BTU (British Thermal Unit)
A unit of heat energy used to rate how much an HVAC unit heats or cools.
Quick Answer
A BTU, or British thermal unit, is the heat needed to raise one pound of water by one degree Fahrenheit. HVAC equipment is rated in BTU per hour (Btu/h), often shown as MBH for thousands. One ton of cooling equals 12,000 Btu/h, which is how schedules link capacity to tonnage.
The Full Picture
A Btu is a small amount of heat. The U.S. Energy Information Administration defines it as the heat needed to raise the temperature of one pound of water by one degree Fahrenheit. On its own it measures energy. Mechanical drawings use it as a rate: Btu per hour, written Btu/h or BTUH, tells you how much heat a coil, furnace, boiler or air conditioner can add or remove each hour. Many schedules write the rate as MBH, meaning thousands of Btu/h, so a column showing 240 MBH is 240,000 Btu/h.
The link to tonnage is what most estimators need. Cooling equipment is commonly sized in tons, and one ton is 12,000 Btu of cooling, so a 5-ton unit is rated at 60,000 Btu/h. The ton has nothing to do with weight. Some schedules show both numbers, some show only tons, and some give cooling in Btu/h and heating input in MBH, so a unit's row has to be read column by column before it is compared with another unit or a bid.
A Btu is not a takeoff unit. Nobody prices a job per Btu. It matters because it identifies equipment capacity, and capacity drives which model, size and price an estimator carries for each counted piece of equipment. It also appears in load calculations, where the DOE sizing guidance describes methods such as ACCA Manual J for heating and cooling loads. The designer has already done that work, and the estimator's job is to carry what the schedule lists, not to re-size the system.
Common pricing slips are unit confusion. Mixing Btu with Btu/h, reading MBH as millions, or treating a heating input rating as heating output can put an equipment line in the wrong size class. A related trap is comparing a schedule's capacity to a substitution proposal at different rating conditions. If a bidder offers an equal unit, compare the capacity at the scheduled conditions, not just the nameplate tons.
Real Examples
Common Misconceptions
People assume: A ton of air conditioning means the unit weighs a ton.
Actually: A ton is a cooling capacity: 12,000 Btu of cooling per hour. A 5-ton unit is rated for 60,000 Btu/h and does not weigh 5 tons.
People assume: A larger BTU rating is always better.
Actually: DOE guidance says oversized cooling systems can be noisy, provide inadequate comfort and cost too much. The right size is set by the engineer's load calculation. An estimator should price what is scheduled, not upsize.
Frequently Asked Questions
How many BTU is one ton?
One ton of cooling is 12,000 Btu per hour, as stated in the DOE guide to home heating and cooling. A 2.5-ton unit is 30,000 Btu/h, and a 10-ton unit is 120,000 Btu/h.
What does MBH mean on a mechanical schedule?
MBH means thousands of Btu per hour. A heating coil listed at 150 MBH is rated at 150,000 Btu/h. It does not mean millions.
What is the difference between BTU and BTU/h?
A Btu is an amount of heat. Btu/h is a rate, the amount of heat moved per hour. Equipment capacity is always a rate, though people often say BTU when they mean Btu/h.
Is BTU used as a takeoff unit?
No. Equipment is counted each (EA), and the BTU or tonnage rating selects the size and price. The rating is a field on the schedule row, not the unit of measure.
How is a BTU defined?
The EIA defines a Btu as the heat needed to raise one pound of water by one degree Fahrenheit. It is a small unit, which is why HVAC capacities run into the thousands.