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Advice: ERV, 400+ CFM range hood and make-up air?

Roger_S39 | Posted in General Questions on

Hi all,

Need input on what to consider when thinking about range hood exhaust cfms for a home that will have an ERV, please.

1. What is the correct way to ensure “things/a home” operates as it should when a house has an erv with dedicated duct lines plus a range hood exhaust that is over 400 cfm?

2. If a 2 story, ~3500 sq ft, new construction home has an ERV installed, should one try to install a range hood exhaust with 400 cfm or less?

3. Would it be ok to install a 600 cfm unit in a home that has an ERV unit? If so, are there any special things to do differently, when considering a make-up air strategy for a home that has an ERV with dedicated duct lines?

Thank you.

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Replies

  1. Nicolas Bertrand | | #1

    I am also curious about this. I was wondering if some of the modern ERV combination units like a CERV2 or Minotair could be hooked up to function as make-up air for a kitchen hood? Do they have a function where they could run as an inlet only for the house and have the exhausts blocked? This way the hood could do the exhaust work and the ERV supply some of the required air?

    I know a dedicated return-air supply is normally required for a tight house, and sometimes with an inline heater if it is cold outside, but maybe there is a way to rig the ERV to take it's place?

    Just thinking out loud and on the same page as Roger.

    Nick

  2. Malcolm Taylor | | #2

    Roger,

    I don't think the presence of an ERV makes any difference to the problems of makeup air for appliances that unbalance ventilation. No ventilation, passive, or mechanical ventilation should all yield similar pressure balance in a tight house.

    For range hoods over 400 cfm you need dedicated make-up air, either mechanical or passive. It's entirely a personal preference as to whether you install a range hood that big. The simplest solution is to select one small enough that it isn't an issue.

    I don't know of any ERVs that can be run in an unbalanced mode to provide make-up air, but maybe other posters have heard of new advances that make it possible.

  3. Roger_S39 | | #3

    Thank you for the responses.

    Here is where the situation is at:

    1. There will be an ERV with dedicated duct lines.
    2. There will be a gas cook top with 6 burners that totals 90,000 BTUs.

    Therefore, the crossroad is:

    A). Stay with a vent Hood of 400 cfm and limit burners used at same time to not go over 40,000 BTUs (based on reading ~100 cfm per 10,000 BTUs).

    B). Go with a vent Hood of 600+ cfm, to allow multiple cooktop burners to be used at same time, and hence incorporate a make up air system.

    The concern, reason to pause, is if option B is chosen, does a make up air system need to be done differently bc a home has an ERV present?

    Also, if multiple burners are on at a medium temp, could one get away with a 400 cfm vent Hood and eliminate the headache and cost of incorporating a make up system into the home.

    Just trying to get it right whilst house is only in the framing stage and want to avoid regrets/mistakes down the road.

    Thank you for reading and all replies.

    1. Malcolm Taylor | | #6

      Roger,

      If I were you I'd size my range hood to provide good ventilation for the stove I want, operated without restrictions on which burners I could use. If that means it needs make-up air, then the two best options are probably a dedicated vent in the kitchen with a mechanical damper, or to crack a window.

      1. Roger_S39 | | #8

        Hi Malcolm,

        Thanks for the reply.

        If one incorporates a make up air system, does it make any difference/ pros v cons of:

        A). Dedicated vent in the kitchen with mechanical damper ( if possible could you elaborate a little on this, please?)

        B). A dedicated duct line from outside to return side of HVAC unit with a damper in the duct line that opens once a vent cfm of 400+ is used?

        Thank you.

        1. Malcolm Taylor | | #9

          Roger,

          Martin's blog summaries the situation and answers your questions much better than I could: https://www.greenbuildingadvisor.com/article/all-about-makeup-air

  4. James Howison | | #4

    I agree that the advice on this is pretty inconsistent. I particularly don't understand the "if it's less than 400 cfm then you don't need makeup air" as Malcolm says above. Is that just a reading of the code? Because I immediately think, I mean that air is coming from somewhere, so it's definitely an issue.

    Long video on this from Corbett Lundsford https://www.youtube.com/watch?v=fOTJmUhghIo on his "Home Performance" channel. He has just about every system, but he also discusses a pressure sensor which can turn on the (I think) ERV and a separate Make-up air system, when needed.

    I guess I like the simplicity of one system that can bring in conditioned supply air, whether for ventilation, or by operating range hood exhaust, bathroom exhaust, dryer exhaust. But perhaps the issue is that ERVs are sized only for ventilation air, and are therefore just not able to keep up with other appliances. So that's why Fantech (and others) sell dedicated make up air systems?

    1. Malcolm Taylor | | #5

      James,

      - Yes it's from our code. The assumption with bathroom exhausts, dryers, and range hoods under a certain cfm is that they don't unduly depressurize a house. Tight houses still have a certain amount of air-leakage and pressure differences caused wind and the stack effect. Unless you get to Passive House levels of air-tightness, these appliances shouldn't cause problems.

      - The problem isn't the sizing of the ERVs, it is that they supply balanced ventilation, so whether turned off, or operated at any level of cfm, they still don't supply make-up air.

      1. James Howison | | #13

        Ah, got it Malcolm, thanks for your note. So you'd have to run the ERV in an unbalanced mode to get any "net positive" air in. But I think some can essentially run the supply fan faster than the exhaust fan, but I'm guessing that's for tuning for balance and not for large make up supply.

  5. Roger_S39 | | #7

    Thank you.

    Really enjoy that YouTube channel and was thinking there maybe some good info there!

    Will rewatch that episode to look for related info.

    Agree that if 400 cfm is pulled from the house, what happens if there is no make up air for it.... As an ERV, as one understands it, is less that 400 cfm but needs to be balanced with in vs out air... Correct?

    It's easy to get overwhelmed once we start to become MORE aware of housing components and basic "what they do; how they work; and why they are needed".

    It gets to be... In isolation things make sense but how do they all fit together to create "balance"/ a home that once all components are installed, results in a "box" that exists in a state of "homeostasis" and not the opposite.

    1. Nicolas Bertrand | | #10

      I have done a lot of reading and research on this for my own house I'm building. My wife is a serious cook and will have a large Thermador cooktop (gas). She has told me that she is having a good hood over it, end of story. I can't argue with her because I don't like smoke or grease fumes in the house, and she cooks like no other! So, I've pretty much compromised on something around 600-800 max CFM (variable) for the hood. Most likely it will have a connected make-up air system with dampers and possibly an inline heater for the colder months. It's not great for a super airtight house that is trying to conserve energy, but it's not like she cooks 8 hours a day or anything. At most it would get used a couple hours a week I'm guessing.

      I know you can get systems that are activated by the switches in the hoods, but I'm sure there are others that can be done manually, but you have to remember to do it. Opening a window in the summer works, but if you need an electronic damper for the winter, just let it do the same thing in the summer.

      My biggest qualm with these systems is to get one that is airtight when not in use. I never have much faith in little flimsy dampers. It would be nice to have a product that has a serious damper that seals tight and is insulated as well.

      Nick

  6. Akos | | #11

    Depressurizing the house is a problem if you have combustion appliances, for those you definitely need a MUA.

    A 3500sqft house would have to be extremely well sealed to have pressure issues with a 600cfm hood.

    Say for example your house is 3ACH @50PA (standard construction). You have a dryer, 600cfm hood and 3 bathrooms with 50cfm fans, max total around 900cfm.

    Assuming 9ft ceilings, that is about 1500cfm at 50PA. At 900cfm you'll be depressurizing the house to around 30PA at most. This would be too high for sealed gas appliances, but not enough to really effect the performance of most range hoods or electric dryer. The amount of depressurization is actually not even high enough to significantly unbalance the ERV.

    1. Malcolm Taylor | | #14

      Akos,

      I think you are right that in the absence of any combustion appliances it probably doesn't matter, but the proportional effect of the range hood is a bit higher than you suggest. Dryers are typically in the 200 to 250 cfm range, so the total of other appliances when all are in use is closer to 500 cfm, not 900. Add to that the 3x tighter envelope most posters here aim for and that 600 cfm range is proportionally a pretty strong factor in depressurization. I don't know en0ugh about ERVs to say if those numbers are a problem.

      1. Akos | | #15

        With a 1ACH 3500sqft house, we are looking 520cfm at 50PA. If we go with 500+600 cfm, you are looking at around 110PA pressure ~0.4" wg assuming the blowers in the units are up for the job.

        Most range hoods are rated for close to 1", for example a Broan 331 (600cfm rated) would still flow 500CFM at that pressure. A Panasonic whisper green set for 100cfm would still be around 60cfm.

        My ERV would loose about 10% flow on the exhaust and gain about the same on the fresh air intake. I don't know how well a dryer would do, my guess is you would definitely see some performance drop.

        I think overall, for a large electric only house, this doesn't feel like a big issue. Range hood don't run for that long, plus you can always crack a window if you need to run all these exhaust fans together at the same time. I guess you can put in a MUA, but I wouldn't do much more than a well sealed barometric damper.

        A house with a combustion appliance is a different story.

        EDIT. One thing I realized that this amount of negative pressure would put a fair bit of pressure on exterior doors. Depending on which way it swing, you might get a surprise when you open it. A barometric damper is a good idea in case one forgets to crack a window.

        1. Malcolm Taylor | | #16

          Thanks Akos, good to have some numbers as confirmation.

          My own 25 year old house, built with not much assiduous attention to air-sealing, needs to have a window cracked when attempting to light the wood stove if either the dryer or (small) range hood is on.

          1. Akos | | #17

            Open combustion appliances are a whole different story. For those, the pressure limit is around 5PA, so in the above case a couple of bath fans running would cause trouble.

            Wood burning stoves and air tight houses are not easy.

  7. Nicolas Bertrand | | #12

    For example, my home I'm building will be around 3,200 sq. ft, but the ACH will be around 1 or less, so that makes a difference when they get that tight.

  8. T Carlson | | #18

    In a tight house the fans starve and the actual CFM exhausted is much less than rated. So to answer your question, unless you provide makeup air your actual fan exhaust output will be much less, so you either up size the fan and it starves down to what you need or give it makeup air. That's all those smart Panasonics are doing, running bigger and harder. The only difference is, if you leave a window or door open they dumb their speed down, how many people in super tight houses do that in the winter?

    Wiring your hood circuit to open your ERV intake....well I'm sure it can be done, IDK, I'd listen to a case for it but it seems over complicated, then you'd need to do bath exhausts and the dryer, possibly a powervent and train it to vary its dampers and fan speed. Mines not that smart.

    As a real world example: house at .55 - .6 ACH50.

    I tested my bath fans with my Alnor hood and the 80 cfm Panasonics were doing around 40 if I recall, my 110 CFM Deltas w/ humidity sensors were around 80 CFM.

    I can retest and get current numbers if you are interested.

    I didn't test the hood output because I didn't feel like making a hood extension, however I did test the depressurization of the hood with the house closed and static with my blower door frame in on a DG700 gauge.

    I have a 350 CFM hood over a propane cooktop and it puts some serious depressurization on the house even with a 6" gravity dampened makeup air intake in the basement. Crack a patio door or a window and you get a blast. Again I can retest if you are curious and get the actual numbers. Keeps me from paperwork.

    In my opinion you should oversize fans and let them work harder to get you what you need. Provide makeup air to the house from the laundry room to give the dryer a little extra to work with. You shouldn't have problems. That's what my house has been doing for just over 3 years. If I burn a motor I'll be sure to post about it and then I'd let my 6" fresh air intake with a gravity damper and filterbox operate into the basement to provide makeup air and leave the HRV as a separate system doing what its supposed to be doing.

    I use my house as an example because it's convenient to monkey with and only has the ability to piss of my wife versus customers which may or may not be worse...whatever...that said I have never had a callback for any type of issue with burned out fans or depressurization complaints or anything of the sort but I build in the 2-3 ACH50 range, maybe some get down to 1.5ACH50 but I don't cater to the customer seeking better than that so my own house is my only test bed.

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