Sizing a furnace
I was reading an article about HVAC sizing with Manual J & D.
Here is my question:
If you have data for an existing home — heating-only therms, furnace run-time (from smart thermostat), HDD, etc for the coldest days of the year for multiple years — can you calculate furnace sizing in a different way?
For example, I used 62 heating therms in Feb 2013 (after subtracting summer non-heating gas use), so I put in 6.2 million btus. I have a basic furnace with one blower speed installed in 1989, so it’s probably 75% efficient (as a guess). So the house required 4.7 million btus of heat (1.5 million lost via combustion).
The furnace ran for 61 hours that month so about 77,000 btus/hr heating input.
The average hourly loss was 7,000 btu/hr (672 hrs that month), but on the coldest day, I know it ran 4.25 hours or 327,000 btus input that day, which is about 13,600 btus/hr per hour heat loss. I could continue this process to create a scatterplot and fit a curve to the record cold temp in our area + 5%.
I imagine I’m not the first person to think along these lines…Is there anything to this? Can it be extended to finding the right sized furnace?