Battery and UPS room HVAC

The uninterruptible power supply (UPS) room is the part of a data centre that keeps everything else running when the utility fails. It is also a room with two very different HVAC jobs: removing steady heat from power electronics, and keeping battery strings at a temperature and air quality that protect both their life and the people who work near them.

Treating the UPS room as just another electrical room is a common mistake. Battery chemistry, room layout, code requirements and the cooling system's own power supply all shape the design. This guide covers the main decisions for lead-acid and lithium-ion installations.

Where the heat comes from

UPS modules

A double-conversion UPS loses a few percent of the power passing through it as heat. Modern units commonly run in the mid-90s percent efficiency range at typical loads, with higher figures in eco or bypass modes and lower figures at light load. The heat rejected to the room is approximately:

Heat (kW) = IT load (kW) x (1 / efficiency - 1)

For a 500 kW IT load through a UPS at 95 percent efficiency, that is about 26 kW of continuous heat, roughly 7.5 tons of refrigeration. Use the manufacturer's efficiency curve at the expected operating load, not the nameplate peak, and remember that redundant UPS configurations often run each module at partial load, where efficiency drops.

Batteries, switchgear and transformers

Batteries themselves produce little heat while floating, but they release significant heat during discharge and recharge. Switchgear, static transfer switches, power distribution units and any transformers in the room add their own losses. Get loss data from the electrical engineer rather than estimating it from floor area.

Battery temperature: the number that matters most

Battery life is rated at a reference temperature, normally 25 C (77 F). Valve-regulated lead-acid (VRLA) batteries are especially sensitive: a widely used industry rule is that service life roughly halves for each 8 to 10 C (15 to 18 F) of sustained operation above 25 C. Cold temperatures reduce available capacity, so a room that runs too cold shortens runtime when it is needed.

That is why many designers separate batteries from UPS electronics. The UPS modules tolerate warmer air, similar to IT equipment under ASHRAE TC 9.9 thermal guidelines, while the battery room is held close to 25 C.

Always confirm ranges against the battery and UPS manufacturer's warranty conditions, since those documents define what the owner is actually entitled to.

Temperature uniformity

Within a battery string, cells at different temperatures age at different rates. Avoid supply air blowing directly onto part of a rack, place racks away from exterior walls and sun-exposed glazing, and aim for small temperature differences across the room.

Ventilation for lead-acid batteries

All lead-acid batteries can release hydrogen, mostly during charging and especially during equalize or overcharge conditions. Hydrogen's lower flammable limit is about 4 percent by volume in air. Fire codes typically require ventilation to keep the concentration below 1 percent of the room volume, either by calculation or by a prescriptive exhaust rate.

In Canada, requirements come from the National Fire Code of Canada and provincial fire codes, the Canadian Electrical Code (CSA C22.1) section on storage batteries, and the local authority having jurisdiction (AHJ). IEEE 1635/ASHRAE Guideline 21 is the most useful design reference for calculating hydrogen evolution and ventilation rates.

Practical design points:

• Exhaust from high in the room, since hydrogen is lighter than air, and avoid ceiling pockets where it can accumulate.
• Supply makeup air low on the opposite side so the room is swept.
• Use exhaust fans with non-sparking construction where the AHJ requires it, and prove airflow with a flow switch rather than relying on fan status.
• Interlock charger alarms with exhaust failure where required.
• Consider hydrogen detection as a supplement for flooded batteries.
• Keep the battery room negative to adjacent spaces.

Hydrogen ventilation rates are usually small compared with the cooling airflow, but they bring outdoor air into a room that otherwise would not need it. In Canadian winters that air must be tempered to protect battery temperature.

Lithium-ion batteries

Lithium-ion UPS batteries do not off-gas hydrogen during normal operation and tolerate higher temperatures, so cooling loads and ventilation for normal use are simpler. The concern shifts to thermal runaway, which can release flammable and toxic gases and propagate between cells.

Relevant standards include NFPA 855 for stationary energy storage systems, UL 9540 for system listing and UL 9540A test data, which describes how a failure propagates and what gases are released. Codes adopted in Canadian provinces increasingly reference these documents, but adoption varies, so confirm with the AHJ early.

HVAC implications can include:

• Gas detection that triggers exhaust or explosion control ventilation, sized from UL 9540A data.
• Dedicated exhaust that discharges safely away from intakes, as covered in exhaust system design.
• Coordination with the suppression system, which affects whether HVAC shuts down or keeps running during an event. See fire suppression interactions with data center HVAC.
• Fire separations and dampers at room boundaries.

Choosing the cooling system

Common approaches for UPS and battery rooms include:

1. Dedicated computer room air conditioners (CRAC) or chilled water air handlers (CRAH) sized for the room load with redundancy matching the data hall.
2. Ducted supply from the data hall cooling system, which only works if the battery temperature target can still be met.
3. Split DX or ductless units for small rooms, with low-ambient kits for Canadian winters.
4. Outdoor air economizers for UPS rooms only, with filtration and humidity limits; generally avoided in battery rooms because of temperature control and contamination concerns.

Whatever the system, its redundancy should match the criticality of the load. A UPS that overheats and transfers to bypass during a utility event defeats the purpose of the UPS.

Cooling during a power failure

During an outage, UPS modules keep producing heat while the chillers or DX units may be waiting for the generator. Batteries discharging also release heat. The thermal mass of a small UPS room is limited, so temperature can climb quickly. Decide whether UPS room cooling fans need to be on UPS power, and include the room in the site's continuous cooling and thermal ride-through analysis.

Commissioning and monitoring

Battery and UPS room HVAC should be verified like any critical system:

1. Measure exhaust airflow and confirm the room pressure relationship.
2. Prove exhaust failure alarms and charger interlocks.
3. Test gas detection setpoints and the response sequence.
4. Load test with the UPS on load banks and record room temperature at the battery racks.
5. Trend room temperature over a season to confirm stability through winter and summer.

Monitoring should include temperature at multiple heights near the battery racks, not just at the return of the cooling unit. Battery monitoring systems often measure cell temperatures directly, and those readings should be visible to facility staff alongside HVAC data.

Frequently asked questions

Can the UPS and batteries share one room?

They can, and many smaller facilities do. The room then has to be held at the battery temperature target, which means more cooling energy for the UPS heat. Separating them lets the UPS run warmer and the battery room stay at 25 C.

Do VRLA batteries need ventilation if they are sealed?

Yes. VRLA batteries recombine most of their gas, but they still vent hydrogen through pressure relief valves, particularly when overcharged or failing. Codes generally treat them as requiring ventilation, although calculated rates are lower than for flooded cells.

Key takeaways

• UPS heat is set by efficiency at real operating load, so use part-load data from the manufacturer.
• Hold lead-acid batteries close to 25 C (77 F); sustained higher temperatures shorten life significantly.
• Lead-acid rooms need hydrogen ventilation to stay below 1 percent concentration, designed with IEEE 1635/ASHRAE Guideline 21 and local code.
• Lithium-ion shifts the risk to thermal runaway, governed by NFPA 855 and UL 9540A, with requirements that vary by province and AHJ.
• Cooling redundancy and power continuity for the UPS room should match the data hall it protects.