EcoRenovator - View Single Post - Heating Degree Days NOT useful for sizing

Xzeption · 04-16-16, 10:48 AM

Actually, I found using heating/cooling degree days PLUS utility data was very accurate for determining my house's loads. For a one month example,

123 therms = 12,300,000 btus at 882 degree days is about 14,000 btu per degree day or 581 btu per degree hour. My design day outdoor temp here in North Carolina is 20*F. So 70*F - 20*F is 50*F delta. Then 50*F * 581 btu / (degree hour) = 29,000 btu/hr at design temps.

Of course it is not exact as it does not include internal heat gain or the fact that those therms also heat water. But I refined the numbers by running a simple linear regression of two years data. The results matched what I expected from using a stopwatch to time how long the system runs on extreme days. The Manual J (with all assumptions made to overstate the load) came back 50% higher at 42,000 btu/hr. At the time, the system output could not go higher than 32,000 btu, and it heated our house without any trouble on an actual design day.

If I had time I would make a simple webpage that could run these calculations for different areas. But I have some toddlers to run after now. Cheers!

04-16-16, 10:48 AM	#14
Xzeption Lurking Renovator Join Date: May 2015 Location: Raleigh, NC Posts: 12 Thanks: 1 Thanked 2 Times in 2 Posts	Actually, I found using heating/cooling degree days PLUS utility data was very accurate for determining my house's loads. For a one month example, 123 therms = 12,300,000 btus at 882 degree days is about 14,000 btu per degree day or 581 btu per degree hour. My design day outdoor temp here in North Carolina is 20F. So 70F - 20F is 50F delta. Then 50F 581 btu / (degree hour) = 29,000 btu/hr at design temps. Of course it is not exact as it does not include internal heat gain or the fact that those therms also heat water. But I refined the numbers by running a simple linear regression of two years data. The results matched what I expected from using a stopwatch to time how long the system runs on extreme days. The Manual J (with all assumptions made to overstate the load) came back 50% higher at 42,000 btu/hr. At the time, the system output could not go higher than 32,000 btu, and it heated our house without any trouble on an actual design day. If I had time I would make a simple webpage that could run these calculations for different areas. But I have some toddlers to run after now. Cheers! __________________ There is an exception to every rule. Including this one. I wish my country would switch to the metric system. My apologies to international readers who see me jump between metric and imperial units.