[jim97006]

I purchased a program that does Manual J calculations (HVAC Calc Residential). I put in all the measurements of the house, the indoor and outdoor temperatures, and the calculations came up with the BTU heat loss. Before I bought the program, I visited a web site that asked for room dimensions, insulation, etc. and came up with a similar number as the program.

I just want to see if my understanding of the heat loss calculations are right, and how they relate to furnace sizing.

Let's say I have 3 furnaces, each 92.3% fuel efficient. Based on their size, the output would be as follows:
60,000 = max BTU output 55,380
75,000 = max BTU output 69,225
90,000 = max BTU output 83,070

When I use the measurements of my house and insulation, I set the design conditions to 72 degrees indoors, and 23 degrees outdoor (Portland, Oregon). According to the program, the heat loss in my house is 40,256.

I read in several places that you don't want to use the number calculated because it doesn't account for coldest days on record. The 23 degrees is the lowest average low temperature, not the lowest temperature on record. So a percentage is usually added (40% is a number I've seen). Using 40%, I could guestimate that the furnace needs to output 56,000 BTU, so I'd guess the 60,000 BTU furnace would work.

However, I don't like guessing that much, so I thought I'd try to use the program to get an actual number. So, I figured I'd use the program to calculate the heat loss at various temperatures. For the following, I keep the indoor design temperature at 72 degrees.

8 degrees = 55,079 (max output of 60,000 furnace)
-1 degrees = 62,833 (coldest temperature on record)
-9 degrees = 69,711 (max output of 75,000 furnace)
-24 degrees = 82,629 (max output of 90,000 furnace)

Based on these numbers, it seems like a 60,000 BTU furnace would be too small. It would work most of the time, but if the temperature outside went below 8 degrees, the indoor temperature could not be maintained at 72 degrees. It seems like the 75,000 BTU furnace would work, since it generates enough heat to overcome the heat loss until the outside temperature goes to -9 degrees, and the coldest temperature ever recorded is -1 degrees. It seems like the 90,000 BTU furnace is complete overkill.

If I change the indoor temperature to 80 degrees and set the outdoor temperature to -1 degree, it shows a heat loss of 69,711, which is what the 75,000 BTU furnace generates. It says that you could keep the inside of the house at 80 degrees if the outside temperature went down to -1.

I realize that the heat loss calculations are not exact, so you can't really rely on the exact numbers above. However, it seems like they're close enough to get a good idea of how close you are on the sizing.

Does my logic above sound reasononable?