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HRV: connect to the air handler or not?

Dave Morgan | Posted in Mechanicals on

We are planning to put an HRV in our new home. My HVAC contractor is proposing that we connect the HRV fresh air supply to the return on the air handler. I have read here on GBA and elsewhere that HRVs work best when they have their own dedicated exhaust and supply ducts. His plan would call for programming the thermostat to run the AH periodically to distribute the fresh air. Please advise me, whether this plan would be a good one for our home (see below), or whether we should spend extra to have supply ducts run from the HRV.

We are building a pretty good, 1,590 sq ft house in climate zone 4c (western Oregon). We are using a conditoned crawl (R21), 2×6 walls with R5 foam, and stuffing as much insulation in the attic as possible (average R50). We are also carefully air sealing. The goal is to get to 2-3 ACH with the blower door.

The floorplan is open–basically two bedroom separated by a large kitchen great room. There will be a wood stove with outside combustion air that we will use daily from November through March. The forced air heat pump will be mostly supplemental, but that may change as we get older and lazier about cutting wood.

The crawl requires 32 cfm by code and this will be supplied by the HRV with a supply and exhaust duct. There will also be HRV exhaust ducts in each of the two baths. The living space will get somewhere between 30-60 cfm of fresh air, depending on whose standard you use. I am concerned that the HRV will be operating continuously, but the AH won’t (don’t want it to), and that the fresh air will not be well distributed in the AH ducting when the AH is off.

Are we better off to spend the extra $’s to put HRV supplies in a couple of places now? I am thinking the great room and master bedroom. Ideas? Thanks!

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  1. Expert Member
    Dana Dorsett | | #1

    On a single-story with a conditioned crawl I'd think it would be relatively easy to install a dedicated ventilation duct system in the crawl space. You could make the crawlspace the supply plenum for the entire house, and install only exhaust ducting. (Or is this a 2-story?)

    Also, I'm wondering what your HVAC guy is even thinking going with forced hot air? An open floor plan 1600' house with insulated foundation and 2x6 R21+5 and an R50 attic is going to have a miniscule heat load relative to the output of even the smallest hot air furnace. Since you're set up for point-source heating anyway, (the wood stove), it may make more sense to use a mini-split heat pump as your backup heat, which would have the benefit of high-efficiency cooling. (Is this Willamette valley, coastal, or ??? Got a ZIP code, for the climate specifics?) If you take that route, supplying the ventilation air to the bedrooms with jump-ducts or grilles into the great room, and having only ventilation exhaust registers for those rooms does a modest amount of temperature leveling when the doors are closed. Lowering the heat loads of the bedrooms by minimizing window area and higher performance windows may be necessary to keep the temp differences bounded, but this is a tried and mostly-true method used in high-R houses in my neighborhood (with outside design temps in the 0-5F range.)

    The 99% outside design temps for most of western OR (except at altitude) is in the mid-to high 20s, and unless you have ridiculously high window/floor fraction your heat load at 25F is going to be well under 20,000BTU/hr.

    Assuming a 15% framing fraction an U0.30 windows the hen-scratch heat load with a 70F inside, 25F outside comes in at something like:

    Windows: 240 square feet x U0.30 x 35F= ~2500 BTU/hr

    R50 attic: 1600' x 1/R50 x 35F= ~1100 BTU/hr

    A couple of U0.50 doors: 45' x U0.5 x 35F= ~800 BTU/hr

    The U-factor of 2x6 R21+5 walls comes in at about 0.050-0.055, and if we assume a 35 x 45' perimeter, rectangular shape and 10' from the foundation sill to the top plate you're looking at a perimeter of 160', and a gross wall area of 1600', less ~300' of windows and doors leaves 1300':

    Walls: 1300' x U0.055 x 35F= ~2500 BTU/hr.

    Add it all up, you're not even quite to 7000 BTU/hr yet!

    So... even if you throw in large fudge-factors for air leakage and floor losses you'd really have to work at it really hard to hit 15,000 BTU/hr (like leaving a window open?). Even 11,000 BTU/hr would probably be conservative, and that is with reach of a $1500 3/4 ton mini-split @ 25F (though you may need to upsize to a 1-ton to manage the cooling load if the house has "sunset views".) If the code inspectors balk at not having separate heat sources in the doored off rooms, a small electric baseboard is cheap and should satisfy them.

    Seriously, this is a low-load house, and most heating equipment is going to be grotesquely oversized (and UNCOMFORTABLE) if you don't plan and design adequately around it. Sure a 40KBTU barely legal efficiency hot air furnace may be $500, with another $500 or so in ducts, but if it ever needs to run, by virtue of being oversized by 4-5x it will be the opposite of comfortable. A mostly DIY install 3/4 ton mini-split comes in under 2-grand, a turnkey pro-install maybe $2.5K, but it better matched to & modulates with load, and it can air-condition (more important inland than at the coast, to be sure.)

    If it's strictly for backup for the wood stove and efficiency doesn't matter, electric baseboards are cheap and fill the bill- you're talking at most 3000-3500 watts of baseboard needed to cover your design-condition load.

    What kind of heat load calculation did your HVAC guy come up with, and what hot air furnace is he recommending?

    How big of a STOVE were you planning on? A 50,000 BTU/hr wood stove will roast you out at your average heat load. Something smaller, maybe even with some thermal mass like a soapstone stove may be in order to keep the sauna-effect damped down, otherwise you'll have to throttle back to a higher-polluting/less efficient burn mode.

    You probably DON'T want to run the HRV continuously, since there is no point to over-ventilating the place. Most people duty-cycle them, and crank the cycle up when you're having a party and doing a ton of cooking, backing it off to a much lower level when just grinding through the work-week. The ASHRAE 62.2 requirements are not well founded in science, and are the subject of much debate- feel free to back off from those considerably.

  2. GBA Editor
    Martin Holladay | | #2

    It's usually a bad idea to use the furnace fan (or the air handler fan) to distribute ventilation air. For one reason, furnace fans are much more powerful than necessary to move the small amounts of air (generally less than 100 cfm) required for ventilation -- so the fan imposes a significant energy penalty.

    Run some dedicated ventilation ductwork -- you won't regret it. For more information, see Designing a Good Ventilation System.

  3. Dave Morgan | | #3

    Wow, thanks for the thoughtful and detailed reply! To answer some of your questions,we are 45 minutes SE of Portland, OR (97004), the HVAC guy calculated a heating load of just under 20K btu/hr. We have a lot of windows in the great room and master, but they all face SE/S, so should give some good solar gains. The heat pump specified is a Bryant 213, 24,000 btu/hr unit. The wood stove is a small Hearthstone (Craftsbury)...they rate it at 40K btu/hr, but those numbers for wood stoves seem all over the map...they say it "heats up to 1,300 sq ft". Yes, the new BSC ventilation guidline puts our house at about 30 cfm, and the 2013 ASHRAE more like 60 cfm. I am betting we end up somewhere in the middle (muddle?).

    It turns out the three supply ducts for the HRV are only going cost $400...that seems like a no brainer now.

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