Tool · Heat loss
What does the house actually lose on a design day?
Enter the building dimensions, an insulation band, the glazing area and quality, and the design conditions. The tool estimates the design heat load in BTU/h, shows the arithmetic with your numbers, and maps the result onto a published boiler output ladder — then hands the decision to a real calculation.
Heat-loss estimator · inputs
Finished, heated floor area. 1 m² ≈ 10.76 ft².
Above-grade storey height, averaged over the heated space.
The R-values behind each band are Natural Resources Canada table rows (effective resistance including framing); the era wording is only a guide to picking one. Not sure? Choose the lower band, or book an assessment.
Total glazing area, estimated from window widths × heights.
Maximum U-factor for windows and doors under the NRCan ENERGY STAR Canada fenestration specification v5.0.
The 2.5% January value is the one ordinarily used in the design of heating systems (Vancouver Building By-law 2019, Appendix C).
The by-law's degree-day definition references 21 °C; your comfort setpoint may differ.
The estimate covers conduction by default. If you know the home's air-leakage rate (ACH), enter it and the tool adds the air-leakage term using the standard sensible-heat equation.
Enter the floor area, ceiling height and window area
The estimate needs the building dimensions, an insulation band and a window area. The result appears as you type, and the formula is shown with your numbers.
How this works
The simplified method, in full
The tool multiplies each surface area by an effective U-value and the indoor–outdoor temperature difference, adds an optional air-leakage term when you supply an airtightness figure, and converts the result to BTU/h. Every constant below is published; the simplifications are stated rather than hidden.
01
Conduction through each surface
Q = U × A × ΔT
U is the effective U-value in Btu/h·ft²·°F, A is the surface area in ft², and ΔT is the indoor setpoint minus the outdoor design temperature in °F. The U-values come from NRCan's effective thermal resistance tables: the frame-cavity row for the selected band plus the published air films, converted with NRCan's 5.678263 factor.
02
Air leakage (optional)
Q = 1.08 × (V × ACH ÷ 60) × ΔT
The 1.08 is the standard sensible-heat factor: 60 min/h × 0.075 lb/ft³ air density × 0.24 Btu/(lb·°F). The tool only adds this term when you enter an air-changes-per-hour figure from a blower-door or EnerGuide result — it never guesses an airtightness value.
03
Vancouver design conditions
ΔT = indoor − outdoor design
The published Vancouver January design temperatures are -6 °C at the 2.5% level (the ordinary heating-design value) and -8 °C at 1%. The by-law's degree-day definition references a 21 °C interior. Other locations need their own published value.
04
Size band, not a model
output ≥ calculated load
The result is mapped onto the CSA output ladder IBC publishes for its four residential SL Series G3 models — the smallest published output that covers the load and the next size below it. It is a scale, not a recommendation, and the hot-water load is not included.
| Band | Wall assembly (effective) | Ceiling assembly (effective) |
|---|---|---|
| Older / little insulation — about R-9 cavity | 2×4 studs, 16 in. o.c. — NRCan table WA-1, R-9 row — effective RSI 1.27 (R-7.2) | 2×4 truss — NRCan table R1-1, R-9 row — effective RSI 1.38 (R-7.8) |
| Mid-era / some insulation — about R-12 cavity | 2×4 studs, 16 in. o.c. — NRCan table WA-1, R-12 row — effective RSI 1.49 (R-8.5) | 2×4 truss — NRCan table R1-1, R-12 row — effective RSI 1.69 (R-9.6) |
| Insulated — about R-20 walls, R-30 ceiling | 2×6 studs, 16 in. o.c. — NRCan table WA-2, R-20 row — effective RSI 2.43 (R-13.8) | 2×6 truss — NRCan table R1-2, R-30 row — effective RSI 3.56 (R-20.2) |
| Well insulated — about R-24 walls, R-33 ceiling | 2×6 studs, 16 in. o.c. — NRCan table WA-2, R-24 row — effective RSI 2.66 (R-15.1) | 2×6 truss — NRCan table R1-2, R-33 row — effective RSI 3.76 (R-21.4) |
| Window quality | U-factor used | Basis |
|---|---|---|
| Single glazing (older windows, no storm window) | 5.56 W/m²·K (0.98 Btu/h·ft²·°F) | NRCan: single glazing loses about 10–20 times as much heat as a properly insulated wall; the tool uses the midpoint (15×) as its central estimate. |
| Double glazing or storm windows (no low-E coating) | 2.78 W/m²·K (0.49 Btu/h·ft²·°F) | NRCan: storm windows or double-glazed sealed units reduce the loss of single glazing by almost half. |
| ENERGY STAR certified window (U ≤ 1.22 W/m²·K) | 1.22 W/m²·K (0.21 Btu/h·ft²·°F) | Maximum U-factor for windows and doors under the NRCan ENERGY STAR Canada fenestration specification v5.0. |
| High-performance triple glazing (U as low as 0.8 W/m²·K) | 0.80 W/m²·K (0.14 Btu/h·ft²·°F) | NRCan: triple-glazed windows can have U-factors as low as 0.8 W/m²·K. |
What this estimate leaves out — and why a Manual J or CSA F280-12 calculation is still required
- · Air leakage, unless you enter an airtightness figure; a leaky home loses more than conduction alone shows.
- · The foundation, basement and slab, and any heated floor over an unheated space.
- · Duct losses, internal gains, solar gains and occupancy.
- · The domestic-hot-water recovery load, which is added to the space-heating load when a tank recovers on a cold morning.
- · The building's actual geometry: wall area here uses a square-footprint estimate (4 × √area), not measured elevations.
Manufacturers require sizing from a load calculation — the IBC SL G3 manual refers to the load calculation versus boiler output — and a firm size comes from a room-by-room Manual J or CSA F280-12 calculation, not from this page.
Tools and your service
An estimate frames the question; the assessment answers it
No web form can see the construction, the leakage paths or the hot-water load. The heat-loss assessment does, room by room, and produces a document a designer or installer can work from.
- On site: rooms, walls, windows, doors and roof construction are measured, not assumed.
- The calculation states its assumptions, so the result can be reviewed rather than trusted blindly.
- The written result feeds sizing and selection — the step where the right boiler output is chosen.
Sizing background is covered in the heat-loss guide, and the running-cost tool handles the fuel side in /tools/running-cost.
If a carbon-monoxide alarm sounds
- Leave the building immediately and get fresh air.
- Call 911. Do not go back inside until emergency services say it is safe.
- Have every fuel-burning appliance inspected before use.
Only after that, call ROMA about the boiler. Official guidance: Health Canada on carbon monoxide.
Sources and verification
- City of Vancouver — Vancouver Building By-law 2019, Appendix C: Climatic and Seismic Information for Building Design in Vancouver (Table C-2 design temperatures) · accessed September 9, 2026
- Natural Resources Canada — Tables for Calculating Effective Thermal Resistance of Opaque Assemblies (frame-cavity tables and continuous materials; method per ASHRAE Handbook—Fundamentals) · accessed September 9, 2026
- Natural Resources Canada — Keeping the Heat In, Section 2: How your house works (heat flow, RSI/R conversion, recommended insulation values) · accessed September 9, 2026
- Natural Resources Canada — Keeping the Heat In, Section 8: Upgrading windows and exterior doors (window U-factors and glazing comparisons) · accessed September 9, 2026
- Natural Resources Canada — ENERGY STAR Canada technical specification: windows, doors and skylights, v5.0 (U-factor criteria and unit conversions) · accessed September 9, 2026
- IBC — SL Series G3 Installation & Operating Manual, doc 120-276E13 (load calculation vs. boiler output) · accessed September 9, 2026
- IBC — SL Series G3 model pages (published CSA output ladder used for the size band) · accessed September 9, 2026
- Boilers.ca — Boiler sizing and heat-loss calculations · accessed September 9, 2026
Facts checked September 9, 2026. Last updated September 9, 2026. Technical reviewer: not yet assigned.
Want the real number for your building?
A room-by-room heat-loss assessment from ROMA Heating & Cooling produces the load the boiler choice should be built on — with the assumptions written down.
On-site service in Greater Vancouver, BC. ROMA lists “Open 7 days” on its site; call to confirm same-day availability.