90,000 BTU's for 900 sq ft Too big???

Discussion in 'HVAC Heating & Cooling' started by Hip2u77, Jul 1, 2011.

  1. Hip2u77

    Hip2u77 New Member

    Jul 1, 2011
    I'm in the process of getting bids for a new condenser, furnace and humidifier after my 21-year-old compressor shorted to ground.

    I've got a 1958 ranch house in Kansas City. Around 900 sq ft. of conditioned space with an unfinished basement.

    Per my choice I was going to have company "A" install a full Goodman setup (my choice) of an 80% 2-stage furnace and a 13 SEER condenser. We figured out I could actually save $250 after rebate if I moved up to a 14 SEER unit.

    The original estimate from company "A" (and 3 others) all called for a 2.5 ton condenser and 70,000 BTU furnace. (I now have a 65% 75,000 btu and 8.5 SEER 2.5 ton setup now.)

    They're now saying to get the 14 SEER unit I'll have to move up to a 90,000 BTU furnance since the coil is bigger. They said it's pefectly fine since it will run on it's 60,000 BTU low setting most of the time. (He earlier said it would run on low for 10 minutes then move to 2nd stage until it hit the set temp.)

    Am I nuts for thinking that's too big for a 900sq ft. house? Not a single company has done any load calcs on the house. "It's a basic ranch house. . . no need to."

    Sorry for rambling.

  2. jimbo

    jimbo Plumber

    Aug 31, 2004
    San Diego, CA
    Most Goodman furnaces are available in 4 widths ( A,B,C,D) for the very reason that it is cooling and heating BTU are not the same and it needs to accomodate varying coil sizes and blower speeds. Have him double check to see if he can get a furnace to match the needed coil. I don't know if this is your exact model, but all have similar variations:

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  4. hj

    hj Moderator & Master Plumber Staff Member

    Aug 31, 2004
    Cave Creek, Arizona
    An undersized unit might run continually to keep up with the heat loss, (or gain in the summertime). An oversized one will cycle the burner on/off more frequentily but that is not as big a factor with heating as it is with cooling.
  5. Runs with bison

    Runs with bison Member

    Aug 23, 2009
    Be careful about that rebate you are counting on if it is from the electric utility. Many systems with nominal SEER ratings don't actually qualify for the utility rebates because of EER rating...not SEER. (This gets really FUBAR with two stage compressors that run 16-18 SEER often not qualifying because they fall just short on max load at max outside temps...never mind that they only run in that mode less than 10% of their operating time.) There is an ASHRAE site to check for certs to see if coil combos qualify and if they don't fit in specific EPA tiers (on another website) the utility won't give the discount. It's rather misleading. This was the same problem that was arising with the Energy Tax Credits on the AC units the past few years.

    Gas utility rebates for high efficiency furnaces were not a problem from what I saw.

    As jimbo noted, the furnace frame size is being set by that cooling SEER requirement. Be certain about that rebate. The 90,000 Btu/hr unit is likely about 3x too large for 900 sq. feet plus basement in KC. I've got a 90,000 Btu/hr 2-stage 95% RUUD in the same climate and 2700 sq. ft...and my furnace is nearly twice what I need at -10 F. But as in your case, it's size was set by the AC (4 ton for me) so I couldn't reduce the furnace size. The low stage is nice because it runs longer and keeps the temp more uniform. The only times the high stage ever fired was when it was in recovery mode after having been turned off or a large setpoint change was made.
  6. Dana

    Dana In the trades

    Jan 14, 2009
    90K would be HUMONGOUS for 900 square feet unless you have no insulation and left a coupla windows open. 2.5 tons cooling might be on the high side too, unless your ducts run in the attic rather than the basement.

    Odds are your +6F (97.5 percentile heating design temp for KC MO) is well within the output of a 2.5 ton 2-stage Goodman split-system heating/cooling heat pump, or a continuously variable 30K Mitsubishi Hyper Heat split system, (Or it could be with a bit of air sealing and spot-insulation on the house.) If your ducts are in the attic, that alone could be literally half your AC load too, making retrofitting a smaller split-system more desirable. (No duct==no duct losses, and no duct heat-gain in a hot attic.) At KC's ~30F mean January temp and local utility pricing, an R410A-refrigerant ductless split system would be cheaper to heat with than 80% gas.

    How much gas (in therms or ccf or whatever) did you use from November-March? With some comparison of fuel use to heating degree-day data it's possible to put a solid upper bound on what the design condition heat load is. It's probably not much more than ~30KBTU/hr @ 6F, and it could easily be under 25K, in which case with 90K of burner output you'd be good to at least -120F (are you moving the house to a high-altitude part of Antarctica? :) )

    At the +6F outdoor design temp R410A most ductless heat pumps still have a COP of ~2, and the Mitusubishis still guarantee 100% of rated output down to +4F. (At -10F the COP has gone to hell, but they still deliver more than 70% of rated output.) I'm less familiar with the Goodman split systems, not sure how they rate at sub-zero temps, but that's not a sustained condition very often in KC.

    With an unfinished, uninsulated basement you can probably cut your whole-house heat load ~20% by putting an inch of XPS foam + and 2x4 studwall with UNfaced batts. (Unfaced to keep from trapping ground moisture in the studwall, and XPS to allow the foundation to dry to the interior rather than raising the moisture content of the concrete and rotting out the foundation sill.) Sealing & insulating the foundation sill to at least R5-R10 with cut up foam board + 1-part can-foam is also cost-effective. The basement would then run warmer winter & summer, reducing mold potential, and reducing overall utility costs.
  7. BadgerBoilerMN

    BadgerBoilerMN Hydronic Heating Designer

    Mar 4, 2011
    hydronic heating designer/contractor
    90,000btuh is just right...for snow melting. The best way to distinguish the professional heating cooling contractor from the imposture is to ask for a sample of his heating/cooling load calculations.

    It would be foolish for a good contractor to size up your house for free (drive-by-sizing excepted) but proof that he knows how and language in the contractor that he will produce a Manual 'J' load calculation before beginning your work is one of the best ways to find the real deal.

  8. jefferson17

    jefferson17 New Member

    Nov 12, 2010
    Bristol, PA
    Consider Ductless Split Heat Pumps

    You should seriously consider a split ductless heat pump system. The rear of our Victorian home really takes a beating from the sun (2 levels, one of top of the other), and after a LOT of research decided on a Dual Ductless Split Heat Pump system from Fujitsu. We only needed a dual system but they are available in single, dual, tri, quad and even more variants.

    They are FAR more efficient than standard gear that uses ducts (you will lose a good amount of efficiency in your ductwork, especially if they aren't properly taped at every seam). Our system gets about 17 SEER. Our AC bill last month was only $50, with some record highs and it set to 76 degrees all day/night. That's how efficient these things run - they use the newest inverter technology (run low and slow to MAINTAIN the set temperature. And they are also heat pumps, so they will keep your house warm in the winter without using expensive gas or oil.

    I paid only $5000 for a dual 12K heat pump system with 24K condenser. I kind of assume that you'll need a tri or quad setup, so that you'll have small units (7K btu or 9k btu in the bedrooms, etc. So your cost be be a bit higher than mine, but you will experience a lot of long-term monthly savings to heat and cool your home.

    While you are at this, make sure that you have a nice thick layer of insulation in your attic! It costs very little to add more on top of what you have, and this also makes a difference.

    Oh - and you may be able to possibly get by with a mere 24K condenser - IF - you only need a tri system. Here's a link that should really help you:


    You just need to figure out for EACH ROOM, what your BTU MAX load will be. If you have lots of trees blocking the sun and wind, then that helps you. This isn't rocket science. A quick calculator is here: http://www.*********.com/webapp/wcs/stores/servlet/THDCalcRoomAC

    There are MANY more, that factor in other things. A good basis would just be to input your measurements here, and raise it to 9' ceilings to provide some wiggle room.

    Bear in mind that these heat pumps have INVERTER technology - so they will run only as hard as they need to to keep your home cool or warm. Conventional systems are either on or off and really quite wasteful by comparison. Those convential ducted systems at are supposedly 13/14 SEER really are not - you will lose at least 10-20% in the ductwork. Perhaps more if they weren't properly taped up at the seams ...

    Good luck to you and please consider the best technology. Even if the system costs a bit more, it will REALLY save you that money back - and FAST.
    Last edited: Jul 16, 2011
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