You’ve finally decided it’s time to replace your old furnace. You’ve narrowed down the field of potential replacement models, but you still have one question left unanswered: What size should I buy? Determining the right size is one of the most important considerations when purchasing a furnace. Not only will this decision affect your monthly utility bills, but it can also have an impact on how much you spend on repair costs over the life of your equipment.

You can’t know for sure what size will work best in your home without considering a few factors. Here are the most important :

Square Footage of Your Home

Furnace replacement experts note that the first step when it comes to determining the right furnace size for our home is to find the square footage of your home. If you’re not sure how much space you have, take a walk around and count up all the rooms that are heated. This includes hallways and stairwells – we don’t want to leave them out. Once you know this number, then it’s time to take measurements.

Here’s how to work out the area of rooms with specific shapes.

  • To calculate the surface area of a rectangular room, simply multiply the length and width.
  • To calculate the area of a triangular room, first, multiply the length and breadth, then divide by two.
  • For circular rooms, start by measuring the radius r, which is the distance from the room’s edge to its center. Then compute πr2. If your calculator does not have a pi (π) function, use 3.14 instead.)
  • For irregularly shaped rooms, divide them into smaller sections and measure them one by one.

Add all the measurements to get the total square footage of your home.

Climate Zone Your Home Is In

You’ll also want to consider your home’s climate zone. For instance, if you live in a climate zone that has harsh winters or experiences many days below zero, it would be wise to install a furnace system with higher efficiency levels and BTU capacity (heating output) than other homes may need. On the other hand, if you live in a hot climate zone, choosing a system with lower efficiency levels and BTU capacity will help you save money on your monthly energy bills.

Here’s a simple chart of the how many BTUs per square foot you need based on different climate zones in the US:

Climate ZoneBTU per Square Foot
Zones 1 (hot)30-35 Btu/sq. ft.
Zone 2 (warm)35-40 Btu/sq. ft.
Zone 3 (moderate)40-45 Btu/sq. ft.
Zone 4 (cool)45-50 Btu/sq. ft.
Zone 5 (cold)50-60 Btu/sq. ft.

Your climate zone matters because it plays a big factor in how much heat or cooling your home will require. It will also impact your energy efficiency.

Calculate the Necessary BTU Output

With a clear idea of the size and the climate zone you’re in, it’s time to determine your home’s heating requirements. For this step, you’ll need to calculate the necessary BTU output. To do this, simply multiply your home’s total square footage by the recommended heating factor in your climate zone. For instance, let’s say you live in a well-insulated one-story 1,900-square-foot house in Zone 2.

You can multiply your square footage by 40, the lower number advised for that zone. This calculation will provide you with 76,000 BTUs, enough to keep your home comfortably warm in that zone. Remember that the area with the greatest exposure to wind and cold will require additional heating. If it is a particularly drafty home, you’ll need more BTUs than if your house was well insulated or in another zone.

How Much Insulation is in Your Walls and Ceilings

f2 What’s the Right Furnace Size for My Home?

The amount of insulation in your walls and ceilings is another important factor to consider when choosing a new furnace. The number of insulation layers you have can impact the size of the heating system needed for your home. It’s also worth noting that homes built before 1980 typically do not have enough insulation, requiring homeowners to rely on their heater more often during cold weather months. Consult a heating expert to learn more about the insulation in your home, and to determine if a bigger furnace is needed.

Consider Special Needs

Finally, when choosing your furnace size, consider special needs or considerations. For instance, if you have a family member with allergies, they may need an Allergen Barrier filter which is only available in larger units. You should also consider the space where your furnace will be installed and ensure that it has adequate ventilation to prevent overheating problems. With these factors in mind, you should be able to choose the correct size furnace for your home.

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