How Single Room Heat Recovery Extractor Units can Reduce Mould and Improve Air Quality

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1. Why do I need to know?

The air quality in a number of homes and offices is poor, especially in cold weather. This can be particularly bad in social housing as was shown by the Energy Performance Challenge project in Salford. Whilst the installation of a MVHR system is the obvious answer for new-builds, the installed cost of such systems can be off putting. For the existing housing stock, refurbishments, retro-fits or where the cost of MVHR is not affordable, an alternative solution is the installation of Single Room Heat Recovery (“SRHR”) extractor units. These with the right Building Management Systems (“BMS”) control system can reduce the risk of moulds by monitoring Relative Humidity (“RH”), reduce carbon dioxide (“CO2) levels and improve air quality.

2. How it works?


SRHR’s extract the stale air from a room, passing the outgoing air through an efficient air-to-air heat exchanger. The better ones use an EC motor to drive the fan with a 0-10 volts DC control input that controls the fan speed from  zero revs. per minute to a maximum of no more than 900 revs. per minute. SRHR’s,

2.1. work best

2.1.1. if they are mounted high on a wall to prevent stratification of the stale air, especially where moulds are likely to form

2.1.2 controlled via a BMS to control the fan speed and the circumstances when the fan is used

2.2. are simple to install by coring a 100mm hole though to the outside wall.

2.3. can be installed for high rise apartments from the inside.

3. Show me real-world examples

3.1.  The pictures below show the installation at the offices of Tensor plc in Cambridgeshire.

3.2. In this example we have used a HeatingSave BMS system to control the CO2 within a room. Although we were particularly interested in CO2 it could also control RH or  volatile organic compounds in the atmosphere. Our objective in this project was lower CO2 to help people stay alert whilst performing repetitive office task on a computer system. Post installation, employees reported that they felt less tired and fresher, especially in the afternoons.

3.3. In a domestic housing situation the EPC project showed that the highest concentrations of CO2 (and VOC’s) occurred in the lounge in the evening and the main bedroom at night. In several examples the CO2 fell dramatically from 2,300 ppm to less than 500 ppm within a 4 minute period at around 6am. This indicated that the occupant opened the window to let in fresh air when they awoke. The point here is that there are some sections of society who live in houses with bad air quality; made worse if they smoke. Whilst asleep at night these people are unaware of the poor air quality, which can only exasperate possible long term health issues. It’s simple and relatively inexpensive to rectify this situation with SRHR.

3.4. We used the smart controls of a HeatingSave BMS to run the SRHR fan speed at the maximum acceptable to the occupants. In the tests in a bedroom and night and a lounge in the evening, the HeatingSave system measured the trigger points of CO2, RH, VOC and sound levels. The latter was measured in dB’s of Sound Pressure Level (“SPL”). The HeatingSave software algorithm triggered the fan speed. Here, the fan ran more slowly in the bedroom at night so as not to wake the people sleeping there, whereas the fan could be run at a higher speed in the lounge when the television was on. When the dB dropped because the TV turned off and people we just talking the BMS lowered the fan speed accordingly.

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