What this calculator does
A fast first-pass load check: multiply each envelope element by its U-value and area and the design temperature difference, add the internal gains, and convert to tons and airflow. It tells you whether the proposed plant is in the right order of magnitude before you commission a full block load.
How to use it
- Enter the U-value and area of each envelope element — walls, glazing and roof.
- Enter the design temperature difference for your climate.
- Enter the internal gain in watts (people, lighting, equipment); 10 W/m² of floor area is a rough starting point for offices.
- Add a 10–20 % margin for the solar and infiltration loads this model omits.
Governing formula
Q_envelope = Σ (U_i × A_i × ΔT) ; Q_total = Q_envelope + 3.412 × P_internal Where:
-
U_i= U-value of element i [W/m²·K] -
A_i= Area of element i [ft²] -
ΔT= Design temperature difference [K]
Worked example — An 8,000 ft² office envelope
The figures below were produced by running this page's own engine with the example inputs (Wall U-value (W/m²·K), Wall area (ft²), Glazing U-value (W/m²·K), Glazing area (ft²), Roof U-value (W/m²·K), Roof area (ft²), Design temperature difference (K), Internal gains (W), Airflow per ton (CFM/ton)). If the calculator returns something different for the same numbers, that is a bug, not a rounding difference.
| Total load | 86456 Btu/h |
|---|---|
| Equivalent capacity | 7.2 tons |
| Matching airflow | 2882 CFM |
| Envelope subtotal | 59160 Btu/h |
| Internal gain subtotal | 27296 Btu/h |
| What this omits | Steady-state envelope conduction only. Solar gain, infiltration, ventilation, and full occupancy latent loads need a full block-load (Manual J) calculation — add a 10–20% margin before equipment selection. |
- Conduction through walls, glazing and roof gives the envelope subtotal.
- Internal gains of 8 kW convert to 27,296 Btu/h.
- Compare the total against a full block load before ordering plant — solar and infiltration normally add 20–40 %.
Assumptions & limitations
Screening-level estimate. Steady-state conduction through envelope elements plus a single lumped internal gain. Solar radiation, infiltration, ventilation and full occupancy latent loads are not modelled.
- Solar gain through glazing — usually the largest single item in a real building — is excluded.
- No infiltration, ventilation air, moisture or occupant latent loads.
- U-values must be converted to consistent units; a full block load (Manual J, or the equivalent regional method) supersedes this tool.
This tool supports professional engineering work; it does not replace a sealed design. Confirm every result against the applicable standard and a licensed engineer before it is built.
References & standards
- ACCA Manual J — residential cooling and heating load calculation.
- ASHRAE Handbook — Fundamentals, non-residential cooling load.
- CIBSE TM52 — the mean summer temperature method for UK buildings.