Furnace size - ballpark estimate


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Old 07-24-17, 11:34 AM
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Furnace size - ballpark estimate

I have a buddy looking at a 2,150 square foot rambler (including finished basement) in Minneapolis. The seller will not let him bring in an HVAC pro to inspect the furnace and perform a heat load calculation. We know the furnace is 60 K BTU and 80% efficiency and my buddy thinks it's entirely too small while the sellers maintain it's adequate.

Anyone able to provide a quick off-the-cuff assessment?
 
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Old 07-24-17, 12:12 PM
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Slightly too small ...maybe. Entirely too small.... doubtful.
How old is this "rambler" ?
 
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Old 07-24-17, 12:16 PM
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Re-built from the foundation up after a fire in 1994. Original home was built late 50's to early 60's.
 
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Old 07-25-17, 07:19 AM
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Really difficult to estimate since there are many variables that can only be determined with a visit. Since half of that area is basement, if they insulated the rim and walls properly then the additional load from down there is minimal. But, if they did it wrong there are both mold and heat loss issues.

Big red flag that they won't allow an inspection from the HVAC guy, but all homes will need a home inspection so insist on using one he hires and talk to them about your concerns, some will have experience with energy issues. If the resulting inspection is not favorable he should be able to cancel the offer.

Buying sight unseen is a scam the banks use. And don't trust the real estate agent they most likely represent the seller.

Bud
 
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Old 07-25-17, 08:21 AM
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Really difficult to estimate since there are many variables that can only be determined with a visit
Yep, totally agree with that and am thankful you and Pete were willing to answer such a vague question.
all homes will need a home inspection
The home inspector he used is a friend of mine and is actually the one who pointed out that the furnace may be under-sized.

Yes, he can cancel the offer and there are additional reasons that may happen but we just wanted to try to get a better feel for this one. Again, thank you both for your insight.
 
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Old 07-25-17, 08:54 AM
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Ideally a heating system will maintain the set temp on the coldest day while the furnace runs full time. Running continuously is not a problem for a furnace as they are built for that and they avoid the efficiency reduction due to cycling. So worst case the temp drops below the set point and the wind is blowing like mad, the furnace runs full time and the temp stops at 60. Uncomfortable, yes, but nothing is going to freeze. At 60 the furnace size could be 35% smaller than the design temp, based upon HDD charting, and the current furnace is certainly larger than that. If you went through the load calculations and selected a new furnace it would probably end up very close to what is there, plus you still have the option to make some energy improvements to reduce the demand.

Bud
 
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Old 07-25-17, 09:21 AM
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Yeah, I reached the conclusion yesterday that it's likely to be adequate on most days, just maybe not the really cold ones. There's a fireplace in the living room which might supplement as well (assuming the piping can deliver enough gas to run both at the same time).
 
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Old 07-25-17, 11:43 AM
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If furnace is from when the house was rebuilt in 1994 then you should consider replacing it due to age. At least budget for it.
Increasing the furnace size usually also means increasing ductwork.
The house should be fairly well insulated and sealed if rebuilt in '94 so 60K might be fine.
Some fireplaces actually send more heat up the chimney from the house than they add.
 
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Old 07-25-17, 12:39 PM
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FWIW, furnace is actually new.
 
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Old 07-25-17, 04:46 PM
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Furnace is new and he won't let you inspect it? Is it a DIY? Maybe seller afraid will get fined if permits are needed in the area?

Google "furnace size calculator" and you will get basic ones that will all say you need something bigger. Have them use that as negotiating point to lower the price?
 
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Old 07-26-17, 06:46 AM
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Seller is being a real PITA over this thing, will not negotiate or budge at all. Our best guess is someone had this lying around and decided to install it. Supposedly was professionally installed but I don't know if the seller will even disclose the contractor's name.

Yeah, I Googled a bunch first and the best answer I got was "Maybe" but there was a bunch of "Too Small" as well.
 
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Old 07-26-17, 10:22 AM
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The only way to tell is to do an accurate load calculation.

Maybe you can get it, saying you want to do some "measurements" and measure the perimeter and all the windows/doors, see if it's 2x4 or 2x6 - do a calc yourself.

I don't think you have a super-cold climate and need a whole lot of heat. This unit outputs 48000 btu/hr - a space heater is 5000 btu/hr on high to put that in perspective.

Probably would have a better margin with a 95% 60k furnace, putting out 57k/hr or so. but i don't think it will have trouble keeping up.

Most furnaces are grossly oversized.

Home inspectors are known to be crooked as heck, so if an inspection is done, make sure your buddy doesn't use one recommended by the agent.

They work for the agents and not for you!
 
 

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