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Community and Q&A

HRV duct set up

Steve Vigoren | Posted in NAHB Class Discussion on

I am getting ready to install my HRV. I have been contemplating exhaust in the two bathrooms, three bedrooms, and kitchen area, with only one supply duct in the great room near my mini-split. 1300 square feet, one level, 3 1/2″ exterior foam, 6″ fiberglass batts, unvented sealed crawl space. The design person at the HRV store is said I should have supply to all bedrooms and just exhaust the bathrooms/kitchen. Climate Zone 7, so I am a little concerned about supplying all that fresh air to the bedrooms in the dead of winter. I am getting for my first blower test tomorrow, after which I will install the fiberglass between studs and get ready to sheet rock. Also need to install my duct work for the HRV soon so I can sheetrock the ceiling. Duct work will be in conditioned chase created by 2×6 joists with OSB then plastic sheeting, then roof trusses with blown insulation in the vented attic. Any comments would be appreciated.

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Replies

  1. User avater GBA Editor
    Martin Holladay | | #1

    Steve,
    Your HRV consultant is right. It's best to exhaust from your bathrooms, laundry room (if you have one), and sometimes your kitchen (but the exhaust grille should be as far away from the kitchen range as possible). Good places for supply air are bedrooms and the living room.

    The idea that unusually creative ways of ducting HRVs might help equalize indoor temperatures has been debunked, because HRV air flows are too low to move much heat around your house. For more information on this issue, see:

    Ducting HRVs and ERVs

    A New Way to Duct HRVs

    Does a Home with an HRV Also Need Bath Fans?

  2. User avater
    Stephen Sheehy | | #2

    Steve: If your HRV is an efficient one, just locate the bedroom supplies so they don't blow directly on people sleeping. I'm in Maine, with HRV supply into bedroom, and it's not a problem, even in really cold weather. Typically, you'll use only about 20 cfm or less per supply, so the airflow is imperceptible

  3. Steve Vigoren | | #3

    Thanks, guys. The blower test went very well, the guy who did it said it was the best he has seen. .05 something at 50 Pascals. I will probably buy the Broan HRV 160TE from them and he will set it when he comes back for the final blower test. Minnesota Power contracts out and pays for the blower test and pays $800 for a tier 3 rating house if it is all electric, which mine will be. He suggested putting the supply air ducts in the bedroom closets, cut the bottom of the doors and use bi-fold which are leaky any ways. My laundry is in the master bath (retirement home). And I will have a supply in the great room (kitchen, dining, living room) also. He said having the simple design helped greatly, one level rectangle. The utility also pays out for 40% of glass facing south, triple pane windows, and energy star appliances, and also the mini-split, so their bonuses will add up. Solar next year, I hope! I'm pumped about the blower test! Start insulating tomorrow and get the sheet rockers next week.

  4. Steve Knapp CZ 3A Georgia | | #4

    Steve. Congratulations! That's better than Passive House performance.

  5. User avater GBA Editor
    Martin Holladay | | #5

    Steve Vigoren,
    Did you mean to write 0.5 ach50 instead of 0.05 ach50?

    0.5 ach50 is quite good -- better than the requirements of the Passivhaus standard.

    0.05 ach50 is an order of magnitude tighter.

  6. John Clark | | #6

    Congrats Steve!

  7. Steve Vigoren | | #7

    Martin, I emailed the guy who did it to clarify. I am a little uncertain of the nomenclature surrounding this.

  8. Steve Vigoren | | #8

    OK, heard back from Doug, he says it was.05CFM/sqft @ 50 pascals. Is that what ach50 means? I should also say I did not have my exhaust fans/dryer vent cut in for this test, which I assume they will leak a little. A friend brought over a new Renewaire EV70 ERV, which says it doesn't need a condensation drain because it shares the humidity back into the house through enthalpy. I thought ERV's were not recommended for colder climates and I am in CZ7 in Northern Minnesota. Does anyone have any advice on this? The price is right, but if it doesn't do the job correctly,.....

  9. User avater GBA Editor
    Martin Holladay | | #9

    Charlie,
    When people talk about cfm50/sq. ft., they are usually talking about "per square foot of exterior building surface area," not "per square foot of floor area." We'd have to know something about the shape of the building to convert this number to ach50.

    That said, 0.05 cfm50/sq. ft. of exterior building surface area is an excellent and impressive result.

  10. Charlie Sullivan | | #10

    0.05CFM/sqft @ 50 pascals is more reasonable. ACH means air changes per hour.

    [Edit: Martin points out below that the sq. feet involved are probably envelope area, whereas my calculation below was based on floor area. I'll leave the calculation below here just for reference, because it might give the right order of magnitude, even though it's probably the wrong number.]

    To convert we need to assume some ceiling height, so I'll assume 8 ft. (0.05 CF/minute)*(60min/hr) = 3 CF/hour. With an 8 ft ceiling, you'd have 8 CF per square foot of floor space, so you have 3 CF being changed per 8 CF total in an hour, so your ACH50 is 3/8 or 0.375. That's well below the 0.6 passive house standard, but within the range of what is sometimes achieved by really good builders.

  11. Jonathan Lawrence CZ 4A New Jersey | | #11

    CFM from a PH perspective also depends on whose standard it is. PHI uses a european formula used in their real estate world that subtracts out closets, utility rooms and the such. PHIUS is basically drywall to drywall.

  12. Steve Vigoren | | #12

    I checked my email. He did say his program goes by square feet of floor space. My house is one level 48 x 28 with nine foot walls and an unvented three foot crawl space that was open to the house during the test, so basically a twelve foot wall. Using Charlie's calculations, 3/12 = .25. I feel my house has to be very air tight, but I can't help but be just a little skeptical that it is this good. I did give my carpenter a copy of the remote manual before we started and he was very interested and put a lot of effort into accomplishing what I wanted to do.

  13. User avater GBA Editor
    Martin Holladay | | #13

    Charlie,
    Sorry! You were right.

  14. Steve Knapp CZ 3A Georgia | | #14

    Steve. On your ERV question, see this article: https://www.greenbuildingadvisor.com/blogs/dept/musings/hrv-or-erv

    I think it will clarify things for you.

  15. Pete Marthaler - Zone 7 | | #15

    "Duct work will be in conditioned chase created by 2x6 joists with OSB then plastic sheeting, ..."

    Mr. Vigoren,
    Do I understand your original post correctly in that you are using essentially joist pan ducts for ducting of your HRV?

  16. User avater
    Dana Dorsett | | #16

    "...conditioned chase created by 2x6 joists..." doesn't read like a panned joist.

    It seems to describe a horizontal service cavity/chase between the finished ceiling and the the bottom chord of the roof trusses, with an OSB layer on the underside of the truss chords. Something along the lines of:

    https://www.greenbuildingadvisor.com/blogs/dept/musings/service-cavities-wiring-and-plumbing

  17. Steve Vigoren | | #17

    48 x 28 home with a center wall (fourteen feet across the 28 foot span). On top of my nine foot walls I put 2x6 spanning to the center wall directly under where my trusses are. I sheeted the 2x6 with osb, covered the osb with poly, taped seams, then set my trusses. From inside the house I screwed the 2x6 to the trusses with 7 1/2" screws every two feet throughout the house. I am using the 2x6 space for wiring, recessed lighting, and ventilation duct. Attic insulation will be on top of the osb/poly in the vented attic (blown celllulose), and sheet rock is on the underside of the 2x6. No insulation in the 2x6 chase, therefore it is conditioned space. The front half of the house (great room, master bedroom have nine foot ceilings, The back half, spare bed rroms, spare bath, master bath, back entry, has eight foot ceilings, so I have more room for duct work. I got the idea from the article referenced by Dana, and a Pretty Good House in Maine that was talked about on this site.

  18. Steve Vigoren | | #18

    I am using round 4", 5", and 6" duct work for ventilation.

  19. Jeff J | | #19

    I have also been thinking about exhausting air from bedrooms, as well as bathrooms, and supplying in only 2 locations. The main location would be right behind a pellet stove in the living room. Significant heat would be picked up here as the air would go right into the stove's return inlet, be heated and sent out to the living room, which is open to most of the house.

    I am concerned about making the current cold air situation worse upstairs. The upstairs has no dedicated heat, it relies on the fairly open design and a central stairway/loft area where the center of the house is an open 2 floors with high cathedral ceiling. Currently, cold air from the bedrooms finds its way out into the central cathedral area and starts a bit of a convection current.

    I could probably help relieve this by adding heat in the bedrooms. My thinking is, rather than add more cold air to those rooms, why not exhaust 20 cfm from each, thereby encouraging the warmed air from the pellet stove to move into those rooms. Thinking this may rob the convection current of its source of cold air.

    I also noted a potential advocate of exhausting from bedrooms in one of the articles sited above https://www.greenbuildingadvisor.com/node/20298

    I could probably set it up this way, if it didn't work well, it wouldn't be hard to move the ducts around to change the 2 bedrooms back to supply air.

    Any thoughts?

  20. Jeff J | | #20

    To be clear, I mean exhaust air into the HRV.

  21. D Dorsett | | #21

    An open doorway at a temperature difference of 5F moves a LOT more than 20cfm. Relying on the 20cfm for temperature balancing would be bailing against the tide with tea spoon.

    20 cfm = 1200 cubic feet per hour. At a 5F temperature difference. moving air with a specific heat capacity 0.018 BTU per cubic foot per degree is 108 BTU/hr.

    Putting that in perspective, a sleeping adult human is good for sourcing about 250 BTU/hr.

    What is the design heat load of the bedroom?

  22. Jeff J | | #22

    sorry I don't know, must be more than 100 BTUs... We are getting heat up there, it just doesn't have a clear circulation path. Maybe we would need to pull more CFM out of there to keep the air circulating the right way, in the door, out the duct. My bigger idea is to do some heating with that new duct...

  23. Jon R | | #23

    Be careful with only a supply into a closed door room. Even 1 sq inch of door undercut per CFM will more than double the amount of moisture that stack effect would push into the walls.

  24. Jeff J | | #24

    These rooms will be return only, the supply will be in the big open part of house. The idea here is to exhaust air from the bedrooms to hrv, to encourage circ of pellet stove heated air from downstairs up into these upstairs bedrooms, in a fairly open house. I could increase the cfm from these rooms until it was enough to keep the flow consistently in the right direction - in the door, and out the exh duct to hrv. The house as a whole would be balanced

  25. D Dorsett | | #25

    A typical hot air furnace runs a few hundered cfm per room at a high temperature difference to fully heat a room at the 99% outside design temp in a climate zone 7 location. Your wall U-factors are about U0.035-ish, and your outside design temp is likely to be -15F give or take a few (probably not much colder than -20F) which would be an 80F difference from a 65F/18C bedroom. An 10' x 8' section of exterior wall is 80 square feet with a heat loss of

    80' x 80F x U0.035= 224 BTU/hr for that much wall area.

    If there is a 10 square foot U0.25 window the window loss would be:

    10' x 80F x U0.25= 200 BTU/hr

    Add up all the wall area and all the window area and run that math, add them up, then subtract 250 BTU/hr per human, that will be pretty close to the design heat load. If you know the manufacturer's published U-factor for the window, use their number. If you know the 99% outside design temp for your area, use that (instead of -15F), and calculate how much colder that is than your desired room temperature.

  26. User avater GBA Editor
    Martin Holladay | | #26

    Jeff,
    Here's the short version: You can't use an HRV to correct room-to-room temperature differences, because ventilation airflow requires small volumes of air (not many cfm), and because you don't have any ducts delivering high-temperature air (as you do with a furnace).

    If you have room-to-room temperature differences that you want to solve, you need to solve them by other means: adding a heat source to the bedrooms; leaving the bedroom doors open for a greater percentage of the day; air-sealing the walls and ceiling of the bedrooms; adding window quilts to the bedroom windows; or thickening the insulation in the bedrooms.

    Forget about solving the problem by making adjustments to your ventilation system.

  27. Jeff J | | #27

    Thank you both D Dorsett and Martin.
    Well, quick calc gives me about 3,800 BTU per hour at 80 deg delta from the larger bedroom, including windows and cathedral ceiling. So, I see that several hundred BTU won't cut it.
    - Except in the coldest weather, that room stays within probably 5 deg or so of the downstairs temp with the door open. Since almost all the heat is coming through the door, it must be a turbulent mess of air movement at that door. So, your point would be it would take enough CFM to provide the required heat to stop that turbulence and allow all airflow to move as I want, away from the door.
    - The turbulent cloud around the 3 upstairs doors continue to feed the convection current in the open 2 story area that is conveying heat from downstairs to upstairs.
    - so, 1 problem at a time, eh? If I were to try to solve both, I would need substantially more CFM and heat. Something like the lifebreath clean air furnace perhaps, and larger ductwork would be one possibility for that. Another would be do the hrv and add baseboard heat to the upstairs rooms. However, I'm afraid the Tstat for baseboards would be set lower, and the problem wouldn't be solved that way. You could try to isolate the bedrooms by making sure they got some internal heat and the doors kept closed (to stop feeding the convection loop). I wonder if undercutting the door for the 20 cfm vent air would be enough to keep that much moving.

    Helpful education, thank you very much.

  28. User avater
    Stephen Sheehy | | #28

    Jeff. I suspect the supply air behind the pellet stove will really be make up air for the stove and not get into the living space. As I mentioned in an earlier comment, a good hrv won't supply air much cooler than ambient air temperature.
    Also remember that you need to balance supply and exhaust.

  29. Jeff J | | #29

    thanks Stephen, my idea was to supply most of the air from the hrv to a register behind the pellet stove where it would then be pulled into the stove return, heated and blown into the house by the pellet stove room supply fan. The pellet stove does have sealed combustion, with a separate combustion air duct and internal fan.

  30. Jeff J | | #30

    I should ask what seem to be the most efficient, higher quality line of HRVs for avg size residential?

  31. User avater GBA Editor
    Martin Holladay | | #31

    Jeff,
    Zehnder HRVs.

  32. Jeff J | | #32

    thanks Martin, will look harder at those... I see there are some in the 90% eff, also a nice duct connection block. What else am I looking for feature wise?

  33. User avater
    Stephen Sheehy | | #33

    Jeff: We have a Zehnder HRV. It was pricey. Zehnder designed the system from our drawings and sent someone out to commission it. Easy change filters. My hvac guy was impressed with how well made every component was and how easy it was to install. It's very quiet and has a nice boost feature when you need extra ventilation from showers or cooking. It's also very efficient.

  34. Jeff J | | #34

    Very good Stephen, I see they want to quote it from the web site. Will give it a try. Glad its working for you. I am planning on using high speed fan for shower exhaust with airflow regulating dampers on the exhaust inlets in the 2 bathrooms. The duct system looks pretty simple

  35. JAMES KREYLING | | #35

    The Venmar E15 ECM HRV, now sold in the U.S. as the Broan HRV160TE is a fine HRV, not quite up to the Zehnder efficiency, but far more affordable. It is a unit worth considering. It is what I will be using in the new home I am building. Note that the Broan HRV 160T has psc motors and the "TE" model has the lower wattage ECM motors.

  36. Steve Vigoren | | #36

    As it turned out, the Broan HRV160TE is what I went with. I don't have it installed yet so can't answer to how well it works. It was recommended by the outfit that did my blower test. Northern MN, 25 - 35 degree F at night, 40 - 55 degrees daytime, I am heating my house to 65 degrees with a 1500 watt thermostatically controlled space heater, nothing else in the house, currently have the sheet rock taper there, :-)

  37. Jeff J | | #37

    Will be interested to hear how the Venmar/Broan works out

  38. Sean W | | #38

    We've had our Venmar E15 ECM ERV (now comes with Broan label) for a little over a year now. Exhaust in 2 baths and 1 near kitchen, supply to living room and 3 bdrms. Around 2100ft² single level with an absolute maze of ductwork. The cfm delivery, in spite of the convoluted ductwork, was very impressive to our installer. Efficient and easy to maintain, cleaning filters and core.

    The Altitude (may have a different Broan name now) wall control allows several options for ventilation rates. From 20 minutes/hr on up to continuous with an option for a max boost level.

  39. Jeff J | | #39

    Sounds good Greg. Do you use the exhaust boost for shower exhaust, etc? Does it provide enough exhaust?

  40. Steve Vigoren | | #40

    Greg, that is my set up also, my home is a little smaller, 1350 square feet. I thought the Broan may be a little oversized, but with the ECM motors there are options to slow down the system as needed. I put in Panasonic bath fans so I am hoping they take care of the shower humidity for the most part.

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