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Making sense of the science of cold draughts

Fresh air rates in many UK classrooms in winter are too low to create an environment conducive to proper concentration. Dr Shaun Fitzgerald explains how natural ventilation systems can increase fresh air without cold draughts and cut energy bills in one go
Following an assessment in 2005 of eight primary schools which were using open windows for ventilation, Building Research Establishment (BRE) findings revealed that, in winter, more than half the fresh air rates were below the minimum required.

There are consequences for those residing in such closed spaces, particularly densely populated ones. Breathing stale air will not supply enough oxygen to keep young minds functioning properly. What's worse, occupied but inadequately ventilated spaces create a playground for bacteria and viruses.

The challenge is how do you ensure a steady flow of fresh air into a room in winter without the ensuing cold draught? To answer this question we need to delve a little deeper into the physics and, rather than just rely on unsupported assertions, see what the experts have to say.

We are often told by consultants, clients and engineers that there are two main problems with natural ventilation in winter - cold draughts or high heating bills which result from incoming cold fresh air being passed over a heating element to warm it up before it reaches the occupants.

This method of overcoming cold draughts is nonsensical in terms of energy use when the heat gains in most non-domestic buildings far outweigh the heat required to maintain an average space temperature of around 21 deg C in winter when external temperatures are in excess of around 6 deg C.

Alternatively, a concept which is being discussed by some in the industry is the use of high level opening vents or windows for winter ventilation. The concept is that cold air enters the building via the vents or windows and is mixed with the internal warm air. A good idea, but how much mixing can you actually get? Not enough would be our assertion in most classroom situations.

In many rooms, the floor to ceiling height is around 2.8m. Any high level vent will have a certain depth to it, so the distance from the floor to the bottom of the high level vent is, at most, 2.3m. Cold air will enter through the bottom of the high level vent when the vent is opened in winter and, if you are sat underneath the window, given typical desk and chair arrangements, the distance from the floor to the top of your head is 1.3m.

This is a long way of saying, the distance between the top of your head and the bottom of the vent is probably 1m. So, the killer question is: 'Does the incoming cold fresh air mix with enough of the room air as it falls 1m for the temperature to be above 16 deg C?'

Fortunately, fluid mechanics experts have provided us with the tools to assess the risk. A window configuration suggested by some designers involves two 0.9m wide high level windows. If we assume that a minimum fresh air rate of 150 l/s is provided, we can use the plume model calculations of Turner and the principle of a virtual origin (Kaye and Hunt) to determine the degree of mixing which can be achieved.

The over-riding conclusion is that if you want to ensure fresh air reaches occupants no colder than 16 deg C, when the classroom is at 21 deg C, opening windows are fine, but only when the exterior temperature is above 13-14 deg C. Alas, as we spend so much of the time in the UK with external temperatures below this level, the opening window strategy won't work - you will get cold draughts.

This is part of the reason many schools which only use opening windows are simply not ventilated in winter.

Something else is needed, which is why my own company, Breathing Buildings, is working with Partnership for Schools to ensure that new regulations for school buildings address both energy and thermal comfort issues. The new regulations require pre-mixing of fresh, incoming air with enough warm internal air to reduce the potential for cold draughts.

// The author is managing director of Breathing Buildings //

19 April 2012

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