Server room cooling requirements: temperatures, sizing and resilience

What temperature and humidity should a server room run at?

A widely used reference is ASHRAE Technical Committee 9.9's thermal guidelines for data processing environments. The 2021 edition sets a recommended envelope for equipment classes A1 to A4, which covers enterprise and volume servers, storage products and similar IT equipment:

Measure	ASHRAE 2021 recommended range (classes A1 to A4)
Dry-bulb temperature of air entering the equipment	18 to 27°C
Moisture (dew point)	-9°C to 15°C
Relative humidity, upper limit	70%, or 50% where testing shows corrosive pollutants

The 50% figure comes from a footnote that many people miss. ASHRAE links the upper humidity limit to air quality: if silver and copper corrosion coupons show corrosion above its limits, catalyst-type pollutants are probably present and humidity should be driven down to 50% or lower. A comms room near a busy road or an industrial process is the kind of space where that check is worth doing.

ASHRAE also publishes wider "allowable" ranges. These are not targets. The reference card explains that facilities should be designed and operated to target the recommended range, while the allowable range is where manufacturers test that equipment will function, which it describes as a statement of functionality rather than reliability.

Two further points help when writing a specification. ASHRAE limits how fast the inlet temperature changes, to 20°C in an hour and no more than 5°C in any 15-minute period for most equipment. And at the dry end, ASHRAE notes that in rooms without anti-static floors, where staff wear ordinary shoes, the risk of generating an 8kV static discharge rises slightly as humidity falls, from 0.27% at 25% relative humidity to 0.43% at 8%. Equipment suppliers set their own temperature, humidity and dew point limits too, so check the warranty conditions for the kit in the room.

Why should temperature be measured at the rack inlet?

The ASHRAE figures describe the air entering the equipment, not the average temperature of the room. A wall thermostat beside the door can read comfortably while the top of a rack is starved of cool air, or while hot exhaust air recirculates to the front of the cabinets.

ASHRAE's technical bulletin on small "edge" IT spaces stresses that the number and physical placement of sensors is very important, and that sensors should be located to measure the environment of the IT equipment. In practice that means inlet sensors at more than one height on the busiest racks, plus at least one humidity sensor, wired into whatever alarm system the building already uses.

Layout matters as much as plant. Blanking panels in empty rack positions, sealed cable openings and a clear hot side and cold side keep supply air going where it is needed, and they make any cooling system easier to control.

How do you size cooling from the IT load?

Start from electricity, not floor area. CIBSE Journal puts it simply: every electron of electricity consumed by servers ultimately converts to heat. So the heat a server room cooling system must remove is, to a first approximation, the electrical power the equipment in the room draws.

A practical sizing sequence looks like this:

1. Measure the real IT load at the rack power distribution units or the distribution board over a normal working period. Nameplate ratings describe a maximum, not typical use.
2. Add the heat from other equipment in the room, such as a UPS (its losses are on the datasheet), lighting and any people who regularly work there.
3. Add heat gains through walls, windows or the roof, which a designer will calculate for the specific room.
4. Add a planned allowance for growth agreed with the IT team, rather than a blanket safety margin.
5. Choose units on their sensible cooling capacity at the room's design conditions, because that is the part of the rating that removes heat rather than moisture.

For example, a comms room whose racks measure 6kW at the distribution board, with a small UPS and lighting in the same space, needs cooling sized around that figure plus the extra gains, not a unit chosen from the room's floor area. Oversizing is not a harmless safety net: plant that is too big for the load can cycle on and off and control humidity badly. For a general method that also covers offices, see the guide to sizing commercial air conditioning.

Close control or comfort air conditioning?

The UK government's Energy Technology List describes close control air conditioning as equipment used to control temperature, and optionally humidity, in rooms and enclosures containing heat-generating equipment such as servers, computers or telecommunications devices. It notes that this equipment typically runs continuously and handles a much higher cooling load per square metre than conventional air conditioning.

To be listed, close control units must have a sensible heat ratio of at least 0.9, meaning at least 90% of their cooling capacity goes on removing heat rather than moisture, and must be able to vary their capacity as the load changes. The list also covers close-coupled cooling, where units sit next to the racks or within them rather than cooling the whole room.

Comfort systems such as splits and VRF are designed around people, who add moisture as well as heat. They can work well in a small comms room if they are chosen for year-round cooling, including cold winter days, and if their controls are not tied to office hours. As the load and the cost of downtime rise, close control becomes easier to justify. The server room air conditioning page compares the options in more detail.

How much backup cooling does a small server room need?

The usual shorthand is "N+1": install enough units to carry the full load (N), plus one more, so a unit can fail or be serviced without the room overheating. Larger or more critical rooms may justify fully duplicated systems.

ASHRAE's bulletin on small IT spaces makes a point that applies to many office comms rooms: smaller data centres may require redundancy for power and cooling, and a small room makes it harder to physically separate redundant equipment. Two units on the same circuit, or side by side in the same ceiling void, share more failure risks than their number suggests.

The right level depends on what an outage costs. A room that only hosts printers and a network switch can live with a single well-maintained unit and a good alarm. A room running payment systems, telephony or production control usually cannot. Readers planning a new or upgraded room can request quotes through the site's form and ask each contractor to state the redundancy level they are pricing.

What should monitoring and alarms cover?

ASHRAE's guidance for small IT spaces says technicians should be able to monitor both the temperature and humidity of the air entering the IT equipment in real time. A sensible monitoring set-up covers:

• inlet temperature at several heights on the critical racks
• relative humidity or dew point in the room
• cooling unit run and fault signals
• filter condition, since ASHRAE suggests filters in small IT spaces have remote monitoring of differential pressure
• water detection under any unit that produces condensate

An alarm only helps if someone receives it. Route alerts to named people out of hours, and test the route as part of planned maintenance.

What maintenance and legal checks apply?

Server room cooling uses refrigerant, so the F-gas leak-checking rules apply in the same way as for any stationary air conditioning. The frequency depends on how much F gas the system holds and its global warming potential. Equipment holding the equivalent of 500 tonnes or more of carbon dioxide must have an automatic leak detection system, and fitting one to smaller systems allows the interval between leak checks to be doubled.

The energy inspection rules can apply too. Government guidance says there is no exemption limiting air conditioning inspections to cooling for the comfort of occupants, although the guidance is not written for dedicated process cooling. If the IT cooling, alone or combined with other units under the same control, is over 12kW, ask an accredited assessor whether it is in scope. The guide to booking an AC inspection, F-gas check or service explains the process.

Continuous plant also needs planned servicing: filter changes, coil cleaning, condensate checks, refrigerant checks and a test that standby units actually take over. A maintenance contract that includes alarm testing and changeover tests protects the room as much as the hardware does. For specifying dedicated cooling, see server and comms room air conditioning.

Sources

• 2021 Equipment Thermal Guidelines for Data Processing Environments, ASHRAE TC 9.9 Reference Card, ASHRAE, checked 20 October 2026
• Edge Computing: Considerations for Reliable Operation, ASHRAE TC 9.9 Technical Bulletin (September 2020), ASHRAE, checked 20 October 2026
• Close Control Air Conditioning Equipment, Energy Technology List, Department for Energy Security and Net Zero, checked 20 October 2026
• Turning waste into warmth: how data centres can heat tomorrow's cities, CIBSE Journal, checked 20 October 2026
• Checking F gas equipment for leaks, Environment Agency and Defra (GOV.UK), checked 20 October 2026
• A guide to air conditioning inspections in buildings, GOV.UK, checked 20 October 2026

Common questions

Is 18°C too cold for a server room?

18°C is the bottom of ASHRAE's recommended range for the air entering the equipment, so a room held well below it is colder than the guidelines call for. Check the IT manufacturers' own limits before changing set points, and change them gradually.

Can a server room run off the office VRF system?

It can, if the indoor unit and controls allow year-round cooling that is not switched off with the office schedule. A separate system avoids a fault or setback on the office side taking the IT room with it.

What humidity is too low for IT equipment?

ASHRAE's recommended envelope starts at a -9°C dew point. It also notes that the risk of static discharge rises slightly as relative humidity falls, so very dry rooms need good earthing and static control during maintenance.

Does a small comms cupboard need dedicated cooling?

It depends on the measured power draw and whether the door stays closed. Because the electrical load becomes heat, even a few kilowatts in a small sealed cupboard can push inlet temperatures above the recommended range without cooling or ventilation.

Related guides

• Commercial kitchen extraction and ventilation: what the rules and guidance expect
• Capital allowances on commercial HVAC: what businesses can claim
• AC inspection, F-gas check or service: what to book and when