You submitted your permit application. It came back with a note asking for a heat loss and heat gain calculation, or a “CSA F280 report,” or “load calculations to support equipment sizing.”
If you’ve never seen that request before, it reads like bureaucratic filler. It isn’t. Here’s what’s being asked for, why, and what a report has to contain to get through review.
Why the calculation is required at all
For decades, residential heating equipment in Ontario was sized by habit. A contractor would look at the square footage, apply a number they’d carried around since their apprenticeship, round up, and install.
Rounding up felt safe. It wasn’t. An oversized furnace reaches setpoint fast, shuts off, and repeats — short cycling. That produces uneven temperatures between rooms, poor humidity control in summer, more wear on ignition and blower components, and higher bills than the equipment’s efficiency rating would suggest. The homeowner ends up with a system that’s technically working and practically unpleasant, and no obvious explanation for why.
The Building Code’s answer is to require that equipment capacity be justified by calculation rather than by rule of thumb. That calculation is done to CSA F280-12, Determining the Required Capacity of Residential Space Heating and Cooling Appliances. The plans examiner asking for a heat loss calculation is asking you to show your work.
(Code editions and referenced standards do get updated. Confirm the current requirement with your municipality’s building department for your specific project.)
What the calculation actually does
A heat loss calculation determines how much heat your building loses on a cold design day. A heat gain calculation determines how much heat it takes on during a hot, humid one. Together they tell you the capacity your heating and cooling equipment needs — not more, not less.
The inputs are the building itself:
- Wall, ceiling, and floor assemblies and their effective R-values
- Window and door areas, U-values, solar heat gain coefficients, and which direction each faces
- Air tightness and infiltration rate
- Ceiling heights and volume
- Exposed surface area — a corner room with three exterior walls behaves nothing like an interior one
- Design temperatures for your specific location
- Internal gains from occupants, lighting, and appliances
None of these is floor area. That’s the whole point. Two 2,400 square foot houses on the same street can have loads that differ by 40% depending on window area, orientation, and how tightly they were built.
The rule-of-thumb problem, concretely
The old shortcut was something like 30 BTU per square foot. Apply it to a 2,400 sq ft house and you get 72,000 BTU/h, so you order an 80,000 BTU furnace.
Run the actual calculation on a reasonably well-built modern house of that size and you might land closer to 40,000. The 80,000 unit isn’t a safety margin — it’s double. It will short cycle from the day it’s commissioned, and no amount of thermostat adjustment will fix it, because the problem is the equipment, not the controls.
This is the failure mode the calculation requirement exists to prevent, and it’s why plans examiners no longer accept a capacity number with nothing behind it.
What a compliant report contains
A report that passes review generally includes:
- Room-by-room loads, not just a whole-house total. The whole-house number sizes the equipment; the room-by-room numbers size the ductwork and distribution. A report with only a house total can’t be used to design the duct system, and reviewers know it.
- Stated assumptions — the R-values, U-values, air-tightness figure, and design temperatures used. If the examiner can’t see the inputs, they can’t verify the output.
- Design conditions for the project’s actual location.
- Recommended equipment capacity for heating and cooling, expressed as a range or a specific match.
- Ventilation requirements where the scope includes them.
- Identification of who prepared it and to what standard.
The document is usually a PDF you attach to your permit submission alongside your drawings.
Can you do this yourself?
Sometimes. Here’s the honest version.
There are free and low-cost online load calculators. For a simple, single-storey, rectangular house where you know your assemblies precisely, some of them will get you a defensible number.
Where they fall down:
- Incomplete output. Most produce a whole-house total only, with no room-by-room breakdown and no documented assumptions. That’s the most common reason a self-prepared calculation gets sent back.
- Garbage in. The output is only as good as your R-values, window specs, and air-tightness estimate. If you’re guessing at those, you’ve produced a precise-looking guess.
- Non-standard geometry. Vaulted ceilings, walkout basements, bonus rooms over garages, large glazing walls, and additions tying into existing structure are where the simplified tools break.
- It’s not F280. Many online calculators use a different or simplified methodology. A number that isn’t calculated to the referenced standard may not satisfy the request regardless of how reasonable it looks.
If your project is straightforward and you’re comfortable documenting your inputs, try it. If your permit is time-sensitive, or your building has any of the complications above, the cost of a resubmission cycle usually exceeds the cost of having it done properly the first time.
Common reasons a calculation gets rejected
- Whole-house total submitted with no room-by-room breakdown
- Assumptions not stated, so the numbers can’t be checked
- Wrong design temperatures — someone used a default from a different climate zone
- Envelope values that don’t match what’s shown on the submitted drawings
- Equipment already selected at a capacity the calculation doesn’t support
- Calculation performed to a different standard than the one referenced
That last pair is worth dwelling on. If your calculation says 42,000 BTU/h and your mechanical schedule shows a 90,000 BTU furnace, the reviewer will notice. The calculation has to actually govern the equipment selection, not sit beside it as paperwork.
How long it takes
If you’re preparing it yourself, budget a few hours to gather envelope specs and window data, plus the calculation itself.
If you’re having it prepared, the working input is your floor plans, elevations, construction assemblies, and window specifications. Most residential reports are a matter of days rather than weeks once those are in hand. The delay is almost always in assembling the inputs, not running the numbers — so gather your window schedule and insulation specs before you engage anyone.
The part that matters after the permit
The calculation is a permit requirement, which makes it easy to treat as a box to tick. But the room-by-room loads are the same numbers your installer needs to size ducts, place registers, and balance airflow. If you’re going to produce them anyway, use them.
A correctly sized system that’s fed by an incorrectly sized duct system will still deliver uneven temperatures. The calculation is the input to the design — it isn’t the design.
If you’re working on a project in Aurora or elsewhere in York Region and need this handled, we prepare HVAC design in Aurora including room-by-room load calculations, duct design, ventilation, and radiant layouts. Send your drawings and we’ll come back with a fixed quote and a delivery date.
Quick answers
Does every permit need a heat loss calculation? Not every one, but most new homes, additions, and renovations that alter the building envelope or the mechanical system will. Your municipality’s building department is the authority on your specific scope.
Is a heat loss calculation the same as an energy audit? No. A load calculation sizes equipment. An energy audit or model assesses consumption and efficiency compliance. They use overlapping inputs and are sometimes required together, but they answer different questions.
My HVAC contractor gave me a number. Is that enough? It depends entirely on whether it’s a documented CSA F280-12 calculation with room-by-room results and stated assumptions, or a capacity figure from experience. The first will generally pass. The second generally won’t.
Do I need this for a furnace replacement? Often not for a straight like-for-like swap, but yes if you’re changing fuel type, moving to a heat pump, or if the house has been altered since the original system was installed. Ask before you order equipment.