Cooling
Comfort air conditioning is built for people: it removes heat and a good deal of moisture, during office hours. Precision cooling is built for electronics: almost all of its capacity goes to dry heat, it holds temperature and humidity tightly, and it runs around the clock. A server room or UPS room needs the second.
What the capacity is spent on, how tightly it controls, and how long it runs. Vertiv's paper comparing the two starts from the load: electronics generate pure sensible heat, heat without humidity, and need continuous cooling. That is why, it says, about 95% of a precision cooling system's energy and capacity is designed to remove dry heat. A comfort system is designed for occupants, so a large part of its capacity goes to removing moisture that people bring.
| Precision air conditioning | Comfort air conditioning | |
|---|---|---|
| Designed for | Critical electronic equipment | Human occupants |
| Sensible share of capacity | 85-100% | 50-70% |
| Control accuracy | ±0.5 K, ±3% relative humidity | Looser |
| Humidity | Controlled, to help avoid electrostatic charge | Unregulated dehumidification |
| Operation | 24 x 7 x 365 | Occupied hours |
The figures are Vertiv's comparison table.
Because the latent part of a comfort unit's capacity is spent removing moisture that is not there. In an IT or UPS room nearly all the heat is sensible: Schneider Electric's cooling paper notes that the power IT equipment draws is essentially all converted to heat. A comfort unit sized on its total capacity therefore delivers much less cooling for electronics than its rating suggests, and, while it works, keeps condensing moisture out of the air.
Vertiv warns where that leads: dehumidifying a room that needs no dehumidifying can lower humidity too much, causing static problems and even electronic failures.
Because the heat never stops. Vertiv points out that most data centres run 24 x 7 x 365, so temperature and humidity must be held around the clock. A UPS room is the same: the UPS loses energy as heat whenever it carries load, and its batteries sit there every hour of the year. Comfort systems are often set back at night or at weekends, exactly when nobody is there to notice the room warming.
Battery life, first. ASHRAE's power equipment paper gives 25 °C as the ideal for lead-acid batteries and halves their life, as a rule of thumb, for every 8 to 10 °C above it. A room that drifts warm every night under a comfort system is quietly shortening the UPS battery string; the UPS battery life guide covers the effect, and the server room monitoring guide covers the sensors that catch it.
The IT inlet target is wider: ASHRAE recommends 18 to 27 °C at the equipment inlet. A room holding the IT inside that range can still be too warm for the batteries if they share it.
Precision units control it; comfort units just remove it. Vertiv's table describes precision humidity regulation as controlled, to help avoid electrostatic charge, and comfort dehumidification as unregulated. Precision units add humidification where needed, which brings its own maintenance: Vertiv notes that a unit with a humidifier may need more frequent maintenance to avoid condensate drain issues that could flood the computer room.
Schneider Electric's cooling paper adds the sizing effect: supplemental humidification puts extra heat load on the unit and can mean oversizing it by up to 30% in rooms with a lot of air mixing.
The battery temperature first, then the UPS intakes. ASHRAE's power equipment paper says the battery room must be held within the narrow range the battery maker specifies, because the life of the batteries is very sensitive to temperature, and it notes that UPS modules, unlike most power equipment, are cooled by forced air. A precision unit's tight control is what makes that possible: Vertiv gives precision units a control accuracy of ±0.5 K, where a comfort unit swings far more.
Two habits help. Put a temperature sensor at the battery string, not only on the wall, so the reading is the one that matters to battery life. And keep the UPS room on the same around-the-clock duty as the IT room, because ASHRAE also notes that IT equipment draws more power during an HVAC failure as its fans speed up, which lands on the UPS just as the cooling stops.
From the nameplate and the controls. Vertiv's comparison gives the tell-tale differences: precision units control humidity, while comfort units dehumidify without regulation, and precision units hold temperature far more tightly. A controller that shows a humidity set point as well as a temperature one, and a unit rated for continuous duty, point to precision equipment; a simple thermostat points to comfort cooling. If the room has a comfort unit and a UPS, check the room temperature at night and at weekends before anything else; that is when a building system is most likely to be set back.
It can carry a small, lightly loaded closet for a while, but it fails on the points above: it runs only when the building wants it, controls loosely, and spends capacity on moisture. For a room with a UPS and its batteries, the safer course is a unit designed for continuous sensible load, sized with the UPS losses included; see the server room cooling sizing guide. Cooling design, installation and maintenance are delivered through specialist cooling subcontractors. For the UPS itself, a UPS maintenance checklist by brand shows what its own service covers.
Air conditioning designed for electronic equipment: most of its capacity removes dry (sensible) heat, it controls temperature and humidity tightly, and it runs around the clock.
The share of a unit's cooling capacity that lowers temperature rather than removing moisture. Vertiv gives 85-100% for precision units and 50-70% for comfort units.
Vertiv notes that over-drying a room can cause static problems and even electronic failures.
It needs continuous cooling sized for the UPS losses and a stable temperature for the batteries, which is what precision units are built to deliver.
Sources, checked 2026-10-04:
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