Practically every SME site I visit that has air conditioning in their comms room or server closet has it running as low as it goes – usually 18°C.  But why?

According to a friendly air conditioning engineer, the IT guys just like to ‘feel’ the cooling when they walk in.  But does running server rooms ‘cold’ have any merit, or is it just money being wasted?

One the one hand, it’s well known that some electronic components (such as capacitors) last longer at lower temperatures – and fan bearings are similarly affected.

But the flip side is that simply running air conditioners flat-out costs, since as well as moving the heat generated by the servers, the systems are moving heat from the surrounding building fabric.  By running server rooms slightly higher than the surrounding building ambient, some of the heat load generated will be sunk into the building fabric, saving money on both parts of the thermal load.

Safe Range

Server quickspecs provide operating temperature ranges, usually 5 to 35°C or more, and a quick look at the DRAC temperature thresholds on a PowerEdge server shows that system board ambient is considered normal up to 42°C, and critical only at 47°C.

Dell and HP just don’t ask for the ambient temperature history in assessing warranty claims – so it seems that within the stated range, reliability isn’t materially affected.  It’s worth noting that the infamous (but out-dated) Google study of disks also found that higher temperatures gave, if anything, longer disk life, and since then disks have moved to fluid bearings that have far greater reliability than their predecessors.

Air conditioners add massively to the electricity bill (and carbon footprint) and excess cooling also dehumidifies the air more (due to the lower coil temperature), and this in turn leaves the environment more vulnerable to electrostatic discharge (which is bad!).

Set Points

The more something provides heating or cooling, the more it costs to run.  So what temperature set point will minimise cost?

Personally, I use a set point a few degrees higher than the surrounding building ambient, so that the comms room coolers are moving only the heat load generated by the equipment (rather than some too from the surrounding building).

For a typical SME or branch office comms room, with a single high-wall mounted cooler, directing the air flow over the front of the rack will generate savings by making use of the front-to-back cooling of the equipment (creating a so-called ‘hot aisle’ at the rear), because the overall ambient can be further increased without affecting the internal temperatures of the servers, thereby sinking much of the heat load to the surrounding building fabric:

Running the air conditioner fans at maximum speed maximises the coiling coil temperature at any particular load and therefore minimises dehumidification.

Working with What’s There

Another consideration is the types of systems installed.

Older R407C systems use at least 30% more electricity and dehumidify more than DC-inverter R410a systems (because of the simple on-off design).

DC-inverters meanwhile tend to be are most efficient running at about 80% stated load capacity.

For Example

Say a comms room had a 5kW R407C system, two 6kW R410a systems, and an electrical load (which can be checked via the UPS management cards) of about 9kW.  In this case, setting the two R410a systems to 25°C and the R407C to 27°C might work well, as the R407C system would ‘kick in’ only if one of the R410a systems packed up (because the 9kW electrical load cannot be moved completely by one of the 6kW R410a systems, resulting in a rise in the ambient temperature).

Because of our friends at VMware, often now I find the cooling systems are way over specified.  In the above example, electrical load might have been reduced to only 3kW and then the R407C system wouldn’t be needed at all, and a 25°C/27°C set point split between the two R410a systems.

As with everything, to maintain a reliable infrastructure monitoring is the key.  Option boards are available for Daikin systems (for example) to connect to environmental monitoring equipment such as APC’s NetBotz range.  Alternatively, a temperature sensor can be simply attached with a cable tie to the air conditioner outlets, and alarms configured on the environment monitor accordingly after some period of observation.

Server Room Air Conditioning Quick Tips

• Understand what you’re dealing with – the electric load (which usually equals the thermal load), the air conditioner type, and their cooling capacity.
• Minimise the use of R407C air conditioners.
• Create hot isles – direct cooled air across the front of the racks as this is what the servers ‘breath’.  Cooling at the rear of the rack is essentially wasted.
• Don’t use comms room air conditioners to cool the building – remove only the heat generated by the equipment.
• Focus on server internal temperatures rather than room temperatures – a 25°C ambient as recorded via the server system board sensors is absolutely fine.
• Be mindful of how quickly and how much temperatures will rise in the event of incoming mains power failure, as UPS shutdown policies may need to be revised (UPS run time could result in overheating, in the absence of any cooling).  This can be tested by shutting down the air conditioners and observing the rate of rise.
• Try to work out the overall room air flow, watching out for hot-spots.  Server racks are cooled front-to-back and can therefore benefit from the use of blanking plates in the racks between servers and of course wire mesh doors, but comms equipment tends to be cooled side-to-side.  Internal division of racks (with side panels) can therefore be advantageous.
• Work out a way of monitoring the health of installed air conditioners and generating alarms when necessary.