What is MVHR?
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 ceiling and are adjusted to balance the system.
The unit incorporates a heat exchanger (see diagram below) 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 vie single or multiple switches
- Automatically, typically via humidity or other sensors
Manual switches should be located in or neat in the wet rooms. Background ventilators in windows are not required with this system.
What is MEV?
Centralised mechanical extract ventilation (MEV) follows the same principle as MVHR except it does not include any heat recovery. A central unit is mounted in a loft or cupboard and ducted to extract polluted air from the wet rooms of the house. As there is no heat recovery, there is no requirement for any ducts to supply the replacement air. Instead, replacement air enters the home through window vents or airbricks. In many houses, however, air naturally enters the building through gaps and cracks in the structure, providing adequate ventilation.
The removal of pollutants, such as moisture, carbon dioxide, and external fumes are all important factors in maintaining good indoor air quality, helping to create a better living environment.
Some MEV units will include a humidity sensor, designed to increase the fan speed in proportion to the relative humidity level, saving energy and reducing noise.