Let it snow, let it snow, let it snow

Snow Scientist by Million Moments

“December 2010 is every bit as cold and snowy as the worst December of the 20th Century – 1981 – and it could well turn out to be the coldest December since 1890” says Philip Eden, Vice President Royal Meteorological Society 2007-09.

The unusually cold pre-Christmas weather is affecting many things, airports and roads have closed, people have been unable to get to work and businesses have been unable to deliver presents. So amidst all the chaos, how is the HVAC coping?

To choose the outside temperatures we’ll use for designing the heating and cooling systems for a building we look at weather data (I, alike many other building services engineers, us the CIBSE & ASHRAE world-wide weather data & design conditions). This advises us of the minimum and maximum likely temperatures in a given area. The nearest weather station to my current project is Manchester & the ASHRAE data for there say that the temperature goes below -3.8 degrees Celsius for less than 99.6% of the time.

So that means that it will be colder than -3.8 Celsius for less than 1.5 days per year. But engineers always like to have a bit of a margin or buffer, so we chose a design temperature of -10 Celsius. All our air-handling units are designed to be able to operate, and provide comfortable working conditions (18-22 degrees Celsius) even at this low temperature. However, with local temperatures plummeting to -16.4 degrees Celcius even our design wont be able to function properly.

But what happens when the temperature is colder than you designed for?

Letting the air handling unit (AHU) keeping sucking in air in extreme temperatures can actually damage it, or its components. If the AHU can’t heat the air up enough, or fast enough, ice can form inside it and destroy filters and sensors.

The coils inside the AHU (these are like radiators, they have hot water running through them and they heat the air up as it passes over the many tiny ‘fins’ on the coil) wont be big enough to heat up the air to the temperatures needed inside the building. If you let the AHU keep going then you’ll be blowing cold air over the people inside…soon you’ll have ‘snow scientists’ like the picture above!
switch AHU off

This leaves you with a dilemma…you can’t leave the AHU on or you may damage it or the people in the building. But if you switch it off then you may not have enough fresh air (people need about 10 litres every second to feel comfortable) and recirculated air may not be clean enough to be able to carry on manufacturing. Also, if your ventilation was providing all of your heating then it will slowly start to get very cold inside.

The usual answer is that during very bad weather, such as the stuff we’re currently getting, it’s not possible to keep running the ventilation systems unless you’ve had the foresight to design them to cope with such low temperatures…but then if none of your staff can safely get to work perhaps the HVAC isn’t your biggest problem!