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Air Conditioning Calculator — Room Cooling Load
Estimate the cooling capacity needed for any room. Sums solar, fabric (sol-air), occupant sensible and latent, ventilation and equipment heat gains using a simplified CIBSE Guide A approach.
Indicative UK summer design dry-bulb ≈ 28-32°C (CIBSE TM49 / Guide A weather data; higher in London/SE)
Total window area on primary facade
CIBSE Guide A Section 1.4 at 22°C — sensible/latent: living 80/35, office 90/50, shop 100/60, restaurant 90/50
Computers, monitors, lighting etc.
Fresh-air / infiltration rate. ~1 ACH typical for an occupied habitable room; raise for leaky or naturally-ventilated rooms
Adds solar-driven roof heat gain to the calculation
Indoor to outdoor unit distance (generic limit ~25m; premium units to 30m)
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How We Calculate This
This calculator gives an indicative room cooling load using a simplified CIBSE Guide A approach. It sums all the heat gains the air conditioning unit must remove to hold the desired indoor temperature on a hot day. It is a sizing estimate, not a substitute for a full design — for installations, confirm with a detailed CIBSE/manufacturer calculation.
Heat gain components
- Fabric gain: Heat conducted through opaque walls and roof. In summer these are driven by the sol-air temperature (sunlit surfaces run hotter than the air), so a mean solar uplift is added to the air temperature difference (≈ +5 K walls, +15 K roof).
- Solar gain: Radiation through glazing, using CIBSE Guide A Table 5.16(g) summer peaks (west highest, then east, south, north) and a derate for double or solar-control glazing.
- Occupant gain: Both sensible and latent heat per person at 22°C from CIBSE Guide A Section 1.4 (heat emission from people) — e.g. living/bedroom ≈ 80 W sensible + 35 W latent, office ≈ 90 + 50, retail shop ≈ 100 + 60.
- Ventilation / infiltration gain: Cooling the fresh and leakage air, Q = 0.33 × air-changes × room volume × ΔT (sensible), plus a moisture allowance for the latent load.
- Equipment gain: Computers, lighting, appliances.
Unit sizing
The total cooling load is multiplied by 1.15 (a 15% sizing allowance) for duct and part-load losses. This is an indicative installer allowance, not a code requirement, and is not a replacement for modelling the loads above. As a cross-check, the result is also shown in W/m² of floor area — UK habitable rooms usually land around 100-140 W/m².
Frequently Asked Questions
Add up the total cooling load: fabric gains through walls and roof (using a sol-air temperature in summer), solar gains through glazing, occupant heat (both sensible and latent — roughly 80W sensible plus 35W latent per seated person per CIBSE Guide A), equipment heat, and the ventilation/infiltration load from fresh outdoor air. Add a sizing allowance and choose a unit with at least that capacity. As a sanity check, UK habitable rooms typically work out around 100-140 W/m² of floor area, so a 20 m² living room usually needs roughly 2.0-2.8 kW.
A summer design dry-bulb of about 28-32°C is a reasonable indicative value for England and Wales — London and the South East run at the higher end, Scotland lower. There is no single fixed figure: CIBSE design conditions come from the Guide A Table 2.24 / TM49 future-weather data sets for your specific location and chosen risk level (for example the 1% design summer year). Use a higher value if you want extra resilience against hot spells.
Glazing is usually the largest single gain. Per CIBSE Guide A Table 5.16(g) (single clear, unshaded, London) the summer peak solar cooling load is highest on west-facing glass (~565 W/m²), then east (~530) and south (~430), with north lowest (~130) — west peaks because the afternoon sun coincides with the hottest part of the day. Double glazing roughly halves these figures and solar-control glass or external blinds cut them by around 70%, which markedly reduces the unit size needed.
Most residential split AC systems allow pipe runs of 15-25 metres between indoor and outdoor units, with a maximum height difference of 10-15 metres. Longer pipe runs reduce efficiency and may require larger refrigerant charges. Check the specific manufacturer data — premium units like Daikin and Mitsubishi often allow up to 30m.
For domestic properties, outdoor AC units usually fall under permitted development if they meet noise limits and are not on a wall facing a highway. Listed buildings and conservation areas require planning permission. Commercial installations typically need planning permission and must comply with local noise regulations. F-gas regulations also apply to refrigerant-containing units.
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Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.