Equating heat loss to natural gas usage
I am trying to equate the heat loss from my house to equate with my natural gas usage.
I have a mid-1950s 11,385 sf ranch home in PA, zone 5. I performed a blower door test to measure my air loss, arriving at CFM50 rate of 1512 or an ACH50 rate of 7.97.
Going by the heat loss formula:
Q = V x ACH x 0.0182 x HDD x 24
Q = Heat loss
V = Volume
ACH = Air changes per hour at 50 Pa
HDD = Heating Degree Days
Looking at January of 2019, we had 1012.4 HDD for my region.
This would mean
Q = 11,385 x 7.97 x 0.0182 x 1012.4 x 24 = 40,126 kBtu
So for the month of January, I had 40,126 kBtu exfiltrate from my house.
Using Energy Star Tech Reference, there are 102.6 kBtu/ccf. This would mean I should be using about 391ccf of natural gas to maintain my 65 degree setpoint.
But I only used 100 ccf in January.
My initial thought was maybe this was due to furnace inefficiencies/unaccounted natural gas use (water heater, etc), but that would mean the ratio should be reversed: I should have used more natural gas than is exfiltrating from the house.
I actually did the heat loss calc for my natural gas consumption for the past two years, using excel, and found that over the winter months (Dec-Mar) the ratio is consistently around 4x what it should be. Accounting for water heater use and other odd variances, I am guessing there is a miscalc or missing variable that would lead it 4 times higher than it should be?
Anyone see what I’m missing?
(Thank in advance for any assistance)