What is a BTU (Really, A British Thermal Unit)?

What is a BTU

by Erik North on July 28, 2011

 

What is a BTU (or British Thermal Unit)? And why are we measuring our heating in British thermal units?

You may have seen the acronym BTU listed on home appliances such as your hot water heater or furnace. BTU stands for British Thermal Unit, similar to the watt and directly related to the joule. But the British Thermal Unit is specifically related to heat and energy.

A Boring Wikipedia-style Definition (And Much Earlier Than Usual):

The British Thermal Unit is measured as the amount of energy needed to heat one pound of water from 39 degrees Fahrenheit to 40 degrees Fahrenheit. Simplified, it’s the amount of energy needed to raise the temperature of 1 pound of water by 1 degree Fahrenheit. Which if you think about it, makes no sense (why is the British unit not in Celsius).

It is equal to about 1,055 joules of energy. Yes, the BTU can be converted to the traditional British measurements of kilograms and Celsius. But, and I believe this in the same American spirit where we’d shoot speed limit signs that were in kilometers, the British Thermal Unit is most often measured in pounds and degrees.

BTUs and Energy Auditing

I always take a look at your home’s heating system during energy audits. It would be strange not to, wouldn’t it? After all, the audit is about energy!

I test to see the maximum heating capacity of your boilerplate and furnace, to make sure it is not exceeding the expected heat load of the house.

The BTU per hour rate of a boiler’s heating capacity determines whether it’s the right fit for your home. Too much heat and your house not only feels too warm, but those extra dollars for heat are flying out the window. Too little heat and, well, we do live in Maine and have winter, don’t we?

BTU is also calculated as part of the energy loss per gallon of your fuel-burning water heater and boiler. This measurement is derived from the BTU heat needed to heat one cubic foot of air.

BTUs and Fuel

BTUs are also a great measure for comparing different fuel sources. Oil for example has approximately 138000 BTUs of energy while a therm of natural gas has 100000 BTUs (I assure you this is a coincidence and not the definition of a therm). Some simple algebra lets you compare prices per BTU of different fuel sources.

Inching Up on BTUs

So, the BTU is the standard measurement calculation I use when dealing with all of the heating appliances in your home. Just like the inch is the standard measurement of the foot and the yard, so the BTU is the benchmark measurement. You want to have the right amount of heat heating your home, so that money is neither wasted or underspent.

Further reference:
Here is a link to a useful fuel comparison calculator: Fuel Comparison Calculator

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