After reviewing the rather complex calculations for determining heat loss, which involve converting heat content of a cubic metre of natural gas to mJ/hour, then to kW, and back to BTU's, I decided if I can find the heat content in BTU of one cubic metre of natural gas, I can simplify the calculation considerably. From a variety of sources, natural gas has a heat content of 36,000 BTU / m3:
Take your total gas consumed for a four month period from November to February, multiply by your furnaces efficiency rating (GFUE%) and divide the volume by 2880 (120 days by 24 hours)
Multiply this by a factor of 2.4, which compensates for actual average HDD for the period to maximum HDD assuming a coldest day of -35 (I used 6360 design HDD for the period - 120 days at 53 HDD per day, versus 2680 actual average HDD from Environment Canada)
Finally, multiply this by 36,000, the approximate BTU energy contained in 1 m3 of natural gas, and the result is the appropriate sized furnace for your house (one that will work non-stop when the outside temp is -35 Celsius):
2490m3 * 0.92% / 2880 HDD * 2.4 HDD * 36000 BTU/m3 = 68,724 BTU
So it looks like the furnace of choice will be 90,000 BTU, oversized just enough to fill demand on the few days that may drop below the -35 design temperature, and if down the road we wish to heat the sunroom in the winter, among other things.
It seems the winner is Dave Davies from Stratford, who (seem to at least) have a fair and reasonable price strategy, a no nonsense approach to installations (they always install a condensate pump to a drain, rather than a hose on the floor). The furnace will be an Elite Series Lennox G61V High efficiency, two-stage with variable speed DC blower motor. Not particularly pretty, but hey, it's a furnace. And with a stroke of good luck, they will be installing a Honeywell Prestige HD 7-day fully programmable setback thermostat, with remote control.
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