About Heat Loss & Gain Calculator
The heat loss and gain calculator computes room-by-room heating and cooling loads per EN 12831, ASHRAE Manual J, and CIBSE Guide A. It is used by HVAC designers and building services engineers to size heating systems, radiators, and cooling plant for each room and the whole building.
Define rooms and their envelope elements with areas and U-values, set the design climate and indoor setpoints, and the tool returns the fabric and ventilation losses, the cooling gains, and a building-level summary that identifies the peak heating and cooling rooms.
How It Works
- Calculate fabric loss for each element as Q = U * A * deltaT against the design outdoor or adjacent temperature.
- Add ventilation loss Q = 0.33 * n * V * deltaT from the air change rate and room volume.
- For cooling, add conductive gains, orientation-weighted solar gains through glazing, and internal gains.
- Aggregate to room and building totals and flag the peak heating and cooling rooms.
Worked Example
A 10 m2 wall at U = 0.3 W/m2K with a 21 C indoor and -5 C design outdoor (deltaT = 26 K) loses Q = 0.3 * 10 * 26 = 78 W of fabric heat.
Formulas
- Fabric heat loss
Q = U * A * (Ti - Te)- Ventilation heat loss
Q = 0.33 * n * V * (Ti - Te)- Solar gain (glazing)
Q_solar = f_orient * I_base * A
Standards & References
- EN 12831
- ASHRAE Manual J
- CIBSE Guide A
Frequently Asked Questions
What is the difference between heating load and cooling load?
The heating load is the heat the system must add to hold the indoor setpoint on a cold design day (fabric plus ventilation loss minus internal gains). The cooling load is the heat it must remove on a warm day, including fabric, ventilation, solar, and internal gains.
Why calculate loads room by room?
Room-by-room loads let you size individual emitters such as radiators or grilles and identify the peak room, rather than under- or over-sizing from a single whole-building figure.
Where do the design temperatures come from?
The tool includes a climate database of winter and summer design temperatures by city, which you can override, so loads reflect the chosen design location per EN 12831 or ASHRAE practice.
Is the heat loss and gain calculator free to use?
Yes, it runs entirely in your browser at no cost and stores none of your input data.