Ravina Project guest blog #3 - Calculating Household Heating Efficiency

The biggest contributor to our household carbon footprint is the use of fossil gas (natural gas) to heat our houses. We all want to be more efficient and use less.

Susan and Gord Fraser have been experimenting with all things "green energy” for years, from solar panels to conservation, and have the data to prove it. This blog will make the geeky details of deep energy retrofits simple. Looking for an even deeper dive? Go to http://www.theravinaproject.org

A yearly efficiency calculation really helps us understand the effects of anything we do to make our household more efficient like upgrading our windows / doors, adding insulation, changing our thermostat, changing our lifestyle and the like.

How efficient are we now and can we measure our efficiency for each year to see if those upgrades are working? We can use our Enbridge invoices to calculate our heating efficiency. What follows is a short method which will allow us to calculate the change in our household heating efficiency over the years.

Heating Degree Days (HDDs) are an important and essential metric used in the calculation of a household’s heating efficiency. You can find Toronto’s total yearly heating degree days online here. The heating season starts on October 1st and ends on May 31st of the next year.

Now for the calculation. Note the HDD total is a yearly total (January to December in the same year) and not a heating season total (October to May of the next year). To compensate for this misalignment subtract the total summertime gas usage (June, July, August, September) from the yearly total. But how do we know that the resulting gas total is all used for heating? We can’t know that for sure but we can estimate our daily non heating usage by looking at our summertime gas usage when the furnace is OFF.

We know the total number of summertime days (122) and we know the total gas used from our invoices so we can calculate our average daily usage. That daily average usage occurs every day, summer AND winter. So we take that average daily summertime usage number and multiply it by the 243 remaining days in the year (365-122= 243) to give us an estimation of the total wintertime gas non-heating usage.

This is a valuable number. It allows us to calculate a more accurate household efficiency. We subtract the total wintertime non-heating usage from the total heating season usage to get our best estimate of the cubic meters of gas we used for heating. And finally to get our household heating efficiency we divide our total cubic meters of fossil gas ("natural" gas) used for heating by the number of HDDs in the year. Less gas used per heating degree day; the more efficient your house. When we first made this calculation in 2004 we were at 0.74 m 3 per HDD.

Here's an example using our own 2021 data:
2021 total natural gas used: 1,655 cubic meters (m 3)
2021 total summertime gas used: 79.5 m 3
2021 average daily gas usage in summertime: 79.5 m 3 / 122 days = 0.65 m 3 per day
2021 total wintertime non heating gas usage: 0.65 m 3 times 243 days = 158.0 m 3
2021 total gas usage for heating: 1,655 - 79.5 - 158.0 = 1,417.5 m 3
2021 total Heating Degree Days: 3,340
2021 heating efficiency is: 1,417.5 / 3,340 = 0.42 m 3 of fossil gas, or "natural" gas, used per heating degree day