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Community and Q&A

ERV fresh air supply: just 1 or 2 supplies, or each room?

BJHuffine | Posted in General Questions on

I feel like this may be a dumb question, but I’m going to ask anyways because the answer may be more experience based.  I’m looking at ERVs manufacturer of the one I’m looking at doesn’t recommend more than one or two fresh air supplies in the house.  In fact, they don’t recommend putting them in bedrooms, but instead in high traffic areas.  The ERV is capable of providing 50-140 cfm and I’m needing just under 90 cfm for my home per the ASHRAE 62.2 standard.  I can do this with a reasonable provision of air in the home and assume they are stating this because maybe there’s been complaints with supplies in bedrooms?  That’s where I figure the community’s experience comes in.  If I do this, then obviously the benefit is less ductwork with larger grills since there would be less splitting.  If I do it the other way, I assume the more splitting also means smaller grills, smaller cfm at outlet and maybe not as much a nuisance?  What’s your experience?  Does it make sense to put them only in the high traffic areas?  Or bedrooms, den, and great room?  Ever wish you’d done something different?

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  1. rockies63 | | #1

    It sounds like you have a small house. I'm in a similar situation but I don't want to install ductwork so I'm going to use one of these.

    Even though it says they are for light commercial they can still be used for residential. This one will do up to 900 sq' and just mounts on the wall with no ductwork.

    If you wanted one just for a bedroom you could use a single room model.

    1. this_page_left_blank | | #6

      The bigger unit you link doesn't even come close to meeting his continuous requirement at high speed, let alone leaving any headroom for boosting. The single room models don't meet the minimum recommended airflow for a capacity of two people in a bedroom. You can't take the square footage allotment provided by the manufacturer seriously, because what is it based on? Nothing, as far as I can tell. Stick with ASHRAE, your local code, or some other recognized standard when it comes to sizing ventilation equipment.

  2. BJHuffine | | #2

    Thanks for the info Scott. I think I'm going to figure out the ERV situation first, but will definitely keep this in my back pocket. I've read good things about this kind of direct ventilation.

  3. Expert Member
    AKOS TOTH | | #3


    You spend a lot of time in a bedroom, it should have either a fresh air supply or a stale air return in there. Relying on your furnace to move the fresh air around works to an extent but does not guarantee enough air exchanges especially during the shoulder season.

    Depending on the efficiency of the ERV and whether or not you have an in-line heater, the air coming out of the fresh air supply would be colder during the winter months. Personal preference, this might not be comfortable for some.

    For my home, I have the stale air returns in the bedroom and the fresh air supply is in the main living space near the peak. This helps break up the stratified hot air plus does a decent job of warming the fresh air without needing an in-line heater.

  4. this_page_left_blank | | #4

    If at all possible, you really want fresh air in the bedrooms. Without registers in the bedrooms, you will get bad air quality when the doors are closed. I think there's an article here referencing a study in that regard.

  5. Expert Member

    I don't know where you are but start by checking your local code requirements. Ours mandates that each bedroom be supplied with fresh air.

    1. tundracycle | | #11

      Malcolm, by fresh air do you mean that your code requires a fresh air supply directly from an ERV/HRV or similar? Or just supply air? Where are you located that requires this?

      When I tried to get just the exhausts for our ERV's to come from bathrooms instead of the return duct for our furnace I got huge pushback from our HVAC folks and builder until I finally relented.


      1. Expert Member
        MALCOLM TAYLOR | | #12

        W. Ramsay,

        Our code here in British Columbia now requires continuous balanced mechanical ventilation with the supply air directed to each bedroom and main living area. There isn't a requirement that it come from an ERV/HRV.

  6. GBA Editor
    Martin Holladay | | #7

    I agree with Akos, Trevor, and Malcolm. You need fresh air in bedrooms. For more information on this topic, see "Ensuring Fresh Air in Bedrooms."

  7. BJHuffine | | #8

    Thanks all! Good comments from all and I've got that article up for my morning read Martin.

    So, I assume one of the reasons the manufacturer may be suggesting one or two places could be due to complaints of cooler air brought directly in the room (since realistically the efficiency isn't 100%) and potentially directly on the occupants. So to reduce this effect, what is best: basic grille (wall or ceiling), register with damper (wall or ceiling), or diffuser (ceiling)? I've been considering the diffuser approach for the fresh air supply with grilles in the wall for the stale air return (except for the kitchen and would use a round vent since there's too much competition for space).

    Also, if you don't mind me asking... if it is best to dump the fresh air in the bedrooms, what about the den, great room, and dining area? I also assume that the smaller amount of air flow into the rooms will balance well with the stale air returns? Right now I'm planning on a stale air return in each of two bathrooms, one in the laundry room, and one in the kitchen (opposite the adjacent laundry room door in case it is closed and far enough away from the range hood to help).

    1. this_page_left_blank | | #9

      I have 20cfm coming into each bedroom. I cannot feel the air unless I stand on a chair and put my hand within one foot of it. It doesn't feel cold, either. (This last part might depend on how efficient your ERV is, and the outside temperature).

      The bedrooms are the most important because of the doors. But having fresh air registers in various spots is a good idea not only for faster diffusion, but also it means lower air flow at each location (lessens the chance for a sensible draft). The locations you suggest sound good. Try to put the supplies at the far ends of the rooms they are in, relative to the direction the air is going to be directed (e.g. well away from the door to the bedroom, in the great room the opposite end to the nearest stale return, etc.).

      I can't guess why the manufacturer might be recommending only one or two supply registers. It may be what you say, it may be they get more sales by making the complete system less complicated and expensive to prospective customers. What is the make of the ERV?

      1. BJHuffine | | #10

        Thanks Trevor. The one I'm considering is the Renewaire EV 130. It has a cfm range of 50-140 and I calculated needing just under 90 cfm. So it seems to be a good fit spec-wise. Plus interchangeable parts, well known product, etc.

        I'm new to the ERV world and am at the planning stages for new construction. Hope to start building in the next month (fingers crossed). Do you put your register in the ceiling or wall? I ask because I've seen illustrations that indicate the wall approach, but to me that seems to take quite a chunk out of the top plate to fit in, so I assume if that was the case then a non-load bearing wall would be used. That's why so far I've thought I'd stick with ceiling registers, though I'll need to make sure they're sealed well. I was even thinking of diffusers to quieten the air flow and disperse the air more horizontally across the room as well. I'd be curious your thoughts and experiences here.

        Also, are there good trunk and branching guidelines? I know the basic 8" branches to two 6", a 6" to two 5", and a 5" to two 4". But the EV 130 has 6" diameter ports. That means at the most optimal branching strategy leaves me two rooms short. Unless I adapt to an 8" at the ERV (not sure if that's allowed though) and then I can get the last two rooms with a standard 4". Since I have little experience here, there may be something I'm not thinking of nor understanding can be done unfortunately.

        1. this_page_left_blank | | #13

          My registers are on the ceiling. There's no reason they can't be on the wall, but I would stick to interior walls. I've even seen diagrams with them on the floor, but I would be concerned about them getting covered up.

          Since you're still in the planning stage, I will suggest having a ceiling service channel (dropped ceiling). This makes it much easier to have leak-free recessed lights, registers, etc.

          I have just the cheapest diffusers I could buy, from Home Depot. Spreading the air around a bit is probably beneficial, but from a comfort perspective I don't even think they make any difference. There could just be a hole there, and other than looking bad, it would work fine.

          I can't offer any perspective on duct branching. My system is a distribution box each for supply and return, and a dedicated "home run" duct to each location. I think conventional wisdom would be to have 4" as your smallest duct size, and balance it with dampers. Hiring an HVAC designer for that is probably a good idea.

  8. BJHuffine | | #14

    Thanks, that's confirming. I saw the distribution box approach with the Zehnder product. Can that be used with other products? And I do plan to work with an HVAC contractor, but wanted to get an idea of the scope of work and functional design so that I can ensure we get it installed correctly.

    1. this_page_left_blank | | #16

      Theoretically, yes. The stuff is awfully expensive, which is enough deterrent for most people. I've heard of people who get a Zehnder ERV/HRV and skip the special ductwork, but never the other way around. There is a more generic option for the duct that I remember seeing, but I can't recall the name. The other thing is you'd want to run it all by your building inspector before spending much time on the idea, as some put up a stink about it.

    2. Expert Member
      AKOS TOTH | | #17


      The easiest is a reducing trunk approach, home running the ducts is a lot of extra piping.

      If you want, you can do a diy distribution box, ACCA manual D has guidelines numbers for sizing. Generally with ~100CFM of air flow, a 6" duct is enough for the main trunk.

      It is a good idea to put a 90 deg flex duct bend (it is bad for losses, so check that your layout allows it) into the ducts going to the bedrooms. This would prevent most of the sound traveling between them along the ducts.

  9. STEPHEN SHEEHY | | #15

    We've got exhaust in bathrooms and kitchen and one for the cat litter box in the mechanical room. Supply in each bedroom and living/ dining area. We've never perceived any drafts. Exhausts are in ceilings, supply on walls.

  10. rockies63 | | #18

    Trevor, don't manufacturers have to conform to ASHRAE and codes in order to design and promote their products. I mean, they come up with a number to say that their unit will be effective for a certain amount of interior square footage. They must do some testing otherwise why state the square footage at all? Are you saying that 71 CFM airflow (highest setting) is insufficient for a room measuring up to 900 sq'? If so, what is sufficient?

    I've also read on this site several articles stating that many experts believe the ventilation rates outlined in ASHRAE are too high. Doesn't this lead to installing systems that are too large and ultimately wasteful?

    1. this_page_left_blank | | #19

      As far as I can tell, there is no regulation for how they market their devices. There are different code requirements in different places. The cfm requirement is based on a combination of floor area and expected occupancy, so any suggestion for application based on size of room has to be only a ballpark figure. 71cfm is probably sufficient for 900 square feet, in most cases. However, you suggested the device to the OP, who had already stated that his design called for 90cfm. I think it's a good idea to size your ventilation unit to run continuously at half to two thirds of the max capacity. You always want some leeway, and running at the max speed is typically going to be rather noisy.

      I would say the single room devices are a poor choice for a bedroom. The air flow is too low even at max speed in recovery mode, and if you look at the noise level it's probably going to be unsuitable to use at max speed.

      There is some debate over whether the latest version of ASHRAE 62.1 specifies too high a ventilation rate. However, the portion disputed is the per area part of the equation. The per occupant part is unchanged from the previous version, and not disputed. That is the relevant part when you're looking at bedroom ventilation.

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