How much water do high-efficiency furnaces need to drain?

• posted on August 9, 2005, 9:18 am
On a typical winter day for those living in cold climates, how much water (condensation) is produced by a high-efficiency furnace per day? I've heard ranges from barely anything to 5 gallons per day.
And would this amount to more or less than what is produced by a central air conditioner?
Thanks.
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• posted on August 9, 2005, 3:50 pm

With out humidity numbers anything would be a wild ass guess.
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• posted on August 9, 2005, 10:01 pm

You might figure about 5% of the fuel's heating value for oil, ie 0.05x140KBtu/1000 = 7 pounds per gallon, eg 42 lb/day of water, if you burn 6 gal/day of oil, and about twice that for natural gas.

Depends on the humidity and house air leakage rate. If wi = 0.00787 (80 F at 50% RH) inside and wa = 0.0130 (Phila in July) and a 2400 ft^2 house leaks 224 cfm (a 0.7 ACH average), the AC needs to remove at least 24hx60x224x0.075(wa-wi) = 124 pounds of water per day.
Nick
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• posted on August 10, 2005, 3:53 am

This from personal recollections. I would guess about a quart per day. Ours discharged over the driveway and I never saw a ice stalagmite over a foot tall after several below freezing days in a row.
The AC output will vary greatly with humidity but something around 5 gallons per day would be realistic here and now.:)
Colbyt
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• posted on August 10, 2005, 9:30 am
Colbyt wrote:

Thanks for the input. Have you noticed any discoloration of your driveway? I've read that the water run-off from condensation of a gas furnace is acidic, with a pH roughly the same as orange juice.
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• posted on August 10, 2005, 11:41 pm

We sold that house about 10 years ago. During the 8 or so years that I saw it drip there I did not see any. It was a blacktop drive. YMMV on concrete because of the chemistry involved.
Colbyt
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• posted on August 10, 2005, 8:54 pm

Ours
gallons
I've never measured it, but one customer with a 5 ton system says he gets about a gallon per hour of run time for his system. This is in 'normal' weather, nit high humidity conditions....
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