Quick answer: furnace sizing in Ottawa
Size by heat loss, not square feet.
A proper calculation looks at walls, windows, attic, basement, air leakage and Ottawa’s −25 °C design temperature.
CSA F280 is the Canadian method.
It is required for new homes under the Ontario Building Code and is the recognised method for sizing replacements.
Most Ottawa homes need 40,000–80,000 BTU/h of output.
Many older houses still have 100,000–120,000 BTU/h furnaces that are far bigger than needed.
Bigger is not better.
An oversized furnace short-cycles, swings temperatures, runs louder and wears out sooner.
Output, not input, is what heats the house.
An 80,000 BTU/h input furnace at 95% AFUE delivers about 76,000 BTU/h.
Check the ducts too.
The blower has to move enough air for both heating and your AC through the existing ductwork.
Why furnace sizing matters more than the brand
A furnace that is sized correctly runs long, steady cycles that keep every room within a degree or two of the thermostat. A furnace that is the wrong size will disappoint you no matter whose name is on the cabinet.
Ottawa makes this a real issue. Our design temperature is about −25 °C, cold snaps can reach −30 °C, and many homes in Nepean, Barrhaven, Kanata and Orleans were built in the 1970s to 1990s and have since been upgraded with better windows, attic insulation and air sealing. Those upgrades cut heat loss, but the furnace in the basement was often sized for the house as it was decades ago.
When we replace a furnace, it is common to find that the right new unit is one size smaller than the old one. That is a good outcome: lower purchase price, quieter operation and more even comfort.
How a CSA F280 heat-loss calculation works
CSA F280 is the Canadian standard for calculating how much heat a house loses on the coldest design day, and how much cooling it needs in summer. It is required for new homes under the Ontario Building Code and is the recognised method for sizing a replacement furnace or heat pump. It replaces guesswork with a room-by-room heat-loss calculation.
What goes into the calculation
Design temperature
About −25 °C for Ottawa, compared with an indoor target of about 22 °C. That 47-degree difference drives everything.
Above-grade walls
Area and insulation level (R-value) of each exterior wall.
Windows and doors
Size, orientation and type. Older single- or double-pane windows lose far more heat than modern low-e glass.
Ceilings and attic
Insulation depth. Many older Ottawa bungalows have been topped up, which changes the result a lot.
Basement and slab
Below-grade walls, whether the basement is finished and insulated, and heated floor area.
Air leakage
Drafts and stack effect. A blower door test gives the most accurate number; otherwise the estimate is based on age and construction.
Ventilation
Heat needed to warm incoming fresh air from an HRV or ERV.
Ducts
Heat lost from ducts running through unheated spaces such as garages or crawl spaces.
The result is a heat loss in BTU/h for each room and for the whole house. The furnace is then chosen so its output covers that load with a modest margin, and the blower is set so each room gets the airflow it needs.
Why room-by-room matters
A whole-house number tells you the furnace size. The room-by-room numbers tell you whether the ducts can deliver heat where it is needed. A cold bonus room over the garage is usually an airflow problem, not a furnace-size problem, and a bigger furnace will not fix it.
Why the BTU-per-square-foot rule of thumb is risky
You will see online charts that say something like “multiply your square footage by 40 to 60 BTU.” These rules are easy, and they are often wrong for Ottawa homes in both directions.
Two houses of the same size can have very different heat loss.
A 2,000 sq ft 2015 build in Findlay Creek may lose half as much heat as a 2,000 sq ft 1965 house in Alta Vista with original windows.
Square footage ignores what matters most
Window area, insulation, air leakage and how much of the basement is exposed above grade.
Rules of thumb usually round up.
Add a contractor’s “safety factor” on top, and you get the 100,000+ BTU/h furnaces we still pull out of mid-sized homes.
Matching the old furnace is not sizing.
If the old unit was oversized, replacing it “like for like” carries the mistake forward another 15–20 years.
Use square footage only as a sanity check on a proper calculation, never as the calculation itself.
Rough furnace size reference for Ottawa homes
The table below gives rough, approximate heat-loss ranges at Ottawa’s −25 °C design temperature. It is meant to help you judge whether a quote is in the right neighbourhood. It is not a substitute for a heat-loss calculation, and real homes fall outside these ranges.
| Home type (heated area incl. finished basement) | Approximate heat loss per sq ft | Rough furnace output needed |
|---|---|---|
| Newer, airtight home (2010+), 1,500–2,500 sq ft | about 15–25 BTU/h | roughly 30,000–60,000 BTU/h |
| 1980s–2000s home with some upgrades, 1,500–2,500 sq ft | about 20–30 BTU/h | roughly 40,000–70,000 BTU/h |
| Older home (pre-1980), original windows, 1,200–2,000 sq ft | about 30–45 BTU/h | roughly 45,000–85,000 BTU/h |
| Large or older home, 3,000+ sq ft, lots of glass | about 25–40 BTU/h | roughly 75,000–110,000 BTU/h |
If a quote lands well above these ranges for your type of home, ask how the size was chosen. If the answer is “it matches what you have,” ask for a heat-loss calculation.
Input vs output BTU: read the label correctly
Furnaces are usually sold by input rating, which is how much gas they burn. What heats your house is the output, which is input multiplied by efficiency (AFUE).
| Furnace input rating | AFUE | Approximate heat output |
|---|---|---|
| 80,000 BTU/h | 95% | 76,000 BTU/h (80,000 × 0.95) |
| 80,000 BTU/h | 80% (older mid-efficiency) | 64,000 BTU/h |
| 60,000 BTU/h | 96% | about 57,600 BTU/h |
| 100,000 BTU/h | 97% | about 97,000 BTU/h |
This is why an old 100,000 BTU/h mid-efficiency furnace can sometimes be replaced by an 80,000 BTU/h high-efficiency one with almost the same heat output. New gas furnaces sold in Canada must be at least 95% AFUE. For a plain-language explanation of the ratings, see SEER2, AFUE and HSPF2 explained.
Is bigger better? The problems with an oversized furnace or AC
No. An oversized system heats or cools the house quickly, then shuts off, then starts again. That pattern, called short cycling, causes most of the complaints we hear about “new furnaces that never feel right.”
Oversized furnace problems
Temperature swings
Blasts of hot air, then cool-down, so some rooms overshoot while others never catch up.
More wear
Every start stresses the igniter, inducer motor and heat exchanger. More starts mean more repairs and a shorter life.
Lower real efficiency
Condensing furnaces are most efficient on long, steady runs, not quick bursts.
Noise
A larger furnace pushes more air through the same ducts, so registers whistle and the blower roars.
Higher cost
You pay more for the furnace and often more for gas.
Oversized air conditioner problems
- Poor humidity control: an AC removes moisture only when it runs long enough. An oversized unit cools the air fast and shuts off, leaving the house cool but clammy on humid Ottawa days.
- Mould risk: lingering humidity in basements and closets encourages mould and musty smells.
- Noise and vibration: a bigger condenser is louder outside bedroom windows, and frequent starts are disruptive at night.
What about undersizing?
An undersized furnace runs nonstop on the coldest nights and may not hold 21–22 °C at −25 °C. It is less common than oversizing, but it happens when an addition or finished basement is added without checking the furnace. The fix is the same: calculate the load.
Airflow, ductwork and static pressure
A furnace is only as good as the ducts it connects to. The blower must move enough air for heating in winter and, usually, even more air for the AC coil in summer.
Static pressure
Undersized return ducts or a restrictive filter raise static pressure, which makes the blower work harder, louder and less efficiently. We measure it on install.
Return air
Many older homes have one or two small returns. Adding a return can solve comfort problems that no furnace size will.
AC tonnage drives blower size
A 3-ton AC needs roughly 1,200 CFM of airflow, so the furnace blower has to support the AC you have or plan to install.
Filters
A thick media filter in the right cabinet lowers resistance compared with a clogged 1-inch filter. See why changing your furnace filter matters.
Two-stage and modulating furnaces: a built-in hedge
A single-stage furnace is either fully on or off. A two-stage furnace runs on a low fire (around 60–70% of capacity) most of the winter and switches to high only on the coldest days. A modulating furnace adjusts in small steps, often down to about 35–40% of capacity.
These furnaces still need to be sized correctly, but they are much more forgiving. On a typical −5 °C day, a modulating furnace runs long, quiet, low cycles instead of short blasts. If your load sits between two sizes, a two-stage or modulating unit lets us choose the size that covers −25 °C without short cycling the rest of the season.
Typical installed prices for a high-efficiency gas furnace replacement (96–98% AFUE) in Ottawa are $4,500 – $8,500, with variable-speed/modulating models up to about $10,500. The final price depends on the home, equipment and venting/ductwork, and Oria gives free quotes on any equipment purchase. See our furnace replacement cost guide for the full breakdown.
What a proper furnace sizing quote should include
- 1
A heat-loss calculation
A whole-house and ideally room-by-room heat-loss number at Ottawa’s design temperature, with the method named (CSA F280).
- 2
A look at your house, not just the furnace
Window types, attic insulation, basement finish, additions and air leakage should all be noted or measured.
- 3
Output capacity stated clearly
The quote should show the furnace’s output BTU/h and AFUE so you can compare it to the calculated load.
- 4
Airflow and duct check
Static pressure, return-air capacity and the blower’s ability to support your current or planned AC tonnage.
- 5
Venting and condensate plan
How the PVC intake and exhaust will be routed and terminated, and where condensate will drain.
- 6
Options at more than one stage
Single-stage, two-stage and modulating options, with the comfort and cost trade-offs explained in plain language.
Get a furnace sized for your house, not your neighbour’s
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Get your furnace sized properly in Ottawa
If your current furnace short-cycles, leaves rooms cold, or is simply nearing the end of its life, the first step is an honest look at how much heat your home actually needs. Oria Heating and Cooling has sized and installed furnaces across Ottawa since 2014, and we give free quotes on any equipment purchase.
Learn more about our furnace installation service, or book a free quote and we will calculate the right size for your home.
Frequently Asked Questions
What size furnace do I need for a 2,000 sq ft house in Ottawa?
It depends on the house more than the size. A newer, well-insulated 2,000 sq ft home may need only 35,000–50,000 BTU/h of output, while an older home with original windows may need 60,000–80,000 BTU/h. Ottawa’s −25 °C design temperature is the starting point, and a CSA F280 heat-loss calculation is the only reliable way to choose.
Is it bad to have an oversized furnace?
Yes. An oversized furnace short-cycles, which causes temperature swings, louder airflow, more wear on the igniter and inducer, and lower real-world efficiency. It also costs more to buy. If your furnace runs for only a few minutes at a time on a mild winter day, it may be oversized for your home.
What is a CSA F280 heat-loss calculation?
CSA F280 is the Canadian standard for calculating a home’s heating and cooling loads. It looks at walls, windows, ceilings, basement, air leakage and ventilation at the local design temperature, and produces a heat loss in BTU/h. It is required for new homes under the Ontario Building Code and is the recognised method for sizing replacement equipment.
Should my new furnace be the same size as my old one?
Not automatically. Many Ottawa homes have had window, insulation and air-sealing upgrades since the old furnace was installed, and many older furnaces were oversized to begin with. A new high-efficiency furnace also delivers more of its input as heat. It is common for the right replacement to be one size smaller.
What is the difference between input and output BTU?
Input BTU/h is how much gas the furnace burns; output is how much heat it delivers to your home. Output equals input multiplied by AFUE. An 80,000 BTU/h input furnace at 95% AFUE delivers about 76,000 BTU/h. Always compare the output rating against your home’s calculated heat loss.
Does a two-stage or modulating furnace help with sizing?
Yes. A two-stage or modulating furnace runs at reduced output most of the winter and uses full capacity only on the coldest days. That means long, quiet, even cycles even if the furnace is sized for −25 °C. It does not replace a proper calculation, but it makes the system far more forgiving.
Does my furnace size affect my air conditioner?
Yes. The furnace blower moves the air for your central AC, so it must be able to deliver roughly 400 CFM per ton of cooling. A furnace chosen only for heating may not support a 3-ton AC properly. We check both loads together, especially when replacing the furnace and AC at the same time.
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