# Community and Q&A

| Posted in Energy Efficiency and Durability on

I have tried several free software packages to calculate the heat load of my 3,100 sq ft home near Boulder Colorado. The numbers from the software (and websites) all vary a lot. So I started thinking why not measure the actual load?

My design heating temperature is 0°F for my area. My gas furnace is 98.7% efficient.

This coming winter when our temperature will drop to 0°F for at least a few hours during the night why not measure the therms used during those hours to keep my house warm and then subtract the 1.3% because my furnace is not 100% efficient?

For instance if the temp is a 0°F outside between the hours of 1AM – 3AM (no solar heat gain) and I measure 1.5 therms during those hours:

1.5 therms X .987 = 1.4805 therms (Furnace efficiency)
1.4805  is 148,014.659 BTU
148,014.6 BTU / 2 hours = 74,007 BTU Heat Load

Does that seem right?

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### Replies

1. | | #1

That's exactly right but you don't need to wait until the coldest morning of this winter if you have last winter's data.

From there, you can add a heat pump and gain some real efficiency!

2. | | #2

Paul thanks for the link to that article.

3. | | #3

I collected lots of hourly data for months. A linear fit to outdoor temp vs furnace on-time using a few weeks of night (no solar gain, wind should average out) values should provide a more accurate design day heat load than a monthly fuel use derived value.

I expect much less data would be more accurate than the typical Manual J. So for new construction, temporary electric heaters for a few nights might be a viable Man J alternative.

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