What furnace size do I need?

What furnace size do I need?

One of the most common heating questions for homeowners is: what furnace size do I need? Since the typical furnace will last 15-20 years, it’s not going to be as simple as replacing your old unit with a new identical one. Innovations in technology — and most importantly, efficiency — means that you might easily be able to get a smaller furnace that performs as well, or better, than your old one.

You can spot an amateur contractor if they give you a quote simply based on the square footage of your home. Furnace size is very important but there are a host of other factors that also need to be equally considered when sizing a furnace:

  1. Type of furnace: single stage, two-stage or modulating?
  2. Efficiency rating – expressed as an AFUE rating
  3. Size of the home (typically measured in square footage)
  4. Layout of the home – open concept or many rooms that restrict air flow?
  5. Climate – we all know what it’s like in Ottawa during January and February
  6. Landscaping – a townhouse in Orleans protected against winter winds by other homes, trees and other natural features may not need the same furnace as a similar-size home on a spacious and open lot in Kemptville
  7. Thickness of the walls and type/amount of insulation
  8. Size, type and number of windows and doors

Furnace Size: bigger is not better

You own a boat or a trailer and you’re looking for a new truck to tow it around. You look at several models that all promise enough towing power. But now you’re thinking about those hills you need to climb on the way to the cottage or that you might upgrade the trailer at some point soon. So even though the brochures say you’ll have enough horsepower and torque with the basic model, you decide on the next-level upgrade package just to be sure you’ll have the power you need when you need it.

Some contractors take the same approach as our boat example, make a rough heat loss calculation and round up to the next furnace size unit “just to be safe” that your furnace will have enough capacity to keep your home comfortable. The bigger-is-better approach is fine when buying a pickup truck but the same theory doesn’t apply to a new furnace. For instance, if your ductwork can’t support greater airflow, you may experience long-term issues such as an overheating furnace or warm and cold spots throughout the home due to the poor air flow. You also may end up spending a lot more in the long run on energy costs. You know you have a properly-sized furnace because it will run almost continuously throughout the coldest days of the year at your desired temperature.

Do Your Own Manual J Calculation

What was that Home Depot slogan? “You can do it, we can help”. Well folks, let us introduce you to the Manual J calculation. This is the one the pros use and you can do it too. See our full explanation on what the calculation is and how you can do it yourself. It takes a bit of time and some knowledge of home construction but you’ll get an idea of exactly of the kind of expertise that you should expect from a reliable HVAC vendor. The Manual J load calculation method was developed by the Air Conditioning Contractors of America (ACCA). It’s not just a calculation of the entire home, but it also determines measurements room-by-room. No house is exactly balanced in every room so it’s essential that you know where the hot and cold spots are in order to get the best furnace size installed.

A proper Manual J load calculation requires professional software but here is a free, handy online tool that will get you started so you’re better informed when meeting with HVAC vendors. Start with the “Select a State” dropdown (scroll down for the Canadian provinces). Then select “Ottawa Airport” as that will be sufficient to apply to the entire Ottawa area from Orleans to Kanata to Hunt Club.

Get a proper quote to determine furnace size

Your furnace is a major investment. Asking a qualified contractor to perform a proper heat loss calculation is a quick and easy process that gives you the confidence that you’re buying the right size furnace for your home.