Estimating heat load based on gas usage
I’m following the advice here https://www.greenbuildingadvisor.com/article/out-with-the-old-in-with-the-new
My numbers are:
Gas usage in Jan: 11.4 GJ
Gas usage in Sep 2020: 2 GJ – I use this to account for usage of stove and water heater that both consume gas
Gas usage by the furnace in Jan: 11.4 – 2 = 9.4 GJ
GJ converted to BTU: 9.4 * 947,817= 8,909,479
Account for 96.7% efficient furnace: 8,615,466
So total BTUs delivered to the ducts is 8,615,466
Using the 60F balance point, in Jan I get 647.6 HDDs.
8,615,466 / 647.6 / 24 = 554 degree-hours
My design load/outside temp is 28F, so 65-28 = 37
37 * 554 = 20,498
Account for 1.4x oversize = 28,700 BTU/h
The above number seems good in the sense that it’s close to Manual J calculations. Through a rough Manual J calculation I get a load of 28379.
I have two questions:
– Is it expected that the number matches Manual J only after accounting for 1.4x oversize?
– I don’t quite understand the math. If 8,615,466 were delivered total in January, then given 30 days in Jan and 24 hours in a day 8,615,466/30/24 = 11,965 BTU/h. This means on a daily average I need 11,965 BTU/h in January. What’s wrong with this way of calculating the load? Why is this number wrong compared to the heating degree days calculation?
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