Heat Load Calculator

1. Definition

A Heating Load Calculator estimates the total amount of heat energy required to maintain a comfortable indoor temperature during cold weather. The heating load represents how much heat must be supplied to offset losses through walls, windows, infiltration, and internal factors. This is essential for sizing furnaces, boilers, heat pumps, and hydronic heating systems.

2. Background Theory

Heating load is determined by the combined effect of conductive heat loss, air leakage, and thermal gains:

• Conductive Heat Loss
Heat escapes through walls, floors, ceilings, and windows due to temperature differences:

Simplified calculators often use global heat-loss factors based on typical construction.

• Infiltration Heat Loss
Cold outdoor air enters through cracks and openings. The heating requirement for infiltration is:

where 0.33 represents the approximate heat capacity of air (Wh/m³·°C).

• Internal Heat Gains
People, lights, and appliances provide heat, which offsets part of the heating demand.
A typical person adds around 100 W of sensible heat—used as a heat offset in this calculator.

The total heating load is the sum of all losses minus internal gains.

3. How the Calculator Works

This calculator estimates the heating requirement using simple, engineering-based approximations:

1. User inputs room area, temperature difference, infiltration rate, number of people, and equipment heat loss.

2. Conduction heat loss is computed using area × ΔT × heat transfer factor.

3. Infiltration heat loss is calculated using airflow and ΔT.

4. People heat gain is subtracted from total load.

5. Internal equipment loss is added back to the load.

6. Results are shown in both kW and BTU/h for compatibility with HVAC equipment ratings.

This method provides a fast, practical estimate suitable for residential and light commercial HVAC design.

While this calculator gives a quick estimate for heating requirements, real buildings often involve complex air leakage paths, room-to-room heat transfer, stratification, and equipment interactions that simple formulas cannot capture. For deeper insight into airflow behavior, heat distribution, infiltration patterns, and system performance, tensorHVAC-Pro offers an advanced HVAC-focused simulation environment built specifically for HVAC engineers—not CFD experts. With an intuitive interface and automated solvers, tensorHVAC-Pro helps you simulate real heating and airflow behavior without the complexity of traditional CFD tools.