What You Need to Know About BTUs

What You Need to Know About BTUs

Winter is just around the corner, and with it comes heating season. Is your furnace prepared? How many BTUs does it have? What is a BTU, and how is it important to your furnace? Let’s take a look at what you need to know.

Measuring BTUs

A BTU (British Thermal Unit) is the standard for measuring heat and heat energy. One BTU can heat a pound of water by one degree Fahrenheit (similar to a calorie on the metric scale).

The average furnace gives off between 80,000 and 100,000 BTUs per hour. However, that’s just a measure of its total energy output, not the actual heat given off. To determine the amount of heat you’ll have to look at the AFUE rating. This measures how much of your furnace’s fuel is turned into heat, versus how much is given off as exhaust in a given season. A furnace with 100,000 BTUs and a 95 percent AFUE will provide 95,000 BTUs of usable heat, under ideal conditions.

Using Manual J

Of course, most conditions aren’t ideal. A variety of factors will affect your furnace’s overall level of efficiency. So how many BTUs do you need to keep your home properly heated? If you have too few, your furnace won’t be able to heat your home properly. But too many, and your house could heat too quickly, causing your furnace to cycle on and off frequently, wearing out the system.

The best way to find the right size furnace for your home is to use Manual J. This is a standard reference for HVAC technicians that helps them measure all the factors that affect your home’s heat, including:

  • The size of your home
  • The amount of insulation you have
  • The size of each room
  • Number and placement of windows and the amount of sunlight your house lets in

By considering all these elements, your HVAC technician can use Manual J to calculate exactly how many BTUs your furnace needs to keep your home comfortable all season.

Visit our website for in-depth information on most HVAC topics, or just give us a call at (408) 868-5500.


Our goal is to help educate our customers in the greater San Diego and San Marcos, California area about energy and home comfort issues (specific to HVAC systems).