When looking at air conditioners, furnaces or any other HVAC units, one term you’ll come across is BTU. This is one of the most important things to focus on as it determines how much heat or cooling the unit can provide. As a homeowner, if you want to learn more about BTUs and what they mean for your HVAC system, read below.

What Are BTUs?

BTU stands for British Thermal Unit and measures how much energy any HVAC unit uses to heat or cool in an hour. Specifically, a BTU is how much energy it takes to raise or lower the temperature of one pound of water by 1 degree Fahrenheit.

The Link Between BTUs and HVAC Equipment

BTUs measure the heating or cooling output of air conditioners, heat pumps, furnaces, mini-splits, and other HVAC systems. The higher the BTUs, the more heat energy the unit can produce per hour for heating purposes. The opposite is true for cooling systems. The higher the BTUs, the more heat energy the system can remove from the building in an hour.

When choosing a new HVAC unit, total BTUs is an important consideration. If the BTUs are too low, the unit won’t be able to heat or cool the space it is intended for effectively. This means your HVAC system will need to run almost constantly, and even then may still be unable to manage your home’s temperature. As a result, you will have much higher energy costs, and your home will be less comfortable than if the unit was the correct size.

If the BTUs are too high for the space, the unit will only run for a short time before reaching your desired temperature and then shut off. While this may not sound like a big issue, it increases energy costs and puts much more stress on your entire HVAC system. Frequent starting and stopping cause regular repair costs and an overall shortened lifespan for your HVAC system.

Determining How Many BTUs Your HVAC Unit Needs

The size of your home and the local climate are two important factors in determining how many BTUs of cooling or heating your HVAC unit needs to produce. The US Department of Energy separates the country into seven different climate zones. Each zone has varying requirements regarding how many BTUs are required per square foot to cool or heat a home effectively.

Oviedo and almost all of Florida are in Climate Zone two. The only exception is the southern tip of the state, which is the only part of the mainland US in Climate Zone one. To heat your home effectively in zone two, you need around 35 to 40 BTUs per square foot. In colder northern climates, you often need as many as 60 BTUs of heating for each square foot.

In Climate Zone two, the DOE recommends that your AC must produce around 45 and 50 BTUs of cooling for every square foot to cool effectively. However, other experts say you only need around 30 to 40 BTUs in the southern US and 20 BTUs per square foot in areas with milder summers.

You can use these numbers to then calculate how large of an AC, furnace or any other HVAC unit you need based on the total square footage of your home. Let’s say the total area of the living space in your Florida home (not including the garage) is 2,000 square feet. This means you would need anywhere from a 60,000 to 100,000 BTU air conditioner, and your furnace or another heating system would need to be around 70,000 to 80,000 BTUs.

The size of a furnace is always measured in BTUs. However, air conditioner size is measured in tons. Each AC ton is the equivalent of 12,000 BTUs. Based on the climate zone calculations, this means that you could need anywhere from five to eight AC tons to cool your 2,000-square-foot home effectively.

The only issue is that the largest AC unit is five tons, which is why many newer homes in hotter climates have two AC units. If your home only had a single five-ton unit, it may be sufficient to keep you cool on milder days. However, your home is likely to feel hot and muggy a lot of the time since that five-ton AC will struggle to remove enough heat to keep you cool.

You can also use the same calculations to determine what size of ductless mini-split, portable air conditioner or window AC you need to cool your bedroom or any other room effectively. Again, you’ll need around 30 to 50 BTUs per square foot. If your bedroom is 120 square feet, then you need a unit that is at least 3,600 to 6,000 BTUs. The general rule of thumb when choosing portable ACs or other similar units is that rooms under 150 square feet need a 5,000 BTU unit.

How HVAC Technicians Calculate How Many BTUs Are Needed

When determining what size of heating or cooling equipment is needed, HVAC technicians use a Manual J calculation. This formula takes into account both climate zone and total square footage. However, these two factors alone aren’t sufficient to determine exactly how much cooling or heating is needed.

This is why Manual J also looks at numerous other factors, such as how well insulated the home is and the number of occupants. It also takes into account the number, size and location of all the windows in the home. Windows influence how much heat gain you get from the sun. A technician also factors in how much shade or direct sun exposure the home receives. By considering all of these things, the technician can have a much more accurate estimation of how many BTUs are needed.

How BTUs Relate to Energy Usage and Efficiency

BTUs are directly related to how much energy an HVAC unit uses. The higher the BTUs, the more energy the unit will use every time it runs. However, estimating the specific energy usage also requires looking at the unit’s energy efficiency.

The energy efficiency of furnaces is measured in AFUE or Annual Fuel Utilization Efficiency, which looks at the total percentage of heat energy that the unit can capture. Standard gas furnaces range between 80 and 90% AFUE, while high-efficiency condensing furnaces can be as high as 98.5% AFUE. When the furnace burns a gas, it turns 100% of the fuel into heat energy, but some percentage of this heat energy always remains in the combustion gases when they flow out of the exhaust vent.

The energy efficiency of cooling units is measured in SEER or Seasonal Energy Efficiency Ratio. This number is directly related to the unit’s BTUs. SEER is calculated by dividing the total BTUs of heat energy the unit removes in one hour by how many watts of electricity it consumes in that time. The higher the SEER number, the less energy the unit will use.

Central Florida’s Cooling and Heating Experts

If you have any questions about BTUs, the knowledgeable team at American Air, Plumbing & Electric has the answers. We are family-owned and operated and have been serving Oviedo and Central Florida since 1987. We are proud to be the most reviewed company in Orlando and the area’s most trusted HVAC service.

Our NATE-certified technicians install, maintain and repair cooling and heating equipment, including air conditioners, furnaces and heat pumps. If you need a new air conditioner, heat pump or furnace, we will perform all the necessary calculations to determine exactly how many BTUs you need. Contact us today to get started.

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