What Size Furnace Do I Need? (Full Guide)

Having the right furnace for your home is essential. A well-heated home is crucial for the occupants’ comfort and safety. During the cold season, the winter climate could cause freezing or bursting of the pipes. Homes that lack enough heat and warmth mean more bills to pay for the homeowner.

Boiler and furnace capacities are measured in British Thermal Units. The units represent the energy required to boost the water temperature by one degree Fahrenheit. Since home heating needs vary, furnaces come in diverse BTU ratings and sizes. That means you need to understand what size furnace do I need for my home. This must also be considered when you need to repair the furnace.

Why Does Furnace Size Matter?

Purchasing the correct size furnace for your home ensures you are comfortable and will ensure you pay fewer bills for heating your home. Undersized or oversized furnaces will cause lots of issues to your home.

Issues with Oversized Furnace

Some homeowners assume an oversized furnace will ultimately warm up their house with less effort or energy. However, it would help if you understood that such a furnace could lead to the following issues:

·         Less energy efficiency – frequent shutting off and turning on the oversized furnace will make it inefficient.

·         They are uncomfortable – these oversized heaters work in bursts and can sometimes heat only certain areas of the home. It may warm up other rooms leaving others cold during the cold season. They are uncomfortable because they could heat the room to become uncomfortably hot.

·         Short lifespan – the frequent shutting off and on could lead to wear and tear, giving the system a premature failure.

·         More repairs – because of your furnace’s rampant wear and tear, you will likely incur more repairs costs. You will have to repair the system more often.


Issues with Undersized Furnace and British Thermal Units

Too small furnaces will also cause lots of problems in your home. Some of them include the following:

·        High energy bills – during the cold season, the furnace is constant cycling all through, which means you will incur higher energy bills. The small furnace will need to keep working continuously to supply the required amount of heat to the home.

·         Uncomfortable – this will be evident on the coldest days. The small furnace will be unable to keep up with the low temperatures and will not produce sufficient heat for the home.

·         Uneven heating – some spots in the house will be warm while others will be cold. That is because the small furnace cannot heat the entire home uniformly.

·         Short lifespan – since you will run the furnace all through, it will wear out sooner, meaning it will have a short lifespan.

The Right Furnace Sizing for Your Home

what size furnace do you needThe appropriate furnace size will gradually warm up your house. It will be much more fuel efficient and last for a long time. If you notice your furnace running 24/7, it is probably too small for the house. If it starts and stops frequently, it might be too big for your home.

You will not experience hot or cold pockets with the right size furnace. However, remember there are some freezing nights when even the right-size furnace may fail to keep you warm.

How Furnace Size is Measured

When referring to furnace sizing, it means more than just the physical size of your furnace. Btu furnace size covers how much heat it generates and the heating output. As mentioned earlier, furnace output is measured in BTU per hour. British thermal unit is the energy measurement required to increase a pound of water by 1 Fahrenheit.

A bigger BTU furnace with 120,000 BTUs per hour is excellent for a larger space or home, while a smaller BTU furnace with a 40,000 BTUs per hour capacity is ideal for a smaller home.

Furnace Sizing with Square Footage

Your home’s size is among the biggest determiner of your BTU furnace size. To be more specific, the square footage to be heated will be a significant determiner. Your home’s square footage should give you a rough idea of the typical size of the BTU furnace.

Estimating Required Furnace Size

Getting the wrong furnace size is a common mistake among numerous homeowners. It would help if you considered other factors since this is a long-term investment, and you want to do it right. They include:

Heating Capacity

Furnace sizing does not just refer to the physical dimensions of the furnace. It also covers the heating capacity of the BTU furnace. Furnace size is estimated by the amount of heat it can produce per hour, measured in British thermal units. The higher the efficiency rating, the more powerful a furnace is and the more warmth it can offer. The typical BTU furnace will produce between 80,000 to 100,000 BTUs every hour.

Your Home’s Sq ft

The square footage to be heated and the size of the home are two crucial factors that determine the furnace size. Bigger homes will need more BTUs, while smaller homes will need fewer. However, the rule of thumb dictates 30 to 60 BTUs for every sq ft. It may vary depending on the climate of your area.

To find your home’s total square footage, find the property’s original listing, appraisal, or lease. If the papers are unavailable, you can measure the width and length of the room you want to heat. After that, multiply these dimensions to arrive at each room’s square footage.

Combine all the sq ft to find the square foot home total. Below are BTU furnaces to consider for specific home sizes:

  • 1,200 square foot home – 35,000 to 75,000 BTUs
  • 1,800 square foot home – 55,000 to 110,000 BTUs
  • 2,100 square foot home – 65,000 to 125,000 BTUs
  • 2,400 square foot home – 70,000 to 145,000 BTUs

Understand that BTU usage for every square foot home is not a specific number but a range. That means the square footage is one of many determining factors in the furnace size. Other factors you need to consider besides house size include furnace efficiency and climate zone. These combined factors will give you a rough idea of the BTUs required to heat your home.

Factors Affecting Your Furnace Size

Furnace Efficiency

Two numbers are involved when getting a new furnace: the efficiency rating percentage and the input rating in BTUs. The efficiency rating percentage is the furnace efficiency. It typically shows how effectively a gas furnace turns air into heat. Most furnaces will have an efficiency rating of 80%, but some newer models could have a standard efficiency of 90%.

For example, if the total heat is 80,000 BTUs and the furnace efficiency is 80%, the heat produced will be 64,000 BTUs. That means the furnace size required to produce 80,000 BTUs should be 100,000 if its furnace efficiency is 80%.

Climate Zones

Another significant factor to consider when getting a new furnace is your environment – where you live. There are established climate zones that can help homeowners determine the size of a new furnace to purchase. These zones will dictate the number of BTUs per sq ft you need. People living in colder climates require powerful electric furnaces. For example, a home in Texas will require less heating power because of mild winters, while the same may not apply to a home in Minnesota.

First, you need to determine what your zone is. Then you need to confirm the BTU output that goes with your zone. Below is a list of zones you can use to know your climate zone:

·         Climate zone 1 (Southern regions): The subtropical and desert areas encompass cities like Houston and Miami. The heating factor here is 30 to 35 BTUs.

·         Climate zone 2 (Mediterranean): The Mediterranean zone includes Southern cities such as Atlanta and California. The heating factor here is from 35 to 40 BTUs.

·         Climate zone 3 (Oceanic and Humid Continental): Here, towns include Missouri, Kansas, and Virginia, with a heating factor ranging from 40 to 45 BTUs.

·         Climate zone 4 (Semi-Arid and Humid Continental): You will find cities such as Chicago, Boston, and New York. The heating factor in this zone is 45 to 50 BTUs.

·         Climate zone 5 (Alpine and Humid Continental): It is the northernmost zone and encompasses Twin Cities in Minnesota, New York, and Buffalo with a heating factor of 50 to 60 BTUs. To get the actual output required, multiply the climate zone BTUs by the sq ft of your home.

For example, if you happen to be in zone 5, the BTU furnace you need should have an output of 50 BTUs per sq ft at minimum. If your square footage is 2,000 square feet, you will require 2,000 X 50 btus= 100,000 BTU output.

Home Styles, Orientation, and Layout

full guide for furnancesYour home’s design is crucial in determining the size furnace you need since there are factors that impact heating requirements. Orientation is a big determiner like; for example, the homes facing south receive more sun. That means in winter; you can maximize the warmth from the sun and reduce the total BTUs per square foot output needed to warm the home.

Technically, these homes will do good with smaller BTU furnaces even during winter since they require less heat output.

The floors in your home will also affect the BTU requirement. A 2,000-square-foot home with two floors requires a smaller furnace than a 2,000-square-foot home with only one floor. That is because the second floor will insulate the lower floor, reducing the heat output required.

The plan of your flooring could also affect the size furnace you need. Open-concept homes bear fewer interior walls meaning they can hold heat. Homes with many isolated rooms will require larger furnaces to heat up.

Windows and Doors

Windows, doors, and skylights could lead to leaks and drafts, especially if they are not well-sealed or old. The more entryways the home has and glass, the more BTUs are required to heat the home. However, if your home lacks these features or is in good shape and energy-efficient, you will need a small BTU furnace.

Furnace AFUE

Furnace AFUE is the energy efficiency rating and can also impact the furnace size you need. For example, if the BTU furnace produces 100,000 BTUs at 100% efficiency, it would generate the exact amount of heat. However, if the AFUE is 90%, this furnace will only chunk out 90,000 BTUs every hour.

If the efficiency rating percentage is low, you will need a larger furnace. Even though you will pay more to secure a high-efficiency furnace, it will be a much smaller furnace meaning fewer energy costs. In the long run, your energy bills will also be relatively low.

You could always talk to a licensed HVAC technician if you are still looking for the furnace AFUE for your ideal furnace. They will assist you with all the calculations and show you how to use the BTU calculator.


Insulation is crucial to homes as it increases their efficiency. Homes with well-sealed thick insulation are much more efficient and require smaller BTU furnaces. Understand that insulation does not just cover walls alone. Insulate the crawlspace, attic, and basement to prevent heat loss. Consider insulation as you find the estimate furnace size needed.

Additional Heat Sources

Some homes with gas fireplaces and wood stoves, among other heat sources, could reduce the heat required for heating and cooling systems. Homes with these secondary heat sources will require less energy from a BTU furnace than ones without the heat sources.

Final Thoughts

The size of furnace for house will depend on different factors. That may include the home’s square footage, home insulation, windows and doors, style and orientation, and even climate zone. Use the furnace size calculator to calculate the furnace efficiency of the ideal heat pump you are looking for and consider all these other factors.



How big of furnace do I need

Understand that there is no one-fit-all furnace. You must assess all these issues, such as climate zone, insulation, your home’s style, orientation, and the sq ft. All these factors will help you find the right furnace for your home.

How many BTU furnace do I need?

That will largely depend on the square feet of the home and the climate zone. Use a furnace sizing calculator to find the ideal BTU required. It is simple as you calculate the square footage of your home and then multiply it by the climate zone you live. That will give you the estimated furnace size and BTU you need.

How many square feet will an 80000 BTU furnace heat?

This furnace will heat 2400 sq ft. A 2400 sq ft home will require a furnace producing 70,000 to 145,000 BTUs.

How many sq ft will a 60,000 BTU furnace heat?

A 60,000 BTU furnace will heat an 1800 sq ft home. The range is between 55,000 to 110,000 BTUs.

How many BTUs do I need for a 1500-square-foot home?

For a 1500 sq ft home, you will need a furnace producing 45,000 to 90,000 BTUs. This home’s furnace would produce the right heating power.

What size furnace do I need for a 2000 square foot home?

To heat your 2000 sq ft home, you will need a 65,000 to 125,000 BTUs furnace. This BTUs per square foot is ideal for that home’s square footage.


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