Correct Furnace Sizing: What to Know

April 12, 2024


Why Does Furnace Sizing Matter?

Knowing which size furnace you need in order to heat your home properly is essential. If your furnace is too small, you won’t have enough heat. But, bigger is not necessarily better, because putting a furnace in that is oversized can lead to it overheating and shutting down, or cycling on and off repeatedly. There are several factors that you need to be aware of when choosing the right furnace size for your home.



Furnace capacity is measured in British thermal units, or BTUs. The higher the BTUs, the higher the heating capacity of the furnace. While there are a number of factors that impact the amount of BTUs needed to heat your home, the size of your home and the climate where you live are the most significant factors to consider when measuring how many BTUs you will need. In the Lower Mainland, our homes generally need between 15 to 30 BTUs per square foot, but a qualified heating contractor will need to take all of the different factors into consideration before calculating the BTUs needed, including how many heating registers you have throughout the house, the duct sizing, and the heat loss from windows and wall/attic insulation.



Cubic feet per minute, or CFMs, is the measurement used to determine the volume of air that a furnace system can move in a given amount of time. The higher the CFM, the faster that heated air will move from the furnace to different areas of your home.


Duct Sizes

The diameter of the ductwork in your home has an impact on how heat travels through your heating system. This is a key factor in determining the size of furnace you need, and how effectively your home is heated.


Heat Loss

The more your home naturally loses heat, the bigger the furnace size you will need. Heat loss is more significant in homes that are not well insulated, have single-glazed windows, have doors that are not properly sealed, or have more exposure to the elements, like waterfront properties.

Your heating technician will do a heat-loss calculation to determine the BTUs needed to heat your home. The high-efficiency furnaces made today are much more energy-efficient than their predecessors and don’t need to be as big as the pre-2000 models. Calculating the new size required will not only save energy and money, but it will also ensure that your furnace is not oversized, which can lead to the furnace overheating, turning off prematurely, and creating greater temperature fluctuations in the home. When the furnace is cycling on and off like this, it can often wear out components prematurely as well. So, before deciding to just swap your current furnace out with the same size, do your homework.


Space Limitations

Simple space limitation is also a factor when it comes to selecting a furnace. Your furnace needs to fit in your mechanical room, and if it is a tight squeeze, the options for equipment that will fit may be limited. An experienced heating contractor will be able to determine the best option if you have space constraints.



The amount of energy an HVAC unit is able to expend, or “tonnage,” is important to consider for furnace systems that also have an air conditioner or heat pump. If your home has a cooling system, make sure that the tonnage of your new furnace is compatible with it.

If this all sounds like a lot to know to correctly size a furnace, that’s because it is! Choosing the correct furnace sizing is a complex task, which is why it is indispensable to hire someone who knows what they’re doing. Ideally, to get the best result, you want a heating professional who is certified in forced air heating design, and who has done hundreds of furnace installations. Our experienced professionals at John Sadler Plumbing & Heating have the expertise required to help you find the best furnace sizing for your home. Get in touch today to book an appointment.

Contact us now to get a quote

Contact Us to Get a Quote

John Sadler Plumbing & Heating

Service Area

  • Surrey

  • North Delta

  • Cloverdale

  • Ocean Park

  • Panorama Ridge

  • Fleetwood

  • Whalley

  • Fort Langley

  • Brookswood

  • Murrayville


Monday – Friday 8:30 AM – 5:00 PM

Methods of Payment