BTU shows up on every piece of HVAC equipment we sell. Furnaces are rated in BTUs. Air conditioners are rated in BTUs and then again in tons. Mini splits list BTUs right in the product name. If you understand this one unit, a lot of HVAC shopping gets easier.
The plain definition
BTU stands for British Thermal Unit. One BTU is the amount of heat it takes to raise one pound of water by one degree Fahrenheit.
That’s a small amount of heat. A kitchen match puts out roughly one BTU if it burns all the way down, which is why HVAC equipment deals in thousands of them.
When you see a furnace or air conditioner rating, it’s really BTUs per hour (BTU/h). A 60,000 BTU furnace can put out heat at a rate of 60,000 BTU every hour it runs. People drop the “per hour” in conversation, and so do a lot of spec sheets.
Heating and cooling use the same unit
A furnace adds BTUs to your house. An air conditioner removes them. Same unit, opposite direction.
A heat pump does both. In winter it moves BTUs from outside air into the house, and in summer it moves them from inside to outside. Our guide on how heat pumps work goes into that.
What a “ton” has to do with it
Air conditioners and heat pumps are usually sized in tons. One ton of cooling equals 12,000 BTU per hour.
The term goes back to the days of cooling with ice. Melting one ton of ice absorbs about 288,000 BTU. Spread that across 24 hours and you get 12,000 BTU per hour. The name stuck.
| Tons | BTU per hour |
|---|---|
| 1.5 | 18,000 |
| 2 | 24,000 |
| 2.5 | 30,000 |
| 3 | 36,000 |
| 3.5 | 42,000 |
| 4 | 48,000 |
| 5 | 60,000 |
Those same numbers usually appear in the model number. A condenser with 36 in its model is almost always a 3 ton unit. Our guide on reading an AC model number shows where to look.
Tons are an equipment size, not an exact output. A 3 ton unit’s real capacity moves around with outdoor temperature, indoor humidity, airflow and which indoor coil it’s paired with.
Furnace input versus output
This trips up a lot of people.
The big number on a gas furnace, the one in the model number, is usually the input. That’s how much heat the burner consumes from the gas. Output is what actually ends up in your ductwork.
The difference is efficiency, which furnaces rate as AFUE. To ballpark output, multiply input by AFUE.
- 60,000 BTU/h input at 80% AFUE is roughly 48,000 BTU/h output.
- 60,000 BTU/h input at 96% AFUE is roughly 57,600 BTU/h output.
- 80,000 BTU/h input at 92% AFUE is roughly 73,600 BTU/h output.
So when you replace an old 80% furnace with a 96% furnace, you may not need the same input size. A 96% furnace with a smaller burner can deliver about the same heat as the old one. The spec sheet lists actual output capacity if you want the exact figure.
For reference, gas is billed in therms, and one therm is 100,000 BTU.
How BTUs relate to sizing your system
Your house gains heat in summer and loses heat in winter. How fast depends on its size, insulation, windows, air leaks, which way it faces and your local climate. Sizing equipment means matching the system’s BTU capacity to that heat gain or loss on a typical hot or cold design day.
The professional method is called a Manual J load calculation. It looks at the house room by room. A good contractor can run one, and it’s worth asking for.
Why bigger isn’t better
An oversized air conditioner cools the house quickly and shuts off before it has run long enough to pull moisture out of the air. You end up with a cold, clammy house, more on/off cycling and more wear. In humid places like Georgia, that’s a real problem.
An oversized furnace has its own issues: short cycles, uneven rooms, sometimes more noise through the ducts.
Why too small hurts too
An undersized system runs nonstop on the hottest or coldest days and still can’t hold temperature. It’s the less common mistake, but it happens, usually after someone adds a room or finishes a basement and keeps the old equipment.
Square-foot rules of thumb
You’ll hear things like “one ton per so many square feet.” Those rules can get you in the neighborhood, but two houses with the same square footage can need very different equipment. Use our sizing tool for a starting estimate, then confirm with your installer. Our guide on how to size an HVAC system goes into more detail.
BTUs on mini splits and window units
Ductless mini splits are usually listed by BTU instead of tons: 9,000, 12,000, 18,000 and so on. Same math. A 12,000 BTU mini split is a 1 ton unit. Multi-zone systems list the outdoor unit’s total and each indoor head separately.
Common questions
How many BTUs is a 3 ton air conditioner?
36,000 BTU per hour. Each ton equals 12,000 BTU per hour, so multiply tons by 12,000.
Is the BTU number on a furnace the heat it puts out?
Usually not. The number in the model and on most listings is input. Multiply it by the AFUE rating to estimate output. A 100,000 BTU input furnace at 80% AFUE puts out about 80,000 BTU per hour.
What does BTU stand for?
British Thermal Unit. It’s the heat needed to raise one pound of water by one degree Fahrenheit.
Should I just buy the same size as my old system?
It’s a reasonable starting point, but not a guarantee. Older systems were often oversized, and many houses have had insulation or window upgrades since. A load calculation from your contractor is the best check.
Want a second opinion on size? Call (877) 843-4822, or try the HVAC sizing tool.