What Size Furnace Do I Need? BTU Sizing Guide for Ontario Homes

October 21, 2026

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Family owned & operatedMulti-generational business since 1975
40+ years of experienceHeating and cooling know-how since 1975
Based in GravenhurstServing homes and cottages across Muskoka

Choosing the right furnace size is one of the most important decisions in a furnace replacement. Too small and your home never gets warm on the coldest nights. Too big and the furnace short cycles, wastes fuel, wears out early, and leaves rooms uneven. Here's how furnace sizing actually works, and why a good contractor calculates it rather than guessing.

Furnace Size Is Measured in BTUs

Furnace size is measured by heat output in BTUs per hour (British Thermal Units). Residential gas furnaces typically range from about 40,000 to 120,000 BTU input. Pay attention to output, not input: a 60,000 BTU input furnace at 96% efficiency delivers about 57,600 BTU of usable heat.

Why Square Footage Alone Isn't Enough

Online charts suggest roughly 30 to 60 BTU per square foot, depending on climate. That range is so wide it's nearly useless. Two 2,000 sq ft homes can need very different furnaces because of:

Insulation levels in the walls, attic, and basement
Windows: number, size, age, and orientation
Air leakage (drafty older homes lose far more heat)
Ceiling height and open-concept layouts
Climate zone. Muskoka's winter design temperature is much colder than Toronto's.
Basement and foundation: finished, unfinished, or insulated
Sun exposure and shading

How Contractors Calculate Furnace Size: The Heat Loss Calculation

A proper sizing uses a heat loss calculation (a Manual J–style load calculation, or the Canadian CSA F280 method). It measures every wall, window, ceiling, and floor, applies insulation values, estimates air leakage, and calculates how much heat your home loses at the local design temperature. The furnace is then sized to replace that heat loss, with a small margin.

At Bridlewood Mechanical, we measure your home and calculate your actual heat loss before recommending a furnace. It's the same approach we use in our Duct Design for New Construction work.

How Many BTUs Do I Need for a 2,000 Sq Ft House?

As a very rough starting point in central Ontario:

Home size	Typical output range*
1,000 sq ft	30,000–50,000 BTU
1,500 sq ft	40,000–65,000 BTU
2,000 sq ft	50,000–80,000 BTU
2,500 sq ft	60,000–95,000 BTU
3,000 sq ft	70,000–110,000 BTU

*Estimates only. Newer, well-insulated homes are often at or below the low end, and older, drafty homes toward the high end. A heat loss calculation gives the real answer.

Is a 60,000 BTU or 70,000 BTU Furnace Better for My House?

Neither is automatically better. The right one is the one closest to your home's calculated heat loss. Many older furnaces were oversized, so replacing a 100,000 BTU furnace with another 100,000 BTU unit is a common and costly mistake. If insulation or windows have been upgraded since the original furnace went in, your home may need significantly less.

How Many Square Feet Will an 80,000 BTU Furnace Heat?

Somewhere between roughly 1,800 and 3,000 square feet, depending on the factors above. An 80,000 BTU furnace can suit a leaky 1,800 sq ft farmhouse or a tight 3,000 sq ft new build.

How Many Square Feet Can a 40,000 BTU Furnace Handle?

Roughly 900 to 1,500 square feet in central Ontario. Smaller well-insulated homes, cottages, and new builds often suit 40,000 BTU furnaces, especially two-stage or modulating models.

Do You Need a Bigger Furnace for a Large Home?

Usually, but not always in proportion. A well-built large home may need less capacity than a smaller, poorly insulated one. Large homes often benefit more from zoning or a modulating furnace than from a bigger furnace.

Furnace Size Also Depends on Your Ductwork

A furnace can only deliver its heat if the ductwork can move enough air. Undersized ducts cause high static pressure, noise, and overheating. When we size a replacement furnace, we check that your ducts can handle it, and we fabricate custom sheet metal modifications when they can't. See Ductwork Installation.

Get Your Furnace Sized Correctly

Bridlewood Mechanical serves Gravenhurst, Bracebridge, and Muskoka with free furnace replacement quotes based on real heat loss calculations. See Furnace Replacement or [contact us] for an assessment.