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

With out humidity numbers anything would be a wild ass guess.

You might figure about 5% of the fuel's heating value for oil, ie

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

Nick

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

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.

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....

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