How to choose the right-sized furnace for your home

We are asked all the time to give quotes for furnace replacement over the phone. We can give you a range based on the type of furnace you want and a general description of what is currently installed, but we can’t responsibly give you an accurate estimate without visiting your residence. In addition to looking for extra install construction requirements, it is essential that we do a heat-loss calculation to properly size your furnace.

What is a heat-loss calculation

A heat-loss calculation (also sometimes referred to as a Manual J calculation or a heat-load calculation) is a measure of how quickly heat escapes the building, measured in BTU/hour or Watts. The general formula for this is:



  • Q is the heat loss
  • U is an aggregate number representing the resistance of the building to heat loss (measured in Watts/m2*°C )
  • A is the surface area of the building being considered (measured in m2)
  • deltaT is the temperature differential between the normal temperature in the building and the 97.5 percentile coldest outdoor temperature during the three coldest months of the year (in °C)

In years gone by this calculation was done with pencil and paper, but now a number of paid software packages are available to do sophisticated modeling, as well as quite a few free ones of varying quality on the internet.

Why is the heat-loss calculation important

Sizing your furnace for your house is extremely important. If you install one that is underpowered then on cold days you will probably never get up to your desired temperature and your furnace will run continuously. The temperature throughout the house will be uneven. Your furnace will also wear out more quickly.

Some people like to install a furnace that is a little overpowered for the space “just to be safe”. This can actually be worse than installing an undersized one, as having too large a furnace can cause your furnace to work in short cycles, waste fuel, turn off as elements get overheated, cause hot spots in some areas of the house and under-heat others, and dramatically reduce the lifespan of the furnace.

Some HVAC contractors have a tendency to replace old low-efficiency furnaces with high-efficiency furnaces that have the same BTU rating, but often lower ratings are more suitable.

What do we need to see and measure for to do the heat-loss calculation

Your contractor will come and carefully measure the size and height of your rooms, take notice of the number and placement of your windows, make qualitative assessments of the construction age, type and quality, and determine the materials for your house, roof and door. They will also attempt to gauge the level of insulation in your house and adjust for modern energy-efficient windows.

How long does it take?

On average we can gather enough information to make a heat-loss calculation in approximately one hour.

Can I get a furnace without one?

In addition to all the energy saving and comfort reasons for doing a proper heat-loss calculation, there is also a very practical issue in Edmonton. In order for your new furnace to pass inspection, the city requires evidence of the heat-loss calculation. If your contractor tells you that you can save on labour by skipping this step, then run the other way!

Do you have any questions or concerns about choosing the right-sized furnace for your home? Leave us a message at (587) 773-2612 and we will get back to you as soon as possible, or call (780) 231-0064 if you need to speak to someone immediately. To schedule an appointment, pick your preferred time on our online booking page.

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