Mechanical ventilation with heat recovery (MVHR) explained
MVHR is a multi-room ducted system that combines supply and extract ventilation in one solution. It continuously provides fresh air to habitable rooms whilst pre-warming it with recovered heat from the extract air which would otherwise have been vented outside and therefore lost.
Warm moist air is extracted from wet rooms such as bathrooms and kitchens through ductwork to a central unit. Supply ventilation air from outside the home is passed through a heat exchanger in the central unit and warmed by the heat in the extract air.
MVHR systems are able to recover around 90% of the heat that would otherwise be lost (measured in accordance with the 2005 SAP Appendix Q test). The warmed air is then distributed throughout the home by ductwork. Air valves at ceiling level ensure that draughts are avoided.
The system runs most of the time at a low background rate but when more rapid ventilation is required because of increased moisture generation (caused for example by cooking, or extra density of occupation), the system is switched to a boost rate, either manually or by sensor control.
How a whole house MVHR works
Typically, 20%-30% of your space heating energy is lost through ‘natural’ ventilation; trickle vents in windows, extractor fans in wet rooms, and gaps in the building fabric. Rather than dumping warm air out of the building in the name of ventilation, an MVHR draws stale moist air from kitchens, bathrooms and utility rooms into a heat exchanger.
At the same time, another fan in the MVHR draws fresh air from outside, which is filtered, then gently fed into bedrooms and living spaces. In a well insulated building, the heat exchanger transfers around 90% of the heat from the old air to the new, so if the temperature of the stale air leaving the bathroom is 20°C, the temperature of the fresh air supplied to the living rooms is around 18°C. Your boiler just needs to top this back up to 20°C rather than warming up a whole house full of cold (unfiltered) air. An MVHR, therefore, reduces your heating bills and improves your air quality. Dry air also costs less to heat than moist air.
As spiralling energy costs start to exceed mortgage repayments, the need to conserve heat and power becomes increasingly important. To really cut the costs, your house needs to become airtight. Ventilating an airtight house by fitting vents (holes) in the window frames and sucking warm air out of wet rooms defeats the object, so the only way to ventilate an efficient home is a balanced (supply/extract) whole house MVHR system. At least that’s our opinion and the Energy Saving Trust seem to agree.
dMVHR Units – single room
In addition to providing whole house MVHR units, we can provide a range of single room dMVHR units to replace your traditional extractor fan. As a balanced unit that supplies and extracts air from any given room, the decentralised dMVHR fan maintains the internal air pressure to avoid drawing cold air in through window trickle vents.
Single room dMVHR units for smaller rooms slot through either a 100mm or 150mm hole in the exterior wall, so are relatively easy to install, provided power is available. dMVHR units for larger rooms typically require a square aperture so require a little more planning.
How ‘green’ are MVHRs?
The MVHR units offered by our company in the Algarve are made largely from recycled materials. Where possible, we source products from UK manufacturers and employ local expert installers. The motors are extremely low powered and typically require less electricity than a regular light bulb. Filters require very little maintenance and only need cleaning with a vac, once or twice a year. Replacement of filters is every 2-5 years. All units have a trickle and boost setting, if required, not that you’ll need to control the system on a day-to-day basis.
An MVHR provides
■ Fresh air throughout
■ Lower heating bills
■ Air filtration and cleaning
■ Less traffic noise
■ Humidity control
■ Cooling in summer
An MVHR removes
■ Water vapour, moisture and mould
■ Dust and airborne allergens
■ Cooking odours
■ Cigarette smoke
■ CO2 and Methane
■ Formaldehyde and VOC’s
By Kevin Conroy
Kevin Conroy is partner of the Sustainable Superstore and owner of MPB Services, offering Insulation Solutions, Construction and Project Management. 915 239 933