What is the correct temperature for a server room?

Recommendations on server room temperature vary greatly.

This guide says that:

General recommendations suggest that you should not go below 10°C (50°F) or above 28°C (82°F). Although this seems a wide range these are the extremes and it is far more common to keep the ambient temperature around 20-21°C (68-71°F). For a variety of reasons this can sometimes be a tall order.

This discussion on Slashdot has a variety of answers but most of them within the range quoted above.

Update: As others have commented below Google recommends 26.7°C (80°F) for data centres.

Also the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) has recently updated their recommended tempature range to be from 18°C-27°C (64.4°F-80.6°F).

However this article agains highlights that there is still no consensus on the subject. As mentioned in the article I would highlight that:

...nudging the thermostat higher may also leave less time to recover from a cooling failure, and is only appropriate for companies with a strong understanding of the cooling conditions in their facility.

IMO most companies would not have such a strong understanding of cooling conditions and thus it would be safer in a small business environment to be running the rooms a little cooler.

NB: It is important to note there are a lot more factors to consider in a server/data room than just the temperature, air flow & humidity for example are also important concerns.

Google's datacenter best practices recommends 80 degrees:

Raising the cold aisle temperature will reduce facility energy use. Don't try to run your cold aisle at 70F; set the temperature at 80F or higher -- virtually all equipment manufacturers allow this.

We run at 72, but then again I don't exactly trust our own room was designed with airflow in mind.

As others have said, somewhere in the low 70's F is good. However, it's even more critical to make sure the rack and the hardware in it can "breathe". If hot air is trapped in the rack - or in a server chassis - then the low ambient temperature won't really do any good.

My server room is set to 69 degrees. We have one air conditioning unit that services that room and it runs 24/7 to keep the room at 69 degrees year round.

All server rooms I've seen usually are between 20°C and 25°C but from experience I've noticed hardware is more sensitive to variations more than a given temperature. I've often seen hardware fail after a bump of, say 4-5°C, even if it is from 20 to 25.

So stability is a key, as well as air flow of course.

18°C (65°F). Its a bit colder than it has to be, but if something fails it gives us a few precious extra minutes to react before it gets uncomforably hot.

https://en.wikipedia.org/wiki/Data_center_environmental_control contains an interesting overview of vendor datacenter temperature recommendations:

We like ours to be between 19C and 23C with alarms at 28C - we're a 98% HP blade place so in the event of ours getting to that alarm level certain servers/enclosures 'turn their wick down' to lower overall power draw (it's called thermalogic).

Remember that Google's advice of 80°F is including virtually no cost to shutting down a datacenter when it overheats due to unexpected load or air conditioning failure. They also include a greater control of airflow over critical components.

I'm in the Navy. On our ship we kept our rooms at less than 65 degrees Fahrenheit. We'd be in the middle of the Red Sea and it would be 120 degrees outside. We'd walk out of the space (at 62 degrees) with a sweater, gloves, a coat and a peacoat and everyone would look at us like we were crazy. Everyone else was striped down to their tee shirts and sweating like crazy.

The problem is, our cooling system does not remove the humidity very well. So, if it's humid outside and the temp goes up to 70 degrees or more it starts to get sticky in the space.