How To Size A Furnace For Your Home

Maintaining a warm, comfortable indoor temperature during the cold season is something that everyone strives for.  You need an environment where you can relax after a long day of work without worrying about the sub-zero temperatures outside.

But for you to achieve this, you need the right furnace – both in size and efficiency. If you install the wrong furnace size in your home, you certainly won’t have the comfort you seek. For instance, a small furnace will not provide enough heating, while a furnace that is too large will lead to unnecessarily high energy bills.

This means you must know how to size a furnace before making that crucial purchase decision. So, how do you do it?

Best Steps to Size a Furnace for Your Home

If you are wondering how to size a furnace for your home, here are three steps to follow.

  1. Determine Your Climate

In Arkansas, winters are not as harsh as in some other states. Whereas states like North Dakota experience temperatures as low as 2°F, Arkansas temperatures rarely go below 20°F.

So, how does this relate to your furnace size?

Well, colder climates require more heat to keep the house warm than relatively warmer climates. This means that you need a furnace with more heating capacity in colder winters. In areas with just mild winters, like Texas, your furnace doesn’t even have to be on all the time.

The table below shows the necessary BTU (British thermal unit) output for a furnace in different climate zones.

Climate ZonesAvg. Minimum TemperatureBTU’s per square foot
Savanna, Desert, and Subtropical42°F30 – 35
Mediterranean59°F35 – 40
Oceanic & Humid Continental32.0 °F40 – 45
Semi-Arid & Humid Continental32.0 – 26.6 °F45 – 50
Alpine & Humid Continental18°F50 – 60

The table shows that colder climates require larger furnaces. Therefore, depending on where you live, you must choose a furnace size that fits the climate there.

  1. Furnace Size for Square Footage

After determining the climatic region you live in, the next step is to establish the size of your home. Your home’s size is directly proportional to the size of the furnace you need. The larger the space that needs heating, the larger the furnace you require. This way, you can heat the entire space effectively without straining the furnace.

Determining your home’s square footage is easy. This information is available on your home’s original appraisal or listing. You can also find it on the lease agreement.

However, if you can’t access this information, you can simply do the calculations yourself. Take a tape measure and measure each room’s length and width, and use this to calculate the total area.

This step helps you determine the ideal gas furnace size per square foot, thus preventing you from buying a too big or too small furnace.

  1. Establish the Ideal BTU Output

At this stage, you know the climate in which you live and your home’s square footage. Now, it’s time to establish how many BTUs per square foot you need. And it’s easy!

Simply multiply the BTUs necessary for your climate by the size of your house (square footage). For example, if you live in the Mediterranean climate that requires around 40 BTUs per square foot, and your house is around 22,000 feet, then your math will be as follows:

40 BTUs ×22,000 ft2= 880,000 BTU output.

This means that you would need a furnace producing 88,000 BTUs to heat the entire space efficiently. However, there are still a few other factors to consider.

For instance, furnaces are not 100% efficient. This means the furnace’s efficiency must also be considered when sizing a furnace for your home.

In the example above, you would need a furnace with 88,000 BTU output. However, if this furnace has 80% efficiency, it means that it won’t be sufficient for the house. Therefore you would need a furnace with a more BTU output to cover the 20% efficiency deficit.

In this case, you would need a furnace that is 20% larger than the 88,000 BTU output. This would be around 110,000 BTU furnace output.

However, if you are not comfortable with all the calculations, you can employ the services of a qualified HVAC expert to help you out.

Problems of an Oversize Furnace for Your Home

While some might assume a bigger furnace size is ideal for keeping the home warm, it comes with its good share of problems.

For instance:

  • Unequal heat distribution: Furnaces that are too large work in quick bursts. This means that they will overheat certain areas while others remain colder.
  • Constant shutting off of the furnace: As some areas get too hot, the furnace shuts off. These continuous shutting on and off cycles cause the furnace to consume more energy.
  • Short lifespan: As the cycling continues, it causes the system to wear faster, leading to a reduced lifespan.
  •  Increased cost: More wear means you might be required to keep repairing the system every now and then. This becomes even more expensive.

All these possible problems outweigh the need to have that oversized furnace. It might be big, but your space hinders it from being efficient. Therefore, buy a size that fits.

Problems with a Small Furnace

Just like an oversized furnace, an undersized one will also have various problems. While you might think that you’ll save on the initial cost, subsequent costs might be too expensive.

If you buy an undersized heater, you will most likely face these issues:

  •  Insufficient Heating: A small heater will not provide the desired heat output to warm your house during the cold season. This means that it will not offer the comfort you seek during those freezing days.
  • High energy bills: As the furnace tries to compensate for its smaller output, it continually overworks. If the furnace runs nonstop, it only means higher monthly energy bills.
  • Shorter lifespan: Because the furnace is always overworking, it results in increased wear. This means that your heater’s lifespan is reduced.

For this reason, if you want the best performance from your furnace, make sure you get the correct size for your home. This way, there will be no energy wastage or overworking, which leads to unnecessary costs. 

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