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Investigation of a Dysfunctional MVHR System

A high number of new homes are now being built with mechanical ventilation with heat recovery (MVHR) systems. These bring fresh air into the property throughout the year and remove humidity, providing high levels of indoor air quality. This is particularly advantageous when the building is close to a busy noisy road as the MVHR system provides adequate ventilation and a peaceful environment with windows closed.

Unfortunately, we are becoming increasingly aware of instances where coordination between the various sectors involved in the design, installation and commissioning has been very poor, causing critical issues with the systems.

September 2018

Enhabit was contacted by a home owner in a newly built apartment block with an MVHR system complaining of high levels of humidity in the bathroom despite the MVHR system being switched on. Enhabit carried out a system assessment, measuring air flows with calibrated equipment, taking photos of the ducting via recessed LED fittings and through the vent terminals. It became clear that the bathroom extract valve was actually supplying fresh air instead of extracting humidity from the bathroom. We opened ceilings to determine where the ducting was missing and found there was no duct run from the extract side of the unit to the bathroom. With the help of our installation team we were able to run a new duct and patch up the holes in the ceiling with barely any trace of an opening being made.

Figure 1 Diagnosed duct run with supply air going to a bathroom instead of extract

On the outside of the building it was impossible to locate where the outlets of the MVHR system were. It only became apparent after accessing the service riser that the outlets where fresh air comes into the building and exhaust air is blown out were installed in a most unusual configuration. They appeared to terminate with vertical rectangular ductwork all placed in a line. Exhaust and fresh air ducts were placed right next to each other. This can cause recirculation, when exhaust air is pulled straight back into the system. This explained why the owner had complained of cooking smells in the bedroom when she was cooking, despite closed doors and the two rooms being separated by a hallway.

Figure 2 Vertical configuration of external ducting, spaced close together causing recirculation

With the vertical configuration in mind, we assessed again the outside of the building and it then became apparent that these terminals where precisely hidden behind a square rainwater downpipe. The rainwater pipe appeared to be no more than 20mm away from the outlets. It would be very difficult for the exhaust air to get out of the building with such a large obstacle blocking the air path. It’s a real surprise to me that anyone with knowledge of a ventilation system would approve such a severe obstruction in the system. What was the reasoning behind this? A lack of planning permission to have grills on the façade? No wonder the systems were noisy in operation.

Figure 3 External ducting covers are blocked by rainwater pipe approx 20mm away from outlet terminals

This case and others like it highlight a real need for improvement in quality control standards for domestic ventilation systems and more education for home owners as to what to expect from a well designed system so that they can recognise when their system doesn’t meet requirements.

The basic requirements of an MVHR system

1. Accessible filters that can be easily changed once or twice a year.

2. External terminals for the MVHR system which are identifiable and separated by a horizontal distance of at least 1.5m or have bespoke cover to bring air flow in different directions.

3. An isolation switch where the system can be switched off for maintenance.

4. A control switch where the fan speed can be increased to boost mode.

5. The system shouldn’t be excessively noisy in normal operation.

6. There should be a commissioning certificate provided by a qualified ventilation specialist company.

Why we’re different

At Enhabit, we spend the required time liaising with the various trades and bodies involved in the project to make sure conflicts between services and issues with the design are ironed out early on in the design stage. We then follow the installation closely to make sure that all aspects of the design are practical and carried out with a final check during the commissioning process.

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