Is my Furnace guy pulling one over on me? (BTU question)


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Old 02-09-07, 04:02 PM
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Is my Furnace guy pulling one over on me? (BTU question)

Hello,

About 7 months ago, I had a new 80% natural gas furnace (forced hot air) installed to replace my 1968 80,000 BTU unit. I also had a new central A/C unit installed - there hadn't been one previously.

I didn't notice until recently, but the new furnace is only 54,000 BTU. Now my house is a 1939 Cape Cod (approx 1000 square feet), wooden siding and without any insulation in the walls at the time of the furnace install. I noticed that the air coming from the ducts feels colder than it did with the other furnace. I've even verified that the fan speed is correct for the "heat" setting.

When I called the furnace installer, he said that 54,000 BTU was fine and that it wouldn't have made sense to put another 80,000 BTU unit in there. He also said that the air coming out of the ducts wouldn't be any warmer with a 80,000 BTU unit.

So what do you guys think? Is he full of it? Am I full of it? Would there be any legitimate reason to replace a 43 year old 80,000 BTU unit with a 54,000 BTU unit?
 
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Old 02-09-07, 04:53 PM
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What did the heat load say to use?
 
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Old 02-09-07, 05:00 PM
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Originally Posted by airman.1994 View Post
What did the heat load say to use?
Honestly, I've no idea. I'm not 100% sure what you are referring to, but if it's some kind of calculation the installer should have done, I never saw it.
 
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Old 02-09-07, 05:03 PM
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"Would there be any legitimate reason to replace a 43 year old 80,000 BTU unit with a 54,000 BTU unit?"

Yes, IF the company that replaced the furnace did a complete heatloss calculation on the house as it existed and determined that the original 80k BTU unit was vastly oversized.

I suspect that neither a heatloss calculation NOR an energy audit with blower door test was performed prior to the furnace replacement. I also suspect that the installer had the 54k BTU unit in the shop and wanted to unload it.

Often a company will install a new furnace of the same size as the original without doing a heatloss calculation if the occupant states that they were comfortable with the old furnace. Any time a furnace is either upsized (rare, but it happens) or downsized a heatloss calc is absolutely necessary and a blower door test is highly recommended.
 
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Old 02-09-07, 05:06 PM
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Originally Posted by furd View Post
"Would there be any legitimate reason to replace a 43 year old 80,000 BTU unit with a 54,000 BTU unit?"

Yes, IF the company that replaced the furnace did a complete heatloss calculation on the house as it existed and determined that the original 80k BTU unit was vastly oversized.

I suspect that neither a heatloss calculation NOR an energy audit with blower door test was performed prior to the furnace replacement. I also suspect that the installer had the 54k BTU unit in the shop and wanted to unload it.

Often a company will install a new furnace of the same size as the original without doing a heatloss calculation if the occupant states that they were comfortable with the old furnace. Any time a furnace is either upsized (rare, but it happens) or downsized a heatloss calc is absolutely necessary and a blower door test is highly recommended.
Thank you for the response. I'm unsure, though doubt, that a heatloss calculation was made, however I am postive a blower door test was not conducted.

What do you guys recommend for a course of action at this point?
 
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Old 02-09-07, 05:08 PM
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If you had a larger furnace with the same air volume the supply air temperature would definately would be warmer.

You only say however that the supply air is cooler.
Does the furnace heat the house properly?
If it does then the lower discharge temperature could be considered normal for a higher efficiency furnace.
You may not have any problems.
 
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Old 02-09-07, 05:23 PM
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Air Temperature

Older furnaces used temperature of the furnace to bring on the fan. New ones do it on a time basis. A certain number of seconds after the thermostat calls for heat, the fan is going to come on. The furnace does not even have to ignite & the fan will come on after X seconds. By doing it this way, the manufacturers can get a couple more efficiency points.
To answer your question, the installer is right. An 80K furnace will not give you any warmer air than the 54K because the fan would be bigger. The ratio of air moved to BTUs produced would remain constant.
 
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Old 02-09-07, 05:25 PM
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To paraphrase GregH, if you are comfortable then you need to do nothing.

You would know if a heatloss calculation was done because it involves measuring every outside wall, every window, checking every opening for type (if any) of weatherstripping, checking the type of window, measuring the doors and checking them for weatherstripping, ascertaining the amount and type (if any) of insulation in the ceiling, walls and floor. Even when using a computer program to do the actual calculation it takes a bit of time to make all the measurements and observations.

As for the air from the registers seeming colder...older furnaces often ran higher air temperatures than do modern furnaces. Also, newer furnaces often run higher volumes (CFM) of air through them so the air from the register may indeed be cooler than it was with the old furnace. Old furnaces often ran about 140 degrees or sometimes higher and newer furnaces may only run 120 to 130 degrees. That lower temperature coupled with the higher velocity will feel considerably cooler.
 
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Old 02-09-07, 07:08 PM
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As long the furnace keeps up on a cold snap and runs non-stop then it's sized right.

I had and older 118k furnace and it was replaced with a two stage 60k, with our temps -10 to -15, it run non stop in 1st stage(39k and once in awhile 2nd stage.)

As other said newer furnace moves alot more air, and can run cooler.
 
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Old 02-09-07, 07:57 PM
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Everyone seems to drive home the most important point, which is "Is it doing the job?" If it is, unless your contract specified a 80,000 btu furnace, there's nothing to be done. Maybe he got lucky on this one.
 
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Old 02-09-07, 07:57 PM
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I suppose it's keeping up. It's managed to maintain, even at 70 degrees when it's been the coldest this year. I was disappointed to see it run about 45-50 minutes per hour. But it sounds like you guys are saying that's not so bad?
 
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Old 02-09-07, 08:06 PM
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Run time

A properly sized furnace will run non stop on a "design day". That is a day on which the outdoor temperature is the same as that for which the furnace is supposed to keep the indoor temperature at the indoor design temperature (usually 70). I don't know what the design temperature is for your area.
 
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Old 02-09-07, 08:07 PM
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Longer run is better than the stop and go... Like in your car.. where are you going to get better gas mileage?? City stop and go, or freeway on cruise?

Also with the long run time the temp in most of the rooms will be even.
 
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Old 02-09-07, 08:25 PM
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Ok, I appreciate the help guys. Thanks a lot! I can certainly sleep easier now.
 
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Old 02-09-07, 08:30 PM
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Smile

A system will run long enough to offset the losses of the home. The perfect system would match the losses perfectly and run 24/7. It's only replacing the lost heat. If it's 50,000 btus, that's what it replaces whether it does it in 15 minutes or 45 minutes. Longer run times means longer equipment life and greater comfort.

Think of this... if you need to save $50, you can either work 5 hours at $10/hr or 10 hours at $5/hr, but it's still $50.
 
 

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