AC size for 43K sensible heat gain

[dan sw fl]

Originally Posted by JLSinPDX

I'm not sure what the average was for the past year. Right now we're paying about $1.30/therm. We used about 1,000 therm for heating, plus another 100 or so with our gas insert in the basement.

Keep in mind that Portland has relatively mild winters for the most part, but temps can drop to the teens. Last winter we were mild overall, though we had a tough December for Portland -- lots of 20-something temps over a period of a couple of weeks.

Station: Airport: Seattle, WA, US (122.31W,47.45N)
____________ Airport: Portland, OR, US (122.60W,45.59N)
_______ KSEA __ KPDX
Month starting
8/1/2008 ____ 88 _ 56
9/1/2008 ____ 165 _ 118
10/1/2008 ___ 408 _ 358
11/1/2008 ___ 468 _ 465
12/1/2008 ___ 864 _ 847
1/1/2009 ____ 797 _ 767
2/1/2009 ____ 649 _ 656
3/1/2009 ___ 714 _ 599
4/1/2009 ____ 473 _ 396
5/1/2009 ____ 300 _ 221
6/1/2009 ____ 115 _ 78
7/1/2009 ____ 72 __ 42
________ 5113 __4603

If you back-fit your Natural Gas usage and temperature bin data ( or HDD) into your heat loss calculation, you will likely find that 60,000 BTUH capacity is sufficient > 97% of the time (< 190 hours per year < 32'F in Portland) in maintaining 68'F.

Yes, it is ENTIRELY acceptable to Not have the added design heating capacity to exactly meet the most extreme conditions ( < 27'F, 75 hours per year).


If you operated a 60,000 BTUH single-stage furnace for 3,000 hours per year you would use 1,800 therms. That is ~50% more than you stated is your annual natural gas consumption.

If you operated an 80,000 BTUH single-stage furnace for 2,000 hours per year you would use 1,600 therms. That is ~40% more than you stated is your annual natural gas consumption.

Back-fitting your Natural Gas usage and temperature bin data ( or HDD) into your heat loss calculation is a method of validating the accuracy of the heat transfer model.

Temperature Bin Data
62'F __ 972 hours
57'F _ 1,299
52'F _ 1,300 hours
47'F _ 1,302
42'F _ 1,270 hours
37'F __ 735 ___ 6,878 93.0%
32'F __ 332 hours 7,210 97.45%
27'F __ 114 ___ __ 189 2.55%
22'F ___ 42 hours
17'F __ 21
12'F __ 10 hours
7'F ___ 2
____ 7,399 hours < 68'F