Furnaces

Buying a new furnace is a big decision because there’s so much choice on the market these days, including different brands, makes and models, available technologies, furnace types, and efficiency ratings.

But one of the most important questions you’ll have to answer is “what size furnace do I need?”

Sizing a furnace isn’t just about calculating the square footage of your home. There are lots of things you have to account for with furnace sizing. Today we’ll explain what those factors are, explore why it’s important to size a furnace properly, and help you calculate furnace size based on your space, your climate, and your comfort needs.

Furnace Size & BTU Calculator

➤ Determine furnace size with our new Furnace Sizing & BTU Calculator to get an instant estimate for your home!

Why Furnace Size Is So Important

It might be tempting to buy a smaller furnace to save money on the purchase price or to buy a larger one to maximize space-heating, but furnaces are like Goldilocks’ porridge: you need one that is just right, for your home.

Oversized Furnaces

A furnace that’s too big will cycle irregularly. This is bad because it will create hot and cold spots throughout the house, impact efficiency, increase maintenance, and reduce the lifespan of the furnace.

Undersized Furnaces

A furnace that’s too small will run continuously and still won’t be able to keep your home at a comfortable temperature. The increased workload will also raise your energy bills, create maintenance problems, and cut the life of the furnace short.

Furnace Size Can Maximize Energy Efficiency

Another problem with having a furnace that’s the wrong size is that it will negatively impact the efficiency of the heating system.

The AFUE (annual fuel utilization efficiency) of a furnace is calculated based on normal operation. When a furnace runs continuously or irregularly because it’s not the right size, then you won’t get the efficiency rating you paid for, and that will result in more energy use and higher heating bills.

To ensure your furnace operates at the efficiency level it’s supposed to, it has to be sized properly for your home.

How Furnace Size is Measured

When you talk about furnace size, you’re not actually talking about the physical dimensions of the unit. Rather, furnace sizing is related to its heating output, or how much heat it can generate.

That’s the BTU of a furnace. A BTU (British thermal unit) is a measurement of energy, and specifically the amount of energy required to increase 1 pound of water by 1 F.

A bigger furnace, such as one with an output of 120,000 BTUs per hour, will generate enough heat for a larger space. A smaller furnace will have a smaller BTU capacity, such as 40,000 BTUs per hour, and be suitable for a smaller space.

Some manufacturers have started measuring furnace size in MBH instead of BTUs. MBH is one thousand BTUs per hour, so you’re still working with the same measurement: 60 MBH is 60,000 BTUs.

Furnace Sizing Starts with Square Footage

One of the biggest things that will impact your furnace size is the size of your home, and specifically how many square feet of space need to be heated.

You can use how many BTU per SF as a starting point. From there, however, there are plenty of other things that can impact the size furnace you need, including:

• The climate where you live
• The style and orientation of your home
• How many exterior walls you have
• The quality and thickness of your insulation
• The number of windows and doors, and their age
• Furnace efficiency
• The state of your ductwork

You’ll have to factor in all these elements, along with square footage, to accurately calculate furnace size.

Factors Other Than Square Footage that Impact Furnace Size

Climate

Canada is a rather large country, and the climate in winter can vary drastically depending on where you live. The climate will also help to determine what size furnace you need.

For example, a 2,000 square foot home in Ottawa would require a larger furnace than that same home in Victoria because Ottawa winters are so much colder.

Get Quotes

How soon are you looking to buy?*

Urgently!

Next few weeks

Not sure yet

Home Style, Layout, and Orientation

The design of your home is important to consider when sizing a furnace because there are so many little things that can impact heating requirements.

Orientation is one of them. South-facing homes get more sun, and in winter you can take advantage of this by letting in the light and allowing the sun to warm your home. In other words, south-facing homes might be able to get away with smaller furnaces than homes that are shaded.

The number of floors in your home will also impact BTU per square ft, but maybe not in the way you think. A 1,000-square-foot home with a single floor will need a slightly bigger furnace than a 1,000-square-foot home with two floors because the second floor actually provides insulation, thereby reducing heating requirements.

Your floor plan can also affect furnace size because open-concept homes have fewer interior walls to hold heat compared to homes with lots of isolated rooms, and this can mean you need a larger furnace.

Furthermore, certain features like sliding doors or sunrooms may leak heat more readily and therefore may increase overall heating requirements, especially in that part of the home.

Home Insulation

Insulation is an important tool that can increase the efficiency of your home. Homes that are well-sealed and have thick insulation are more efficient, and efficient homes can typically get away with smaller furnaces.

Insulation isn’t just important in the walls: your attic, crawlspace or basement, and roof all need to be insulated to prevent heat loss.

Windows and Doors

Windows, skylights, and doors can be notorious for leaks and drafts, especially older ones that aren’t energy-efficient and properly sealed. The more glass and entryways you have, the more BTUs you’ll need to compensate for the heat loss.

By contrast, if you don’t have a lot of these features, they’re very small, or they’re well-insulated and energy-efficient, then a slightly smaller furnace might be sufficient.

Additional Heat Sources

When you’re trying to answer “how many BTU furnace do I need,” that will change depending on whether you have secondary heat sources like wood stoves or gas fireplaces that can reduce the demands on a furnace.

If you have additional heat sources that you use regularly in the winter, then you won’t need a furnace with as many BTUs of heating output. You’ll need to include that information in your furnace sizing estimator.

Ductwork

Like with windows and doors, leaky ductwork can increase heating demands throughout your house and increase the furnace size you need, along with increasing your energy bills.

It might be worthwhile to perform a visual inspection of your ductwork to see if there are any problem areas. By fixing leaks, you can improve the overall efficiency of your home and your heating and cooling systems.

Exterior Walls

Homes with fewer exterior walls have less exposure to the elements, and will likely have lower heating requirements. This can include:

• Apartments
• Condominiums
• Semi-detached homes
• Townhouses

Furnace AFUE (its energy efficiency rating)

We already mentioned how size can impact AFUE, but the AFUE of the furnace can also impact the size you need.

Let’s use a 100,000 BTU furnace as an example. At 100 percent efficiency, that furnace would indeed generate 100,000 BTUs of heat per hour. However, if the AFUE is only 90 percent, then that same furnace will only generate 90,000 BTUs of heat per hour.  

The take-home here is this: the lower the efficiency rating, the larger the furnace will need to be.

You will pay more for a higher efficiency model, but you’ll also be able to get away with a smaller furnace, and that will offset some of the cost. Furthermore, you’ll save more on energy bills in the long run with a high-efficiency furnace.

BTU Per Square Foot: How to Estimate Furnace Size Based on Square Footage

We’ve already talked about the myriad factors that can impact furnace size, but you can get a basic estimate using the square footage of your home.

While you shouldn’t use this furnace sizing calculator to make a final decision about furnace size, you can use it to get an idea of the size range to look at when you’re shopping for a new model.

The general rule of thumb is that you need between 30 and 60 BTUs per square foot, depending on your climate.

Calculating Furnace Size for Different Climate Zones

Climate zones can help you determine how many BTUs per square foot you need. You can find more information about climate zones and insulation requirements from the North American Insulation Manufacturers Association.

Determine Which Climate Zone You Live In

As a general guideline, the Vancouver region and similar nearby locations would be considered mild climates. The prairies and other northern regions would be considered cold climates. And most of the rest of the country like Southern Ontario and the Maritimes would be considered moderate climates.

Furthermore, many homeowners in colder climates opt to have dual furnaces, especially for larger homes.

Here’s a list of general BTU per square foot requirements you can use as a starting point to estimate furnace sizing:

Let’s look at an example of how these estimates can help you.

Say you live in a cold climate like Winnipeg in a home that’s 1,800 square feet. The chart would indicate you need a furnace that’s around 108,000 BTUs. But let’s say your home is older, doesn’t have a lot of insulation, and has drafty windows. That could add an additional load to your heating requirement, so you might want to look at furnaces between 110,000 and 130,000 BTUs.

But then say you recently upgraded the insulation, installed energy-efficient windows and doors, and were looking at a furnace with an efficiency rating around 99 percent. In that case, you might want to look at furnaces between 85,000 and 100,000 BTUs instead.

These are just examples with estimated numbers to give you an idea of how the additional factors will change the furnace size you need.

Getting a Furnace Professionally Sized

The best way to ensure your furnace is the right size for your home is to have an evaluation from a professional HVAC expert.

An experienced HVAC technician will be able to accurately calculate the square footage of your home, factor in all the other elements that can impact furnace size, and give you a precise BTU capacity for your furnace.

10 Common Mistakes to Avoid in DIY Furnace Sizing

1. Only using square footage to calculate furnace size. You must consider all the factors that go into the furnace sizing calculation.

2. Forgetting to count skylights. These count as windows!

3. Not accounting for the direction of your windows. Windows that face south will get more sunlight and heat, making your home warmer.

4. Not paying attention to the quality and amount of insulation throughout your home. The more insulation you have and the more efficient it is, the smaller your furnace can be.

5. Failing to account for air leakage around the home. If your home isn’t properly sealed and lets air in/out, then you’ll need to factor that in (you’ll need a larger furnace). Another thing to consider is leaky ductwork, as that can reduce the efficiency of your furnace.

6. Not accounting for the number of people living in the house. More people equals more body heat, so the number of occupants should be factored into your furnace sizing calculation.

7. Choosing a furnace size based on the cost. Smaller furnaces will be more affordable, but that won’t necessarily save you money in the long run if it’s not the right size for your home.

8. Ignoring your climate zone. Climate plays a huge role in furnace sizing. The colder the climate, the larger the furnace; the warmer the climate, the smaller the furnace.

9. Forgetting about plans. Are you planning to renovate soon? Build an addition? Combine two rooms? Make sure you take these plans into account when sizing your new furnace.  

10. Choosing a furnace size based on your existing furnace. Who’s to say your existing furnace was sized properly in the first place? You could drastically improve efficiency and comfort by completing a furnace sizing calculation when it’s time to replace your old furnace.

Sizing Considerations for Vacation Homes or Seasonal Properties

When sizing a furnace for a vacation home or seasonal property, all the same considerations apply, as well as a few more:

Seasonality: Consider how often and when the property will be occupied. If the home is used only during certain seasons (like spring, summer, and fall but not winter), you may not need the same heating capacity as a year-round residence.

Fuel type: Vacation homes and seasonal properties are sometimes in more rural and remote areas, and that can mean the property might use different heating fuel compared to at home. Some fuels are more efficient than others, and you must take this into account when sizing the furnace.

Usage: Properties like these often have different occupancy patterns than regular homes, and it’s important to keep that in mind when determining HVAC requirements. For example, there might only be one or two occupants some weeks, whereas other weeks there might be a full house.

Supplemental heating: Many cottages and similar properties have woodstoves, baseboard heaters, and other types of secondary heating sources that can take some of the load off a furnace.

Expert Insights: Tips from HVAC Professionals on Furnace Sizing

• Square footage is an important part of your load calculation, but don’t forget that if you have high ceilings, then there’s more space to heat
• Consider using Manual J® for your load calculation
• Don’t forget to take the colour of your roof into consideration (dark roof = smaller furnace)
• Pay attention to whether your windows are single-, double-, or triple-paned
• Homes with open floorplans will have different heating requirements compared to traditional layouts
• If your home was built prior to the 1980s, it probably has less efficient insulation and isn’t as airtight, so you might need a bigger furnace

FAQ

What is BTU in furnace calculations?

In furnace sizing, a BTU is how much heat a furnace can produce in one hour. For a larger space, you’ll need a bigger furnace with a higher BTU.  

Can a furnace be too big for a house?

Yes, a furnace can absolutely be too big for a space. When a furnace is too big, it will come on and shut off before completing a heating cycle, potentially leaving hot and cold spots around the house, creating humidity issues, and putting unnecessary wear and tear on the system.

Does Furnace Size Affect Air Conditioning?

Furnace sizing can impact air conditioning in a home with central air/central heating because the two systems share components, including the ductwork, blower, and air filter.

When the furnace is too large or too small, it can cause problems for the A/C, such as insufficient cooling, airflow problems, reduced dehumidification, reduced efficiency, longer cooling cycles, and more wear and tear.

Furnaces aren’t the most exciting thing to think about until yours stops working on a freezing winter night. But you’re ahead of the game if you know how your furnace works. 

You’ll be able to spot issues early, keep it running efficiently, and even save some bucks on those energy bills.

In this article, we’ll break it down in simple terms. You’ll learn how your furnace heats your home and the parts that make it tick.

How Does a Furnace Work?

A furnace works by burning fuel (such as gas or oil) or using electricity to heat air. A blower distributes the heated air through ducts to warm the home. A thermostat regulates the furnace, turning it on and off to maintain the desired temperature.

Here’s a closer look at how it all works:

Step 1: Ignition and Combustion

If your furnace runs on gas or oil, it starts by igniting the fuel. This happens in the combustion chamber, where the burning fuel creates heat. For electric furnaces, it’s a bit different—electric heating elements warm up instead of burning fuel. 

Generally, it takes about 30 to 90 minutes for a furnace to warm up. 

How well this heat is used depends on the furnace’s efficiency, measured by something called AFUE (Annual Fuel Utilization Efficiency). It tells you how much of the heat generated goes directly to warming your home. For instance, a furnace with 90% AFUE uses 90% heat for your comfort while the rest escapes as exhaust.

Step 2: Heat Distribution

Once the air is heated, the blower fan works. It pushes the warm air through ducts that lead to the vents in your rooms. This distribution makes every corner of your home comfortable, even when it’s freezing outside. A more efficient furnace means less wasted heat during this stage, lowering energy bills.

Step 3: Temperature Regulation

The thermostat constantly checks the temperature in your home. When the air gets colder than you’ve set, the furnace turns on. Once your desired temperature is reached, the thermostat tells the furnace to shut off, saving energy and preventing overheating. 

Types of Furnaces by How They Work

The type of furnace you have can affect how evenly your home is heated, how efficient it is, and even how much you’ll spend on energy bills.

Let’s discuss the different types of gas furnaces, starting with one of the simplest options.

Single-Stage Furnaces

A single-stage furnace operates at one consistent heat output level, either fully on or off. It uses a fixed-speed blower and burns fuel or electricity to heat air. The heated air is distributed through ducts to maintain the thermostat’s set temperature.

It works well in smaller homes and apartments where consistent heating isn’t a problem. But because they can only operate at one power level, they aren’t great at adapting to changing conditions, like when the weather shifts during the day. 

Single-stage furnaces are slowly becoming obsolete as more efficient, adaptable options are now available.

Two-Stage Furnaces

A two-stage furnace operates at two heating levels: low for mild days and high for colder days. It uses a variable-speed blower to adjust airflow, improving efficiency and comfort. The furnace runs longer at the lower stage, maintaining more consistent indoor temperatures and reducing energy use compared to single-stage models.

They’re quieter, better at maintaining comfort, and help reduce those annoying temperature swings that can happen with older furnaces. 

Modulating Furnaces

A modulating furnace adjusts its heat output in precise increments based on real-time indoor temperature needs. It uses a variable-speed blower and a modulating gas valve to deliver consistent and efficient heating. 

Modulating furnaces are the most efficient and comfortable option on the market. They’re great for larger homes or anyone who values energy savings and top-notch performance. 

Types of Furnaces by the Fuel They Use

The fuel type a furnace uses affects efficiency, cost, and environmental impact. Let’s start with one of the most common choices in Canada: gas furnaces.

Gas Furnaces

Gas furnaces are a popular option for many homes because they’re efficient and cost-effective. They run on natural gas or propane to heat air. Gas furnaces still need electricity to run specific components like circuit boards, relays, and blower motors)

Here’s a step-by-step process of how they work:

1. Thermostat Sends a Signal: The thermostat detects a drop in temperature and signals the furnace to turn on.
2. Ignition Starts the Burner: The furnace’s ignitor lights the burner, heating the air.
3. Heat Exchanger Warms Up: The burner heats the heat exchanger, which transfers heat to the air without exposing it to combustion gases.
4. Blower Distributes Air: The blower fan pushes the heated air through ducts, spreading warmth throughout the house.
5. Exhaust Gases Are Vented: Combustion gases created during the process are safely vented out of the home through the flue pipe.
6. Thermostat Turns Off Furnace: Once the desired temperature is reached, the thermostat signals the furnace to shut off.

Essential Parts of a Gas Furnace

• Burner: Creates the flame to heat the heat exchanger.
• Ignitor: Starts the combustion process, replacing old pilot lights.
• Heat Exchanger: Transfers heat from the burner to the air.
• Blower Fan: Pushes warm air through the ducts.
• Thermostat: Regulates when the furnace turns on and off.
• Flue Pipe: Safely vents combustion gases outside.
• Air Filter: Traps dust and debris to keep the air clean and protect the furnace.

Electric Furnaces

Electric furnaces are another popular option, especially in areas without access to natural gas. While their operating costs can be higher, they’re often easier to maintain due to fewer moving parts.

How an Electric Furnace Works: Step-by-Step

1. Thermostat Sends a Signal: The thermostat detects a drop in temperature and signals the furnace to turn on.
2. Electric Heating Elements Warm Up: The furnace uses electric resistance coils to generate heat. These coils get red-hot, warming the air around them.
3. Blower Fan Pushes Warm Air: The blower pulls cool air from your home, passes it over the heating elements, and distributes the warm air through the ducts.
4. Heat is Delivered Evenly: The warm air circulates through the house, maintaining a steady temperature.
5. Thermostat Turns Off Furnace: Once the room reaches the desired temperature, the thermostat shuts off the heating elements and blower.

Important Parts of an Electric Furnace

• Heating Elements: Coils that produce heat through electric resistance.
• Blower Fan: Pushes air across the heating elements and distributes it through the house.
• Thermostat: Controls when the furnace turns on and off.
• Air Filter: Cleans the air entering the furnace to keep the system running efficiently.
• Sequencers and Relays: Ensure the heating elements don’t all activate at once, preventing electrical surges.
• Breaker Panel: Manages the high electrical demand safely.

Oil Furnaces

Oil furnaces run on heating oil, stored in a tank, and are known for their ability to produce consistent warmth, even in harsh winters.

How an Oil Furnace Works: Step-by-Step

1. Thermostat Sends a Signal: The thermostat detects the indoor temperature dropping and signals the furnace to turn on.
2. Fuel Pump Delivers Oil: Heating oil is drawn from the storage tank and sent to the burner.
3. Burner Atomizes the Oil: The burner sprays the oil into a fine mist and ignites it, creating a controlled flame.
4. Heat Exchanger Transfers Heat: The flame heats the air inside the heat exchanger, transferring the heat to the circulated air.
5. Blower Fan Distributes Warm Air: The blower pushes the heated air through ducts to warm the home evenly.
6. Exhaust Gases Are Vented: Combustion gases are vented safely outside through the flue or chimney.
7. Thermostat Shuts Off Furnace: When the desired temperature is reached, the thermostat signals the system to shut down until more heat is needed.

Important Parts of an Oil Furnace

• Oil Tank: Stores the heating oil used as fuel.
• Fuel Pump: Transfers oil from the tank to the burner.
• Burner: Atomizes and ignites the oil to create heat.
• Heat Exchanger: Transfers heat from the combustion process to the air.
• Blower Fan: Distributes heated air through the ducts.
• Thermostat: Regulates the furnace operation.
• Flue or Chimney: Safely vents combustion gases outside.
• Air Filter: Traps dust and debris to keep the air clean and the system efficient.

Common Components for a Furnace

Here are the main components that are necessary to run a furnace:

Thermostat

The thermostat monitors the indoor temperature and signals the furnace to turn on or off to maintain your set temperature. Without it, the furnace wouldn’t know when to start or stop, leading to a freezing house or wasted energy.

Manual Thermostat

Manual thermostats are simple to use but require you to adjust the temperature manually. They lack scheduling features, leading to higher energy bills if you forget to lower the heat.

Programmable Thermostat

Programmable thermostats allow you to set schedules for heating, making them more efficient than manual ones. Their precise temperature control helps save energy and reduce costs.

Smart Thermostat

Smart thermostats learn your habits and adjust automatically for maximum comfort and efficiency. With remote control via Wi-Fi, they’re ideal for managing heating on the go.

Heat Exchanger

The heat exchanger is responsible for creating the heat that warms your home. It transfers heat from the furnace’s fuel combustion or electric elements to the air, ensuring your home stays cozy.

Heat exchangers also serve as safety devices by separating harmful combustion gases, like carbon monoxide, from the air circulating through your home. This makes them critical for both comfort and safety.

In condensing furnaces and boilers, there are two heat exchangers. The primary one captures heat from combustion, while the secondary exchanger recovers additional heat from the exhaust.

Blower Motor

The blower motor circulates warm air throughout your home. It pushes air through the furnace’s heat exchanger and into the duct system, ensuring every room gets evenly heated.

Modern blower motors come in two types: 

• Single-speed run at one constant speed.
• Variable-speed motors adjust airflow based on your home’s heating needs.

Final Thoughts

A furnace’s operation involves a complex cycle of heating, airflow, and temperature regulation. While understanding how it works can help you spot issues, attempting DIY repairs can void your warranty or render your furnace unusable. If your furnace is acting up, it’s always best to have a professional take a look.

We provide fair and transparent quotes from trusted contractors in your area.  Let the experts handle it and keep your home warm and safe. Get a free quote today—no obligation!

Frequently Asked Questions