Getting a first-pass cooling load before running a full block load

A screening calculation from envelope U-values and internal gains, and the margin it still needs.

The engineering question

Do you need a full block-load calculation to know whether the proposed plant is in the right ballpark?

Worked example

The figures below were produced by running the Cooling Load Calculator with the same inputs — during the build, from its authoritative engine.

Results
Total load	86456 Btu/h
Equivalent capacity	7.2047 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.

Each envelope element contributes U × area × design temperature difference.
Internal gains convert from watts to Btu/h.
Convert the total to tons, then to airflow at the chosen cfm per ton.

Sanity check

This omits solar gain and infiltration, which together often add 20–40 %. Treat it as an order-of-magnitude check only.

Tools for this problem

These are the EngiMetric calculators that compute the numbers in this topic. Each one shows its formula, its units and its limits rather than returning a bare result.

Engineering review required

This topic supports professional engineering work; it does not replace a sealed design. The result depends on site data, the governing standard and engineering judgement, and should be confirmed by a competent engineer before it is built.

References & standards

ACCA Manual J — load calculation
ASHRAE Handbook — non-residential cooling load
CIBSE TM52 — mean summer temperature method