BTUs or British Thermal Units is a measurement of heat (cooling) energy. More technically, it is the amount of heat needed to raise the temperature of one pound of water one degree Fahrenheit. Some relate it to about the amount of heat that can be produced by burning one wooden kitchen match. A typical home needs about 50,000 BTUs of heat every hour to maintain it's desired temperature on a very cold day.
Please take time to read how we can determine exactly what YOUR HEAT LOAD is. Most gas furnaces are rated in input BTUs which is what you have to buy, but what you actually get is output BTUs. The difference is what goes UP THE CHIMNEY. . Many old
furnaces waste about 50% while newer CONDENSING FURNACES have an AFUE of about 95, meaning they only waste about 5%.
Twelve thousand (12,000) BTUs equals one ton. This term is more often used when talking about Air Conditioners, Refrigeration Systems, Heat Pumps, and Geothermal systems. A typical home would need somewhere around 36,000 BTUs, or 3 TONs, of cooling per hour on a very hot day. An old air conditioner with an EER of 8 would need about 45 cents worth of electricity to do this, where new one with an SEER of 16 could do it for 22.5 cents, and a Geothermal system could do it for 13.5 cents. You can see how this can save a lot of money in a year.
This has a very interesting answer. The term BTU was around long before the invention of mechanical refrigerated air conditioning. In the early 1900s many public use buildings were air conditioned with ice. The old downtown theatre in Dodge City was one of these. Men would bring truckloads of ice, which had been cut from ponds in the winter and stored until summer, and dump it into the basement through an outside trap door. A very large fan would simply move air across this ice and then into the seating area. One pound of ice has the ability to absorb 144 BTUs of heat as it melts. Every TON or 2000 lbs of ice would remove 288,000 BTUs (2000X144) of heat from the building. Since they would only bring ice once per day or once every 24 hours, each ton of ice would remove 12,000 BTUs per hour (288,000/24)
Thus todays commonly accepted yet little understood term TON equals 12,000 BTUs
Weber Refrigeration Heating and Air Conditioning