Heat Loss From Usage

JofG

I am in the process of converting from oil to to natural gas for home heating and hot water (I live near Philadelphia). I have decided to go with a high efficiency mod/con boiler but have received wildly divergent estimates for sizing from potential contractors (59k to 211k!). I have attempted to estimate the heat loss but am not confident in my estimates because I am missing accurate insulation information.



It hit me that I should be able to do better than estimate the heat loss like this by finding my heat loss from actual usage. I know exactly how much oil I have used and the number of degree days for this period. I can reasonably estimate the efficiency of my old oil boiler (Columbia FT-6165C) and make a guess at how much of the usage is for hot water (I have a summer/winter hookup). Worst case I can ignore the hot water and get an upper bound estimate.



Listed below is the information from the spreadsheet I created for this. I have never done anything like this before and want to know if I am missing something? Also am I correct in assessing that this is actually better than a traditional heat loss calculation since it is measuring my actual usage?



Thanks to anyone who can shed some light on this.



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Measured Heat Loss



The tank was full on 12/30/2010 and then again on 3/5/2013. So the oil delivered between these dates is exactly what we used.



Total Gallons Used..............2,921

BTU's Per Gallon.............140,000

Gross BTU's.............408,884,000



Boiler Efficiency.....................0.82 (estimate)

Net BTU's.................335,284,880

% BTU's For HW...................0.05 (wild estimate!)

Heating BTU's..........318,520,636



Degree Days.....................11,206

Degree Hours.................268,946

BTU's Per Hr Per Degree...1,184



Delta T For Coldest Hour........54 (68 - 14)



Max Heat Loss Hour........63,954