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Community and Q&A

ERV Best Practices for Ventilation and Radon Mitigation

drafthunter | Posted in General Questions on

I live in the Northeast and plan on adding two Panasonic Intelli-Balance cold climate ERVs. This is an existing house, not new construction, hence the decision to go with two units, rather than trying to duct throughout the 3 floors.

Each unit will be set to 50 CFM and use one integrated wall cap for supply and return:

For ducting, a short run 6” insulated flex duct would go to a Zhender 4 port manifold to comfotube for both supply and returns (4 supply and 4 returns).

Unit 1 will ventilate the 1st and 2nd floors (1700 sq ft, 3 bedrooms). It will be mounted in a master bedroom closet on the second floor.

Question for unit 1, is it ok to run the comfotube in an unconditioned attic, but under 17” of blown cellulose and above fiberglass batts that fill the joists?

Unit 2 will be in the basement (500 sq ft finished, 250 sq ft utility and laundry room). One Mitsubishi 12k BTU heats and cools the finished basement, no heating or cooling in the utility room which is also where the passive radon vent is located. Finished basement temp is 65 degrees during the winter, unfinished is 50ish. Radon peaks at just under 5 picocuries. I use a RAdon Eye RD200 to constantly monitor. If I crank open a basement casement window (typical size) about a 1/2”, levels will drop to as low as under 1 picocurrie.

Questions for unit 2: Do you think 50 cfm would be enough to replicate the open window scenario? For maximum radon mitigation does supply and return register placement matter? In addition,  typically you would put a return in a laundry room, my plan is to instead put a supply. The thinking is to add tempered warm air to the room for comfort but also to assist the passive radon vent by making it warmer in the room during the winter. Does that sound reasonable?

general question: besides a zhender manifold system, how do people typically 4 way split out flex duct?

Any suggestions would be extremely appreciated. I have read every ERV/HRV article I could search for on this site and watched tons of YouTube videos. I’ve contemplated this install for months now trying to plan it correctly.

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  1. charlie_sullivan | | #1

    Q1. Probably fine. The concern would be condensation on the duct during hot humid weather, but the supply air won't be that cold--warming than the air in the house, and much warmer than in an HVAC duct. But maybe be more specific about the location, so we can look at how bad your summer humidity gets.

    Q2. There are two things that window does. One is it gives you some extra air changes. The other is that it equalizes the basement pressure with the outdoor pressure, so that stack effect suction isn't sucking radon into your basement from the outdoors. We don't know which effect is more important in your observed behavior, so we can't know for sure. So it's hard to predict what will happen. On the one hand, it's generally not recommended that one an ERV to try to solve a radon problem. But on the other hand, that unit allows you to independently vary supply and return flow rates, and you could set the supply higher than the return to counteract stack effect suction and perhaps set a similar effect to the open window. The hazard is that you could get a tweaked just so, and then forget to keep monitoring and something changes and it doesn't work anymore. But your levels aren't dangerously high anyway so maybe that's fine.

    1. drafthunter | | #2

      Q1 Long Island

      Q2. Great point about the pressure equalization. The Panasonic has a dial for both supply and return. If I set the return to 50, how much of a difference do you suggest to set the supply to? Oh and I’m OCD between the radon meter and my IQ air monitor lol. Plus I have it set to alarm if it hits 4.0 for more than 10 minutes.

      Thanks for your help.

  2. drafthunter | | #3

    Just an update for anyone who is looking to do an ERV for this purpose. I’m getting fantastic results, beyond my expectations, and better results than I’ve seen with active radon fans and vents going into the slab.

    See attached screenshots from my RadodnEye monitor. I installed the IntelliBalance 100 (Cold Climate), in April. The red line indicates the time period it was installed. Before that I would open a basement window during the day and close it at night. You see the big peaks of radon. Before that with the Windows always closed. I had a peak of 9 on the meter and constantly went over 5.

    Just mounting the unit on the wall and only venting fresh and stale are to the outside (I.e. no interior ducting yet), with the inside air ports facing down and about 2’ off the ground, radon was reduced to an avg of 1.5ish (maybe even lower). Once I installed small duct work of an exhaust vent just a little lower at about 6” off the ground, and 8’ flex duct going to a HEPA and carbon filter just laying on the ground and angled slightly up into the air, I get great air mixing and the super low radon. Unit is only set to 50 CFM total and self balanced.

    Note the only peaks you see after the ERV install, are very large rainy days. The largest peak being remnants of hurricane Ida. One of my next projects will be a French drain of some sort to get water away from the house.

    In addition, I have pristine air quality in the basement. An additional plus.

    1. chicagofarbs | | #12

      How is the non basement application working?

      I have a post a few down below yours about how to ventilate on an existing home without major retrofit.

      We have 3 levels, about 1,000 sf each.

      I’d love to duct all toilet exhaust and a general kitchen back to a single ERV but it’s not really feasible.

      I was thinking 1 per floor, maybe the Panasonic, just outside each bathroom to try and reroute the exhaust, and a general supply to the closest common area.

      1. drafthunter | | #13

        For the “non-basement” I’ve been pushing it off going into the attic. Hopefully soon. I ordered 4” R-8 flex duct. It will be buried in cellulose.

        I mounted it in a bedroom closet, which allows me to vent through the wall for that bedroom and the bedroom on the opposite wall. I then have to go through the attic for one other bedroom and two bathrooms.

        I have 3n floors as well. could go through the closet floor to vent down to the living room/1st floor.

        One thing I wish I would have know about before I installed mine is, check out the Broan dual port vent cap. It allows you to vent up to 100 CFM with one exterior hole to the outside. The Panasonic dual vent cap only allows up to 50CFM.

        1. chicagofarbs | | #14

          Thanks for the update.

          If we install one of these, I dont think we will be able to duct out to multiple rooms. Will have to prioritize where the supply goes since we don’t have ceiling exposed or an attic space to run ducts.

          1. drafthunter | | #15

            If you are NOT in a cold climate, you could always get the Panasonic spot ERVs. You could place one in each room. They are only rated down to around 40 degrees though.

            Otherwise I would prioritize the primary bedroom for the supply. I use an iQair air monitor, which is supposed to be very accurate. Two people sleeping in our bedroom spikes CO2 well over 2000 ppm over night.

          2. chicagofarbs | | #16

            @Drafthunter -

            Thanks for the suggestion. We are in Chicago, CZ5. Looks like we will need an option with defrost just in case. I think Panasonic has an option with that.

            We have 3 levels, approx. 1,000 sqft per level.

            We typically use 1 room on each level most frequently (basement = rec room, 1st floor = living/kitchen, 2nd floor = master bedroom).

            My hope/plan was to get a dedicated ERV for each level and supply to the spaces listed above. And if possible, route the bathroom on each level to its respective ERV unit.

            The alternative is something like the Lunos standalone through wall HRV (

            Currently, our home is pretty leaky with no mechanical ventilation, our air barrier is our drywall.... see attached image for our CO2 tracking in our bedroom. Very alarming at the sheer difference between our CO2 levels just from air tightness!

          3. drafthunter | | #18

            For some reason, won’t let me reply to your last post.

            Your CO2 levels actually fine, especially for two people sleeping in there. Looks like it’s well under a 1000 even at peak. I would be more concerned at your VOC’s. While not that high overall, it is high for a bedroom IMO. Mine is at 0 most of the time.

            Did you just paint, get new furniture or carpet?

          4. chicagofarbs | | #19

            Can’t figure out why our VOCs are that high.

            No paint or finishes.

            Could be from random products, like cleaners or beauty stuff, since we don’t have ventilation and it’s getting cold out, our windows aren’t opened much.

  3. forcedexposure | | #4

    Thanks for sharing!

  4. rondeaunotrondo | | #5

    Draft hunter,

    Did you buy the comfotubes/manifold from Zehnder directly? Did you duct to any other floors? What box did you use for your hepa filter?

    1. drafthunter | | #6

      I wound up using 4” flex duct for the supply and exhaust in the basement and 4” solid duct to exhaust from the kitchen. I installed a second unit in my bedroom closet.. It’s not ducted yet, just supplying fresh air to the primary bedroom. I plan on using 4” R8 flex duct to each bedroom (3) and two bathroom.

      Easier to add a second unit rather then ducting through 1st floor, to the second floor from the basement. Not an ideal layout but best for a retrofit without tearing open walls.

    2. drafthunter | | #7

      Oh and the HEPA I got from here, although I don’t know how great it is as I can still smell smoke from outside come through it:

      I think the Panasonic Merv-13 filters that I switched to, do a better job.

      1. brian_wiley | | #9

        Thanks for the update, drafthunter. It'd be interesting to see comparative pm2.5 for the inline HEPA filter box and the Panasonic Merv-13 if you happen to have it.

        1. drafthunter | | #11

          See my update below about the deficiencies of the HEPA box I have. Wouldn’t be great comparison. Right now it’s in addition to the MERV-13 filter.

  5. rondeaunotrondo | | #8

    Thanks, I would also be doing to help radon and smoke from wood burning from our neighborhood. Was hoping to do the CERV2 to help redistribute loads to a very small bedroom and thus would need to duct to the second floor. Interesting to hear that the hepa didn’t help with the smoke. The carbon filter didn’t help? I’m assuming it’s the hepa box plus merv 13 in the ERV so 2 levels of filtration?

    1. drafthunter | | #10

      Correct 2 levels of filtration. I think the problem is the HEPA box is not well designed and sealed well. The filter they make for it isn’t the exact size of the box and they use styrofoam inserts to make up the gaps. The opening at the beginning of the throat is too wide and barely covers the filter. The case it self leaks as well through the seams so I have to tape it. The carbon filter is a good one too, 1 pound of carbon.

      I’m hoping find a solution to the smoke. For now I have to turn the units off at night when the smoke is bad.

  6. 88cch | | #17

    Regarding your ducting of unit #1, you should check the static pressure specs on the ERV unit to be sure it can move the desired air through the ductwork. Be a shame to install all that ductwork and find out you have no/little airflow.

    1. drafthunter | | #20

      Unit 1 is only supplying fresh air to the basement. I actually use the radon levels as a guide to air balance and flow. Previously I had the stock Panasonic MERV-8 filter, combined with t he HEPA and average radon levels were around .40. Once I added the MERV-13 instead of the 8 to reduce smoke, it is now 1.1. Still way below what it would be without the ERV.

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