An office fills with heat from people, computers, screens, lighting and sunshine through the windows. Older buildings with large glazing and newer, well-sealed ones can both overheat. Air conditioning helps, but the best results come from getting the sizing, layout, fresh air and controls right together.
What the law says about office temperature
HSE’s guidance on what the law says is clear that there is no maximum workplace temperature. Employers must provide a reasonable indoor temperature, assess the risk of heat and act on it. The Approved Code of Practice suggests a minimum of normally at least 16°C for indoor work. HSE’s list of ways to manage workplace temperatures includes air cooling or air conditioning with adequate ventilation, alongside blinds, moving desks out of direct sun and other measures.
Sizing: why there’s no simple rule
You’ll see rules of thumb for office cooling per square metre or per person. They’re a starting point at best. Two offices of the same size can need very different systems, depending on:
- glazing area and orientation, and whether there are blinds;
- how many people use the space at peak and how much IT they run;
- lighting type and hours;
- insulation, roof exposure on top floors, and ceiling heights;
- meeting rooms that fill up suddenly.
We work out the heat gains for each zone at survey. Our sizing guide explains the approach.
Open plan, cellular offices and meeting rooms
Open-plan floors with suspended ceilings usually suit cassettes, spread out so no one sits in a cold draught. Small private offices and meeting rooms work well with their own indoor unit and controller, because their loads swing more than the open floor. For whole floors or buildings, VRF serves many indoor units from shared outdoor plant, with central control and timers; some heat recovery systems can heat one zone while cooling another.
Controls cut down disputes. A sensible set-point range, timers so the system isn’t running all weekend, and a central controller for the office manager stop most of the thermostat wars.
Want it done properly? AirCon Doctor installs, repairs and services air conditioning and heat pumps across London and the South East, from our base in East Grinstead. We’re an F-Gas certified company and NICEIC approved. Get a fixed-price quote.
Fresh air is a separate job
Cassettes and wall units recirculate the room air. They don’t bring in fresh air. HSE’s guidance on improving ventilation says recirculating systems, including air conditioning, should be designed with fresh air inlets. HSE also suggests CO2 monitors as a broad guide: levels consistently above 1500ppm in an occupied room indicate poor ventilation. If your office gets stuffy as well as hot, mechanical ventilation with heat recovery may be needed alongside the cooling.
Installing in an occupied office
At survey we check ceiling voids, routes for refrigerant pipes and condensate, positions for outdoor units, and the electrical supply. In a leased office, the landlord’s consent is usually needed for plant and pipe routes, and multi-let buildings often have rules on where plant can go. We then agree a programme with you: phased by area, or timed around your working hours, so the office keeps running.
F-Gas and TM44 duties
The organisation that controls the system is the operator under the F-Gas rules. Systems with 5 tCO2e or more of refrigerant need leak checks at least every 12 months, every 6 months from 50 tCO2e, with records kept. To work out tCO2e, multiply the charge in kg by the refrigerant’s GWP and divide by 1,000: 3 kg of R410A (GWP 2088) is about 6.3 tCO2e.
If the total effective rated output of the air conditioning in the building is more than 12 kW, an accredited energy assessor must inspect it at least every five years under the air conditioning inspection rules. We prepare systems for TM44 inspections and fix what they find.
Maintenance and costs
Office systems run long hours, and a breakdown in a heatwave affects everyone. A planned maintenance contract keeps filters, drains and leak checks on schedule. A single small office with one wall-mounted split falls in our standard range of £1,200 to £2,200 + VAT. Larger offices, cassettes and VRF are priced after a survey with a fixed-price quote. See our office air conditioning page.
Quick answers
Is there a maximum temperature for working in an office?
No. HSE says there is no legal maximum, but employers must provide a reasonable temperature and manage heat as a risk. The code of practice suggests a minimum of normally at least 16°C.
Does office air conditioning provide fresh air?
Usually not. Cassettes and wall units recirculate air. Fresh air needs a ventilation system or openable windows.
Does our office need a TM44 inspection?
If the building’s air conditioning has a total effective rated output over 12 kW, yes, at least every five years by an accredited energy assessor.
Related services
- Office air conditioning
- Commercial air conditioning
- VRF air conditioning
- TM44 air conditioning inspections
- Ventilation systems and MVHR
More advice
- Air Conditioning Maintenance Contract: What It Includes and Is It Worth It?
- Air Conditioning F-Gas Regulations: What Businesses Must Know
- What Is VRF Air Conditioning? Guide for Business Owners
- Can Air Conditioning Improve Air Quality?
- All advice
This guide is general information, checked against the sources linked on 30 October 2026. It isn’t legal, tax or financial advice. Rules, grants and product ranges change, so check the current position before you rely on it.