Cooling
Cooling is sized from heat, and in a server room the heat is easy to find: the IT equipment turns essentially all the power it draws into heat, the UPS and power distribution add their losses, and lighting and people add a little more. Schneider Electric's worksheet adds those up in watts, and the UPS term is the one most often forgotten.
As much as the power it draws. Schneider Electric's White Paper 25 explains that the power IT equipment sends out through data lines is negligible, so the power drawn from the mains is essentially all converted to heat, and the thermal output in watts equals the power consumption in watts. A BTU-per-hour figure on a data sheet is not needed. If you know the IT load in watts, you know its heat.
For North American equipment ratings, the paper gives the conversions: one BTU per hour is 0.293 watts, one watt is 3.41 BTU per hour, and one ton of cooling is 3,530 watts.
A fixed part tied to its rating plus a part proportional to the load. Schneider Electric's worksheet gives the heat of a UPS with batteries as 0.04 times the UPS rating plus 0.05 times the IT load, and power distribution as 0.01 times the UPS rating plus 0.02 times the IT load. For a redundant UPS system, the rating used is that of the working UPS only; the redundant capacity is left out.
The fixed part is why an oversized, lightly loaded UPS adds proportionally more heat. The paper notes that in its example the UPS and distribution contributions are amplified because the system runs at only 30% of capacity. Sizing the UPS close to the real load, which the UPS sizing guide covers, therefore lowers the cooling load too.
About half as much heat again as the IT load. Schneider Electric's example is a 250 kW rated data centre of 465 m² with 150 racks and up to 20 staff, loaded to 30% of capacity, a typical level, so the IT load is 75 kW. Its total heat output is 105 kW, approximately 50% more than the IT load. In that example IT is 71% of the heat, the UPS 13%, lighting 10%, power distribution 4% and people 2%.
| Item | Heat output (Schneider Electric worksheet) |
|---|---|
| IT equipment | Same as total IT load power in watts |
| UPS with battery | (0.04 x UPS rating) + (0.05 x IT load) |
| Power distribution | (0.01 x UPS rating) + (0.02 x IT load) |
| Lighting | 14.32 x floor area in m² |
| People | 100 x maximum number of people |
| Total | Sum of the above |
The worksheet ignores heat through outside walls, windows and roofs. Schneider Electric notes that this is fine for rooms inside an air-conditioned building; a room with outside exposure needs an HVAC consultant to add that load.
The heat load plus margins for humidification, redundancy and growth. Schneider Electric lists the factors: the equipment load, power equipment included; the building load; oversizing for humidification; oversizing for redundancy; and oversizing for future requirements. Humidification alone can require a CRAC to be oversized by up to 30% in rooms with a lot of air mixing.
For smaller rooms, under 4,000 ft² (372 m²), the paper gives a general rule: a CRAC rating of 1.3 times the anticipated IT load, plus any capacity added for redundancy. Larger rooms need the wall, roof and recirculation effects examined for the installation.
Often, because its limits are tighter than the IT room's. ASHRAE's power equipment paper notes that UPS modules, unlike most power equipment, are cooled by forced air, and that a UPS battery room must be held within the narrow range the battery maker specifies, because battery life is very sensitive to temperature. ASHRAE recommends 18 to 27 °C at the IT inlet, but gives 25 °C as the ideal for lead-acid batteries, with life roughly halving for every 8 to 10 °C above it.
A UPS sharing the room with the IT is counted in the worksheet above. A separate UPS and battery room is sized the same way, from its UPS and distribution terms, and held at the battery temperature rather than the IT inlet range. The server room UPS guide covers the UPS side.
It rises. ASHRAE notes that during an HVAC failure the IT equipment's power draw goes up as its internal fans speed up to cool it. That extra power is extra heat in a room that is already losing its cooling, and extra load on the UPS. Redundant cooling capacity, sized on the full heat load including the UPS, is what keeps one failed unit from becoming a room-wide event. Cooling design, installation and maintenance are delivered through specialist cooling subcontractors. For the batteries the room protects, replacement batteries sized for data-centre UPS are listed by UPS model.
One to one: the IT equipment's power draw in watts is its heat output in watts. Add the UPS, power distribution, lighting and people terms from the worksheet.
By Schneider Electric's worksheet, 0.04 times the UPS rating plus 0.05 times the IT load, for a UPS with batteries.
Schneider Electric gives one BTU per hour as 0.293 watts, one watt as 3.41 BTU per hour, and one ton of cooling as 3,530 watts.
For rooms under 4,000 ft² (372 m²), Schneider Electric gives it as a general rule, plus any redundant capacity. Larger rooms need a detailed assessment.
No. Schneider Electric's worksheet uses the rating of the working UPS system and leaves out the redundant UPS.
Sources, checked 2026-10-04:
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