In short
A BTU is a small unit of heat: enough to warm one pound of water by one degree Fahrenheit. Equipment is rated in BTU per hour. A furnace's input BTU is the gas it burns, and its output is input times AFUE, so an 80,000 BTU/h furnace at 96% delivers about 76,800. Twelve thousand BTU per hour equals one ton of cooling.
A BTU is the unit behind nearly every size number in a mechanical room. The furnace, the water heater, the air conditioner and the gas piping are all described with it. One BTU is about the heat given off by burning a single wooden match. The related rating terms are collected in the glossary.
What a BTU rating tells you about a furnace or air conditioner
A BTU rating on equipment is a rate: BTU per hour, written BTU/h. On heating equipment it is how much heat the appliance produces each hour, and on cooling equipment it is how much heat it removes. A few conversions cover most of what appears on a quote.
- 12,000 BTU/h equals one ton of cooling, which is how AC tonnage is counted
- 1 kilowatt equals 3,412 BTU/h, the bridge between electric and gas equipment
- Furnaces in houses around the GTA and Hamilton typically run 40,000 to 100,000 BTU/h of input
- A whole-home gas tankless water heater is commonly 199,000 BTU/h, more than double most furnaces
Input BTU vs output BTU on a furnace
The big number on a furnace is its input: the gas it burns per hour. Output is input multiplied by AFUE. An 80,000 BTU/h furnace at 96% AFUE delivers about 76,800 BTU/h to the house, while an older 80,000 BTU/h furnace at 80% delivers 64,000. New gas furnaces must be at least 95% AFUE, so a replacement with the same input as the old one puts out noticeably more heat.
Both numbers are on the rating plate, a metal or foil label inside the burner compartment door of a furnace and on the side of a water heater tank. An air conditioner's nameplate is on the outdoor unit, with the capacity usually coded into the model number in thousands of BTU.
Why BTU input decides your gas line and meter size
Every gas appliance's input adds to the home's total connected load, and that total is what the piping and meter must deliver. Under CSA B149.1, the gas code used in Ontario, pipe is sized from the BTU/h it has to carry and the length of the run. Adding a 199,000 BTU/h tankless, a pool heater or a standby generator to a line sized for a furnace and a tank can leave appliances starved for gas. The fix is a larger pipe run, and sometimes a meter upgrade by Enbridge Gas.
BTU also sets who may do the work. A G2 gas technician is licensed for appliances up to 400,000 BTU/h, which covers residential equipment, and larger commercial appliances need a G1.
More BTU is not better when sizing a furnace
A furnace should match the house's heat loss, not exceed it by a wide margin. An oversized furnace heats the air near the thermostat fast, shuts off, and starts again minutes later. That short-cycling is noisy, leaves far rooms cold and wears the igniter and heat exchanger. The correct output comes from a CSA F280 heat-loss calculation, and typical figures by house size are in what size furnace you need.
High-efficiency furnace installs run $4,500–$7,500, and BTU size moves the price less than efficiency tier and blower type do. If your quotes disagree on BTU, ask each contractor how they arrived at the number. ZK Mechanical sizes each furnace installation from the house and writes the input and output on the quote; request one here.
Questions people ask about BTU
How many BTU do I need to heat my house?
It depends on the home's heat loss, which is driven by insulation, windows, air leakage and size. Houses in this area usually need a furnace between 40,000 and 100,000 BTU/h of input. A tight 2000s build sits near the bottom of that range, and a large or uninsulated pre-war brick house sits near the top. A heat-loss calculation gives the figure for your address.
Is BTU the same as BTU per hour?
Not strictly. A BTU is an amount of heat, and BTU per hour is a rate of delivering it. Equipment labels often print 'BTU' when they mean BTU per hour. The same difference exists in electricity: a kilowatt is a rate and a kilowatt-hour is an amount. One kilowatt-hour equals 3,412 BTU.
What does MBH mean on a furnace or boiler rating plate?
MBH means thousands of BTU per hour. The M is the Roman numeral for one thousand, so 80 MBH is 80,000 BTU/h. It appears on boiler and commercial equipment plates and on gas piping tables. If a plate shows both an input MBH and an output MBH, the output is the heat delivered after combustion losses.