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Community and Q&A

Keeping ERV, makeup air and range hood ducts AIRTIGHT

mpsterner | Posted in Mechanicals on

I am 2 days away from my blower door test and 8 months into our custom home build in Northern Wisconsin. We’ve (I think) impeccably detailed a contiguous air barrier and covered all our bases. There’s no combustion in the home and we’ve selected appliances with as little venting as possible (heat pump dryer and water heater, etc.). 

That said, I have (4) 8″ ducts coming through the wall: 
– ERV intake
– ERV exhaust
– Range hood makeup-air
– Range hood exhaust

The HVAC team installed mechanical dampers today on all 4 as that was going to be my solution to keep these as airtight as possible, however, upon inspection, well, see the picture… Each damper has two nickel to quarter size gaps around the shaft that are going to contribute greatly to infiltration/exfiltration. 

Is the ERV system actually already airtight at the ERV unit itself? Meaning, if I seal the ductwork up to the ERV, will the box itself provide airtightness? 

Are there better airtight mechanical dampers? What are my options to shore up this potential major leak? 

Thank you!

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  1. Expert Member

    A few small pieces of hvac tape?

  2. Expert Member
    BILL WICHERS | | #2

    You should be able to find better dampers. If you're OK spending the money, there are dampers that are rated to seal off ducts in fires (no, I don't have any part numbers handy, sorry), and those type seal very well -- but you pay a lot for those.

    I think your best option would be to go to the HVAC supply house and look at a few dampers to see which ones have the best design. Note that you'll probably have better luck having your contractor do this, or bringing your contractor with you. Supply houses tend to not be very helpful with this sort of thing unless you have an account with them.


    1. mpsterner | | #3

      Hi Zephyr,
      Thanks very much. After a quick search, I see that you're right... They're very expensive:

      Looks like they could be as much as $650/piece and I have 4. That seems probably not feasible.

      Could there be other less expensive but also more airtight options? I don't have any kind of supply places around me–everything has to be ordered online since we are far from any cities.

      Also, if I seal the ductwork up to the ERV, is the ERV airtight itself or does it allow free flowing air straight through into the distribution?

  3. Expert Member
    Akos | | #4

    Search for FAD-8. These have a leakage spec and has a rubber gasket around the gate which should get a tighter seal.

    ERV will (or should be) running 24/7, so those dampers are always open. How well they seal doesn't matter much. For blower door testing, you can cap them.

    The range hood/makeup ones will leak a bit, provided it has a decent seal over the rest of the area, I would guess the leaks will be near the noise floor of the flow meter. You can always quantify the leakage rate during testing by also capping these while the blower is running and comparing flow rates. If they are a significant leak, easy enough to swap them out.

  4. mpsterner | | #5

    Hello all,
    Just wanted to provide an update. We had our blower door test today and needed to hit 1.1 ACH50 for our load calculations to be accurate and planned heat system to work well and we hit .39 ACH50!

    The measurement was initially taken with the above described ducts and dampers taped off. We then ran the test again with the tape removed to see what leakage these dampers contributed and found that they only contributed an additional .02 ACH50, putting us at .43. With that, I think these dampers should suffice given their purpose.

    We still have many more measures that will be taken that will further tighten the home but I was pleased that these dampers didn't make as much as a difference as I thought they may.

    Still, I can see that the FAD-8 dampers that Akos suggested would be tighter with the rubber perimeter seal. They are on backorder though and I already have these installed so I plan to proceed as-is given the results.

    1. Expert Member
      MALCOLM TAYLOR | | #6


      Well done!

  5. Expert Member
    Akos | | #7

    Great job with the air sealing!

    Depending on how your make air damper is plumbed, you might want swap that out. Leaky dampers mostly an issue if connected to the return on your air handler especially if the return ducting is restrictive. I've seen returns that were close to 100pa at max flow, which can pull a fair bit of air if the damper is not tight. With a modulating unit, this would be happening 24/7, so it can add up to a fair bit of loss.

  6. tundracycle | | #8

    "ERV will (or should be) running 24/7, so those dampers are always open. How well they seal doesn't matter much."

    This is the first I've heard this. Ours run 20 mins / hr and with up to maybe 6 or 8 people in the house that keeps CO2 levels below 700 ppm and often below 550 ppm. If we have more people in the house (or during Covid, people from outside our immediate family) then we turn them on continuous.

    1. Expert Member
      Akos | | #9

      In the context of a new install with ECM ERV connected to a modulating heat pump, the ERV should be running 100% of the time.

      ERV efficiency increases as the flow rate through the unit decreases. Running an ERV at 80CFM all the time delivers the same amount of fresh air as at 160CFM 50% of the time. If you look at something like a Panasonic IB200, that is about 10% efficiency loss at the higher flow rate.

      On a modulating unit the air handler blower will be running all the time already, so there is no extra electrical cost for that.

      For most applications, I doubt this makes a big difference, but it is pretty close to free energy savings. This is how I set up the ERVs when paired with a ducted mini split. I don't even interlock the blower to the ERV, if the mini split is ever turned off, the fresh air is simply supplied to the house through the return duct.

      This math all changes if you are talking about a single/two stage air handler with PSC blowers, in that case cycling is usually the better answer.

      1. aunsafe2015 | | #12

        "This is how I set up the ERVs when paired with a ducted mini split. I don't even interlock the blower to the ERV, if the mini split is ever turned off, the fresh air is simply supplied to the house through the return duct."

        So you connect the supply from the ERV to what part of the ducted mini split?

        Or are you saying it doesn't even matter where you connect to the ERV to the mini split, because when the mini split is running, it will suck all the ERV supply air into the supply ducts, and if the mini split isn't running, then the ERV air can flow through either the supply or the return ducts with the same end result?

  7. mpsterner | | #10

    Hello Akos,
    We're pretty far along with our HVAC install and are about a week away from intending to begin drywall.

    The inspector came out today and was concerned that we didn’t have "balancing dampers" on all supply ducts so that the system could be commissioned and fully balanced out to deliver proper CFM.

    I do see that on my mechanical plan it stipulates:
    “Provide volume control dampers on all duct systems. Volume control dampers are not shown on plans. Dampers shall be factory-fabricated with zinc plated, die cast control hardware, control hardware shall include heavy gauge dial and handle with elevated platform for insulation duct mounting.”

    I assume these balancing dampers must be essential, correct? Why would the HVAC guys have not installed them or are there ones that you install after drywall?

    Does the system need to be commissioned pre-sheetrock in order to adjust these volume control dampers or are these still accessible in some way afterward?

    Thank you! HVAC guys are coming to fix a bunch of things on Tuesday so I'll try to have them address this then.

    1. Expert Member
      Akos | | #11

      There should at minimum be a set of balancing dampers to each trunk from the main plenum. These are needed as runs are never equal pressure and there is no other way to make sure even the longest runs get enough flow. The zoning dampers can also work for this by adjusting the travel stops.

      You also need one for each register, if this is a home run type of setup, the balancing dampers can be right at the unit. Otherwise just behind the register so you can take off the grill, reach in and adjust the damper as needed. You do have to be careful with these in a bedroom, especially if you really need to throttle one of the feeds as they can create noise.

      Register grills can also have their own adjustment but that is for fine tuning for individual comfort.

      Part of initial commissioning is making sure that each room gets the as designed air flow from the HVAC design. All these dampers and blower speed needs to be adjust until the you see the required flow. Make sure this is done before drywall goes up in case you have any kinky ducts.

      Short of designing the ducting EXACTLY to create just the right resistance at each location, there is really no other way of doing this, so yeah, the balancing dampers need to be installed. Preferably in a location where they can be adjust down the road if needed.

  8. mpsterner | | #13

    Hi Akos,
    Thank you for your feedback, as always. I met with the HVAC contractor today and reviewed everything and am a bit more concerned. We're only a week out from starting drywall and fear there may be changes required. A lot was said and I am not sure who to rely on here and you've been the only one that has been right all along...

    In summary, the HVAC contractor said that the ductwork can easily delivery the low CFM needs of each room. From there, you would just dial back each room by using the registers after everything is done. He indicated that they never do any steps beyond that (and no one in the area does). He indicated that he is willing to do "whatever I want" given that it is drawn into my mechanical design with balancing dampers and a commissioning process but is saying that he can't imagine why it is necessary.

    When asked about initial commissioning to make sure each room gets the designed airflow he said that you can't do an accurate commissioning before drywall and doors are in as those will change the air delivery. Is this true or should you always do a pre sheetrock and final commissioning?

    He also said that you can't shut a damper away in a closed space that is inaccessible. If we put dampers in we would have to add access hatches in our drywall. Is that right or is there a damper that is meant for this that is fine to close away in a floor joist cavity? What type of damper is used for balancing?

    He has indicated that we can at least do balancing dampers at the main trunks, including the specified zone damper that separates the upstairs. That said though, the only thing controlling the 95 cfm in the 2nd floor bedroom from the 35 cfm in the bathroom would be the registers at the floor.

    You had previously suggested that we use filter grills to keep our ductwork clean. I brought this up today and he said "we have a filter cabinet in the basement so we can't also do filter grills because of the static pressure." Is this okay or should I demand the filter grills?

    He also said that the design shows no air return in the basement and he thinks we must add one. It appears that the designers had basement return air covered by the "main" air return at the top of the basement stairs and then a louvered basement door, however, we actually turned the air return into the hallway with no louvered door. Can you advise on if it is okay to add a return in the basement?

    Most of the ductwork is accurate, however, there are some areas where our design shows a 7" where they used 6" ductwork, or a trunk goes from 10x8 to 8" round and they just did 10x8 the whole way. Furthermore, the design has grills that are 2x10, 2x12, 4x10, 4x12, 4x14, 6x12, etc. and instead I noticed that all of the ducts at the floor are the same size. None of them vary. Does that matter substantially?

    All of this said, what is important and what is not? I am confident that he'll do whatever I demand be done but I feel a bit like at this point he's followed the design but not super closely (duct sizing, register sizing, . Should I just have him own it and commit that when finished we'll be within 10% of the desired CFM air flows?

    Let me know what you think on how far I should take this. I am a little concerned that in a high performance home all of this may be more important and his "conventional experience" with no initial commissioning process might work fine for many homes but won't work well here.

    I've attached the design in case that's helpful at all.

  9. Expert Member
    NICK KEENAN | | #14

    "He also said that you can't shut a damper away in a closed space that is inaccessible. If we put dampers in we would have to add access hatches in our drywall. Is that right or is there a damper that is meant for this that is fine to close away in a floor joist cavity? What type of damper is used for balancing?"

    Any electrical device has to be accessible so it can be repaired or replaced.

    1. mpsterner | | #16

      What about if it is not electrical? These balancing dampers don’t have electricity, they’re just mechanically set for proper flow.

  10. rondeaunotrondo | | #15

    Seems like if you’re building a high performance home from scratch you should be getting all of the things you specified. This is very frustrating dealing with contractors who aren’t building to spec. I would want large return filter grills with a big box to help with reduce pressure drop. Would be nice to have 2-4” thick MERV 13 filters detailed previously by energy vanguard or home performance. I think Corbett only has one return grill in his home. I believe there are different opinions on 1 vs. multiple. J. Semmelhack will put in one return with similar features. Can you talk to your designer about these issues?

  11. joshdurston | | #17

    Commercially we use Tamco insulated dampers on most air handling units. They're spendy, but insulated and low leakage.

  12. Expert Member
    Akos | | #18

    No home owner will or should be expected to know to slightly close the supply register in the living room to get sufficient airflow to the corner bedroom. So it can't be just adjusting grills, there needs to be something that sets the system to about work from the start.

    From experience, I can tell you there is always at least one vent with airflows well bellow design, you generally spend most of the time adjusting the rest to get the required flow to these.

    You'll never get all the airflows just right, but it should at least be in the ballpark. If in the end all the vents end up flowing above their designed value, even better, you can now run your blower at lower speed which uses less power and is quieter.

    Even in the 40 year old hvac I'm currently working with had dampers for each vent. Most were on the main supply trunk, the rest at the duct boots. These should be in a location where they can be adjusted down the road or reachable by taking the register grill off.

    Return in the basement is not really needed, a well insulated basement has very low cooling and heating load, as long as you get some flow there, it should be comfortable. If you are in radon land, a stale air pickup for your ERV would be a good idea.

    Filter grills are your friend. A good filter grill with well sealed return ducting means your ducts and air handler stays pristine for many years to come. There are filter grills that take even 4" filters. If possible I would size both return filter grills to take the exact same filter as your air handler. A bit oversized, but now you only have to stock one type of filter.

    I can locally order with 2 day turn a Dayus DARH in any standard size that can take 1",2" or 4" filter (the local shop might tell you they only make a 1" one, but that is incorrect). If the HVAC guys won't supply them, have them roughed in for the correct dimension and order them yourself. They are easy enough to install. I prefer to get them with a wire hinge on one side and spring pins on the other side for easy access.

    As for the exact duct size and register size, as long as they can supply the correct flow, as is fine. Generally as long as the register is larger than designed it should be fine. I do like to keep floor register small and tucked close the wall, the 2" ones are great for this but they might be restrictive especially with a wood grill.

    Sheetrock won't effect flow much neither will doors with sufficient undercut or transfer grill. A well designed system would have less than 3 PA pressure difference between hallway and room which is WAY less than what pressure your air handler is pushing.

    You want to commission the system now as it is easy to fix issues. After sheetrock is up, any re-ducting will get messy very quickly.

    Just because it is not "commonly done" it doesn't mean it is not the right way. I was in a fairly expensive gut reno open house a while back and the master suite on the third floor was easily 15F above the rest of the house. Somebody took some shortcuts on the heating and cooling. It did sell to a lucky new owner but sure as heck I would not want to live with that.

    1. mpsterner | | #20

      Thank you very much. Hopefully they don't give me pushback on getting this done in the next 3 days before sheetrock! Might be impossible...

      A few things:
      - Any recommendations on best type of balancing damper? They had previously said that they can end up rattling and need replacing over time.

      - If I am using filter grills, should I also use the filter cabinet in the basement that the contractor mentioned? He said we can't use both due to pressure drop but is that true? Is the filter cabinet in basement the air handler filter that you mentioned using the same size as? Sounds like you suggest using both?

      - How far along should the system be to do this initial commissioning? I would assume that the filter grills and everything else need to be in place? Registers? This testing doesn't involve the heat pump though, just the air handler, correct?

      As noted, most of the ductwork is correct but the last piece where the register goes, they're all 2x12 where the system calls for 2x10, 2x12, 4x10, 4x12, 4x14, 6x12, etc. It sounds like this could potentially be problematic but its okay if they are able to deliver the CFM. That's the critical piece.

      Lastly, do you have a Venmo?

      1. Expert Member
        Akos | | #21

        At minimum have them put a flow hood over each register as is. This way you can see if there are any major misses.

        The indoor unit won't run without the outdoor unit at least wired, so if this is not the case, you might be SOL.

        The filters don't need to be in as long as you account for the change in static pressure. You can set the static pressure measured from the system as is to check flow rates. Once the filters are in, you can re-check the static pressure and adjust the setting to match the final install.

        The furnace filter is only really needed if you have leaky returns (mostly for retrofit). If you don't have any panned returns and the rest is well sealed, you don't need one. Two larger filter grills will generally be lower pressure drop than a single furnace filter.

        Flow rates for most of your registers will be low enough that I doubt you'll get much if any noise from dampers. I've never used anything more than the standard disk ones.

        A 2x12 is WAY smaller cross sectional area than a 6x12. It might deliver the flow but could also start to whistle with the grill on or the velocity might be too high if you are sitting near it.

  13. tundracycle | | #19

    What constitutes good airflow? How much of a temperature differential between bedrooms should be expected/acceptable? What CO2 levels in bedrooms at night should be acceptable? Other?

  14. qofmiwok | | #22

    Zehnder doesn't design any dampers into their intake/exhaust grates. They told me the air-tightness is all controlled by the ERV. But I'm looking for good ones for my make-up air systems for hood and for dryer. Checking out the FAD ones now.

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