HVAC Load Calculator

About the HVAC Load Calculator

Provides a simplified HVAC load estimate (BTU/h) from floor area, insulation quality, window area and occupancy. A full Manual J is needed for design.

How to use this calculator

1. Enter the room or house area in square feet, then choose the appropriate load factor for the climate and insulation level.
2. Add the window adjustment for glass area, since windows can increase heat gain or heat loss.
3. Add the people adjustment for the number of occupants, because people add internal heat.
4. Read the final load estimate and use it as a starting point for system sizing, not as a substitute for a full Manual-J design.

The formula explained

This calculator uses the simplified load formula \(load = area \times factor + windows + people\). It estimates the total heating or cooling load from floor area, window effects, and occupant heat.

• load = estimated heating or cooling load
• area = floor area of the space in square feet
• factor = load factor based on climate, insulation, and building type
• windows = extra load added for window area and window type
• people = extra load added for the number of occupants

Step by step method

1. Find the floor area of the space in square feet.
2. Multiply the area by the factor that matches the building conditions.
3. Add the window adjustment and the people adjustment.
4. Use the total as the estimated load.

Worked example

Problem. Estimate the load for a \(1{,}500\) square foot home with a factor of \(18\), windows adding \(2{,}400\), and people adding \(1{,}200\).

1. Start with the formula \(load = area \times factor + windows + people\).
2. Substitute the values: \(load = 1{,}500 \times 18 + 2{,}400 + 1{,}200\).
3. Compute \(1{,}500 \times 18 = 27{,}000\), then add the adjustments: \(27{,}000 + 2{,}400 + 1{,}200 = 30{,}600\).

Tips and common mistakes

• Use the correct factor for your climate and insulation level, because this changes the result a lot.
• Do not forget window and people adjustments, since leaving them out can understate the load.

Frequently asked questions