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Humidity in Data Centres – Part 1

Humidification – do we still need it?

I’ve heard it said recently that, with the more relaxed guidelines for humidity now being adopted for data centres, humidification is no longer a necessity but is this really the case?

It is true that ASHREA’s latest guidelines for data centres (2008/2011) allow much wider ranges for humidity than was previously the case, and that they do give scope for energy savings by reducing the amount of humidification required. The lower limit is now defined as a “recommended” minimum of 5.5°C dew point (DP) or an “allowable” minimum of 20% relative humidity (RH) where previously there was a recommended minimum of 40% RH.

So just what do these terms mean and how do they affect data centres in the real world?

“Humidity” is the amount of water vapour (moisture) in the air. Any given mass of air has a maximum amount of moisture it can hold which depends upon its temperature and pressure. For data centres the pressure is atmospheric pressure which changes very little for each location and so the amount of moisture the air can hold is for all practical purposes directly related to its temperature.

For example, air at 21°C can hold a maximum of 0.0156kg of water vapour per kg of dry air: at this condition it is said to be “saturated” and it has an RH of 100%. If it contains only 0.0078kg of water vapour i.e. half of what it could hold then it has an RH of 50%. Air at 10°C can hold only 0.0076kg/kg of moisture so if the air at 21°C and 50% RH with its moisture content of 0.0078kg/kg is cooled to 10°C some of the moisture will start to condense out and this is said to be the “dew point” (DP).

If the air is cooled further, to the recommended minimum DP of 5.5°C at which the maximum moisture content is 0.0056kg/kg, then even more moisture will condense out. If this air is then reheated to 21°C with no moisture added back it will now be at 36% RH (0.0056kg/kg actual divided by 0.0156 kg/kg maximum). So the ASHREA recommended minimum of 5.5°C DP is equal to 36% RH at 21°C. At different data centre temperatures the same 5.5°C DP will give different levels of RH, as follows:

Air temperature °C

RH to equal a 5.5°C DP
















Alternatively we can use as our minimum the much easier to measure “allowable” figure of 20% RH.

And how does this affect our data centre?

In autumn, when the outside temperature starts to go to low single figures most of the moisture is condensed out and the RH is frequently at 100% with droplets of condensation in the air (i.e. fog). Once the air gets below freezing the amount of moisture it can hold is very low and at -10°C, which is usually taken as the minimum design ambient for the UK, it is just 0.0016kg/kg. When this air is brought into a data centre and warmed to 21°C then unless moisture is added the RH will be 10%, below even the “allowable” minimum of 20%.

One way around this problem is to run the data centre cooler in winter which, because RH is dependent on temperature, means the RH will be higher. In fact air brought in at -10°C and 100%RH would have to be kept below 11°C if is not to fall below 20% RH and would therefore be below the allowable minimum temperature. Of course -10°C is very uncommon, (although it does occur) so it is perhaps better to look at the problem the other way around and say that if we keep to the ASHRAE allowable minimum temperature of 15°C then for how many hours a year will the RH drop below 20%? The answer for central England, based on Met Office records taken over 10 years, is an average of 40 hours a year.

So the answer to whether you can do without humidification and still stay within ASHREA allowable guidelines is: not quite. You may of course take the view that only 40 hours per year is not enough to worry about and as long as this doesn’t give you any warranty or hardware support issues, and you are prepared to operate your data centre at 15°C for part of the year, this may be acceptable.

In my view 15°C is pretty chilly if you have to work in it for any length of time and a better solution is to use energy efficient humidification, which will be the subject of Part 2.

If you’d like more information about data centre humidification and how its management can improve your facilities energy efficiency contact us at

Writer bio: My name is Frank Wilman and I was one of the founders of Future-tech back in 1982. I worked on my first “data centre” in 1973 and have design and installed almost every cooling solution available over the years. My passion is mechanical cooling systems as they give us an opportunity to be truly innovative and continually push the boundaries of resilience and efficiency