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Breathing Buildings - Adaptive Thermal Comfort

PSBP - Adaptive Thermal Comfort - BB101 - BB93 - Natural Ventilation

Goodbye BB101 Hello Adaptive Thermal Comfort

Priority Schools & Future Design Regulations

The Priority School Building Programme: Facilities Output Specification (FOS) has changed the world of schools. The Education Funding Agency plans to publish new guidelines for ventilation, thermal comfort and indoor air quality to replace BB101 with guidance based on the standards in the current facilities output specification.

Having helped in the development of the output specification, we are uniquely placed to help contractors and engineers ensure that their school design project delivers a low energy project with a superb indoor climate.

Weather Files

FOS stipulates the use of the Design Summer Year (DSY) weather files, which makes it more challenging to avoid overheating than BB101, which accepted the use of the cooler Test Reference Year (TRY) weather files.

Operative Temperature

FOS sees the use of operative temperature as a measure of thermal comfort. In CIBSE Guide A, operative temperature is defined as:

operative temperature = ½ (air temp) + ½ (mean radiant temp)

Breathing Buildings' dynamic thermal modelling software automatically generates output in terms of both air temperature and operative temperature. It is therefore easy to assess designs against the new thermal comfort criteria.

Adaptive Comfort

One aspect of the adaptive comfort philosophy is based on the premise that past thermal history modifies building occupants’ thermal expectations and preferences. An example of this would be that during a warm weather spell, occupants would feel comfortable in a building that would otherwise be considered too warm.

Air Freshness

For naturally ventilated buildings, the maximum daily average CO2 levels are still 1,500ppm (the same as BB101) although the maximum acceptable value for a maximum of 20 consecutive minutes has been reduced to 2,000ppm.

For mechanically ventilated buildings, this requirement has been lowered to a maximum daily average of 1,000ppm with a maximum of 1,500ppm for a maximum of 20 consecutive minutes per day. This has implications involving the amount of heat lost in cold weather, and the fan power required to provide nearly twice as much outside air for a mechanical system compared with a natural system.

Thermal Comfort

There is much more emphasis on the conditions in the space being comfortable and in the importance of mitigating cold draughts. There is now a requirement for pre-mixing of outdoor air, ruling out simple openings close to the occupant, which then leaves MVHR, NVHR® or E-Stack®. MVHR is penalised by not only higher specific fan power but the requirement for more outside air to meet the new lower CO2 restrictions which results in higher energy consumption over the year compared with NVHR® or E-Stack®.

The Science of Cold Draughts

We are often told by consultants, clients and engineers that the two main problems with natural ventilation are cold draughts or the high energy usage previously associated with alleviating the draughts. We agree, and that is where the E-Stack® natural ventilation system comes into its own.

Using our patented mixing system, we introduce turbulence to promote mixing and thereby mitigate the cold draughts in a low energy way, utilising the heat gains in the space.

But many in the industry ask just how much mixing you can actually get from a louvre or opening window at high level.

Realistically you can hope to achieve 4 degrees of mitigation at best, so when it is 5 degrees outside you would still get incoming air at 9 degrees, which is too cold. We would encourage you to ask your natural ventilation advisor to share their science and if they can’t or won’t, come and talk to us.

Cold draughts are a real problem with natural ventilation, and we know that curing this problem by passing the incoming cold fresh air over a heating element is just nonsensical in terms of energy use. The heat gains in most non-domestic buildings far outweigh the heat required to maintain an average space temperature of say 21°C in winter for any external temperature above 6°C.

The problem is that if air cooler than around 16°C falls onto you then you will know it! It is just too cold to handle.

One concept which is being discussed in the industry is the use of high level opening vents for winter ventilation. The idea relies on the incoming cold fresh air mixing naturally with the interior warm air before it reaches occupants. A nice idea – but does it work?

The first problem we have in many rooms is that the floor to ceiling height is around 2.8m.

Secondly, 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.

Finally, if you are sitting 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 that 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°C?”

What is the result? See the graph below for two limiting cases of a cooler office with minimum ventilation requirements and a single window, and a warmer classroom with lower fresh air rates and a wider window.

The over-riding conclusion is that if you want to ensure fresh air reaches occupants no colder than 16°C, opening windows are fine, but only when the exterior temperature is above 13-14°C.

Alas, as we spend so much of the time in the UK with external temperatures below this level, then unfortunately the opening window strategy won't work – you will get cold draughts.

This is part of the reason many schools which just have opening windows are simply not ventilated in winter. The research findings in 2005 of the Building Research Establishment, who studied 8 primary schools with opening windows, found that in winter more than half the fresh air rates were below the minimum required.

Something else is needed – hence why the E-Stack® natural ventilation system was developed.

Click the PDF image to view the technical information in full

natural ventilation

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