Article — Heat Loss Calculator
Heat loss calculator: sizing boilers and heat pumps with Q = U × A × ΔT
A heat loss calculator computes how much heat a building loses to its colder surroundings on the design day, using Q = U × A × ΔT summed across the building envelope plus infiltration loss. Units are watts (W) or BTU/h. A typical 100 m² insulated home in a mixed climate (ΔT = 30 K) loses about 3 to 4 kW of heat — equivalent to a 14,000 BTU/h heating load. This is the minimum capacity for the boiler or heat pump that will replace that lost heat.
Heat sizing was once the realm of HVAC contractors with paper worksheets and rules of thumb that added 30 to 50% “safety margin” on top of every calculation. The result was decades of oversized boilers running short cycles and burning fuel inefficiently. Modern code requires Manual J or equivalent room-by-room calculation precisely because the old rules of thumb wasted energy. The math is simple; getting the inputs right takes care.
The heat loss formula
Two physical mechanisms drive heat loss. Transmission moves heat through solid building materials — walls, windows, roof, floor. Infiltration replaces heated indoor air with cold outdoor air through cracks and ventilation.
Q_trans = U × A × ΔT per element, wattsQ_inf = 0.34 × V × ACH × ΔT infiltration, wattsQ_tot = Σ Q_trans + Q_inf total loadBTU/h = W × 3.412 US sizing unitsFor each envelope element, multiply the U-value (W/m²K) by the surface area (m²) by the temperature difference between inside and outside (K). Sum the results. Add infiltration: 0.34 times room volume in cubic metres times air-change rate per hour times ΔT. The total is the heating load at design temperature.
U-values for walls, windows, roof, floor
U-value is the rate of heat transfer through a 1 m² section of building envelope per 1 K temperature difference. Lower U means better insulation. Typical modern construction values: wall 0.25 to 0.35, double-pane window 1.0 to 1.5, insulated roof 0.15 to 0.20, floor 0.20 to 0.30 W/m²K. Single-pane windows hit 5.8 — an order of magnitude worse than modern double-glazed.
The Passivhaus standard limits annual heating energy to 15 kWh/m², which forces all envelope U-values to 0.15 W/m²K or lower — about half of typical code requirements. A well-built Passivhaus in a cold climate can be heated by a hair dryer-sized resistance heater for the coldest hour of the year. The standard predates inverter heat pumps; many new Passivhaus homes now use the heat pump for hot water alone and skip dedicated space heating entirely.
U-values come from window labels, insulation manufacturer specs, or assembly calculations. For mixed assemblies (insulated stud bays with framing thermal bridging), use the area-weighted average. R-19 fiberglass batt in a stud wall has cavity U of 0.30 but assembly U of 0.40 after counting the 2×6 studs at 8% of the wall area.
Heat loss from air infiltration
Cold air leaking in through cracks, vents, and door gaps replaces warm indoor air. The energy to reheat the replacement air is infiltration loss. The formula simplifies to 0.34 × V × ACH × ΔT, where 0.34 is air density times specific heat divided by 3,600 seconds per hour.
Air-change rates vary from 0.3 ACH (modern airtight, blower-door tested) to 3.0 ACH (pre-1960 single-pane home with weatherstripping problems). For older homes, infiltration loss can match transmission loss; for new homes, it is under 20% of the total. A heat-recovery ventilator (HRV) reduces effective infiltration to near zero because the incoming air is preheated by the outgoing.
Outdoor design temperature
Design temperature is the 99th-percentile coldest hour for your region, not the all-time record cold. ASHRAE Fundamentals publishes the design data. Houston is 0 °C, London −3 °C, Chicago −18 °C, Stockholm −20 °C, Anchorage −28 °C. Picking the 1950s record cold instead pads the heating load by 30 to 50% — exactly the bad practice modern Manual J calculations correct.
Indoor design temperature is your comfort target — usually 20 to 21 °C for living rooms and 18 to 19 °C for bedrooms. The calculator uses one indoor temperature throughout; for room-by-room calculations, repeat the calculation with the target per zone.
Sizing a boiler from heat loss
The heating system capacity must equal or slightly exceed the calculated heat loss at design temperature. Match exactly when possible — modulating boilers and inverter heat pumps run most efficiently when sized close to the design load. The 1.5× safety factor of past decades produced systems that short-cycle, lose efficiency, and wear out faster.
- 1 kW = 3,412 BTU/h, the conversion factor between metric and US heating ratings
- 1 ton of cooling = 12,000 BTU/h = 3.52 kW
- Heat pump COP at design temperature ranges 2.5 to 3.5 for cold-climate models
- Modulating boiler turndown = 5:1 typical, so a 30 kW boiler modulates to 6 kW minimum
- Sizing target = next standard size above calculated load, not 1.5×
- Design-day capacity = heat-pump nameplate is at +8 °C; cold-climate units derate 40% at −15 °C
Heat loss by building age and zone
Pre-1960 homes lose 100 to 150 W/m² at typical design conditions because the walls have no cavity insulation and windows are single-pane. Code-built 1980s homes lose 50 to 75 W/m². Current new construction lands at 25 to 50 W/m². Passivhaus targets 10 W/m². A 200 m² floor area times the per-m² loss gives a rough total — the real calculator does the room-by-room work.
How to reduce heat loss
Attic insulation is the cheapest improvement per watt of heat saved. Going from R-19 to R-49 in an attic that started uninsulated cuts heat loss by 60% on that surface. Wall insulation is the second priority, especially in older homes with empty stud cavities — blown cellulose retrofit costs $1 to $2 per ft² and cuts wall loss by 70%.
Air-seal before insulating. Caulk all penetrations through the building envelope (attic hatches, recessed lights, top plates, chimney chases) before adding new insulation. R-60 of attic insulation does nothing if a 4 ft² hole around the chimney vents your heat to the sky. A blower-door test costs $300 to $500 and identifies every leak in 90 minutes.
Windows are the slowest payback for upgrade, but the biggest comfort improvement. Replacing single-pane with modern triple-pane drops window U from 5.8 to 0.6 — nearly 10× improvement. The energy payback runs 15 to 30 years; the comfort payback is immediate because the surface temperature of the glass jumps 10 °C in winter.
Common heat loss calculation mistakes
Using gross wall area instead of net is the most common error. The wall area should subtract windows and doors — those are calculated separately at their own U-values. A 100 m² wall with 15 m² of windows has 85 m² net wall area; counting 100 m² inflates the calculation by 15%.
If the home has a heat-recovery ventilator providing fresh air, infiltration approaches zero — the HRV is the ventilation. Counting both produces a heating load that is 20 to 40% too high. For HRV-equipped homes, set ACH to 0 (or 0.1 to be safe). For homes without mechanical ventilation, use 0.3 to 0.7 ACH depending on airtightness.
Ignoring thermal bridging is the third common miss. Steel beams, concrete slabs, and aluminium window frames carry heat much faster than the surrounding insulation. ISO 10211 covers thermal bridge calculation; for residential work, multiply the calculated load by 1.05 to 1.10 as a thermal-bridge allowance.