How to Pick the Right Furnace Size for Your Home

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Every winter, we get emergency calls from very surprised customers who didn’t realize their furnace wasn’t going to make it through another winter.

Suddenly, they discover they have a major outage on their hands and their home is cooling down rapidly. They have to decide right away which furnace to buy and go with whatever the best price is at that moment.

Life happens and sometimes this is just what it hands you. Needless to say, though, this isn’t the ideal way to make a significant investment into a new furnace you may have for the next 15 to 20 years.

If you have some time and a heads-up that your current furnace isn’t going to last forever, this handy guide can help you start thinking through the best new furnace for your home.

Sizing Your Furnace for Your Space

There are a number of different approaches to match the size of the furnace to the size of the space that needs to be heated. These are two of the most common methods.

Geographic zone

One method is to identify what geographic zone you live in and follow the output criteria for heating in that zone.

Canada has three main zones: 1, 2 and 3. Each zone correlates to a different HDD value derived from the Canadian National Building Code. HDD stands for heating degree day.

For example, Zone 1 includes areas under 3,500 HDDs. Zone 2 includes areas with 3,500 to 6,000 HDDs. And Zone 3 includes areas above 6,000 HDDs. Zone 1 is the warmest and Zone 3 is the coldest.

Each furnace will be rated for use in a different zone.


The next method of sizing a furnace to a space is using BTUs, or British Thermal Units. Basically, BTUs measure how much energy it takes to increase heat by one degree.

BTUs correlate to square footage. The bigger the space, the more BTUs will likely be needed to heat it.

However, how the square footage is apportioned also matters when it comes to correctly matching the furnace’s BTUs rating with the space being heated. If a certain portion of the home is not ducted and will not receive heat, that amount of square footage should be subtracted from the calculation.

Warning Signs Your Furnace Is Too Big

It wasn’t so long ago that the governing school of thought in the HVAC industry was to always upsize the furnace capacity (BTUs) versus the size of the space.

This “bigger is better” mentality did no one anyone favors, as HVAC professionals now know.

When you install a furnace that is too big (powerful) for the space being heated, it is not unlike trying to drive a high-performance race car through a little town full of stoplights. The engine just isn’t built for slow, stop-start driving, and before long, you may have a major repair on your hands.

In the same way, a furnace too big for your space is likely to generate all of these warning signs and more:

Rising energy costs

A more powerful furnace will draw in more energy/power than it needs every single time it powers up. This will run up your winter utility bills without measurably improving the heating in your space.

Uneven heating

Because the furnace is putting out more heat than the space needs, an “on” cycle is likely to heat your space beyond what feels comfortable. An “off” cycle will then cool down your space past what is comfortable.

You may also experience hot and cold spots in different parts of your home.

Constant cycling

Remember our race car analogy? An overly large furnace idling in a small space is going to cycle on and off continuously because it simply puts out too much power to get the heating balance right.

Frequent repairs

Any motor that works in a sub-optimal environment is going to experience more wear and tear and break down more frequently than an optimally sized motor.

Shorter useful life

Along with frequent repairs, you can also expect a shorter useful life out of an oversized furnace.

Warning Signs Your Furnace Is Too Small

Installing a furnace too small for your space can actually give rise to similar issues as what occurs when the furnace is overly large for the space being heated!

Actually, the same issues we just outlined above will show up when your furnace is not sufficiently powerful to heat the size of your space.

Why Square Footage Alone Can’t Predict the Right Furnace Size

With so much information readily available on the internet today, you may be wondering if you can simply do your own calculations, determine the optimal furnace size for your space and get down to the business of buying your furnace.

We wish it was that simple! There is quite a bit more that goes into correctly matching the furnace size to the size of the space than just square footage.

Other important factors can include the height of the ceilings, the number of windows and the direction they face, the size of the rooms, the state of the ductwork and insulation, the number of occupants, how many levels your home has, the type of roof, the building materials used to construct your home and even your choice of exterior house paint!

This is why it’s smart to involve a trusted HVAC professional when you’re shopping for a furnace that will heat your space for the next two decades.

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