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Building Science

Preparing to Install an ERV in a Not-So-Old House

My 60-year-old house is about to get a Zehnder ERV

The Zehnder ComfoAir Q600 ERV with ComfoTube and other components arrived at my house last week.

You may recall that I bought a ranch-style house a couple of years ago and have been doing some work to improve it. The house was built in 1961, so it turns 60 years old this year. That may seem old to some people, but I think it’s actually still young and sprightly!

How leaky is my house?

Home builders in 1961 didn’t pay much attention to making homes airtight. In fact, they probably hadn’t even started claiming that “a house needs to breathe” because pretty much no one was arguing otherwise, especially in Atlanta, Georgia. I don’t know what the air leakage in the house was before 2012, but I know it had to be a lot leakier than it is now. Here’s a little history of the air-sealing and air-leakage measurements over the past decade:

  • 2012 – My father-in-law, from whom we bought the house, had the attic encapsulated with open-cell spray foam insulation. It was a pretty bad job, and I’ll have articles and videos on that and what we did to fix it coming this year.
  • 2015 – My father-in-law had the crawlspace encapsulated and some other air-sealing work in the basement done by PV Heating & Air.
  • 2019 – I bought the house and did a blower-door test. The result came in at 6650 cubic feet per minute (cfm) with the pressure difference between the house and outdoors at -50 Pascals (cfm50), which translates to 11.3 air changes per hour at 50 Pascals (ACH50). The encapsulated attic was almost equally connected to the living space and the outdoors.
  • 2019 – Woodman Insulation came in and added more open-cell spray foam insulation (made by SES Polyurethane Systems).
  • 2019 – Blower-door test results: 5220 cfm50, 8.8 ACH50. That’s 2.5 ACH50 lower than before I had more foam sprayed in the attic. Nice decrease! And the attic is now much more connected to the living space than the outdoors, with zonal pressure diagnostics showing only a 5 Pascal difference with house-outdoor pressure difference at 50 Pascals.
  • 2019 – I had all the soffits, fascias, and gutters replaced, and seized the opportunity to do more air-sealing and put in more insulation above the exterior wall top plates.
  • 2019 – Blower-door-test results: 5018 cfm50, 8.5 ACH50.

So yeah, my house turns 60 years old this year, but at 8.5 ACH50, it’s not that much worse than the Georgia requires for new homes (5 ACH50). I’ve tested homes younger than this one that acted much more geriatric, with air-leakage numbers twice as high as my house. And when I get to gutting and remodeling my basement, I’ll make it even tighter.

Ventilating dehumidifier or enthalpy recovery ventilator (ERV)?

Until recently I hadn’t decided which way I’d go for ventilation. We have an Ultra-Aire ventilating dehumidifier at the Energy Vanguard office, and I like that approach. I’ll be adding some dehumidification later, but I’m going with an enthalpy (or energy) recovery ventilator (ERV) for our mechanical ventilation system.

And which ERV am I getting? Actually, it arrived yesterday, and it’s the Zehnder ComfoAir Q600. Most people who deal with high-performance homes will tell you that Zehnder makes the most efficient and elegant mechanical ventilation systems. It’s the dream system for people who know residential ventilation.

And it’s not cheap. I’ll be honest. I’m not a rich guy and am behind on saving for retirement (not that a young person like myself is thinking about retiring), so even though the Zehnder ERV is a the Rolls Royce—sorry, Tesla—of ERVs, I might not have gotten one on my own. But the folks at Zehnder America gave this system to me, knowing that I’d tell you about it and you might decide to get one for yourself. There’s my disclosure. Still, ask around and you’ll find to be true what I said about Zehnder being the most esteemed name in balanced ventilation systems.

Balanced ventilation arrives…sort of

Last week three pallets of Zehnder ERV equipment arrived at my house. You can see some of it in the lead photo of this article. The ERV itself will go into my basement mechanical room, next to my heat-pump water heater. Since I’m going to gut and remodel the basement, I’ll be using the ERV just for the main floor at first. I’ll run the ComfoPipes up to the encapsulated attic and use the Comefotubes (the round white rolls in the photo) to distribute the ventilation air to the rooms. I’ll have boost-switches to ramp up bathroom and kitchen exhaust when we need more ventilation.

Here’s a little video I made of the “unboxing” after I got the delivery.

It’s going to take me a little while to get the system up and running because I’ve got a bunch of people waiting for a book they pre-ordered, and that I have to finish writing it. (Here’s my book update page.) But I’ll provide occasional updates here as things progress.

Meanwhile, I’m counting on good indoor air quality in my young and sprightly house to help keep me young and sprightly because, well, I just turned 60 years old myself!


Allison Bailes of Atlanta, Georgia, is a speaker, writer, building science consultant, and founder of Energy Vanguard. He is also the author of the Energy Vanguard Blog. You can follow him on Twitter at @EnergyVanguard.

Related Articles at Energy Vanguard:

My New Project: A 1961 House With Home-Performance Angst

Seizing an Air-Sealing and Insulating Opportunity

My Undersized Ducted Mini-Split Heat Pump


  1. brian_wiley | | #1

    I was surprised and interest to see that an ERV is being added with the house at 8.5 ACH50. I was under the assumption that the air tightness needed to be much lower before it was necessary. Is there a typical threshold where it becomes helpful, or at least isn’t a waste?

    1. charlie_sullivan | | #5

      My reading is that the plan is to get that down a lot further in the future. I don't know whether this is being installed now because there was an opportunity to do so at low cost, or because of sequencing considerations with all the other work.

      1. brian_wiley | | #6

        Thanks for the follow-up, Charlie. When I first read that I started thinking that I’d have to sequence everything differently if ERVs were necessary at 8.5 ACH50. It being preemptive makes a lot more sense.

    2. GBA Editor
      Allison A. Bailes III, PhD | | #7

      Brian, I have a two-part response to your question. First, CO2 levels in bedrooms can be a problem when people are sleeping, even in a leaky house. Martin Holladay wrote about this in 2018:

      Ensuring Fresh Air in Bedrooms

      I just bought two Awair Element IAQ monitors and have been watching the CO2 levels in our house. We haven't gone over 1,000 ppm yet, but for the past week we've been sleeping with the bedroom door open and two windows cracked because spring has arrived. But we're still seeing nighttime CO2 levels hit 800 ppm. And the low almost never goes below 500 ppm, even though it's been less than 400 ppm outdoors. (Because the baseline was so high, I put both monitors outdoors this morning to calibrate them, and both quickly pegged at 400 ppm, the bottom end of the Element's CO2 scale.) My guess is that this summer I'll see the bedroom CO2 levels go above 1,000 ppm if we don't have the new ERV running, especially if we have someone in the guest room and our bedroom door is closed.

      The second is what Charlie Sullivan already pointed out. I'm planning to make the house much more airtight. Before the end of this year, I should have the basement gutted and air-sealed, and I hope to bring the air leakage down to less than 7 ACH50 then. When I replace the windows on the main floor, I hope to be able to get down below 5 ACH50, which is the point where the IRC says you need mechanical ventilation. (Sadly, Georgia home builders got their way with our last code update, so mechanical ventilation isn't required for new homes here until the air leakage is below 3 ACH50.)

      1. brian_wiley | | #10

        Thanks so much for the follow up, Dr. Bailes. That's really helpful to think of the reasoning behind the need. I'm sure we're in the same boat as you relative to a high ACH (our house was built in 1955) but with elevated levels in our bedrooms when we close the door.

        Because of it's draftiness, I had always assumed that we'd be okay with CO2 levels, but those articles you mentioned suggest otherwise. Looks like it's time to do a bit of data collecting to see where our house is at.

        1. GBA Editor
          Allison A. Bailes III, PhD | | #11

          Yep, I recommend monitoring CO2 and PM2.5. The Awair Element is a good device for that. Airthings has one coming out this summer that does everything the Element does plus radon and pressure. The Airthings Wave Plus does most of what the Element does plus radon and pressure. Unfortunately, they left PM2.5 out of that device.

          Another reason to have ventilation even in a leaky house is to deal with a pandemic like the one we're still in now. Just because you have a house that leaks a lot when the blower door induces a pressure difference doesn't mean you'll have much air moving through when you have people over.

  2. BirchwoodBill | | #2

    Curious on how it will be ducted, exhaust of air from bathrooms? Is there a 6 inch version of HDPE smooth wall pipe that we can use with other ERV?

    1. GBA Editor
      Allison A. Bailes III, PhD | | #8

      William, yes, we're exhausting from the bathrooms and kitchen, as well as the encapsulated attic and encapsulated crawl space. The supplies will go into the bedrooms and a couple of other places to balance the flows.

      I don't know the answer to your second question. The large Zehnder ducts are made of expanded polypropylene, which looks like expanded polystyrene with graphite in it and has about the same R-value, 4.2 per inch.

  3. AntonioO | | #3

    How much would all that have costed if the folks at Zehnder were not so generous? Also, I got the impression that you would be tackling this job yourself. Is this something a four out of five hammers handy person could tackle on his/her own or should it be left strictly up to pros like yourself?


    P.S. Don't know if you realized it, but if you're looking for your other watch, it's on your other wrist. 😊😊😊

    1. Trevor_Lambert | | #4

      I'm not sure how the hammers rating system works, but I installed my own system. I'd say if you're reasonably competent in building things and have some common sense, it's a simple job.

    2. GBA Editor
      Allison A. Bailes III, PhD | | #9

      Antonio, I believe the total cost would have been about $12k. I'm going to be doing some of it myself. PV Heating & Air here in Atlanta will help with the hard parts, like punching through my brick wall and a few other things. I'm definitely not a pro installer and have never installed one before, but in looking at videos online and other stuff, I don't think running the duct and installing the vents will be that hard. I'd say someone with 4 out of 5 hammers skill level could do it.

      The hardest part for me may be the prep work. I may have to move my heat pump water heater to the other side of the mechanical, and I definitely have to do some of the basement demolition to get started.

      ~ ab3

      PS Dang! I've been looking for that thing! Thanks, Antonio.

  4. Yeldog | | #12

    With VS equipment -- why not use the HVAC system?

    Have been using VS equipment for years -- they have very low fan settings for continious circulation. Even with zoning (I always do) ... they work great keeping fresh conditioned air in all the rooms. I run them in the winter as well w/ my radiant and radiators.

    My current house uses a dehumidifier to introduce the fresh. Debating on my new one that is just finishing up .... with the new VS equipment being so good at removing humidity ... my last one never needs the dehumidifier. Also with radon -- I like the idea of a slightly pressurized house vs what often happens with HRV/ERV.

    12k buys a lot of VS equipment upgrades and comfort

    1. GBA Editor
      Allison A. Bailes III, PhD | | #14

      Yeldog, I assume you you're asking why I don't use a central fan integrated supply ventilation system, which brings outdoor air into the return side of the air handler with a controller to determine when and how much air to bring in. That is indeed an inexpensive way to bring in outdoor air, but it doesn't give you any heat exchange. Also, the amount of outdoor air you bring in varies with the blower speed, so you may not be getting enough when you need it and at other times you may get too much. I can use a good ERV to exhaust the bathrooms, thus eliminating the conditioned air lost without recovery there as well as the penetrations through the roof for their vents, which I'll do when I replace the roof in the next few years.

      We spec ventilating dehumidifiers in some of our design jobs, especially in really humid climates, and we have one in our office. It solves the problems of outdoor air and dehumidification, and it's easier to know how much outdoor air you're getting with it than with a CFIS on a variable speed air handler.

      Yes, $12k can buy a lot of other equipment. But you can do an ERV that has the same functionality as the Zehnder at lower cost, so it doesn't have to cost $12k. If you want a whole ventilation system (equipment, distribution, controls) that will perform beautifully and do exactly what you want, Zehnder appears to be the best. Right now, I'm saying that based on years of seeing them installed and hearing about them from people who spec and commission them. Soon, I'll have my own personal experience to confirm that.

  5. SuesMan | | #13

    In regards to your 'heat pump water heater': in April 2017 I had an AO Smith HPTU50 installed in our 15 year old house. The heater was set to 'efficiency' mode until it began to have problems keeping up and then I set it to 'hybrid' mode. I have used a hot water loop with a ball valve to adjust the flow for the lowest flow for many years to provide warm water quicker to the furthest bathroom. In November 2019 the water heater failed to heat water, throwing error codes indicating that the heat pump was failing. Our plumber responded to our call and worked with AO Smith to determine that the heat pump was failing. To their credit AO Smith replaced the water heater for free with us being only responsible for the plumber's installation fee ($200+). In December 2020 the water heater again began throwing error codes indicating heat pump failure. Again our plumber worked with AO Smith to determine that the heat pump was failing. Again AO Smith replaced the heater at no cost except the plumber's fee ($200+). I complained to the customer service dep't about having to pay $200+ every year or so for a water heater which was supposed to save energy and money and that we paid a premium for in the first place. I asked for a refund of our plumber's fees. After much back-and-forth, they informed me that I should not be using a hot water loop with a HPWH as it made the HP run too much and lead to premature failure. This is clearly stated in the current owner's manual, but was not in the first manual or on the website when I purchased the HPWH. AO SMith has generously refunded the latest plumber's fee. I am of the opinion that the 1/4" PEX line that I use for the hot water loop with the ball valve turned almost closed is not wasting very much water at all compared to running the hot water tap for almost 1 full minute several times each day, but I have not measured the flow. On the latest HPWH I have disabled the heat pump and am using electric water heating only as AO Smith has indicated that they are not going to honor their warranty any more.

    1. GBA Editor
      Allison A. Bailes III, PhD | | #15

      Michael, that sounds like a horrible HPWH experience. I haven't heard about that problem before and wonder if it's specific to AO Smith. (Mine is a Rheem.) When I gut my basement, I'll be redoing all the plumbing for the house since it takes about 3 minutes to get hot water to the kitchen sink with the current system. If I do install a recirculation loop on the hot water, it would be the demand type. It sounds like yours is continuous, which I could see being a problem with a HPWH.

    2. kevinjmaas | | #16

      I put in a Geospring HPWH about a year before you, retaining the existing Grundfos recirculation pump. The heat pump did seem to run a lot, but I made the same calculation you did: heat radiated through the recirc line won't put much more of a load on the HPWH than running faucets for a minute to get hot water. So far the Geospring is holding up!

  6. trigonman3 | | #17

    Hi, Dr. Bailes.

    In an earlier comment section, you mentioned a switchable two-core HRV/ERV system (for an Asheville, NC client). Could you link to that product, or perhaps more usefully, give us search terms to find products from multiple manufacturers? I'm in climate zone 3 (north Texas) so can see advantages of both.

    Scott ALKB (trigonman3)

  7. qofmiwok | | #18

    How did you get Comfotube? I ordered Comfotube but they gave me Comfoflex; said they weren't selling Comfotube in the US anymore.

    BTW the total cost of my Zehnder system is over $25k, and that's with us doing some of the work ourselves.

  8. gawdzira | | #19

    I have installed a few systems. One piece of advice for the ComfoPipe, tape with Zip tape (other tapes will fail) at the joints as you assemble. It is a wiggly snake without stabilizing the joints as you go. Also, don't snap the black plastic fittings into the manifold plate until you connect the Comfotube. That way you can spin the black dealibobs on first and then snap them onto the manifold. Crap, I just realized this is an old article. So, maybe not advice for you.

    1. yeldogt | | #20

      Did you use them for bath exhaust ?

      I only did it once where I piped it using the bath as an inside source and thankfully my architect advised that I make provisions should I not like it. He was correct .... I did not like it it two ways. 1 - it pulled colder air into the bathrooms and the boost setting was not enough.

      What about make up air for a stove hood?

      There are so many variables when building house --- size and number of people can change so many aspects. Location is a big thing . Take my area of PA where I have always had a second residence of some type. High radon area -- do you pull air from under the slab (they all leak) -- or pressurize the house and have an open system w/o fan.

      Most people with 25k to toss at a secondary ventilation system are not building simple small houses

      1. qofmiwok | | #21

        Mine is almost passivehaus and we're exhausting 4 baths. Not sure why you were pulling cold air into the baths. The baths are supposed to exhaust so you should be pulling air from neighboring rooms which are warm.

        We expect to have a separate make-up air for the hood. But we have to see how the house performs. You're right, having only 2 people in 4000 sf changes things compared to 8 people in 2000 sf. I'm also in a high radon area and it took my architect a minute to get past his desire for positive pressure, but we have radon pipes running through the conditioned crawl under the slab, and I'm told we might not even need a fan on it due to the Zehnder ventilation also helping out. Again we'll see once everything is commissioned.

        1. yeldogt | | #22

          I used it in a deep retrofit of an 1810 federal in Philadelphia -- baths were kept warmer in the winter. The one bath off of a colder hall was most affected as it was always bringing that colder air into the room

          I am in the "positive pressure" camp in a radon area. IMO it's better to keep it out vs trying to move it out once it gets it. I can't comment on sub slab systems other than to say the guys who do the blower door tests will tell you they affect the house. Some really leak. There is no choice but to install a sub slab system when building in an area with the potential -- you don't want the radon to get it. My current rehab project was a 4 prior to the start .... now we are getting an occasional trace ... but --- I'm still finishing it up.

          In PA we need AC and unless very small house ... it's going to have a ducted HVAC system. Add in radiant if you have the $$ ... I'm not sure what you gain from the complex HRV/ERV. It's the old George Carlin skit about the cold box (refrigerator) inside the warm box (house) inside the cold box (Earth) .... what about the butter (now add another warm box).

          I often wonder how much conditioned air the typical radon systems takes from the house ... especially those with no foundation drain systems. Lots of systems that fight each other .. some very expensive

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