furnace size / efficency

[dddon]

I just read a thread that was about furnace size. The science of that question is this: for best "match" , the furnace should run all the time at the lowest usual outside temperature.. this was determined by a research group at Honeywell some 35 years ago, by using a custom unit that integrated time , number of cycles, inside and outside temps. At that time most furnaces in the Mpls. area that were tested , were 200 to 500 percent TOO big!!
my question is this: what method do installers use today to find the heat-loss number??
I also read a number of the threads on AC and heat pumps, and wonder why some of the members here get SOOoo emotional about their point of view, and don't give any factual basis for that viewpoint...
BUT .. all in all guys ..great job!! thanks ...

[bigtime]

I would say that most furnaces installed today are still way oversized. A load calculation can be made to determine the size furnace required. This calculation has built in safety factors and then the calculator throws in some fudge also. There is not much difference in the cost of a 60,000 and a 100,000 bth furnace. If a furnace is 10% to small you get a lawsuit, but if its 100% to big no one seems to care.

I replaced a 140,000 btu furnace in my house with a 60,000 several years ago. The new furnace runs maybe half the time during a cold night.