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    Buildings / heating load

    Heat loss calculator

    A heating load is two numbers added together: conduction through the envelope and the energy needed to warm incoming air. This calculator applies the U·A·ΔT and 0.33·ACH·V·ΔT expressions, splits the two contributions, and sizes the plant with a distribution allowance.

    U-value0.30ΔT30 KIntensity21 W/m²
    Design heat loss
    4.28 kW
    4275 W at ΔT 30 K
    Fabric loss
    1800 W
    42% of the total
    Ventilation loss
    2475 W
    0.50 air changes per hour
    Boiler size at 90%
    4.75 kW
    Allowance for distribution losses
    Loss breakdown
    Fabric and ventilation, the two terms that make up the design load.
    4.28 kW
    Loss breakdown
    TermWatts
    Fabric U·A·ΔT1800
    Ventilation 0.33·ACH·V·ΔT2475
    Total4275
    Per m² of envelope21

    Steady-state at design outdoor temperature. Thermal bridging and solar gains are not included.

    Inspect the model
    Choose a quick result view, a scaled comparison, or the complete diagnostic set.
    Load against outdoor temperature
    Straight line: every extra degree of ΔT adds the same loss.
    Load against outdoor temperature0.0001.442.874.315.757.1800559914141818232327273232363641414545kW
    Canonical equations
    The exact relationships implemented by this engine.

    Q = U·A·ΔT · Steady-state conduction through the envelope.

    Q = 0.33·ACH·V·ΔT · 0.33 Wh/m³K is the volumetric heat capacity of air.

    Q_total = Q_fabric + Q_vent · Add distribution losses for plant sizing.

    engineering methodology

    A clear path from inputs to results.

    A heating load is two numbers added together: conduction through the envelope and the energy needed to warm incoming air. This calculator applies the U·A·ΔT and 0.33·ACH·V·ΔT expressions, splits the two contributions, and sizes the plant with a distribution allowance.

    Step-by-step methodWorked exampleAssumptions explained

    How to use

    1. Enter the area-weighted U-value and envelope area.
    2. Set the design temperature difference.
    3. Enter the heated volume and air change rate.
    4. Read the design load and the plant size.

    The formula

    Q=UAΔT+0.33 ACH V ΔTQ = U A \Delta T + 0.33 \, \text{ACH} \, V \, \Delta T

    Reference: CIBSE and ASHRAE steady-state heating load method.

    Worked example

    200 m² at U 0.30, ΔT 30 K, 500 m³ at 0.5 ACH

    Inputs
    • Envelope 200 m², U 0.30 W/m²K.
    • ΔT 30 K.
    • Volume 500 m³, 0.5 air changes per hour.
    Calculation path
    1. 1

      Fabric: 0.30 × 200 × 30 = 1,800 W.

    2. 2

      Ventilation: 0.33 × 0.5 × 500 × 30 = 2,475 W.

    3. 3

      Total: 4,275 W, or 4.28 kW.

    4. 4

      At 90% efficiency the boiler needs about 4.75 kW.

    Result

    4.28 kW design load · 1.80 kW fabric · 2.48 kW ventilation.

    Interpreting the result

    Ventilation often exceeds fabric loss in a well-insulated building, which is why airtightness and heat recovery matter more than extra insulation once the envelope is good. The split chart makes that visible immediately.

    Assumptions & limitations

    • Steady-state at design outdoor temperature.
    • No solar or internal gains.
    • Thermal bridging included in the U-value.
    • No thermal mass or intermittent heating effects.

    Frequently asked questions

    Should I use the external or internal area?

    Use the area the U-value was calculated for, usually the external envelope.

    Does this size a heat pump?

    It gives the design load; heat pump selection also needs the flow temperature and emitter output.

    What is a typical ACH?

    0.3–0.5 for a modern airtight house, 1–2 for older housing.

    Methodology last reviewed 2026-10-21. Results are informational and not financial or engineering advice — see our disclaimer.