Thermostat wars – what if they’re not all in your head?

So we’re coming into the office air conditioning season after a warm dry spring. And that means it’s time for the annual crop of headlines about thermostat wars. Like this one, and this one and this one. Notice something similar with these?

That’s right – all of the ‘discussion’ about office temperature seems to be about the difference between people. Specifically, that men generally like it colder than women. Oh, and for some strange reason the ladies in marketing always feature as needing it hotter than anyone else.

Folks, I smell a giant, slightly sweaty, rat here. Of course, there are differences in how people perceive temperature. However, if you’ve had the pleasure of ploughing through the ASHRAE and CIBSE literature on temperature and comfort in offices, then you know that an immense amount of research effort goes into identifying a range of temperatures that works for the largest number of people.

So when you get surveys that suggest 60% of people are unhappy with the temperature in their office – cooling in summer or heating in winter – my conclusion has to be that its much more likely the problem is the HVAC.

Too hot and too cold. In the same office?

People disagreeing about office temperature and fighting over the thermostat makes good headlines. Truth is, they may both be right. Its completely normal for office temperatures to vary across the room. So Karen sitting under an air conditioning vent may well be freezing while Mick starts to sweat in the sun at the other side of the room.

And while humans aren’t very good at estimating absolute temperature, we have quite high levels of sensitivity to changes in temperature – anything over 1 degree in 15 minutes is generally detectable. So even quite small variations throughout the day tend to get noticed, particularly if they coincide with peaks and troughs in metabolism.

Why does my office temperature vary so much?

Temperature – and how it affects comfort – is a more complicated subject than it might first appear. If we break it down to its component parts it begins to be obvious why variations happen in buildings.

Perfect office temperature

Perfect office temperature – they don’t seem worried by convection

The basic physics of how heat moves around is probably familiar. Heat can be transferred from one thing to another by radiation (heat energy being transferred directly – like in sunlight), convection (heat being carried around in air or other fluid) or by conduction (transfer along a physical object like a metal spoon in coffee). In an office, radiant heat and convection are normally the two most significant mechanisms for transferring heat around, and these create the first set of challenges.

Because heat will move around a bit differently, depending on the mechanism. Unless you’re sitting in an office made of warm water convection is going to rely on warm air circulating around your office. On the other hand, any ‘body’ – a window, a wall, a desk, a laptop – that you can see will be radiating heat directly at you. If they are at a similar temperature to you this effect will be small, but once the mean radiant temperature gets more than 5 degrees out of whack to air temperature, you are likely to feel uncomfortable.

Since the biggest ‘bodies’ in any office are generally windows and walls, and since they are generally exposed to external temperature, being close to a hot window or a poorly insulated wall makes you very vulnerable to fluctuations in their temperature. And you might be freezing while Frank near the kitchen is toasty.

But wait, there’s more! Why your office’s temperature varies over time.

Do you like graphs? Of course you do. Here’s a fun one:

Measured temperatures in open plan office

Office temperatures Cambridge 7th – 27th April.

This is a graph of the temperature measured at multiple points across an open plan office for 19 days. Spot the two weekend periods (here’s a clue – look at the minimum temperatures). Why are they significantly cooler? It’s not just because there is no heating on. In fact the main difference is the lack of people. Occupation adds heat. A lot of heat. Each person gives out about the same about of heat as a 100 Watt lightbulb, and that’s before you add in the lighting and equipment that they need.

On top of that, there are long term trends that change how hot offices get, from cooler technology – computer monitors that use LED screens – to hot desking. So over the course of the day and over time you can expect to see the heat load (how much heat is generated) change a lot.

Surely my office heating and ventilation is designed to deal with all this?

Indeed it is. And in future blog posts we’ll have a look at some of the work on how zoning, building management systems and natural ventilation is helping.

However, as you can see, how comfortable you are during the working day is actually the sum of some complicated factors, and that’s before we get into activity levels, clothing, humidity, air speed and all the other issues that have significant impact on the working environment. A good building should compensate for many of these, but there are multiple reasons why they change over time, so regular reviews of whether the building’s heating and cooling is doing its job is always a good policy.

Not just telling the lady in marketing to put on another coat.

This is just the first in what we hope will be a series of posts about HVAC and comfortable buildings. Tell us what problems you’re having and we’ll make sure we cover them. 



Hotspots. Real data on rack cooling.

Since last year we have been running a few pilots investigating cooling in data centres across the UK. We now have nearly 15 months of highly detailed data on cooling in racks from different data centre locations that include standard servers, blade servers, hard discs, switches and even a bunch of Mac Minis and it seems like a good idea to share a bit of what the data is telling us.

Health warning: these are pretty preliminary ideas on what is, after all, quite a small sample of data. We’ll need more sites and more time to confirm these hypotheses. But now seems like a good time to take stock and see if we can answer some of the simple questions and think about where we could go with this approach.

It’s also likely data centre professionals are going to want to ask more sophisticated questions than we have about cooling and environmental monitoring from this data: we’ve listed a few  ideas for investigation below and as we add more sites and data we’ll come back to these.

Rewind: what’s Purr all about?

Here’s an Internet of Things recipe we love. Take some very dull data (let’s say temperature). Add a few bits of context (try time, exact location and location relative to other sensors). Sprinkle it with Internet Sauce and voila! Information…to inform decisions and help people decide what action to take next.

For Purr, these actions are about making HVAC more efficient. We think it’s a bit crazy, in the age of the IoT, that most data centres are using three times more cooling capacity than they need. We also think that looks like an expensive problem, or maybe an opportunity.

Temperature in server rack 1

Context for the data: What and where are our pilots?

For practical and ethical reasons (we have an absolute ban on publishing identifiable data without customer clearance) we won’t go into where these pilots are.

There are five test racks, instrumented with 6-8 temperature sensors and a gateway. All of them are in co-location facilities in the south-east of England sitting alongside other customers’ racks.

Data centre temperatures: what does the data tell us?

A few big stories jump out: after several months, the most noticeable feature is the difference between the best and worst performing racks over time. Our champion rack held a mean temperature of 20.6 degrees with less than a degree of variance throughout monitoring. On the other hand we had one rack whose mean temperature of 20.27 obscured swings of over 16 degrees during the trial including peaks over 31 degrees.

Our sensors also revealed the gradients in the racks – ambient intake temperatures change at different levels. Again this was much more pronounced in some facilities than others, with one rack showed a gradient with a 4.4 difference on the intake side. Others had a much tighter range – down to 0.2 of a degree.

Here’s a quick summary of the data from the trial:

Temperature for server racks

Why mean data can be mean – the stories behind the numbers.

Averaging or looking for max/min data obscures some of the interesting stories, which can easily been seen from a heatmap and graph. What, for example, happened on this afternoon?

Server rack temperature showing open door 

In this case, we know: that sharp fall in temperature is characteristic of someone opening the door of the server rack while they work inside.

This one is more mysterious: why does this cold side suddenly develop a hot spot? Is it a variation in air pressure in the facility? Some equipment slipped inside the rack?

Server rack temperature in datacentre 1

What’s next – more questions. More data

Clearly we have a long way to go before we have enough data to undertake the sort of detailed analysis that say, Backblaze can do. (Oh how happy that would make me!) 

But already there is a queue of things on our bucket list for investigation. Which include: 

  1. Can the delta between intake and exhaust tell us anything useful about type of equipment and the type of work it is doing?
  2. Can it give us a useful indication of where a rack has excess capacity?
  3. How many short excursions out of recommended operating range should be tolerated?
  4. Is there a characteristic pattern of temperature change that shows when air is mixing?

If you have questions about the effects of temperature on your server, hard disc or switch please let us know, we’d love to add to the list.

Better yet, if you have a site where you would like to know more about temperature changes then join our beta programme and we’ll install Purr and work with you to analyse the data.


This is the first in a series of posts called ‘Data (Centres), meet Information’ in which we share what we’re learning from our installations. We’re hoping to foster some discussion about the best ways to use information to improve Data Centre efficiency. We’d love to hear your thoughts – whether it’s on better ways to do this or if you think we’re barking up the wrong tree.