… In my brain – and Sonia, maybe you can tell me if this is crazy – I feel like there are layers of filters in there, and it just spins there. It’s like the air gets mixed in there… so much. See? I sound like a child.

Sonia: Let’s just call it a magic box.

Chris: it’s a magic box with a wheel, but I don’t know…

Phil: So, the basic difference between the ERV and the HRV…

Chris: Yeah.

Phil: The ERV transfers the moisture. It’s the enthalpy-recovery ventilator. So: moisture in, and some of that moisture gets transferred back into the building through the outgoing air.

Chris: Yeah.

Phil: And HRV doesn’t do that. It pulls the moisture out, and there’s a condensing line.

Sonia: And you’re thinking of it from a cold weather climate.

Phil: Mm-hmm.

Chris: True!

Phil: That’s right! Thank you.

Sonia: In an air-conditioning climate, it could be the reverse.

Phil: Aha! Okay.

Chris: But, do you still want that humidity recovery in reverse? Will an ERV shed its moisture to that…? No, what am I saying?

Sonia: Will it dry?

Chris: Yeah. Will it dry?

Sonia: So, kind of the rule of thumb is that the ERV helps maintain whatever humidity you have in the house. So, regardless of whether it’s dry or wet outside, it will either dry the air or moisten the air, depending on your moisture gradient.

Chris: Wow. What are we looking to maintain in the house?

Sonia: Typically you want to stay between 30% and 60% [indoor relative humidity]. So, less than 30% relative humidity…

Phil: Pretty sad, yeah.

Sonia: Yeah, right. You tend to dry out… right? So, nose bleeds, it’s uncomfortable, dry skin, static electricity. And above 60%, now we’re getting into building science issues where we’re looking at condensation, potentially, on cold surfaces and also, in a cooling climate, it starts to get uncomfortable…

Chris: Clammy, yeah.

Sonia: Clammy.

Chris: Which no one likes.

Phil: No. Interestingly, on NPR they were giving the weather report the other day and I heard the weatherman refer to the weather as clammy, as if it was a technical term.


Chris: That’s fantastic.

Phil: I just thought that was outstanding!

Chris: On NPR, straight-faced?

Phil: On NPR.

Sonia: Everything’s straight-faced on NPR.

Phil: “Warm and clammy today.” I just loved it!

Chris: Wow! So then, the real question I think that some people have there and they’ve been waiting for us to say it now is, “So, which is better? What should I use, Sonia?”

Phil: “ERV or HRV?”

Chris: “Which one’s better?”

Sonia: It depends!

Chris: Okay.

Sonia: So, it depends who your clients are. So everything, I know everyone wants a rule of thumb, but…

Chris: Yeah. What’s your rule of thumb?

Sonia: But the rule of thumb is you really have to assess the situation. So, if I’m building a 4,000-square-foot house for a single, older couple, I probably want an ERV because they’re not making a lot of moisture.

Phil: They’re pretty dry anyway.


Sonia: They’re dry. They’re desiccated.

Chris: You and I are this close to being there, too, buddy.

Sonia: And they’re also susceptible; they’re more susceptible to dry skin, etcetera. Whereas, like Phil had said earlier offline, “If I have a small house that’s got three teenage boys who shower twice a day, and my wife is Italian and she likes to cook pasta all day, I might actually go with an HRV.”

Phil: And teenage boys take really long showers, by the way. That’s all I’m saying.

Chris: That’s all you need to say.

Phil: Yeah.

Chris: Yeah, I gotcha. So, it depends on your client, and also your climate? Do you think climate plays into that choice? Or do you think it’s really about the humidity you’re generating inside that makes you make that decision?

Sonia: Typically, it’s the humidity you’re generating inside. I would say, probably 80% of the time, an ERV is the right choice. You just need to be ready for those times when you have a high-humidity generating household…

Chris: Right.

Sonia: …and consider an HRV might be the right course of action.

Chris: Right. And now we’ll probably get into the different ways to do makeup air and exhaust air. This is about tying your bathroom exhaust into an ERV or HRV system. It is one of those things that people either despise or they love, depending on how energy-efficient they are trying to be. What’s your take on that?

Sonia: Tie it in.

Chris: Tie it in! Alright. Very good.

Phil: So, Sonia, are there situations where you would just say, “We need a dehumidifier”?

Sonia: Absolutely. Typically not where we are. But, when you get down South. And the other thing to think about, as we’re building these more-efficient [buildings] – and Joe Lstiburek has a great presentation on this – we need very little cooling now. But the problem is, when we’re not cooling, we’re not getting dehumidification. So, if you’ve oversized your air-conditioning system, it’s going to cycle on and off, and in between you’re not going to get dehumidification. So really, a separate dehumidification system is really warranted as we start to get into more tighter and efficient houses.

Chris: Right.

Phil: Sounds expensive.

Sonia: Not necessarily.

Chris: So, I’m in a super-tight house (1 ach50, whatever). So you’ve got this three-bedroom house, and you’ve got an ERV system in it. How much do you think that costs? You know… tight house.…

Phil: That’s a good question. I’m going to throw out a number.

Chris: Do it!

Phil: Five grand?

Chris: Five grand. That felt good. That felt alright. Yeah, we’re nodding. So, alright. That passed the peer test there.

Phil: Yeah. There’s labor. The actual boxes – that’s not that expensive.

Chris: Right. You’re running ductwork, which probably, in these systems (if you’ve got a nice system), we’re talking about those cool Zehnder tubes…

Phil: Well, if you’ve got the Zehnder. I mean, the Zehnder boxes start at $2,200 or something like that.

Chris: Yeah.

Phil: Over two grand for those. But you can get a simpler box from Venmar for half that price.

Chris: Sure.

Sonia: But, you’re also talking efficiency, right?

Chris: Right!

Sonia: So here’s the big deal with efficiency… right? So, if I get a sweet-ass Zehnder…

Chris: Yeah!

Phil: That’s right!

Sonia: …which we’re putting in our passive house… (We haven’t fired it up yet.) I’m talking… what, 92% efficient? So, the air coming out is almost the same temperature as the air going in.

Chris: Which is amazing!

Sonia: It’s totally amazing. But, I’ve done some forensic investigations in some houses that decided to, at the last minute, go with a much less expensive heat-recovery ventilator. So what’s happening, on those cold Maine days, we’re getting air that could be 45, 50 degrees – which is not necessarily a problem if you put it somewhere where it’s not going to fall right on you.

Chris: Right.

Sonia: So, if you have a basement and you bring it in, or you put it somewhere where it’ll mix before it hits the occupants. Not necessarily a problem. But if you have 45-degree air coming in, in a 70-degree house – and it’s dumping right on you watching TV – that’s going to be unacceptable.

Chris: While you’re trying to sleep in your bedroom.

Sonia: While you’re trying to sleep.

Chris: Right on your head.

Sonia: So you really need to give some thought [to equipment that is] more expensive, higher performance, more comfort – depending on where you can put it in the house.

Chris: Right. Oh, maybe we should talk about the Coanda effect.

Sonia: Coanda.

Chris: Oh, is that how you pronounce it?

Sonia: That’s how I pronounce it, but I could be wrong.

Chris: Coanda effect. This is what I’ve been told: that you are supposed to – almost all of your supply should be up high, so that the air can…

The Coanda effect is the air’s tendency to cling to the surface area adjacent to that volume of air that is moving. If you just blow it out into the center of the air, if it’s cool, it’ll just drop. But, if you blow it next to a ceiling’s surface, it will kind of run along the surface, so it will spread out more. So, you’re supposed to supply high.

Phil: Aha!

Chris: Is that too geeky for this podcast?

Phil: That’s really good.

Chris: Alright.

Phil: Great visuals.

Chris: So air clings to surfaces, so as you’re thinking about where you’re putting these ducts (diffusers – that’s what I meant to say – where you’re putting these diffusers), you’ll want to think about how it’s going to move. Right?

Sonia: That’s perfect. You want to think about how it’s going to mix.

Chris: Right. And you were talking about supplying in bedrooms. And then Phil, before we went on air, we were discussing someone else’s alternative point of view, which was supplying…

Phil: We’ve been working with Robb Aldrich from Steven Winter Associates. And I love Robb — super-smart guy; he does the testing of this in real time. We’ve been using him for one of our projects and he believes in something a little bit different. He does exhaust-only from all the bathrooms. Supplies in the hallways, but exhausts out of the bedrooms. And his theory is that it helps with the mixing of the air and the thermal comfort. If we’ve got a single point source or if we try to heat the entire upstairs in a small house that’s got three bedrooms upstairs – maybe there are some holes in the walls or vents or there’s some way for the air to get through it – that this will actually help with the thermal mixing.

Sonia: So when you say, “bathroom exhaust-only,” you’re saying direct exhaust.

Phil: Direct exhaust. Thank you, Sonia. Right. That has nothing to do with the HRV, it just goes directly out. It is not tied into the system.

Chris: Gotcha.

Phil: There are contaminants and there is a lot of moisture. Do not put that back into the system at all.

Chris: Interesting. So, he’s supplying near the point source of heat, and using an exhaust-only…

Phil: Not an exhaust-only, no. Just using exhaust for the HRV or ERV in the bedrooms.

Chris: Gotcha. So, supply carries that point source out…

Sonia: And that’s a great technique for mixing. So we’ve talked about: if you have a lower-performance ERV or HRV – so the air’s coming in cooler – if you bring that fresh air in right next to your heat source.

Chris: Right.

Sonia: It’s going to condition that air before it gets to the people, so it’s actually a great technique.

Chris: Alright.

Phil: I have one thing that I want to ask Sonia about, because I know there’s a simple way to do this. (I still have to do the math in my head, and maybe you do, too.) But when I look at the efficiency of the ERV or the HRV, I always think, “What does 92% mean versus, you know, 85%?” That sounds pretty good to me.

Chris: In terms of money, Phil?

Phil: No! In terms of comfort.

Chris: Yeah. Oh, okay.

Phil: If you can do the simple math and say: Hey listen, it’s 70 degrees inside, and it’s zero outside. When that air mixes, is it 35 degree air (if it’s 50% efficiency)? And if it’s 92% [efficient], suddenly is it 92% of 70 degrees and that’s what we’re measuring? If that’s the case, we’re about 62-degree air coming in. Does that sound right?

Sonia: It’s actually called the mixing of air streams. What you have to account for is, sometimes it’s not perfectly balanced… right?

Chris: Right.

Sonia: So, for example, if you run your monster range hood…

Chris: Yeah.

Sonia: …and you’re actually pulling more outside air in than you’re exhausting out…

Chris: Right. We’re going to take a break and then we’re going to talk about…

Sonia: That balance changes… right?

Chris: Yeah.

Sonia: So does the temperature coming in. The other thing is, you have to look at the performance curve of your energy-recovery ventilator. And sometimes the performance changes based on airflow or outside temperature. So what might be at its best performance for an 85%-efficient ventilator might be closer to 65% at a certain performance point.

Phil: Hmmm.

Sonia: So you really need to think about where you are operating your equipment and then do the math. Because a 65%-efficient ventilator on a minus-5-degree day is going to bring in 40-something degree air.

Chris: Chilly.

Sonia: Yeah.

Phil: This, Chris, is why we hire engineers.

Chris: I know. Because I don’t trust your fuzzy little…

Phil: I know. My rule of thumb… right?

Chris: “If it’s a 90%, and it’s a 60-degree day…”

Phil: Sonia has many more thumbs than I do.

Chris: “…and there’s a train traveling from Cincinnati to…”


Alright. I can tell… I think we ought to go refresh our drinks. Call this the end of Part One. Come back with Part Two, and come back with the awesome Sonia Barrantes.

Sonia: That’s very sexy.

Chris: Oooh! Thank you. She’s flattering me. That was probably awful. Anyway, so don’t go anywhere. We’ll be right back.

Here is the link to Ventilation for Your Tight House — Part 2.


  1. jhrockwell | | #1

    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

  2. GBA Editor
    Martin Holladay | | #2

    Response to John Rockwell
    Thanks for your important comments. (By the way, you posted your comments on the wrong page. I think you really meant to post them here: Ventilation for Your Tight House — Part 2.)

  3. Christopher Briley | | #3

    Safety First
    Thanks John for that important clarification! I'll make a note in the body of the transcript.

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