Every trade has its rules of thumb. Ours include “one ton per 500 square feet,” “40 BTU a square foot,” and the classic “put in what was there before.” They are quick, they sound confident, and they are why so many Ontario houses have furnaces twice the size they need.
Here is why the rules fail, the one shortcut that is actually useful, and what the code asks for instead.
A square-foot rule assumes every house of the same size loses heat the same way. They do not. Here is what it ignores.
Glass loses heat many times faster than an insulated wall. A house with a wall of glass facing the lake is a different animal.
A 1960s bungalow with nothing in the walls and a new house with full insulation can be the same size and need very different heat.
A drafty house can lose as much through leaks as through the walls. A tight one loses very little.
Toronto is about -18 C on design day. Orillia is about -25 C. Same house, different load. See design temperature.
A room with a 9-foot ceiling holds a third more air than one with 7-foot ceilings. Vaulted rooms more again.
ICF, SIPs, double walls and framed walls all behave differently. A rule of thumb treats them all the same.
For an existing house, your actual gas or oil use over a winter tells you a lot about what the house really needs. Energy people call it a fuel-use or billing analysis. It is a recognized way to estimate the load, and it very often shows the old furnace is far bigger than the house needs.
It has two limits. It gives one number for the whole house, not room by room, so it cannot size ducts. And it is not the CSA F280 calculation a permit asks for. Use it to sanity-check a quote, not to replace the heat loss.
If a contractor quotes a furnace twice the size your gas bills suggest, ask him to show you the calculation.
Room by room, at your design temperature. Everything else is a guess.
The old furnace was probably oversized. Matching it repeats the mistake.
A right-sized system runs long, steady cycles. That is good, not a sign it is struggling.
If you are upgrading insulation or windows, get the heat loss after the upgrade, not before.
Ontario permits for new houses and new heating systems ask for a CSA F280 heat loss. The current edition, F280-12, sets the floor: the equipment has to cover the calculated load. An upper limit from older editions was dropped, but examiners and good designers still push back on big oversizing because of what it does to comfort and equipment life.
See why F280 is mandatory and what size furnace you need.
Got a quote that seems big? A $395 heat loss tells you whether it is. We sell no furnaces, so we have no reason to inflate the number.
Get Free Quote →It is a rule of thumb, not a method. It ignores your windows, insulation, air leakage, ceiling heights and design temperature. Two houses with the same square footage can need very different equipment. Ask for the room-by-room heat loss behind the number.
There is no honest single number. A new tight house and a drafty 1950s bungalow of the same size can be miles apart. That is exactly why the code asks for a CSA F280 calculation instead of a per-square-foot figure.
Using your actual gas use is a recognized way to estimate what an existing house really needs, and it often shows the old furnace is far too big. It does not give room-by-room numbers, and it is not the F280 a permit asks for. Use it as a cross-check.
The current CSA F280-12 mainly sets a minimum: the equipment must cover the calculated heat loss. Older versions also had an upper limit, which was dropped. Good designers and plan examiners still question big oversizing, because it causes short cycling and comfort problems.
Because the old one was probably cycling on and off all winter, and fine is not the same as right. A right-sized furnace runs longer, steadier cycles, with fewer hot and cold spots and less wear. See oversized furnace problems.
Even worse. Heat pump output drops as it gets colder, so a square-foot rule tells you nothing about what it can do at your design temperature. See heat pump sizing.
Send a floor plan or sketch and tell us about the walls, windows and insulation. We calculate the real heat loss so you can check any quote.