Here is a sample calculation, using a three-month meter reading for a typical house. You can use any period (but at least two weeks of winter weather is necessary). You can read the meter yourself for the information, look at your furnace bills or phone your utility to see if they have appropriate records. The natural gas usage of other gas-fired appliances in the house is estimated from gas utility data and subtracted from the total for the period in question, so that the gas requirement for heating can be isolated. (Oil furnaces are harder to size using this method, but it may be possible using oil fill-up intervals and the number of litres delivered.)
The goal is to find a relationship between the gas consumed and the heating degree days (HDD). A heating degree day is essentially the number of degrees of heating required over the course of 24 hours, compared to a reference temperature of 18°C. For example, if the average daily outside temperature is 10°C, then the number of heating degree days for that day is 18°C - 10°C = 8 HDD. You can get the approximate HDD for your calculation period from the Environment Canada website. Use the data from the "Degree Days: Below 18°C" row.
Once the relationship of the HDD and gas consumption is established, then you can calculate gas consumption for the design temperature in your area. This temperature is usually available from a mechanical contractor or your local building officials. It is not the extreme minimum temperature; it can be estimated from the average temperature over 24 hours on the coldest day of the winter. To approximate the design temperature: go to the historical weather data for your community on the Environment Canada website; find the coldest January over the last several years; then pick out the lowest daily average temperature in that month; and use that as the design temperature. Being a degree or two out will not make a huge difference in the calculation.
The example below uses a design temperature of -35°C. At that temperature, the maximum HDD per day is equal to 53, which is the difference between 18°C and -35°C. Calculating the size of the furnace necessary on the coldest day of the year will mean that the furnace has the capacity to handle any expected local temperature. You can find a furnace’s efficiency rating on its EnerGuide label or in the product documentation.
Total gas consumption from December to March = 1,320 m3
Estimated consumption for other gas appliances (data from utility) = 306 m3
Therefore, gas consumption during the period for heating = 1,320 - 306 = 1,014 m3
Heating degree days for that period (from Environment Canada data) = 2,840 HDD
Heating consumption by degree day = 1,014 m3/ 2,840 HDD = 0.3570 m3/HDD
Heating consumption at 53 HDD/day = (53 HDD/day)(0.3570 m3/HDD) = 18.9 m3/day
Where gas has an energy content of 37.5 MJ/m3, and the existing furnace has an efficiency of 72 per cent, then:
Heat loss at 53 HDD/day = (18.9 m3/day) (37.5 MJ/m3)(0.72) = 510 MJ/day or 21.3 MJ/h*
According to the energy content of electricity, 3.6 MJ/h = 1 kW, then 21.3 MJ/h = 5.9 kW
This heat loss would require a furnace that produces an output of 5.9 kW or about 20,100 Btu/h (1 kW is approximately 3,412 Btu/h).
If we allow the CAN/CSA F280 permissible oversizing of 40 per cent, then the proper furnace sizing would be (1.4)(20,100 Btu/h) = approximately 28,100 Btu/h.
If you are calculating for an oil furnace, heating oil has an energy content of 38.2 MJ/litre.
* Note: This calculation is correct, although many people think the efficiency factor is in the wrong place. It is not. We are calculating the house heat loss based on fuel used and furnace efficiency. A more efficient furnace will have delivered more heat to the house, and the heat loss will be higher.