Heating and Cooling Load Calculator

Calculator Inputs

Floor area, m²

Ceiling height, m

Wall area, m²

Wall U value, W/m²K

Roof area, m²

Roof U value, W/m²K

Window area, m²

Window U value, W/m²K

Floor loss area, m²

Floor U value, W/m²K

Indoor heating setpoint, °C

Outdoor winter design, °C

Indoor winter RH, %

Outdoor winter RH, %

Indoor cooling setpoint, °C

Outdoor summer design, °C

Indoor summer RH, %

Outdoor summer RH, %

Ventilation airflow, CFM

Infiltration, ACH

Heat recovery efficiency, %

Occupants

Sensible heat per person, W

Latent heat per person, W

Lighting power, W/m²

Equipment power, W

Equipment use factor, %

Solar irradiance on glass, W/m²

Solar heat gain coefficient

Unshaded solar factor, %

Sizing safety factor, %

Formula Used

Envelope load: Q = U × A × ΔT. Total UA equals the sum of wall, roof, window, and floor UA values.

Air sensible load: Q = ρ × Cp × V × ΔT. The calculator converts CFM to m³/s and adds infiltration from room volume and ACH.

Solar load: Q = glass area × solar irradiance × SHGC × shading factor.

Latent load: Q = ρ × V × ΔW × hfg. Humidity ratio is estimated from temperature, relative humidity, and pressure.

Cooling total equals sensible plus latent load, multiplied by the safety factor. Heating total includes transmission, outdoor air, and winter humidification.

How to Use This Calculator

Enter the room size first. Add envelope areas and U values for each surface. Use realistic indoor and outdoor design conditions.

Add ventilation airflow, leakage rate, and recovery efficiency. Enter people, lighting, equipment, window solar data, and a safety factor.

Press Calculate Loads. Review the result summary above the form. Use CSV or PDF to save the report.

Example Data Table

Input	Example Value	Meaning
Floor area	120 m²	Conditioned room area
Wall U value	0.55 W/m²K	Wall heat transfer rate
Ventilation	180 CFM	Fresh air supplied to the space
Infiltration	0.6 ACH	Uncontrolled leakage rate
Solar irradiance	500 W/m²	Solar gain falling on glass

Heating and Cooling Load Guide

Why Load Calculation Matters

Heating and cooling load calculation links building physics with comfort design. A load is not the same as energy use. It is the rate of heat that must be added or removed at a design condition. This calculator separates that rate into envelope, airflow, solar, occupant, lighting, and equipment parts. The split helps a designer see the largest source quickly.

Envelope and Airflow Effects

Transmission load comes from heat moving through walls, roof, glass, and floors. It uses area, U value, and temperature difference. A high U value means heat passes more easily. Better insulation lowers this part. Air load comes from ventilation and infiltration. Ventilation is planned fresh air. Infiltration is unwanted leakage through cracks and openings. Both streams carry sensible heat. They may also carry moisture, which creates latent cooling load.

Cooling and Heating Differences

Cooling load usually has more parts than heating load. Sun through glass can be large in warm climates. The solar heat gain coefficient and shading factor control this item. People, lights, and machines also release heat indoors. Some gains are sensible. Others are latent because moisture is added to the air. The sensible heat ratio helps compare these two effects.

Heating load is often dominated by envelope loss and outside air. In dry winter weather, humidification can add demand. This calculator estimates that moisture load when indoor winter humidity is higher than outdoor humidity. Heat recovery reduces planned ventilation load, but it does not remove infiltration losses.

Using Results Carefully

The result should guide early sizing, comparison, and teaching work. It is not a replacement for a full certified building survey. Real projects may need wall orientation, thermal mass, schedules, duct losses, local codes, and safety checks. Still, a clear component calculation is very useful. It shows why sealing leaks, shading windows, improving insulation, or reducing internal gains can change equipment size. Use conservative design temperatures, realistic U values, and measured airflow when possible. Then review both watts and BTU per hour. Cooling tons are also shown because many air systems are sold that way.

For best results, run several scenarios. Compare summer and winter cases. Test better glass, lower leakage, added shading, or higher recovery efficiency. Small changes can reveal practical upgrades before costly equipment decisions are made during early planning work stages.

FAQs

What is a heating load?

It is the heat rate needed to keep a room warm at the winter design condition. It includes envelope loss, outdoor air, leakage, and possible humidification demand.

What is a cooling load?

It is the heat rate that must be removed from a space. It can include sensible heat, latent moisture, solar gain, occupants, lights, equipment, and outdoor air.

What does U value mean?

U value measures heat transfer through a building surface. Lower U values mean better insulation and lower transmission load for the same area and temperature difference.

Why is humidity included?

Moisture creates latent load. Cooling equipment must condense or remove moisture when outdoor air contains more water vapor than indoor air.

What is sensible heat ratio?

It is sensible cooling divided by total cooling. A lower ratio means more moisture removal is needed compared with temperature-only cooling.

Does heat recovery reduce all airflow load?

No. This calculator applies recovery only to planned ventilation. Infiltration is leakage, so it remains untreated unless the building is sealed better.

Can this size real equipment?

It supports early estimates and comparisons. Final equipment sizing should use local codes, detailed survey data, manufacturer tables, and professional judgment.

Why add a safety factor?

A safety factor covers uncertainty in weather, leakage, occupancy, and materials. Avoid very large factors because oversizing can reduce comfort and efficiency.