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Finding a furnace small enough ~22,000btu/h

user-917856 | Posted in Mechanicals on

A 1,100 sf home in Seattle I am working on needs a new furnace. The gas line is in place, the metal ducts were all sealed and insulated to R-15 and the walls and floor were also insulated to R-13 and R-30. Depending upon how tight the envelope becomes in the last wave of air sealing, the home will require ~22,000 – 24,000 btu/h, given my calculations with Manual-J 8. Now the trick is to find a nice furnace to put in there. Project’s priorities for the furnace:

Proper sizing/capacity
Sealed combustion condensing
AFUE of 90% or greater
Dual/multiple capacity
Will accommodate a return-side fresh air intake or similar

Can anyone suggest a current line of furnace to accomplish this?

Current water heater is electric, but will probably need replacement in the next 5 years or so. Combined hydronic is theoretically possible, but finding the right contractor and design on this type of thing would be really important. Maybe that contractor is you.

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Replies

  1. Hugh | | #1

    I've just renovated a 1967 apartment complex with 120 units and we've blown the walls with cellulose, added 1-inch of roxul insulation board, windows, and siding. We also ripped out gas furnaces and water heaters and installed a Laars II on demand gas heater with a closed loop exchanger in an airhandler. The units are as much as 3 bed 2 bath and do well. We anticipate heat requirement at 15,000 btu.

  2. GBA Editor
    Martin Holladay | | #2

    Kris,
    As far as I know, no manufacturer makes a furnace that small. You might want to consider using one or two direct-vent space heaters.

    I suggest you read Heating a Tight, Well-Insulated House.

  3. user-716970 | | #3

    You might also consider putting in a good condensing water heater and using it for double duty as your hot water source, and as your heat source via a fan coil and an air handler...

  4. Armando Cobo | | #4

    In NM, we've been using 40 gal. water heaters tied to HW coil in the air handlers to supply heat for small, well insulated homes up to 1700-2000 sq ft. Its very economical.

  5. GBA Editor
    Martin Holladay | | #5

    Kris,
    As noted in the article I linked to, a lot of Passivhaus designers are heating and cooling their houses with ductless minisplit units.

  6. David Meiland | | #6

    It sounds like the existing ductwork is driving a decision to continue using ducted air. I would also investigate the ductless minisplit options but barring that, look into removing the gas furnace and replacing with an air handler and heat pump.

  7. J Chesnut | | #7

    Kris,
    So this is a rehab? For budget reasons do you have to stay with a furnace?
    What is the ventilation system?
    What do people think of using the existing ductwork for a balanced HRV ventilation only system and as Martin suggests heating with a minisplit?

  8. Kevin Dickson, MSME, P.E. | | #8

    Seattle is the PERFECT climate for a minisplit.

    Even more first cost can be saved with a PTHP which are available for $500 and just plug in.

    https://www.greenbuildingadvisor.com/community/forum/passivhaus/18636/heating-small-house-pthp

  9. Kevin Dickson, MSME, P.E. | | #9

    And don't forget that with the heat pump options, air conditioning comes "for free". Even a home in Seattle will benefit from A/C for a few weeks in summer.

  10. Michael Chandler | | #10

    A Packaged Terminal Heat Pump might be able to tie into the existing ducts but the need to have it on an outside wall limits it's applicability. We are using an 11,600 btuh unit on a 1040 sf rehab in a similar climate here in NC. The unit were using has a heating COP of 3.2 at 40 degrees exterior but a cooling EER of only 10.2.

    Depending on the situation with your floors you may want to look at putting a few small coils of radiant heat in a few select locations. We recently did a house with radiant in mudroom, kitchen, entry hall and bathrooms only. It functioned like a "virtual woodstove" the warmth in those areas warmed the entire house.

    The small house mentioned above has radiant in the master bath, part of the master bedroom and the living and mudroom. It should be plenty to keep the house warm. We'll program the radiant as primary heatt on the PTAC T-stat with secondary being the heatpump and third the strip. the radiant runs off a small heat exchanger on the 145 kbtuh Rinnai condensing Nat gas water heater.

    Assuming subfloors are in you could add radiant zones by stapling 3/8" PEX to the sub floor, filling between with strips of backer board and tiling over the top of that on a crack membrane. Not the big fancy, expensive radiant solution you might be thinking of but great for a small, well insulated home.

    I'm writing this on a phone so please pardon any typos.

  11. Doug McEvers | | #11

    I would consider a Lennox G61V 36B-045. This is a 94% efficient, two stage furnace with a variable speed fan. This is the smallest unit they make and will work nicely, especially if you use a nightly setback of the thermostat.

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