Server Room Temperature: What to Hold, and How to Hold It

A recruitment firm in the City rang us on the Tuesday after the August bank holiday. Their comms room sat at the end of a top floor corridor, the door was warm to the touch, and two access switches had stopped answering over the weekend. The wall split inside was in perfect working order, wired into the same out of hours schedule as the rest of the floor when the office was fitted out, so at seven on the Friday evening it switched off with the meeting rooms and stayed off for three days.

Nothing had broken. The room had been told to stop cooling, and it did as it was told.

The reference this industry works to is ASHRAE's Technical Committee 9.9, and its recommended envelope, which the guidelines state is "suitable for Classes A1 to A4", is 18°C to 27°C dry bulb at the equipment air inlet, with a separate upper dew point limit of 15°C and a relative humidity ceiling (2021 Equipment Thermal Guidelines for Data Processing Environments, ASHRAE TC 9.9 reference card). Design for the middle of that band, not its edges. The number is the easy half. What decides whether a server room survives is whether the temperature is held continuously, whether a second machine exists for when the first one stops, and whether somebody is told within minutes instead of on the Tuesday.

Where 18°C to 27°C comes from, and what it does not mean

ASHRAE publishes two different things and they get mixed up constantly. On the first, the reference card is direct: "Recommended: Facilities should be designed and operated to target the recommended range." That is the 18°C to 27°C figure, and it applies to Classes A1 to A4. The allowable ranges are wider, and they are a manufacturer's test result rather than a design target. The card explains that IT makers test their equipment to verify it will function within the allowable conditions, and says plainly that this is a statement of functionality and not of reliability.

So when somebody tells you their server room sits at 30°C and the kit is fine, they are quoting an allowable number at you. The equipment is working. Nobody has told you what it is doing to the life of the hardware, and the guidelines deliberately decline to promise anything on that.

ASHRAE class	Typical space	Allowable dry bulb	What tends to live there
A1	Data centre, tightly controlled	15°C to 32°C	Enterprise servers and storage
A2	IT space with some environmental control	10°C to 35°C	Volume servers, storage, workstations
B	Telecommunications spaces, offices, minimal control	5°C to 35°C	LAN switches, office computers, printers
H1	Zone cooled lower for high density kit	15°C to 25°C, recommended 18°C to 22°C	High density servers

Two details there matter more than the headline range, both from Tables 2.1 and 2.2 of that card. The recommended band is defined for Classes A1 to A4 only, so a room full of switches is not covered by it in the way people assume. And high density equipment in an H1 zone tightens the recommendation to 18°C to 22°C, which is a different specification and a different amount of plant.

One more thing, and it is what most site surveys miss. These are inlet air figures. The card's note on rate of change measures the maximum air inlet temperature minus the minimum within a time window, and limits it to 20°C in an hour with no more than 5°C in any fifteen minute period, tape equipment excepted at 5°C in an hour. A wall thermostat by the door reading a comfortable number tells you very little about the air going into the top of a rack.

A comms cupboard is not a small server room

Most of the rooms we are asked to look at in London offices are not server rooms at all. They are cupboards holding a patch panel, a couple of switches, a router, a UPS and whatever else needed somewhere to live. In ASHRAE's classification they sit in Class B, described on that card as telecommunications spaces, offices and homes with minimal control of environmental parameters, holding LAN switches, office computers and printers. No recommended range is defined for Class B, only an allowable one, so anybody quoting 18°C to 27°C at a switch cupboard is borrowing a figure from a different class of space.

The bigger problem is that a cupboard has no air path. A server room gives air a route to leave the equipment, mix and come back. A cupboard has a closed door, a heat source that never stops, and one sensor at head height by the light switch while the heat collects at the top of the rack. Cupboards also accumulate: a switch and a patch panel, then a UPS, then a NAS, then the CCTV recorder because the cupboard already had power. Nobody recalculates anything, and the first sign of that is a summer of nuisance reboots which get blamed on the network.

Why comfort air conditioning is the wrong machine

An office split keeps people comfortable during office hours. A server room needs a machine that does one job every hour of every day, including Christmas. The differences are not subtle.

It is designed to stop

Comfort cooling assumes a building that empties. Timers, occupancy setbacks, seven day schedules and BMS zoning exist to switch cooling off when nobody is there, which is exactly the wrong instinct for a room whose load has nothing to do with occupancy. The City job above was not a plant failure, it was a control strategy failure, and I have watched the same thing arrive from a cleaner's isolator, a rewired lighting circuit and a landlord's out of hours setback on a shared floor.

It removes moisture you wanted to keep

A comfort unit is rated to do a mix of cooling and dehumidification, because that is what makes a room full of people feel pleasant. An IT room needs nearly all of that capacity spent on temperature. Mitsubishi Electric state the figure outright for their IT range: the M Series MSY-TP includes "an SHF (Sensible Heat Factor) of up to 0.98", because "a high sensible cooling system is able to significantly cool the room without removing too much moisture" (A new R32 air conditioning range targets the IT cooling sector, Mitsubishi Electric). Put a comfort unit on a dry sensible load and you buy capacity you cannot use, then dry the room out while you do it.

Drying a room out has a cost. The ASHRAE card cites committee funded research showing the risk of generating 8 kV of static rises from 0.27% at 25% relative humidity to 0.43% at 8%. Static discharge does not announce itself. The upper limit has moved as well, from 60% relative humidity in the 2015 edition (ASHRAE TC 9.9 reference card, fourth edition) to a 2021 position that permits 70% only where silver and copper coupon testing shows corrosion below 200 and 300 angstroms per month respectively, and says it should otherwise be driven to 50% or lower.

It may refuse to cool in winter

This one surprises facilities teams. A room with a constant internal load needs cooling in January as much as July, and a standard comfort system is not necessarily happy running in cooling mode near freezing. Units built for IT duty say so on the datasheet: Mitsubishi Electric list the MSY-TP as "Cooling down to -25C outdoor air temperature" and "ideal for small computer rooms or areas that require a greater degree of sensible cooling" (MSY-TP R32 Inverter Heat Pump, Cooling Only, Mitsubishi Electric). If the unit in your comms room carries no low ambient cooling rating, you have a system with a season it does not work in.

Where the duty is tighter than a wall unit can manage, the step up is close control, which the same manufacturer describes as "specifically designed for computer rooms, laboratories etc, where a need for high sensible cooling and close control of temperature and humidity is required" (IT Cooling, Mitsubishi Electric). The indoor unit format is the least interesting part of this decision, whatever the split and cassette comparison suggests.

Working out the heat load without guessing

There is a shortcut here that is genuinely reliable. Schneider Electric's white paper puts it plainly: "The power transmitted by computing or other information technology equipment through the data lines is negligible. Therefore, the power consumed from the AC power mains is essentially all converted to heat. This fact allows the thermal output of IT equipment in Watts to simply equal its power consumption in Watts." (Calculating Total Cooling Requirements for Data Centers, White Paper 25, Neil Rasmussen, Schneider Electric). Its worksheet covers the rest of the room, and is written for data centres and network rooms alike.

Heat source	Figure you need	Heat output calculation
IT equipment	Total IT load power in watts	Same as the total IT load power
UPS with battery	UPS rated power and IT load	(0.04 x power system rating) + (0.05 x total IT load)
Power distribution	UPS rated power and IT load	(0.01 x power system rating) + (0.02 x total IT load)
Lighting	Floor area in square metres	21.53 x floor area
People	Maximum number of people in the room	100 x number of people

Every row comes from Table 2 of that paper and every result is in watts. It then gives a sizing rule for rooms of this size: "A general rule is that a CRAC system rating must be 1.3 times the anticipated IT load rating plus any capacity added for redundancy," which it says works well for network rooms under 372 square metres. That is a specification you can hold a contractor to, and a long way from how most comms room units get picked, which is that somebody decided the room was about the size of a small meeting room.

Redundancy, and what N+1 actually buys

N+1 means enough capacity to carry the load with one unit out of action. In a small server room that usually means two units each capable of the full duty, not two units each sized at half.

What gets missed is that redundancy is a running arrangement, not a cupboard of spare kit. If the second unit only starts when the first fails, you will discover on the day you need it that its condensate pump seized eighteen months ago. Units should alternate duty on a rotation so both run regularly and both fail visibly, and they should sit on separate electrical circuits, because one breaker taking out both machines is not redundancy. Sizing has to allow for it from the start, which is why that white paper lists oversizing to create redundancy as a sizing factor rather than an afterthought. It also lets plant be worked on without taking the room offline, which is the difference between planned air conditioning servicing on a sensible weekday and a filter change nobody can schedule.

The bank holiday problem

Ask a straight question of whoever looks after your buildings. If the server room cooling stops at two on a Sunday morning, who finds out, how, and what do they do?

Most sites cannot answer it. There is often a sensor, sometimes an alarm, occasionally an email to a distribution list including two people who have left. Monitoring is only worth having if it produces an action, which means a threshold that triggers early rather than at the point of damage, an escalation path with a named person at each step, and a route that survives the building network going down with the room.

The rate of change limit matters here too, because that ASHRAE figure of 20°C in an hour and no more than 5°C in any fifteen minutes is about air going into the equipment. It is why the instinctive response to a hot comms room, propping the door open and aiming a fan at the rack, is less harmless than it looks. Better to log and trend the temperature rather than only alarm on it, so a room drifting warmer across two summers is visible before it becomes an incident. That discipline is the one that makes scheduled refrigeration maintenance worth paying for on a food site. A visit producing a record you can act on is an investment. A visit producing a signature is a cost.

Two compliance points that catch people out

Air conditioning inspections come first. Regulation 17(1) of the Energy Performance of Buildings (England and Wales) Regulations 2012 states: "This Part applies to air-conditioning systems with an effective rated output of more than 12kW", and regulation 18(1) requires inspection by an energy assessor at intervals not exceeding five years (SI 2012/3118, Part 4, legislation.gov.uk). The trap is aggregation. Government guidance is explicit that this "will include systems consisting of individual units which are less than 12kW, but whose combined effective rated output is more than 12kW", and puts the penalty for failing to hold a report at £300 (A guide to air conditioning inspections, GOV.UK). A server room split bought separately from the office system still counts towards the building total, which we cover in our guide to TM44 air conditioning inspections.

Refrigerant is second. GOV.UK is unambiguous about a leak: "If you find a leak during a check, you must repair it as soon as possible and repeat the test within a month to check the repair worked." Check frequency runs on charge size in carbon dioxide equivalent, at least every 12 months for 5 to under 50 tonnes and at least every 6 months for 50 to under 500 tonnes (Checking F gas equipment for leaks, GOV.UK). In a single unit room a slow leak is a capacity problem long before it is a compliance problem, and both arrive eventually. Our summary of the F-Gas rules for businesses sets out where the duty sits.

The questions worth asking before you sign anything

• What is the measured IT load in kilowatts, and what was added for UPS, distribution, lighting and people?
• What is the sensible heat ratio at the design condition, not the headline capacity?
• What is the lowest outdoor temperature at which it still cools, and does the manufacturer state it?
• Is the room on any timer, occupancy or BMS setback schedule, and can that be locked out in writing?
• If one unit stops, what carries the load, and how do the two share duty in normal running?
• Does the temperature sensor read equipment inlet air or room air, and where is it?
• Who receives an alarm at three in the morning on a bank holiday, and who is second on that list?

A proposal that cannot answer the first three is a price for a box, not a specification for a room.

Frequently asked questions

Is 24°C too warm to run a server room at?

No. It sits inside the 18°C to 27°C band ASHRAE TC 9.9 recommends for Classes A1 to A4, and running in the upper middle of it is a deliberate energy choice many operators make. Two caveats: the figure applies at the equipment inlet, not a wall sensor, and a room at 24°C has less headroom when cooling fails than one at 21°C.

Does a comms cupboard need cooling, or is ventilation enough?

It depends on the load and the space, and it is answered by calculation rather than opinion. Ventilation only works if the air it draws on is reliably cooler than the room needs to be, which in a sealed office in July it is not. A cupboard holding a switch and a patch panel may be fine. The same cupboard once a UPS and a CCTV recorder have moved in usually is not.

Does a server room need humidity control as well as cooling?

The recommended envelope covers moisture as well as temperature, with an upper dew point limit of 15°C, so humidity is part of the specification rather than an extra. In most small London server rooms the practical risk is unwanted drying, caused by a comfort unit spending capacity on latent load, and plant with a high sensible heat ratio deals with most of it.

Should server room cooling run through the winter?

Yes. The heat comes from the equipment, not the weather, so the load barely changes between January and July. What changes is whether the outdoor unit will operate at low ambient temperatures, and that is a question to ask before installation rather than discover in a cold snap.

Does the air conditioning in a server room need F-Gas leak checks?

If it holds fluorinated gas above the threshold charge, yes, on the frequency GOV.UK sets by carbon dioxide equivalent, and the work must be done by a certified engineer. It is worth doing regardless, because in a single unit room a slow refrigerant loss shows up as a room that stops holding setpoint on the hottest afternoons.

Can a portable unit cover a comms room while the fixed system is repaired?

As a short bridge, sometimes, and it needs planning rather than panic. A portable has to reject heat somewhere, and a hose through a doorway into an occupied corridor moves heat around the building rather than out of it. Condensate needs a route, and the supply should not be shared with the rack. Treat it as a way to buy hours, not to cover a long weekend.

The room does not care that it is a Sunday

Almost every server room problem I have attended came down to one of three things: a machine specified for a different job, a control strategy nobody had read, or an alarm that went to someone who no longer worked there. The temperature target is the part anyone can look up. Continuity is the part that costs money when it is wrong, and it is worth understanding what it costs to run AC continuously before the decision is made rather than after.

Most of our server room work in London starts with somebody who has already had one bad weekend and would rather not have a second. Our page on server room air conditioning explains how we survey these rooms, and we are happy to walk yours with you and say plainly whether what is installed matches what the equipment in it needs.

About the author: Ali Elm runs Be Cool Refrigeration & Air Conditioning, a family-run London firm that has been installing, servicing and repairing commercial refrigeration and air conditioning since 2004. His F-Gas certified team has delivered more than 2,000 projects across London and the South East.

Last updated: 15 September 2026