Server rack and network closet temperature: what is too hot, and how to monitor a rack without air conditioning
A single rack in a closet should keep its air inlet between 18 and 27 °C, the ASHRAE recommended range for standard IT equipment. The room average tells you little: the problem is the hot pocket at the top of the rack and the UPS sitting in it, whose battery life halves for every 8 °C above 25 °C.
In short
- ASHRAE class A1 equipment is designed for an inlet of 18–27 °C recommended and 15–32 °C allowable. Measure at the inlet, not on the wall.
- A closed rack in a closet without cooling can run 10–15 °C hotter at the top than at the floor. One sensor at the top front is the minimum that tells the truth.
- The UPS battery is the first thing to die: VRLA batteries are rated at 20–25 °C and lose half their life for each 8 °C above that.
- Power problems cause 45 % of serious outages and cooling 19 % (Uptime Institute 2025). In a small closet the two are the same failure.
What temperature a rack may reach
The reference is the ASHRAE TC 9.9 thermal guideline. For class A1 equipment, which covers ordinary servers, switches and storage, the recommended inlet range is 18–27 °C and the allowable range is 15–32 °C. The number that matters is the temperature of the air entering the equipment, measured at the front of the rack.
Manufacturers usually quote a wider operating range on the data sheet, often 0–40 °C for a switch. That is the limit at which the device still works, not the temperature at which it will last. Fans run faster, the CPU throttles, and capacitors and batteries age faster. Running at 35 °C is not a fault. It is a shorter life and a higher chance that a hot day ends in a reboot.
| Inlet temperature | Status | What to do |
|---|---|---|
| 18–27 °C | ASHRAE recommended | Nothing. Log it. |
| 27–32 °C | Allowable, shortened life | Warning alarm, check airflow and door |
| 32–40 °C | Outside A1 envelope | Critical alarm, act within the hour |
| over 40 °C | Beyond most data sheets | Shut down non-essential load |
Why the room average lies
Warm air rises and a rack is a chimney. In a closed cabinet with no forced ventilation the difference between the bottom and the top of the rack is routinely 10–15 °C. A thermometer on the wall at eye level reports a comfortable 24 °C while the switch in the top unit is breathing 36 °C.
The second lie is the door. Many small racks live in a cupboard or a storeroom that is cooled by the building air conditioning only while the door stands open. Somebody closes the door on Friday evening, and the temperature climbs all weekend. This is the most common story behind the Monday morning alarm from an otherwise healthy closet.
The third is the seasonal one. A closet that behaves all winter starts to fail in July, when the ambient rises by 8 °C and the rack follows. A month of history from a logger shows this trend weeks before the first reboot.
The UPS is the first casualty
The device that suffers most from a hot rack is not the server. It is the UPS. Nearly all small UPS units use valve-regulated lead-acid (VRLA) batteries, and their design life is specified at 20–25 °C. The rule of thumb used by Schneider Electric, Eaton and Riello is the same: every 8 °C above 25 °C halves the life of the battery. A battery good for four years at 25 °C lasts about two at 33 °C.
This matters because the UPS is usually at the bottom of the rack, where it is coolest, but its own heat and a closed cabinet push it well above 30 °C. The failure shows up at the worst moment: the mains drop, the UPS takes the load, and a tired battery holds it for ninety seconds instead of fifteen minutes.
The Uptime Institute puts power at the top of the outage list, 45 % of serious incidents in its 2025 analysis, most of them UPS-related. Cooling is second at 19 %. In a small closet these two numbers describe one event: heat kills the battery, the battery fails to bridge an outage.
A closet without air conditioning: what actually helps
Not every closet justifies a split unit. Before spending on cooling, these four steps often bring a rack back inside the envelope, and each of them can be checked with a single temperature sensor before and after.
- Open the airflow path. Fit blanking panels in empty units so hot exhaust cannot loop back to the front. Remove the glass door, or replace it with a perforated one.
- Fit a roof fan kit in the rack. It costs little and typically removes 5–8 °C from the top of the cabinet. Fans need a temperature reading to prove they work, and a sensor to tell you when one has died.
- Ventilate the room, not just the rack. A louvred door or a transfer grille to a corridor is often enough for a load below 1 kW.
- Move the UPS. If the rack cannot be cooled, put the UPS at the bottom, outside the exhaust path, or outside the cabinet altogether.
If after these steps the top-of-rack inlet still sits above 30 °C in summer, the closet needs active cooling. The sensor data makes that a short conversation with the budget holder rather than an argument.
How to monitor a single rack
One rack needs one logger with two probes: one at the top front of the rack, at the inlet of the highest device, and one at the bottom near the UPS. The difference between the two is the number that tells you whether airflow is working. Add a door contact if the closet relies on an open door for cooling, and a leak sensor if there is any plumbing in the wall.
The logger should send an alarm on its own, by e-mail directly from the device, so that a dead monitoring server does not mean a silent closet. A local web page and SNMP readout let you pull the values into whatever you already run. A PoE-powered logger such as EnviSensor ETH PoE takes one patch cable and no spare socket, which in a closet is the difference between installing it today and installing it never.
Set two thresholds per probe. A warning at 27 °C at the top inlet, a critical alarm at 32 °C, and a separate warning at 30 °C for the UPS probe. Keep at least a month of history: the trend before summer is worth more than any single reading.
Frequently asked questions
What temperature is too hot for a server rack?
Above 27 °C at the equipment inlet you are outside the ASHRAE recommended range; above 32 °C you are outside the allowable range for class A1 equipment. Most devices keep working to 40 °C but age faster.
Where should I put the temperature sensor in a rack?
At the front of the rack, at the air inlet of the highest device. That is the hottest inlet in the cabinet. A second probe near the UPS at the bottom shows whether airflow is working.
How does heat affect a UPS battery?
VRLA batteries are rated for 20–25 °C. Every 8 °C above 25 °C halves the battery life, so a four-year battery lasts about two years at 33 °C.
Can I run a rack in a closet with no air conditioning?
Often yes, for loads below about 1 kW, if the rack has blanking panels, a roof fan and a ventilated door. A sensor at the top inlet will show whether it stays under 27 °C in summer.
Do I need a cloud service to get rack temperature alerts?
No. A network logger can send e-mail alarms directly from the device and expose readings on a local web page, over SNMP or as JSON. The cloud is useful for viewing several sites, not required for the alarm.
Related pages
EnviSensor ETH PoE
Rack logger powered over the patch cable, up to four sensors.
Server room monitoring
Temperature, humidity, smoke and leaks: what to monitor first.
Server rooms & IT
Environmental monitoring for racks and server rooms.
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