HVAC Heating Load Calculator

Inputs

Enter design conditions and envelope details. Use consistent units within the selected system.

Form layout adapts: 3 columns (large), 2 (medium), 1 (mobile).

Project / Zone name

Unit system

Indoor setpoint temperature

Outdoor design temperature

Floor area (zone)

Ceiling height

Envelope heat loss

Walls area

Walls input method

You can use R or U; the calculator converts automatically.

Walls R-value

Walls U-value

Roof / ceiling area

Roof input method

Roof R-value

Roof U-value

Floor area (loss path)

Floor input method

Floor R-value

Floor U-value

Windows area

Windows U-value

Doors area

Doors U-value

Thermal bridge allowance (%)

Accounts for framing, fasteners, and junction losses.

Duct / distribution loss (%)

Use 0% for room heaters without distribution.

Air leakage, ventilation, and gains

Infiltration rate (ACH)

ACH = air changes per hour at design conditions.

Ventilation flow

Occupants (count)

Sensible gain per person

Lighting density

Equipment density

Gains usage factor (0–1)

Use 0.5–0.8 if not all loads run during peak heating.

Sizing controls

Safety factor (%)

Covers uncertainty in weather, envelope, and operation.

Heating system efficiency (%)

Used to estimate required input capacity.

Example data table

Illustrative scenarios to sanity-check your inputs and outputs.

Scenario	Area	ΔT	ACH	Vent	Design Load
Office zone (well insulated)	120 m²	21°C	0.4	12 L/s	~9–13 kW
Retail (average envelope)	200 m²	18°C	0.8	25 L/s	~18–28 kW
Warehouse bay (leaky, tall)	500 m²	20°C	1.5	60 L/s	~60–100 kW

Formula used

Transmission losses: Q = U × A × ΔT for each envelope element.
Thermal bridges: Qtrans = Qbase × (1 + TB%).
Infiltration (SI): Q = 0.33 × ACH × V × ΔT.
Infiltration (Imperial): Q = 0.018 × ACH × V × ΔT.
Ventilation (SI): Q ≈ 1200 × Vdot × ΔT, with Vdot in m³/s.
Ventilation (Imperial): Q = 1.08 × CFM × ΔT.
Net heating load: Qnet = max(0, (Qtrans + Qair) − Qgains).
Design and input: apply safety %, then divide by efficiency %.

How to use this calculator

Select SI or Imperial units and enter indoor/outdoor design temperatures.
Enter floor area and height to define the zone volume.
Provide envelope areas and either R-values or U-values for walls, roof, and floor.
Add window and door areas with U-values from specifications.
Set infiltration (ACH) and ventilation (L/s or CFM) based on design assumptions.
Include people, lighting, and equipment gains with a realistic usage factor.
Review the breakdown, adjust safety/efficiency, then export CSV or PDF.

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