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Heating & Air Conditioning

matching a new gas furnace to an existing A/C unit?

5 years ago

I live in a house with a narrow 14" goodman gmp075-3 75k/60k btu output gas furnace that needs replacing, the a/c unit is a 2.5 ton goodman ckt30-1b, the problen is finding a furnace with similar specification most of the furnace past 60k btu in the market are 17.5" and wider. The tech offer a 14" carrier 58STA070-1-12 70k/56k btu output, 1200cfm 3-3.5 ton cooling size. Would this be a mismatch unit since the a/c unit is 2.5 ton? Our home is 1200 sq ft 1940's cape cod .

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Comments (8)

klem1
5 years ago
Matching WxLxH isn't necessary (and seldom possible) since furnace require's new transition anyhow. Most furnaces have multi-speed blower that can be set to match ac coil requirement. If you don't trust your installer,you surely shouldn't trust people on the net.
mike_home
5 years ago
Where are you located? I think a 75K furnace for a 1200 sq foot house is over sized. If you installed a 60K furnace like the Goodman GDS8 or GMS8 you can get it in a 14 inch width and adjust the blower to accommodate the 2.5 ton AC.
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Based on the information you've provided, sounds like the York quote is the best option, but I don't agree with the 80% modulating furnace. That's a high-end furnace, and going with the 80% version doesn't make sense to me, especially in our region where it can get cold at times (I'm in Maryland). Is PVC venting an issue? The York Affinity is a nice line of equipment. American Standard isn't bad, either, but the equipment that the York rep. is quoting is better in my opinion. 3 tons is a bit much for your cooling load, but it's what you'd have to go with if you went with the Affinity line. The 2.5-ton Latitude unit would be the better fit. With the low stage on the Affinity approximately meeting the cooling load at design conditions (which won't be met on all days), I don't think you'll be getting those long run times that you should with a 2-stage system. Ask the York rep. if he can quote the CZB or YZB 2.5-ton single-stage Affinity unit with a 90+ efficient v/s furnace (modulating or 2-stage). Not sure where the American Standard rep. came up with his sizing. Do you like it cold in your house (68-70)? Did the York rep. do the load calculation?
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Alex Lopez
Original Author
5 years ago
Mike i'm located in bergen county
mike_home
5 years ago
When the current furnace was operating properly, did it short cycle ( cycle on and off frequently) on cold days? If it did then that is an indication it is over sized. The right way to do this is to do a load calculation to verify the correct size.

You should also consider a 95% AFUE furnace. It will be more money, but it will be less in fuel costs. You may also qualify for a utility rebate.
Alex Lopez
Original Author
5 years ago
last modified: 5 years ago
Mike I dont think it short cycles, with thw temp set a 72° and a swing value off 2°f, on a 30° day the furnace turns on every 45 to 90 minutes for a period of 18 minutes to raise the temperature 4° degrees .
sktn77a
5 years ago
Based upon the above information, the small difference between 60kBTU and 56kBTU shouldn't be significant.
mike_home
5 years ago
Alex, if the thermostat is set to 72 then you would like to see it call for heat at 71 degrees. Waiting until it is 70, and shutting off at 74 is on the high side, but if you are comfortable then that is what is important. That large swing is helping to reduce the amount cycles per hours.

Raising the temperature 4 degrees in 18 minutes on a cold day also indicates the furnace is putting out a lot of heat compared to the heat load. I think you can safely go to a smaller furnace.
tigerdunes
5 years ago
last modified: 5 years ago
existing furnace is probably oversized...raising temp 4 degrees sounds good but a swing like that does nothing for comfort...I would change out thermostat when you install new furnace...you want an even temp setting to be maintained...add a filter cabinet at furnace return if you don't already have one...for a 1200 sq ft home besides the physical footprint needed, I would get a smaller furnace that can handle your 2 1/2 ton AC condenser...even a 60 KBTU furnace would be oversized.

IMO

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