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Community and Q&A

Fresh air distribution for hydronically heated house?

jchas | Posted in Mechanicals on

Currently have a leaky hydronically-heated house.

I plan on removing the siding, installing blueskin VP peel-and-stick, then 2 layers of Roxul R6 ComfortBoard IS from the roof right down to the footings.

Because this should really improve the air leakage, how should I get fresh air in/out of the house to control humidity / air quality / etc?

Do people run a set of air ducts throughout the house from a HRV?

One of the appeals of using hydronic heating is that sound / dust transmission is significantly reduced.

What would you recommend??


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  1. GBA Editor
    Martin Holladay | | #1

    It's very common for HRV installers to run ventilation ductwork in homes with hydronic heating systems. The ducts have a smaller diameter than ducts used for forced-air heating; most ventilation ducts are 4 inches in diameter.

    Even if your home had a forced-air heating system, you really wouldn't want to use your heating system ducts for ventilation. It's always better to have dedicated ventilation ductwork.

    For more information on this topic, see Designing a Good Ventilation System.

  2. slopecarver | | #2

    Don't forget to air seal the attic, much of the air leakage is from the stack effect of air in your house and walls leaking through openings in your ceilings and top plates of the walls. Depending on the complexity and age of your roof you could consider doing a PERSISIT roof retrofit or similarly with less destruction/construction a REMOTE type attic floor.

  3. user-2890856 | | #3

    Small ducts as Martin has stated . Only as big as the amount of air you need to move but not so small that noise is an issue . Well balanced and DON'T Skimp on insulating them if they are in unconditioned space , you cannot have too much insulation . Make sure they are very well sealed also .

  4. jchas | | #4

    Thanks for all the replies.

    It's a vintage 1942 house with plaster & lathe walls and cast-iron radiators. One of the really nice features is how quiet it is.

    It's sounding like, no matter what, I'll need to give up some of that quietness by running air ducts (albeit small ones) throughout the house if I air-seal the building envelope too much?

    Or is there a way to do it on a floor-by-floor basis without a central HRV?

  5. GBA Editor
    Martin Holladay | | #5

    I suggest that you read the article I linked to in my first answer. One option mentioned in the article (the use of Lunos fans) is a room-by-room solution that doesn't require ducts.

  6. jchas | | #6

    Thanks. Just working my way through 5 of your articles now...

  7. charlie_sullivan | | #7

    Since Martin's article was published, Lunos has added a new product that is a better choice for bathrooms and kitchens as it can work as an exhaust fan and when it is run as an HRV system, it doesn't spread the smells from those rooms around as much. With that and their regular ones in other areas I think it's a terrific system especially if you like quiet and want an easy retrofit.

    There's also a much cheaper copy-cat product available through Home Depot.

  8. jchas | | #8

    Joe, the thinking was to bring the entire structure within the air barrier (PERSIST?) by cutting off the roof overhangs and applying peel-and-stick over the entire roof, down the walls right to the foundation footings.

    Then applying external mineral wool board insulation outside the entire structure (roof & walls) and then furring/sheathing/shingling the roof and furring/siding the walls.

    I'm in a coastal climate (Victoria, BC) and the intent was to stop the rot problems that we've had due to air leaks/condensation in this 1942 house.

    Now I'm wondering whether ventilation is going to become an issue once I install a continuous air barrier over the entire outside surface. The house (before being wrapped) is currently 5.0 ACH50Pa


  9. Expert Member
    Dana Dorsett | | #9

    Ventilation is always an issue, even in leaky houses, since there's no guarantee that the air leakage is feeding the ventilation air where it's needed, or even that it is leaking in from a healthy place. (Eg, cracks in the foundation leaking air filtered by soil, or air leaking in from an attached garage isn't exactly spore-free contaminant-free stuff you prefer to be breathing.)

    Building it tight is always right, as is setting up the ventilation to take only clean air and putting it where it's most needed.

  10. jchas | | #10

    So it used to be that hydronic heating was promoted as a way to get a quiet, draft-free house.

    Sounds like those days are over.... With spot/central ventilation, you're still going to get the drafts and the sound conducted from one part of the house to another--no?

  11. STEPHEN SHEEHY | | #11

    John: heating and ventilation are two different needs, with two different systems designed to meet those needs. Your house heated with a hydroponic system still needed ventilation, but maybe relied on a leaky envelope. By tightening up the envelope, you now need mechanical ventilation. The good news is that spot ventilation, via Lunos or whole house via an HRV, are both quiet and the volumes are so low that there are no drafts. My HRV supply lines are only 24 cfm or less. They run 24/7. It's nothing like the sounds from hot air furnaces.
    Your boiler will be much noisier than your ventilation system.

  12. jchas | | #12

    Just to play devil's advocate here....

    Let's say I go crazy and insulate my house to R50 all over (yes I know that's not attainable if I have windows) and perfectly air seal it. Then my heat loss would be roughly 1/50 (=2%) of what a non-insulated house (just perfectly-sealed plastic tarps for walls) would be.

    If I then were then to add an HRV with 85% efficiency and 1.0 ACH, then I would be losing 15% of my house's heat per hour which would be equivalent to R6 even though the walls/roof were R50.

    I assume that HRVs run nowhere near that air volume (what do they normally run at?) but to me it illustrates that there's a point where the gains of tightening/insulating your building envelope is offset by losses due to the required increase in mechanical ventilation.

    Or am I way off in the weeds on this one?

  13. STEPHEN SHEEHY | | #13

    If your house is perfectly airsealed and insulated to R50, you could heat it with a few warm bodies. Fine tune it by adding or subtracting kittens until you are comfortable.

    I believe the typical HRV air exchange is around .30 per hour. Obviously, there is a point where your envelope is good enough and it makes no sense to spend anything to make it better. It's a subset of the Pretty Good House rationale.

  14. jchas | | #14

    At 0.3 ACH @ 85% efficiency, a house would lose 5% of its heat per hour (equiv to an R20 envelope), so it looks like ventilation is not a huge issue with the R12 I'm thinking of adding to the house's exterior.

    So that's good -- thanks!

    Right now, humidity is not an issue at all (even though we're in a coastal region 4 climate) probably because the stucco / tar paper / shiplap / plaster & lathe envelope is so leaky (5.0 ACH50Pa).

    In terms of deciding *when* it's necessary to add dedicated ventilation mechanicals (e.g. HRV), would it be prudent to do all the building envelope upgrades first, then do either a blower door test or see whether indoor humidity levels are consistently too high before deciding whether to add an HRV and all the associated ductwork?

  15. LucyF | | #15


    I really think you have to plan your ventilation along with the building upgrade because you've got to plan for the duct work, or in the case of the Lunos where the penetration in the building envelope is going to be.

    The Lunos is quiet. It's not really noticeable at all.

    What do you think your house is insulated to right now?

  16. jchas | | #16

    When I divide actual power consumption by heating degree days and envelope surface area, I get an overall effective R-value of R4.5.

    The blower door measured 5.0 ACH50Pa.

    The windows are 15 y-o double-pane vinyl so I'd assume they're no better than R2. Ditto for the 7 skylights.

    So my plan is to add two 1-1/2" layers of Roxul ComfortBoard IS @R6 each to the outside with a rain-screened Hardi-plank siding.

    The interior is all plaster and lathe with hot water cast iron rads so I don't want to start running bulkheads for ventilation if I don't have to. But I really like how there's no ductwork carrying noise through the house (noisy teenagers).

    But maybe I'm being naive...

  17. GBA Editor
    Martin Holladay | | #17

    Some of your posts are based on muddled thinking and an erroneous understanding of heat loss and R-values.

    You wrote: "Let's say I go crazy and insulate my house to R50 all over (yes I know that's not attainable if I have windows) and perfectly air seal it. Then my heat loss would be roughly 1/50 (=2%) of what a non-insulated house (just perfectly-sealed plastic tarps for walls) would be. If I then were then to add an HRV with 85% efficiency and 1.0 ACH, then I would be losing 15% of my house's heat per hour which would be equivalent to R6 even though the walls/roof were R50."

    The heat loss formula for determining transmission losses through floors, roofs, and walls is
    Q = A • U • ΔT.
    In other words, the rate of heat flow through a building assembly (in Btu/h) is equal to the area of the assembly (in ft²) times the U-factor (in Btu/ft² • hr • F°) of the assembly times the ΔT (in F°).

    R-50 = U-0.02

    If you wanted to imagine a badly built building with a heat loss rate (remember, we are talking about heat flow over time, which is a rate) that was 50 times faster than the rate of heat loss for a building with R-50 walls and ceiling, that badly build building would have R-1 walls.

    R-1 = U-1.0

    U-1.0 assemblies allow heat transfer at 50 times the rate of U-0.02 assemblies.

    If you want to see the effect of an HRV on your heating or cooling bills, you have to stop thinking in terms of "a percentage of my home's heat" (after all, it's a long way down to absolute zero -- even if every object in your home were at 0 degrees F, the objects would still have heat) and start thinking in terms of BTU (a measurement of energy) or BTU/h (a measurement of power).

  18. jchas | | #18

    Hi Martin,
    I agree with everything you wrote up until the R-10 part. I'm pretty sure that U-factor and R-values are direct reciprocals of each other.

    i.e. that R-1 equals U-factor 1, and R-50 equals U-factor 0.02. And, by extension, R-10 walls would have a U-factor of 0.2, not 0.02.

    Where I'm off is in my assumption that a non-insulated air barrier (like a poly sheet) would have a U-factor of 1. Instead, many single-pane windows are rated at U1.22 (R0.8) which, although close to U-1/R-1, is not exactly.

    I agree that U-1 assemblies would lose heat at 50 times the rate of U-0.02 assemblies. I'm just pretty sure that those assemblies would be rated R-1 and R-50 (not R-10 and R-50).

  19. GBA Editor
    Martin Holladay | | #19

    Thanks for catching my error. I have corrected my post.

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