MVHR (mechanical ventilation with heat recovery) is a whole dwelling ventilation system that supplies and extracts air continuously at a low rate with the facility to be boosted as required meeting Building Regulations Part F System 4.

The unit is normally installed in the loft space or cupboard and rigid ducting supplies fresh filtered air to the habitable rooms and extracts stale polluted air from the ‘wet’ rooms. Supply and extract diffusers are fitted to the ceilings and are adjusted to balance the system.

The unit incorporates a polymer heat exchanger that tempers the incoming air before it is delivered to the habitable rooms. The efficiency of the exchanger can vary from 70% to 95%  heat recovery depending on whether it is of cross-flow or counter flow type, counter flow being the most efficient. Building Regulations are now calling for increased thermal
efficiencies therefore the 95% heat exchanger is the best option.

The system can be controlled to boost speed when moisture is being generated by bathing or cooking either; 
- Manually via single or multiple switches
- Automatically, typically via humidity or other sensors
These should be located in or near the wet rooms. Background ventilators in windows are not required with this system.

For housebuilders energy efficiency is still the most important specification factor when it comes to ventilation as they seek the most cost-effective way to achieve carbon reductions and reduce dwelling emission rates (DER). Specifying an MVHR unit which boasts low energy usage and high thermal efficiency is key to reducing a home’s DER.

As consumers have become more informed of the health implications of IAQ MVHR is coming to the fore as the ventilation solution of choice. “The Future of Indoor Air Quality in UK Homes and its Impact on Health”, a recent report by Professor Hazim Awbi, at The University of Reading, states the health risks associated with poorly ventilated homes in no uncertain terms. The research cites MVHR as the most cost-effective solution for achieving an energy efficient air exchange rate and a healthy environment for building occupants.