Empirical Heat Loss Calculation using Gas Bill

HomeownerDustin

I was having a tough time following this post on Green Building Advisor to find my empirical heat loss, so I put together an easy-to-follow formula that actually only takes 10-15 minutes to complete. https://www.greenbuildingadvisor.com/article/out-with-the-old-in-with-the-new
The formula below is for my 2x4 studded house with gas boiler and hydronic heating system, with my indoor temp set at 67-68F all the time. Many people reading this article have similar homes. For more info and for further customization in the formula, read that article.

What is heat loss?
Roughly, heat loss is the amount of energy that escapes your home that must be replaced by a heating system to keep the temperature steady.

How do you calculate heat loss?
Many online calculators have you plug in home measurements and windows/doors and enter (guess) the U factor of all exterior surfaces. By contrast, this method asks how much energy you used to keep your house at a comfortable temperature during a winter month. The end number is the BTU/hour that your heating system output to keep your home comfortable (approx 68F). This is your heat loss.

Preparation:

• Get a gas bill from the middle of winter. Note the dates and therms used in that period (thermsUsed).
  
• Go to DegreeDays.net, download the 65 degree temp table from a nearby weather station that gives the temps for the dates on your bill. Sum the degrees for the days on your gas bill. This is the sumDegreeDays.
  
• Look at your boiler. Divide output (e.g. 80,000 BTU) by input (e.g. 100,000 BTU) to get the boilerEfficiency.
  
• Go to https://www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/County Level Design Temperature Reference Guide - 2015-06-24.pdf, and find your county’s “99% Design temperature”. This is designTemp
  

Heat Loss Calculation:
heatLoss = [(thermsUsed*boilerEfficiency*100,000)*(65-designTemp)]/(sumDegreeDays*24)

You can now use your calculated heatLoss to appropriately size a new boiler or to benchmark insulation improvements.