Many UK data rooms were habitually run very cold. Published guidance used for IT equipment, including the ASHRAE recommended envelope people quote for a wide set of hardware classes, is broader than that habit: often discussed from the high teens Celsius up into the high twenties at the air entering the equipment, with humidity kept inside limits so you are not condensing in the racks or drying the air to an extreme. That is a conversation with your hardware vendor and a designer. It is not a setpoint to copy onto a wall controller, and it is not a design specification.
Inlet air, not a thermometer on the door
Industry envelopes usually have a recommended range, where most operators aim, and a wider allowable range, where equipment may still run for a time. Sitting inside the recommended band gives you margin. Sitting at the edge of allowable because you want to save energy is a vendor question, not a forum question. Warranty wording for your servers outranks a blog. Measure at the equipment inlet if you can do that safely. A thermometer on the door of the room can read several degrees away from the air the servers actually swallow. Hot recirculation makes a nonsense of any target. Talk about inlet air, not a random wall sensor. Recommended bands are tighter than allowable bands. Your hardware vendor's warranty is the local rule. Holding 18C or below because that is how a previous decade felt uses electricity, can push humidity into awkward places, and still fails if the airflow path is wrong. A room at a milder inlet temperature with a clean front-to-back path often protects kit better than a cold room with exhaust air spilling into intakes. Many recommended IT inlet envelopes discussed in ASHRAE-style guidance run from the high teens into the high twenties. Confirm the current guidance and your hardware warranty. Do not copy 18C from habit. This page is not a design specification. Targets are bands, measured at the kit, and a designer and the vendor set them for your room. A warmer setpoint often reduces cooling energy, until airflow problems or a warranty limit stop you. Raise setpoints only inside the band your designer and vendor accept, with inlet temperatures checked. People rarely sit in these rooms all day. When they do, HSE's workplace temperature guidance applies to the person, which is a different aim from the server inlet. A visiting engineer can cope with a warmer aisle. They should not be asked to work in an unsafe space. Those are two judgements.
Humidity bands and a brief that names the sensors
Very dry air raises static concerns for some environments. Very damp air risks condensation on cold surfaces and corrosion. The published envelopes express that as a humidity or dew-point band. Humidity has limits as well as temperature. You do not need to memorise the band to brief a designer. You do need them to state the band they are working to, and the sensors they trust. A comfort split controlled only on temperature will not hold a tight humidity specification. If your kit or your insurer wants a controlled band, say so at survey. Dehumidification and humidification are extra scope. This page will not pick the numbers for you. Give the installer the equipment list, the heat load if you know it, the redundancy you want, and any vendor inlet limits in writing. Ask them to propose a target inlet band and to say what happens when one unit fails. Confirm anything you hear about ASHRAE against the current publication and against the servers on the floor. F-gas certification covers refrigerant work. GOV.UK publishes that certification. If the plant is large, check GOV.UK for air conditioning inspections. As a 2026 illustration, the fee for a proper survey and monitoring is small beside a day of thermal shutdown, and it is money you can describe in hundreds to low thousands of pounds including VAT depending on how many rooms and how much logging you want.
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How this guide was prepared
This guide was written by the Local AC Installers Editorial Team. We compare official guidance, legislation and established consumer information, separate general information from project-specific advice, and show publication dates and sources. We do not accept installer accreditation claims at face value.