About HVAC Load Calculator
The HVAC load calculator estimates multi-zone heating and cooling loads for buildings per ASHRAE MJ8, EN 12831, and CIBSE Guide A. It is used by mechanical engineers and HVAC designers to size plant, select equipment tonnage, and check the sensible heat ratio before specifying systems.
Define each zone with its floor area, occupancy, glazing, and envelope data; pick a design city; and the tool returns sensible and latent cooling, heating loads, equipment sizing, and a zone-by-zone breakdown that updates as you edit.
How It Works
- Choose a design city to load summer and winter design temperatures, then set indoor design conditions and a safety factor.
- For each zone, sum envelope conduction, solar gain through glazing, occupant, lighting, equipment, and ventilation gains for cooling.
- Split cooling into sensible and latent components and compute the heating load from fabric loss plus ventilation loss.
- Apply the safety factor, convert totals to equipment tons and kW, and report the sensible heat ratio and peak zone.
Worked Example
A 100 m2 office at 24 C indoor and 33 C outdoor with U_avg = 0.4 and 200 m2 envelope gives an envelope gain of 0.4 * 200 * (33 - 24) = 720 W; adding solar, internal, and ventilation gains and a 1.1 safety factor sizes the cooling plant at roughly 1.5 tons.
Formulas
- Envelope (conductive) gain
Q = U_avg * A_env * deltaT- Solar gain through glazing
Q = A_glazing * SHGC * I- Ventilation sensible gain
Q = 1.23 * Q_fresh * deltaT- Heating load per zone
Q = U_avg * A_env * deltaT + 0.33 * ACH * V * deltaT- Sensible heat ratio
SHR = Q_sensible / Q_total_cooling
Standards & References
- ASHRAE MJ8 (Manual J)
- EN 12831
- CIBSE Guide A
Frequently Asked Questions
Which load calculation standards does this tool support?
It references ASHRAE MJ8 (Manual J), EN 12831, and CIBSE Guide A for residential and commercial heating and cooling load estimation, with a 24-city design climate database.
What is the difference between sensible and latent cooling load?
Sensible load is the heat that changes air temperature (envelope, solar, lighting, equipment, and people); latent load is the moisture that must be removed (occupant respiration and outdoor air humidity). The sensible heat ratio (SHR) is the sensible fraction of the total.
How is equipment tonnage derived?
Total cooling is multiplied by the safety factor and divided by 3517 W per ton of refrigeration; heating is divided by 1000 to give kW, so you can match plant capacity directly.
Is the HVAC load calculator free to use?
Yes, it runs entirely in your browser at no cost and stores none of your input data.