Guide · Heat-loss calculations
What a heat-loss calculation is, and why radiant starts with one.
A heat-loss calculation works out how much heat each room loses on the coldest day your home is designed for. Every good radiant system is designed from it, and in Ontario the Building Code says heating capacity is based on it.
The short version
- It works out heat loss room by room at your municipality’s design temperature, the cold a system is sized to hold.
- Ontario’s Building Code bases heating capacity on the calculated heating load and points to CSA F280 for it (Articles 9.33.4.1 and 9.33.5.1)1.
- For radiant, it sets loop spacing, water temperature and zoning, not just the size of the boiler.
- It isn’t an energy assessment: one sizes the heating system, the other rates the whole home.
01 · The idea
What a heat-loss calculation measures
Heat always moves from warm to cold. On a winter night your home is losing heat through its walls, windows, ceiling and floor, and through every gap that lets cold air in. A heat-loss calculation adds all of that up, room by room, for the coldest conditions your home is designed to handle.
The result is a number for each room and a total for the house: how much heat the system has to deliver to hold a comfortable temperature on that design day. Everything else in a radiant design is built on those numbers.
That coldest condition is the outdoor design temperature, which the Building Code sets by location in Supplementary Standard SB-11. That’s why we size to your municipality’s design temperature rather than one number for everyone.
02 · The inputs
What goes into a heat-loss calculation
On a new build, most of this comes from your drawings. On an existing home, it comes from a site walk: measurements, what’s in the walls and attic, the windows you have and how the basement is built.
03 · Doing it properly
Room by room vs rules of thumb
04 · Why it matters for radiant
Why radiant depends on the numbers
With forced air, a room that runs cold can sometimes be fixed with a bigger register. A radiant floor is laid, covered and lived on, so the design has to be right before the floor goes down. Three things come straight out of the heat-loss numbers:
- Loop spacing. Rooms that lose more heat get tighter spacing, so the floor delivers what that room needs.
- Water temperature. The design sets the lowest water temperature that keeps every room comfortable, and outdoor reset follows the weather.
- Zones. Rooms with different needs, like a bright living room and a basement, go on separate zones at the manifold.
There’s a physical limit too. A floor can only get so warm before it stops being comfortable to stand on. ASHRAE Standard 55 puts the range for floors people stand on at 19 to 29 °C3. If a room loses more heat than its floor can comfortably deliver, the design catches it on paper.
The same numbers size the heat source. A boiler sized from a calculation runs long, steady cycles, and one sized from the old label usually short-cycles. For an air-source heat pump, whose capacity falls as the weather gets colder4, sizing to the heat loss matters even more.
05 · What you get
What a heat-loss design gives you
Documented so it can be built exactly as drawn, whether we install it or your own installer does.
A number for every room
Room by room, to the CSA F280-12 methodology, sized to your municipality’s design temperature.
Layout & manifold schedule
Every loop with its length, spacing and zone, ready for your installer or your designer’s permit set.
Water temperatures & sizing
The water temperature schedule and the boiler or heat-source size, matched to the heat loss.
06 · Heat-loss questions
Heat-loss calculation questions
Do I need a heat-loss calculation for a building permit?
In most cases, yes. The Ontario Building Code bases heating capacity on a calculated heating load, points to CSA F280 for the calculation and for how far equipment can be oversized or undersized, and sets outdoor design temperatures by location in Supplementary Standard SB-11. Radiant as a primary heat source is the kind of project where municipalities ask for full mechanical drawings. We produce the heat-loss calculations, loop layouts and manifold schedules, and coordinate directly with your BCIN-registered designer so the stamped submission set goes together without back-and-forth.
Is a heat-loss calculation the same as an energy assessment?
No. An energy assessment, like the one a registered energy advisor does for Ontario’s Home Renovation Savings program5, looks at how your whole home uses energy and is the starting point for some rebates. A heat-loss calculation works out how much heat each room needs on the design day, so the heating system can be designed and sized. They use some of the same information but answer different questions.
Can you do one for an existing home?
Yes. We walk the home, measure, and look at the insulation, windows and basement. Retrofits and boiler replacements start the same way new builds do.
Why not size the new boiler from the old one?
Because oversized boilers short-cycle and waste fuel. We run a room-by-room heat-loss calculation on your actual home and size to that number. It’s the single most important step in the whole project, and it’s where every one of our quotes starts.
What should I have ready?
Drawings if you have them, or a site walk if you don’t. Anything you know about insulation, window upgrades or past renovations helps, and so does knowing which rooms feel cold now.
Can I get the design without the installation?
Yes. Builders, designers and homeowners with their own installer can order a design-only package: the heat-loss report, loop layout and manifold schedule, water temperature schedule and boiler or heat-source sizing, documented so your installer can build to it.
Sources
- Ontario Building Code 2024 (O. Reg. 163/24), Article 9.33.4.1 and Article 9.33.5.1.
- Natural Resources Canada, Keeping the Heat In, Section 8: Upgrading windows and exterior doors.
- ANSI/ASHRAE Standard 55, Thermal Environmental Conditions for Human Occupancy, summary of the floor surface temperature range.
- Natural Resources Canada, Heating and cooling with a heat pump.
- Home Renovation Savings, Energy assessment required upgrades.
Sources checked September 2026. Programs, amounts and codes change, so confirm current details with the source before you plan around them.
Keep reading
More radiant guides
- Guide
What drives radiant floor heating cost
What moves the number, and what a good quote includes.
- Guide
Radiant floors and heat pumps
How they work together in an Ontario winter, and the rebates available now.
- Service
Boiler installation
Condensing boilers sized from a heat-loss calculation, never from the old label.
Start with the math
Get your heat loss done right
Book a free consultation and we’ll walk your home or your plans, run the heat-loss numbers, and tell you exactly what radiant would take. No pressure, no guesswork.
