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HVAC Load Estimate

Free HVAC Load Estimate calculator with clear step-by-step results.

About HVAC Load Estimate

Splits the load into the three components that dominate: fabric conduction through the envelope, glazing which loses far more per square metre than wall, and ventilation air that must be heated from outside to inside temperature. Cooling adds occupant and equipment gains plus solar through the glass.

Formulas

Fabric loss

Watts = envelope area x U-value x design temperature difference

Ventilation loss

Watts = volume x air changes per hour x 0.33 x temperature difference

Worked example

With the default values below, HVAC Load Estimate returns Design heating load of 6.624. Change any input above and every figure here is recalculated in your browser.

Inputs used

Conditioned floor areaminimum 1 m2
120 m2
Ceiling heightaccepts 1.5–6 m
2.5 m
Insulation standardone of Poor - uninsulated, single glazed, Average - partial insulation, double glazed, Good - well insulated, Excellent - near passive standard
Average - partial insulation, double glazed
Design temperature differenceaccepts 1–60 K
22 K
Occupantsaccepts 0–50
4
Glazed areaminimum 0 m2
18 m2

Results

Design heating load
6.624 kW
Indicative cooling load
5.080 kW
Ventilation and infiltration loss
1.525 kW
Heating load intensity
55.2 W/m2

Step by step

  1. 1Values used
  2. 2Fabric loss
  3. 3Ventilation loss
  4. 4Design heating load= 6.624 kW
  5. 5Indicative cooling load= 5.080 kW
  6. 6Ventilation and infiltration loss= 1.525 kW
  7. 7Heating load intensity= 55.2 W/m2

Assumptions and limits

  • The result is derived only from the 6 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Simplified estimate only. Heating and cooling equipment must be sized using a recognised load calculation method and installed to applicable codes and manufacturer requirements.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Simplified estimate only. Heating and cooling equipment must be sized using a recognised load calculation method and installed to applicable codes and manufacturer requirements.

Frequently Asked Questions

Is this good enough to size a boiler?

It is a sanity check, not a design. A proper room-by-room calculation to a recognised method is needed before specifying equipment, particularly for a heat pump where oversizing causes cycling.

Why is the load intensity figure useful?

It benchmarks the building. Above roughly 100 W/m2 suggests a poorly insulated envelope; a well-built modern house sits nearer 30-50 W/m2.

Related Calculators

HVAC Load Estimate – Calculate Design heating load