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ERV/HRV design questions

TorBu | Posted in Green Building Techniques on

I am looking to spend a few weekends to install an ERV or HRV in my house – I’m considering the PanasonicFV-10VE2.

There is also the desire to have an A/C installed at some point, and I have a related question on that.

My house – 2,411 SF; 5BR, 3.5BA – has an attic that I can (almost) stand up in, but it is unconditioned. It has a ridge vent along with soffit vents. The soffit vents, however, are only every three bays (16″ o.c. rafter spacing).

My house is located in ZIP 94941 in Marin County, just north of San Francisco. (Climate zone 3(c?) )

As part of a recent full remodel, the house has been fully insulated with Rockwool Confortbatt (R-23 in the lower floor, R-15 on the first floor), incl. the interior walls, which have ‘Safe’n Sound’ in them. Each bathroom has a bathroom fan installed already. And the house has radiant floor heating throughout. 

My questions are:
1. What’s the best size for the duct in the attic, 6″ or 8″?
2. Can I have the EFV’s exhaust vent into the attic, since the attic is vented?
3. Should I use different sizing for the registers?
4. Should I use different type of registers for return vs. supply?
5. When I get to the A/C, could I get away with a single mini-split wall unit on the upper floor, and rely on the ERV to help distribute it into the upstairs bedrooms? (I’m only interested to have A/C serve the upper floor.)

Thank you, I look forward to everyone’s comments and feedback!

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  1. Expert Member
    Akos | | #1

    With the flow rate you are looking at, 6" duct is more than enough as long as you don't have sharp bends in it.

    Generaly you supply fresh air needs to go to all bedrooms, this might be an issue if you are trying to get to the guest room, and stale air is picked up from common areas.

    Vented attic is not the best spot for HVAC equipment, if you must install it there, make sure you insulate the lines the ERV, more important, air seal all ceiling penetrations.

    ERVs don't move enough air to provide any appreciable amount of cooling, so if you want to cool bedrooms with closed doors, you need to go with a ducted mini split (preferably not in the attic) and run ducts to the rooms. You can mount one up against the ceiling in the hallway between the bedrooms have a couple of short runs to the bedrooms and living space. This would be much easier if the space there is not yet finished.

    The outside air connections on the ERV need to go outside the building, venting into an attic of any kind is never a good idea. Panasonic has a nice tandem hood that you can use for venting through a gable end that makes this very simple.

  2. TorBu | | #2

    Thank you Akos for the response.

    For the ERV's outside air connections I will need to use an existing roof penetration or create a new one. I could find a tandem one that would support vertical through-the roof set up. Any recommendation for such set up?

    re: A/C, since we see only a few weeks during the year high temperatures here, I could deal with keeping the bedroom doors open, so the colder air from the dining and living area could 'spill into' the bedrooms. Would that be sufficient, or still not enough to move the needle?

    Lastly, does anyone have a list of tools to get - preferably with brand/product recommendations - to balance the ERV system myself?

    1. Jon_R | | #3

      > Would that be sufficient...?

      What is the bedroom load? And how much warmer can the bedroom be and be tolerable?

      You might look into radiant heating/cooling with an air to water heat pump.

  3. Trevor_Lambert | | #4

    A tandem vent is a bad idea, as you will get a significant amount of contamination of incoming and outgoing air. Ideally you want intake and exhaust separated by at least 6 feet, ideally 10 feet. Exiting vertically might also be non-optimal; it seems to me that either stack effect or just the difference in pressure between intake and exhaust could be a problem. Hopefully an expert can weigh in on that.

    Leaving the doors open to bedrooms can be an acceptable situation. It depends on your tolerance for temperature difference, the insulation and air sealing level of the house, and exact layout. My house currently has this setup, and as long as the doors stay open the temperature difference is minimal. We have a super insulated and sealed house, which helps, but the mini split is positioned near the top of the stairs, which probably hurts.

    1. Expert Member
      Akos | | #5

      Air from tandem vents exit at pretty high velocity, so unless these are in a very protected area like big overhangs, there is very little chance of mixing. They do work. I rather have a single hole through the building envelope and small chance of recirc.

      There are no roof tandem vents that I know of. You could potentially re purpose a 4" or 5" one designed for venting combustion equipment but now you are dealing with PVC in the fresh air system. Short length, so not a big issue, but the inspector might disagree.

      Most ERVs have a very steep flow/pressure curve, a couple of extra pascal from stack pressure won't effect the balance at all. The more problematic issue with roof vents is you can sometimes draw in smells from things like asphalt shingles or flat roofs. I would stick to gable or soffit connections at least for the fresh air intake. You can use a standard roof vent for the exhaust.

      The intlellibalnce unit is self balancing, you won't need any tools to commission it. Just set the dials to the desired flow rate.

  4. TorBu | | #6

    Thank you for all the advice and replies!

    @Jon, I don't have a Cooling load calculation.

    @Trevor, since your setup is similar to what I'm considering, I'm wondering whether you would mind sharing what type of mini-split - brand & unit - you have, since that is working well in your/that situation?

    @Akos, I agree with you on reducing the number of holes in the building envelope! Looks like though, I'll need to route the intake through the soffit though. I am thinking this rectangular wall vent will fit, right underneath the overhang:

    I would prefer to get the air intake in that case from the other side of the house - my neighbor on the nearer side sadly is making heavy use of his fireplace during winter times - however, that would mean that the fresh air pipe to the Panasonic ERV will easily end up being 40+ ft. Is such a long run for the fresh air intake going to be a problem?

    1. Trevor_Lambert | | #7

      I just have the cheapest unit I could get, since I only need it for cooling. It's a Senville SENA/09HF/ID (slim duct model, but not connected to any ducts at the moment). It's ok, but one thing I dislike is that it never really turns off. When it's "off", the fan is still quite audible and drawing about 25W. By comparison, the Fujitsu I have on the main floor drops to inaudible levels and about 3W. So I end up turning the Senville one on and off manually pretty frequently.

      1. TorBu | | #10

        I just learned that, because the wall where I wanted to mount the wall unit is not an exterior wall, I would need to make a tight bend that fits into a 2x4 wall AND run the lines up into the attic first, incl. the condensate line.
        That seems to complicate things significantly, because I'd need to add a condensate pump and the because of the tight bend.

        I may need to reconsider a horizontally ducted unit in that case in the attic. Perhaps I should rather create a separate post for my A/C needs...

        1. Expert Member
          Akos | | #13

          It is not an issue to run lines inside 2x4 stud walls. For bends, what you need to do is bring up the line about one foot away from where it would connect to the back of the head, instead of bending it stright out and having a hard 90, you bend it to go horizontal first and then gently pop it out of the wall to connect up to the line on the back of the mini split.

          If you measure things well, it will be in-line with the lineset connections on the back of the wall mount and no hard bends needed.

          Drains can also be run down inside the wall as long as you have floor or washer drain in the basement to connect it to. I always use black polyethylene irrigation pipe and fittings for these. Don't use any of the corrugated stuff that comes with most units.

          1. TorBu | | #14

            This probably wasn't clear from my original post, but my remodel was already completed last year. Running something in the walls down to the basement would require to rip everything open. All walls, including interior walls, have Rockwool insulation. And, there is no floor or wash drain anywhere remotely close.
            Going up would be the only way.

    2. Expert Member
      Akos | | #8

      You definitely want the fresh air on the side with least chance of pollution. Away from stoves and bbqs.

      At 100CFM the friction loss through a 6" duct is pretty small, as long as the flex is pulled tight and well supported, 40' is no problem.

      A regular wall vent in a soffit works no problem. I've also used ceiling diffusers painted to match.

      P.S. If you are in an area with forest fires, I would also install a filter cartridge that can take a HEPA filter. As long as you oversize this, the ERV should be able to pull enough air through it. A nice benefit of the filter box is the filters in there last much longer than the built in ones and are cheaper to replace.

      1. TorBu | | #11

        Sounds good. I will just have a longer run in the attic.

        And since I also have plans to add a makeup air connection using, e.g., a Field Controls FAD-6, I could bring in fresh air via an 8" duct into a MERV-13 filter, and the after that split into the makeup air line (6" duct) and the line to the Panasonic (6" duct).

        Would there be any recommendation for what type of manufacturer for filter cartridge box would be a good recommendation here?
        And it sounds like that, in that case, I can remove the filter that is in the Panasonic?

        BTW, because I have enough space in the attic, I was going to use rigid duct instead of flexible duct, except for the last 4-5 ft to the registers, unless that doesn't make sense?

        1. Expert Member
          Akos | | #12

          That could work as long as you size the filter properly. Most MUA need a lot more flow than the ERV, so it would be a very big filter if you want a high MERV unit in there.

          Pre-made in-line filters exist (search hvacquick) but tend to be expansive and require odd size filters. If you don't mind a bit of DIY, the best is to take a standard 16x25 4" filter box, cover each side with sheet metal and put on the 6" or 8" takeoff. I use a fantech FB6 but it doesn't support more than a merv13 filter, definately not large enough for MUA duty.

          I would not remove the stock filter, keep it there in case anything makes it past the pre-filter. With a good pre-filter, most likely you'll never have to change it.

    3. exeric | | #9

      I also have the same Panasonic unit you have. Since it has independent controls for both intake and exhaust fans you really don't need fancy equipment to balance it. I just got a CO2 meter and moved it around the house and found out what settings for both fans equalized the CO2 reading I wanted from room to room. It isn't a precision measurement but I felt comfortable doing it that way..

  5. Expert Member
    Akos | | #15

    Take a look at one way and two way ceiling cassettes for the mini split. These are narrow enough to be mounted between ceiling joists and have a condensate pump built in. It is a much easier install as you can run everything in the attic.

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