Furnace Sizing: You probably do not need a furnace with the output of your current furnace. Most furnaces in Canadian houses can provide far more heat than the house requires. A properly sized furnace will be running almost continuously during the coldest day of the winter. Having a furnace of a correct size will result in efficient operation during the whole heating season. A grossly oversized furnace will run only for a short period, never coming up to peak efficiency. Note, however, that sizing may not be a big issue with high-efficiency, condensing gas furnaces. Due to the design of condensing appliances, they are efficient even when oversized.
So, how do you size your furnace? You can have the contractor use a home heat loss calculation that is available from Canadian Standards Association (CAN/CSA F280) or a sizing procedure from the Heating, Refrigeration and Air Conditioning Institute of Canada (HRAI). Having a proper sizing will cost you $150 – $300 from a qualified contractor.
Those who keep their heating bills, and who are mathematically inclined, can try the calculation in the text box entitled “Calculating house heat loss from utility bills.”