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The Ventilation Debate Continues: Interview with Dr. Iain Walker

Iain Walker Lawrence Berkeley National Lab Lbnl

The debate over how much to ventilate a home has been going on a long time. Last year, Building Science Corporation introduced their own standard to compete against ASHRAE 62.2 because of problems they felt weren’t adequately addressed in the ASHRAE standard. I’ve written about the standard and interviewed Dr. Joseph Lstiburek and ASHRAE 62.2 committee chair Paul Francisco (links at bottom).

Recently I interviewed building scientist and ASHRAE 62.2 committee member Iain Walker, PhD, about the issues. We had a great discussion, but make sure you read to the end. You don’t want to miss what he says about the ERV flaw you’ve probably never heard about. Also, be sure to attend the ACI national conference, where Iain, Joe, and several others will be on a panel debating this issue.

Allison Bailes III What do you think of the great ventilation debate that’s happened over the past year and Joe Lstiburek going rogue and creating his own residential ventilation standard?

Iain Walker I don’t know that there’s much debate happening really. You know, we have one national standard that is being used more and more, and I think that’s the way it’s going to continue. So I’m not sure there’s very much debate going on.

ab3 Do you see any value at all in the issues that Joe’s brought up?

Walker Well, it depends on what you mean by issues. Most of the issues have been discussed at length by the 62.2 committee on and off. Certainly some of them, the 62.2 committee is investigating in great detail.

I’ll give you an example. One is this idea of maybe the standard should require filtration. Currently it has a very weak requirement for something like a MERV 6 filter. But MERV 6 filters don’t really filter out the size of particles we’re concerned about for health. So maybe we should have a requirement for that and we haven’t figured out what that would look like and how to make it equitable across different systems and different climate zones and different homes, so it’s complex to find something in that area that’s suitable for a national standard, but certainly we’re looking into it.

We’re not dismissing Joe’s concerns, and to be fair they’re not just Joe’s concerns. We all know what to do to improve indoor air quality. The question is always how to get it to work right in the standard.

ab3 OK, getting into what’s probably the biggest difference between 62.2 and Joe’s standard, what do you think of his idea to require different rates for different ventilation types, central-fan integrated supply vs. exhaust only vs. balanced?

Walker Well there’s been a lot of work done in this area, and the difficulty is that different systems are better in different circumstances. This makes it very difficult to say can we always credit one system or another, and I would say, no, we can’t. This is what the 62.2 committee decided also because you can create a scenario in which whatever ventilation system you like is better or worse, but overall the answer is there is no one better system from the point of view of achieving the indoor air quality that we’re looking for here.

ab3 In Joe’s standard, the system that has to ventilate at a greater rate, because he feels like it’s not as effective, is exhaust-only. He has cited Armin Rudd’s study of two homes in Texas as evidence to support that, but you’ve said there are other studies contradicting that. Can you go into that?

Walker Sure. Well, on the issue of what’s better, there’s a whole bunch of papers that were published several years ago—Max Sherman and I wrote several of them and a few other people were involved, too—looking at this exact issue: Should you mix? Shouldn’t you mix?

We all agree that there’s certainly an advantage to running a central system that filters the air. I think we all agree on that but the question is, now how do we compare that to the filtration provided to the building by an exhaust system and people have researched that and found out that the building envelope is something like MERV 9, but it depends a lot on how leaky the envelope is. The tighter the envelope, the more filtration you get. The looser the envelope, the less filtration you get. And indeed that’s what the 62.2 committee has been saying that if you’re going to give a credit for filtration, we have to figure out the effective filtration for all systems, and it is a little tricky to deal with.

There are other things like the difference between a balanced system and an unbalanced system. In a balanced system, you can simply add the infiltration rate to whatever ventilation rate you’ve got, but you can’t do that with unbalanced systems. The 62.2 standard has been silent on that issue. It just says, you know, here’s the flow rate. We don’t care if you do balanced or unbalanced. Frankly, I think it’s probably going to continue that way, mostly for simplicity. There’s a big interest in keeping it simple from the users of the standard, builders and contractors.

As we’ve said many times, the standard is just a minimum performance. You can always do better. And the implication there is if you say you use an HRV or ERV that’s balanced, and you provide quite a bit more ventilation, well you go ahead and that’s great. You’ve complied, and you’ve got a better system. You’re providing heat recovery.

That’s great also because the standard doesn’t say anything about requiring heat recovery either. And that’s a better system and the standard says, go ahead and use it. No problem at all. We don’t have to have anything in the standard that deals with that for that very reason. You can always do better if you want because the standard is focused on, what’s the worst you can do and still have a reasonable indoor air quality. That’s the basis of the standard. You can always do better than that. That’s why it’s silent on this balanced vs. unbalanced issue.

ab3 Well that gets me to my next question. A few weeks ago you said that it’s “dangerous” if you ventilate less than the 62.2 rate. Why do you think it’s dangerous? I think you were talking about less because you kind of confused better and more. You said you can always do better than ASHRAE 62.2, but does that mean more?

Walker Well, OK, let’s answer that question first. If you’re concerned about diluting indoor air pollutants, you do that by bringing in more outdoor air. But you can also remove pollutants by deliberately filtering for particles. There are also filters that will remove some of the chemicals, but they’re a little obscure and we don’t see them very much.

We certainly see filters all the time, and it’s almost trivial these days to go into a store and buy yourself a better inline filter and stick into your forced air system. Or you can get units that just service a room. You can go out and buy these things now, and that will provide better indoor air quality because you’re deliberately doing something extra to remove those pollutants. So there are all sorts of ways to get to better indoor air quality.

You can also—we do this in California—we say you can’t use high VOC emitting materials in buildings. That’s a great idea for improving indoor air quality, much better than trying to ventilate it away or to scrub it with a filtration system. If you don’t bring polluting things into the home, that is obviously a great idea.

The final point on this is that sometimes the outdoor air quality isn’t good. It’s very difficult to dilute indoor air pollutants if the outdoor air quality isn’t very good. But that’s something that’s very difficult for us to deal with in an IAQ standard because there’s a lot of regulation for outdoor air that is really beyond the scope of what we’re trying to do with the ASHRAE standard.

Frankly I think in the US, the actions of the EPA are keeping our outdoor air pretty good. It’s not always great, but it’s pretty good in most places. That’s actually great, and I think that the EPA and lots of other people are doing outdoor air research and making that cleaner, and we’ll let them continue to do that.

I don’t think the ASHRAE 62.2 standard is going to try and deal with outdoor air pollutants in any significant way as a minimum standard. I mean you can always do better. You can envision some sort of control system that if you know there’s high ozone outdoors, you might want to turn off your ventilation for an hour or two and control that way. You can certainly do that, but I don’t think we would require that as a minimum performance specification.

Now that was the second part of your question. So there are many aspects to achieving good indoor air quality. It’s not just ventilation. There are all these other possibilities, like source control and filtration and that sort of stuff.

Remind me again what was the first part of your question.

ab3 The first part was about your comment about it being dangerous to…

Walker Oh, yeah, yeah, yeah. Well, OK, dangerous might be a little bit of a strong word, but I think that the objectives are manifold for the standard. Indoor air quality is sort of a broad term, so I just want to break it down a little bit.

I think we’re getting much more of an emphasis on health than we used to have. Traditionally, we used to ventilate mostly for things like odors and moisture control. They were the prime drivers historically. And by historically, I mean going back thousands of years up until twenty-odd years ago. Just because health is a very difficult thing to deal with because there’s a lot of things that affect health and everybody responds differently. We’re only just getting enough knowledge to make some reasonable health decisions.

The danger, if you will, of not ventilating to the 62.2 level, sort of combines all these things together. We know historically about odor control and we know about moisture control, and you could impact things like mold growth, which is a health problem or degradation of surfaces, which is a structural problem. But also, we’re ventilating for chemicals we know are problems. I should speak more broadly – pollutants we know are problems, because it’s not just chemicals.

The most significant pollutant from a health perspective is particles, in fact. There have been studies done that have measured concentrations of these things in homes. And we know enough about some of these components of indoor air quality and what their health effects might be and the levels we’re setting in 62.2 are at about the point where we think we’re reasonably dealing with most pollutants and most circumstances.

Now we’re obviously going to be able to deal with extreme events or unusual things but all a standard can do is work most of the time for most homes for most people, and we’re never going to capture every single possibility. That’s just unreasonable.

The problem with going below that is that the levels of various pollutants could build up to the point where we could not say this protects most people most of the time anymore. This is the consensus of a whole bunch of people on the committee with all sorts of backgrounds representing all sorts of interests.

Our European friends, of course, are occasionally critical of our standard because they say our ventilation rates are actually much too low. They like much higher rates, and their ventilation standards are considerably higher than 62.2.

But, you know, there has to be some sort of judgment applied here and this is where we are now. This is what the knowledge of all the people who have worked in this area for a  long time is a reasonable number to go for to protect against indoor air quality issues that affect a substantial number of people a substantial amount of time.

I think that if you want to ventilate a home, that’s a good target. If you go below that, you’d better have some pretty good evidence that it’s OK. You better have some pretty good large scale studies or a reasonable consensus among a lot of people representing a lot of points of view that actually it’s OK to go to say half that air flow rate. Currently we can’t say that.

The danger in picking lower numbers without any backing is, is that even real? If you think of yourself as a contractor installing ventilation systems, and you install us a very low flow one that is below 62.2 and there is an indoor air quality problem and somebody sues you, the danger there for a contractor is you would be successfully sued because 62.2 basically represents the current recommendations of good practice in our industry. If you’re going to go significantly below that level, you’d better [garbled recording]. If you get sued, you’d be in a lot of trouble because you’re not taking the advice of your industry. It just creates a minefield, frankly, that I don’t think anybody should want to walk into.

ab3 But the minefield is mainly because there’s this standard that says you need to ventilate at this rate, and the standard isn’t based on a lot of hard health data and epidemiological studies. Right?

Walker That’s right, but it’s based on a lot of knowledge about air flow in buildings and pollutant emission rates and pollutant concentrations and the health impacts of them. We can say a lot of good stuff without needing large-scale epidemiological studies that would answer things with a great deal of certainty.

And frankly, we’ll never have those studies. You’d have to cover so many climates, so many different pollutant generation rates, such a wide group of people, and you’d have to do intervention studies where you ventilate and didn’t ventilate. We’re never going to have that large scale epidemiological information so we have to do the best with what we know.

And what we know is we have some pretty basic physics going on about dilution of pollutants if you know their emission rates. And even if you know their emission rates vary, like with formaldehyde, we have good models of how that works. We know the science of how that works. And we have some pretty good health data on, not all the pollutants, I would say, but certainly the significant ones that show up a lot in many houses that have health impacts like particles, oxides of nitrogen, and some things like formaldehyde that people are more familiar with.

Then there are some marginal things like water vapor, which isn’t  necessarily directly a health impact, but the effect of water vapor can be a health impact as a second order thing. So I think we know enough to be making reasonable decisions and that is what the ASHRAE committee is all about. It’s about knowing enough about the science of this to make a reasonable decision.

ab3 One more question about 62.2: What do you think would make the 62.2 standard better and where do you think the committee is going with it?

Walker The committee—I mentioned this a few minutes ago—is actively thinking about, should we have some kind of air filtration requirement, mostly to account for particles, because particles are so important from a health perspective. The committee is actively working on this right now. I can’t predict when we’ll have something concrete in the standard. That’s always a very difficult and a dangerous thing to do, but I can tell you that this is a subject that the 62.2 committee is very interested in and is working on very hard at the moment.

We’re bringing some new people onto the committee and getting people to come and give presentations. So I think the issue of filtering air is going to be an interesting topic in the future for the standard. I think that the major change we’re likely to see going forward is that because I think there may be some other detailed things that the standard…

I personally don’t think the standard isn’t going to have to act on because of other things happening but it’s important to talk about and that is issues about combustion safety and this idea of unvented combustion in a home. The reason why this is an issue for the standard is because combustion produces a lot of pollutants that we don’t want to have in our home, a whole broad range of them.

In the home performance community there’s a great interest in this because when we go into homes and tighten them, we do combustion safety testing, for example, or when homes are rated, there’s combustion safety testing also. I think that this is, from an indoor air quality perspective, burning stuff in your home is crucial.

This leads me to the next point that I think we might see coming in 62.2, and that is a better way of dealing with kitchen exhaust. Here I’m talking primarily about range hoods because cooking, whether you’re burning gas or cooking with electricity, produces all sorts of pollutants and it’s probably the major source of pollutants in your home for most people, unless you’re doing something really extreme.

Venting those pollutants correctly, as in you want to exhaust them directly to outside—that’s the preferable way of dealing with it. Right now the standard says you’ve got to have a range hood with 100 cfm of exhaust or you can ventilate the kitchen at a higher air flow rate overall but we’re learning more on how they work.

And range hoods don’t capture all the cooking pollutants when you turn them on. So we’re working with ASTM, not with the ASHRAE 62.2 committee. We have an ASTM committee looking into, could we develop a capture efficiency test method so range hoods could be rated for their capture efficiency.

Then what the 62.2 standard could do, or any other ventilation standard or even building codes for that matter, they could say, yes, you’ve got to install a kitchen range hood with a minimum capture efficiency of say 75%. But until we have the test method in place that actually rates the range hoods, standards like 62.2 or mechanical codes can’t require a capture efficiency because there’s no test method. I think we will see something along those lines happening in the next couple of years.

We have a really good group of people together developing the ASTM test method for capture efficiency. The industry as a whole—and by industry, I’m including the people who manufacture kitchen ventilation products as well—is very interested in seeing this go forward, and I think we’ll see that change in ventilation standards within a couple of years that deals a lot better with how we deal with capture efficiency of exhaust and does a much better job of ventilating kitchens.

A little add-on to that is this issue of, well, do people even turn these fans on, whether it’s in kitchens or even in bathrooms, too. The standard does require that you install exhaust fans in kitchens and bathrooms, but they’re user-operated, so you’re there cooking and you turn it on or you’re having a shower and you turn it on.

Another area that’s developed in 62.2 is maybe we should automate these things. I’m thinking more particularly about kitchen ventilation and how range hoods operate. If people don’t turn on their high capture efficiency range hood, it doesn’t help them at all. We have to know it operates.

There are a few manufacturers out there making automated range hoods. This is another possibility for a requirement. Not only will we have to have good capture efficiency but there will be some specification about how you automate a range hood so that whenever you turn on a burner to boil a pot of water or turn on the oven to bake some bread, the ventilation system in the kitchen turns on automatically.

We’re currently at the beginning steps of that. We’ve done laboratory testing of the current control systems, and there’s a big range of performance. Some are good and some are poor, so we may have to have a standard for that, too, given the range of performance. That’s certainly something I think could have a really good impact on indoor air quality in homes, whether it’s in the 62.2 standard or in the plumbing and mechanical codes.

I’ll be honest with you, when people ask me what’s the single biggest thing they can do to improve the indoor air quality in their homes, I always tell them to use their range hood. That’s so easy to do. You flip a switch. You don’t have to purchase anything. You’re not buying a fancy filtration system. That’s the first thing to do, and I think that the ASHRAE standard and these codes are very interested in these sort of things that are relatively simple but could have a substantial effect on indoor air quality without having to increase ventilation rates. We could really have some big effects there.

Those areas are where I see the whole business of indoor air quality advancing in the near future: active filtration and doing better with kitchen exhaust on several levels. That’s where there’s a lot of activity right now. That’s where I see us improving our indoor air quality efforts in the near future.

ab3 At the Dry Climate Forum, I think you talked about ERVs recycling formaldehyde and sending it back into the house rather than outdoors because it behaves like water. Can you talk about that?

Walker Yeah, this is an interesting perspective. I just want to be clear — we’re talking here about ERVs, which exchange moisture as well as heat, not HRVs. This is an issue for ERVs. The issue for ERVs is that as you’re exchanging moisture between the two air streams, maybe other things get transferred, too. The particular pollutant that we’re concerned about is formaldehyde because formaldehyde, from a chemical point of view, looks a lot like water. It’s quite a similar molecule.

We’ve done some testing to look at this in our laboratory to see how much formaldehyde might get transferred between the incoming and outgoing air streams. In other words, if you operate this ERV, maybe you’re not getting rid of the formaldehyde like you think you are. That was our question.

Indeed we found that there can be substantial transfer, but it depends a lot on the specific materials that are used in the ERV to do that transferring of the moisture. It’s very specific to the particular way of doing it. Also, there are differences between ERVs.

There are basically two ways of exchanging the moisture. Some do it through a membrane, and the second one is with a wheel that spins between the two air flows. The wheel ones we’ve always had some concern because there’s always some bypass about where the wheel goes from one air stream to the other. There’s always a little bit of air leakage there. In fact to be fair to ERV ratings, that bypass is included in the ratings so the ratings are good. This is all included in that.

The concern from an indoor air quality point of view is what is that bleed air and the fact that you’ve got a wheel rotating from one air stream to another is actually transferring pollutants. The answer is, yes, it is, but unfortunately, I can’t just tell you, well, it’s 10%. There isn’t a number.

It depends a lot on the materials used and how the ERV is designed and how well it’s sealed to prevent that sort of transfer. We’re shortly going to have a paper on that. There was some work done by other people at LBNL. I was only peripherally involved and I did not write the paper.

The ERV manufacturers are taking this very seriously. I can’t make any promises, but we’ve had some very good discussions with them about this issue. Where I think we’re going to end up is, because we can identify the various materials that are used to transfer the moisture and unfortunately some formaldehyde, we can also pick materials that are much better at transferring the moisture and not formaldehyde, and the manufacturers are going to switch the materials they use. This is where I believe we will be shortly because the manufacturers are very aware of this. They’re concerned and they want to do the right thing, so I think that’s what’s going to happen.

Currently it’s a bit of a caveat emptor for ERVs. If you want some advice, I would say try and avoid the ones with the disk wheel if you’re looking at an ERV for now. We believe this issue is going to go away because we think changing the materials will be able to solve this problem. But right now it is a concern. There are some ERVs that are better than others and no way for a purchaser to know because they’re not rated for this.


Make sure you register for the ACI national conference in Detroit because Iain, Joe Lstiburek, Paul Francisco, and several others will be on a panel debating the issue of residential ventilation. Duncan Prahl of Ibacos and I will act as moderators. Register now! The Early Bird deadline is 31 March 2014.

ACI 2014 62.2 Debate


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Interview with Dr. Joe Lstiburek — The Ventilation Debate Continues

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ACI reason #5 image courtesy of OmStout Consulting.


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This Post Has 22 Comments

  1. Great job, Iain! When I have
    Great job, Iain! When I have asked builders if they think tight houses need mechanical ventilation, there is general agreement that it is a good idea. I recommend that they simply use the 62.2 Standard as a reference on how to accomplish that.

  2. Great interview, Iain and
    Great interview, Iain and Allison. Where I see the greatest challenges for implementing 62.2 are with low-income (read as “much older”) housing stock. Applying a standard to non-standard housing stock brings up issues that I don’t believe the 62.2 committee have considered fully.  
    Installing exhaust fans where vermiculite (ACM) is in the wall and ceilings, or located in interior spaces makes implementation quite challenging. Even if one is successful, pulling makeup air through vermiculite or 50 year old fiberglass insulation may actually compromise indoor air quality.  
    Exhaust only ventilation may increase whole house depressurization and possibly be detrimental to atmospheric combustion appliances, especially if air-sealing measures were quite successful. Having a dirt floor basement under a depressurized home certainly doesn’t help IAQ, either. Do you expect to see prescribed isolation of atmospheric combustion appliances at some point? 
    While there is a lot of commonality between new and old housing stock with respect to indoor pollutants caused by contents, typically older homes have a different indoor pollutant make-up than new homes. How is LBNL looking at these factors?  
    I’m surprised I haven’t seen more about the difference in ventilation rates between exhaust only and balanced ventilation as it impacts 62.2. Is it there is not enough data about the Half Fan Rule? How does the 62.2 committee plan to address this?  
    I look forward to the CAZ testing and 62.2 discussions at the ACI National Conference in Detroit. 

  3. Allison, I think you asked
    Allison, I think you asked but I can’t see if this was answered:  
    “You can always do better if you want because the standard is focused on, what’s the worst you can do and still have a reasonable indoor air quality.” 
    Specifically, if you have a system that does “better” (filtration and/or better mixing), will there be a credit allowing a lower rate?  
    I think that’s the issue us mere mortals would like to see addressed. Makes it much easier to sell better fresh air systems as also having efficiency benefits, and much easier to install smaller systems and control comfort.

  4. I find it interesting that
    I find it interesting that Iain says the building envelope provides ‘something like MERV 9’ filtration for exhaust-only ventilation without regard to  
    the additional particulates/odors/chemicals that might be added to ‘fresh air’ as it’s pulled into the building through cavities, garages, crawls, etc.

  5. I agree with David, and
    I agree with David, and further to his point: With the exception of particularly polluted locations think under a busy freeway) or people with pollen hypersensitivities, filtration of outside air seems trivial in contrast to filtration of recirculated indoor air. Forced air heating/cooling systems certainly need good filtration, but stressing the filtration of outdoor air is like saying people should only be going outdoors with MERV 9 filters sealed to their face. So much for my Mother’s wisdom of “go outside for some fresh air”. I’d be interested to know if my logic is incorrect, and that outdoor air is typically far more polluted than indoor air, but I’m pretty sure at least the EPA has my back on this.

  6. Yeah, I wasn’t even
    Yeah, I wasn’t even considering the issue of why it is we need to filter outside air to begin with.  
    To me, the reason for an intake filter is to minimize mold and dust intake. Otherwise the fan and HX core would quickly get dirty, as well as floors and furniture. MERV 6 is more than adequate. 
    Homes with special considerations need better IAQ management, not something 62.2 should be concerned with, IMHO.

  7. You folks are raising some
    You folks are raising some good questions in the comments, so let me have a go at answering a few of them.  
    There is definitely a question about what pollutants are in the air brought in due to operating a ventilation system. Unfortunately, we have a lot more speculation than data on this topic! For example, for an exhaust system, what if the filtration capability of an envelope has a greater impact than the extra formaldehyde brought in form building components? What are we to recommend? So, Dale, I can’t give you good answer about what to do with older homes with specific sources you want to avoid. If you are really worried, I think you need to have the air tested or use a supply ventilation system with good filters. A concern with supply ventilation is always that they are only as good as the maintenance – particularly with regards to regular filter cleaning and replacement. So I hesitate to recommend supply systems despite recognizing their potential advantages. In answer to Dale’s question about sealed combustion – it is possible that one day we will see it in furnaces and water heaters due to codes specifying minimum energy performance. But this is a tricky topic given that we allow gas to be unvented for cooking (yes, I know you should always use a range hood) and other appliances, such as woodstoves, may never be as sophisticated. So combustion venting issues may be with us for awhile yet – but in fewer and fewer homes. In the new homes vs. old homes for pollutant sources – this tends to be overwhelmed by several factors: 1. Old homes are leaky, 2. Peoples activities are the same in both (cooking, cleaning, bathing, breathing,) and people keep bringing new stuff into homes to bump up emission rates over time. On the effect of balanced vs. unbalanced ventilation, Dale is correct to point out that we have know that these combine differently with infiltration and we even have good ways of estimating the difference (it’s been in the ASHRAE handbook for years). However, there are a few reasons that 62.2 does not currently account for this. 1. Pressure to keep the standard simple. 2. 62.2 is just a minimum – you are allowed to do better with a balanced system. 3. Many balanced systems we see in the field exceed the 62.2 minimum flowrate any way because people want to operate them that way, 4. In the tighter homes where balanced ventilation is more typical, the smaller quantity of infiltration reduces the differences between balanced and unbalanced systems. The 62.2 committee has considered more issues than this and the current consensus is to not differentiate. But this could change in the future. One last point on the particle filtration question. The reason why this is so important is because particles are the number one health issue in homes by some distance. There are both indoor and outdoor sources we want to minimize. There is research underway at LBNL looking into optimizing ventilation systems for particle control. We will know more and be able to provide more concrete advice in a year or two. Hopefully, I have addressed some of your questions and note that the 62.2 committee discusses all these issues, and more, at great length and we always welcome input for ways to improve the standard. And to close my comment, thanks to Allison for setting up this forum where we can chat about these issues.

  8. What is the logic in the
    What is the logic in the differences in 62.2 for new and existing houses? Here in the mid-Atlantic, we are achieving under 3 ACH50 in 2009 IECC houses regularly. If we go back to retrofit these houses, they will require a larger 62.2 fan than a 2012 IECC that is essentially identical.

  9. The ERV issue has been
    The ERV issue has been publicized before:  
    Home Energy Magazine 
    September/October 2010 
    ERVs Get The Yellow Flag 
    by Max Sherman 

  10. I’m not sure how big a deal
    I’m not sure how big a deal that bypass transfer should really be. if the moisture transfer vector is the cause of concern with ERVs, well even ERVs that are really good at moisture transfer top out at 50-60% recovery efficiency. I believe that implies we’re still dumping at least half as much of that stuff as we thought, and that presumption is that the transfer is exactly as good as the moisture itself. 
    at least in cold climates I find the specification for HRVs problematic, especially at the higher CFMs of the new 62.2 compare to, say, the 2010 version of the standard. historically in cold climates the problems I’ve seen with HRVs is overdrying of the home, leading to end users turning them off and leaving themselves with zero ventilation. 
    without speaking to all climate zones and all situations, I can say that in my cold climate at least I would accept “less” dilution to “no” dilution very quickly… I’m still very likely to prefer ERVs for this reason.

  11. @Rob, I agree that ERV’s are
    @Rob, I agree that ERV’s are more appropriate for high performance homes, even in cold climates. As long as we don’t rely on them as the primary bath exhaust.

  12. I can’t agree with you there
    I can’t agree with you there David. My observations of using an ERV for bath exhaust is that after a shower, it results in a temporary bump in the housewide relative humidity (on the order of something like 2% for a typical home) and then it reduces again, since that latent transfer is far from perfect. 
    I’m quite satisfied with that result and see no reason to skip energy exchange on what can easily be 1/3 or 1/4 of the total ventilation air brought in over time simply on the basis of that effect. plus the initial first cost advantage of not needing bath fans is nice to help offset the cost of the ERV in the first place. 
    I’m definitely open to information to the contrary, but I’m not seeing the issue with ERVs doing bath exhaust at all at this point in time based on what I’ve seen and calculated, and I haven’t seen anyone make a compelling case with rigor yet. But if anyone can, I’m sure it’s you!

  13. @Rob, whether or not this
    @Rob, whether or not this ends up being a problem for any given home depends on size of home, how tight it is, the amount of moisture generated, and temperature variations within the home. These things cannot be modeled or predicted so the prudent course of action is to avoid the risk. 
    This risk is not just theoretical. For example, my brother’s home had an ERV as the only exhaust in master bath. His home is not small but he has two shower heads. He recently had to spend ~$20k for mold remediation. The nominal RH during winter was as high as 50%!  
    I’ve read other anecdotal reports of tight homes with winter RH issues due to inadequate bath exhaust. The mold show typically shows up in closets with exterior exposure and behind furniture due to imperfect heat distribution. 50% RH can easily become 70% in those areas. 
    Since the designer has no control over how much moisture a family generates or how they manage temperature settings, it seems prudent to be conservative here, even if the problem only manifests in a small percentage of homes. If course, if an individual designer understands the issue and is confident enough in his or her assessment and willing to assume the risk, then fine. But most ‘green building’ specifiers are not qualified to do this sort of analysis. So I think there’s an obligation to push back against mainstream practice when risks like this are uncovered. 
    BTW, Panasonic WhisperComfort prohibits installation in laundry or baths. As homes become tighter, other ERV manufacturers would do well to follow Panasonic’s lead.

  14. that’s valuable feedback,
    that’s valuable feedback, thanks David. your ongoing engagement and contributions to the industry and my personal education are always appreciated here. 
    Given the issues I’ve seen with HRVs, I think my initial response would be that your statement is a fine argument for a humidistat control on the ERV to ensure it is run properly. I find it… unlikely… that any commonly sized ERV would be incapable of removing enough moisture to prevent mold if controlled adequately. 
    But, as I continue to noodle this, I realize that my experience is also primarily with homes involved enough to higher a specialty mechanical designer… and the ventilators have always been sized to exhaust multiple baths at once on a high speed setting with intermittent or continuous low speed as the baseline operation, so the problem is reduced to basically only “how much do we have to run at high speed to reduce the moisture”.. we’re simply not going to exceed the max cap of the ERV to remove moisture.. if we did, even individual bath fans wouldn’t be adequate anymore (the ERV has at most 60% exchange efficiency remember). but in other homes with smaller ventilators sized for IAQ only I could see how that would not be true. 
    I’ve learned something today, great way to start the day. thanks David. I do wish the statement about ERVs and baths were slightly more nuanced though. ERVs with high rate exhaust capacity… and a dehumidistat controller!… shouldn’t be a problem anywhere if controlled properly. it seems that the issue should, as I currently understand it, only really apply to homes with minimally sized/fixed speed units.

  15. David: 

    Could you tell us a little more about the problem in your brother’s house? 
    What climate is it in? How much insulation is in walls and ceilings? What is the Infiltration rate? Where did the problem manifest? What was the repair that cost so much? Was the repair successful? 
    Must be lucky, but here in the mid-Atlantic we haven’t seen this sort of thing. 

  16. @Rob, an ERV system
    @Rob, an ERV system (including controls) can certainly be designed so that recycled moisture doesn’t lead to unacceptable RH levels. But we must first recognize the potential problem. Otherwise status-quo installation practice exposes super-tight homes to this poorly understood risk. 
    I have a client in northern Maine at present who’s building a home to near-passive house standards. He intends to use 2 ductless mini-splits for h&c; and a (ducted) recovery ventilator. This issue (ERV vs HRV) came up in a conference call last night. With an envelope approaching 1 ACH50, an ERV will no doubt raise RH by recycling perhaps 50% of internally generated moisture. This is uniformly considered a benefit of using an ERV in cold climates. 
    Here’s the problem in homes like this… During the coldest weather he can expect significant temperature variations in bedrooms and other isolated areas such as closets. This is an acceptable tradeoff to folks wanting to take advantage of low cost and high efficiency of ductless heat pumps. So with a family of 4 in such a tight enclosure, the question we pondered is whether the recycled moisture would be enough to create mold-producing RH in nooks and crannies that don’t receive direct supply air from the heat pump. The truth is, I don’t know the answer, but based on simple psychrometric analysis, this home seems to be a high risk if he relies solely on an ERV to exhaust the baths. I recommended two options: install dedicated bath exhaust fans or go with an HRV and forfeit more comfortable RH levels. Perhaps I’ll suggest your approach (multi-speed ERV) as a 3rd option. 
    Thanks for the conversation.

  17. @Ed, my brother lives in
    @Ed, my brother lives in central NC (CZ-4A). Although I don’t know the R-values or blower door results, his envelope and mechanicals were engineered and verified by a energy efficiency specialist (cira 2000) whose name I will not mention. The ERV is a Honeywell with enthalpy wheel. The two exhaust inlets are in the baths. His shower has two heads, with non-code flow rates. Lots of steam.  
    Mold remediation was expensive because of the extent of the damage. I think he overpaid, but that’s another story. He had to move out for about a week as I recall.  
    He did not take my advice, to install a dedicated bath fan. Instead, his mechanical contractor reconfigured his heat pumps as dehumidifers on RH calls. He has a Voyager water heater and hydronic supplemental heat fan coils, which made it simple to use them for reheat. But his gas bills last summer were higher than his typical winter gas bills! The problem with that is that the mold manifested mostly on walls behind furniture and in closets with exterior exposure, which clearly indicates high winter RH. 

  18. David: 

    Is it a winter or summer problem? If winter, could the mold behind the furniture be due to high RH plus maybe weak insulation in the wall?? If the house was built so recently, then maybe not. 
    I am soooo glad we don’t have to deal with that a couple of hundred miles north of you.

  19. @Ed, this was clearly a
    @Ed, this was clearly a winter RH problem as noted in my previous comment. I can’t see how mold could form in those locations in summer.  
    The fact that you haven’t seen this problem (yet) doesn’t mean much. If anything, the risk would be higher in colder climates. Again, it all depends on convergence of several factors – envelope tightness and size, amount of moisture generated and temperature variations. Voids in wall insulation could certainly be a contributing factor to the latter.

  20. David 

    Thanks. If we see this problem every 15 years, I can live with that.

  21. It always has and always will
    It always has and always will be more lucrative to treat symptoms than to solve problems, ask the pharmaceutical industry! 
    ASHRAE 62.2 seems to be effectively a one size fits all treatment of what are a myriad of problems, their infinite number of combinations & symptoms they produce.  
    The issue that Cinderella’s sisters had with wearing her shoe was that it gave some bunions, others blisters & some both. That’s an old story & this approach will someday be seen as incredibly primitive and myopic. 
    Until the problems/causes of symptoms are identified and understood solutions are a guess. 
    Every locations exterior air quality varies seasonally, even hourly. 
    Every building interior air quality varies by occupancy, interior finishes, furnishings, equipment. 
    Some of these varying conditions produce problems which require filtration &/or ventilation. Given the exterior & interior variables the need for filtration or ventilation varies. Note that the nature of output of VOCs (one of the polluter types of greatest concern) is high after manufacture and lower as times passes.  
    Air quality may be improved best & most efficiently by operating ventilation from ‘low to high’ or to preserve it by turning ventilation off. 
    I submit there is no way to avoid the blisters (under achieving good air quality) & bunions (driving the cost of construction higher and further from reach of more and more people with little return) with a one size fits all approach. 
    I wish far more time was spent understanding, measuring & mitigating the problems individually than trying to find the right shoes size. 
    There’s more money to be made in doing & redoing the latter and it is a far steeper hill to climb to do the former.  
    Reliable protocols & standards to measure the risks for each potential air quality defect inside & out are needed. 
    Devices to monitor air quality for these defects inside & out are needed. No the plural, devices! 
    Until then prescriptive ventilation and filtration solutions are a “try this & call me in the morning approach”, read GUESS. 
    Primitive is only one word that describes that! And yes, lucrative is another!

  22. What are your thoughts on
    What are your thoughts on whole house air purifiers?

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