Choosing a Mechanical Heat Recovery Ventilation Unit

This guide covers the fundamental questions that people ask when looking at purchasing a Mechanical Ventilation with Heat Recovery unit for their property. 

What is MVHR?

Benefits of MVHR

Retrofitting an MVHR unit

Installing an MVHR unit

Is MVHR suitable for you?

What is MVHR?

Mechanical Ventilation with Heat Recovery (MVHR) is a whole house ventilation system that both supplies and extracts air throughout a property. A heat recovery system is designed to extract moist stale contaminated air and supply  fresh filtered air to the occupied space to continue the desired level of comfort. The heat recovery system keeps the house fully ventilated by recovering the heat which  being extract from the inside environment. Heat recovery systems typically recover  in excess of 85% of the heat  being extracted and have significantly improved the energy efficiency of buildings.  MVHR is generally the most efficient ventilation system.

• Habitable rooms are provided with fresh air while stale moist air is extracted from wet rooms and kitchens on a continual basis.
• Before the extracted air is exhausted out of the building, the heat in the air is transferred by means of a heat exchanger into the fresh filtered air, which is introduced into the building.
• The pre-warmed fresh air is introduced into all habitable rooms on a continual basis. Thereby the need to completely heat the fresh air as it enters the building is eliminated. Efficient systems typically heat cold outside air from 0ºC to 18ºC through the heat transfer, when the extract air is 20 ºC.
• No extract air is re-introduced or re-cycled, thus extracted germs and pathogens will not spread through the system.
• A gentle cooling effect in the warm season is achieved by a potential summer bypass. The cooling effect is limited and needs to be supplemented by cross-ventilation if there are excessive solar gains. However, some systems can be combined with active cooling (Zehnder ComfoAirQ cool).

All new-build properties require sufficient ventilation: that is, a system which works to remove any excess moisture by creating airflow. Typically, ventilation systems work by removing some of the air from a property and replacing it with air from outside., traditional ventilation systems remove heat, severely reducing the property’s energy efficiency and increasing the cost of heating.

Mechanical Heat Recovery Ventilation addresses that shortcoming by recovering the heat from any air extracted from a property. A typical MVHR system comprises a heat recovery unit, ideally installed in a central location, which is connected to every room in the property via a network of ducts, which open into the rooms via ceiling or wall vents.

Benefits of MVHR

• Continuous supply of fresh filtered air which provides good indoor air quality.
• No drastic CO2 peaks
• Elimination of bad odours due to constant circulation of fresh, filtered air around the property.
• Filtered air: the filter traps pollen easing hay fever symptoms of the property’s occupants. 
• Keeps damp, mould, fungus and dust mites in check.
• Active dehumidification in the cold season.
• There’s less noise as you can keep windows closed.
• Keeps outdoor pollutants and bugs out.
• Approximately 30% of the heating energy can be saved in airtight buildings with highly efficient MVHR systems compared to naturally (uncontrolled) ventilated buildings.
• Little energy is used to operate the system. 

Retrofitting an MVHR Unit

• One storey buildings/ bungalows are often the easiest ones to retrofit, as ductwork can possibly be installed in the loft area with minimal disruption.
• One and a half storey buildings with eave spaces also present a suitable service zone for ducting.
• For two storey buildings with a loft it is sometimes possible to run risers through in-built wardrobes or service shafts into the ground floor.
• Unlike engineered timber joists, solid timber joists cannot be drilled for duct runs within the floor zone. However, sometimes parts of the ceiling can be suspended.

Installing an MVHR unit

Heat recovery systems are relatively site-specific. It’s important to choose the right system for a property and to ensure that it’s installed correctly. The Building Research Establishment recently conducted a study into the installation of MHVR units. Their findings suggest that as many as 9 out of 10 heat recovery units in UK homes may have been imperfectly installed. Researchers found that many systems required subsequent changes to air inlet valves, ducting or insulation.

Most new build properties are highly suitable for MHVR, but it’s best not to take suitability for granted. While the ventilation system should perform well regardless of a property’s airtightness, optimum heat recovery performance requires an airtightness of less than5m³/(h.m²) @50Pa. The effects of MVHR are reduced if the property are above this. To a certain extent this is simple common sense: if your building is leaking heat, there’s relatively little point in retaining the heat extracted by ventilation alone.  There are requirements for minimum airflows within a dwelling depending on size, location and number of wet rooms.

Heat recovery systems are dependent on effective filtration. Filters help to maintain the efficiency of the heat exchange unit as well as ensuring consistently clean air inside the property. Over time, filters clog with the dirt and other substances they filter out. A clogged filter can no longer effectively clean the air, and, just as importantly, the heat recovery system will be unable to run at maximum efficiency. It’s exactly the same as a hoover: when the filter clogs, it can’t clean.

The first question for developers is how much filtration is required. The UK Pollution Map is a great resource for checking the levels of air pollution in your area. The higher the pollution, the more important your filtration system will be, and the more often filters will need changing. The second question is about building use. How can you ensure that your building’s occupants know how to change filters, and that they’ll make the change as often as necessary? If filters are simply allowed to clog and left clogged, your building may as well not have a heat recovery system at all.

MVHR units can usually be sited in a cupboard, attic or ceiling void, although best practice suggests that the unit should be installed within the building’s heat envelope. The most compact units can fit into a space the size of a kitchen cupboard. Aside from space, the most important consideration is noise. If properly installed, MVHR units are not noisy, but common sense is advised: a unit directly above a bedroom might disturb a light sleeper. Another noise factor is ducting: if the ducts are not the right size for the volume of air they handle, they will cause noise.

Is MVHR suitable for you?

Mechanical Heat Recovery Ventilation System will require some upkeep by building occupants. Unlike an extractor fan or a stack ventilation system, an MHVR is essentially a white good, a piece of technology that rewards careful use and maintenance. An important question for planners and developers is how to ensure that building occupants know how to use the system properly.

This consideration also affects installation. MHVR systems typically incorporate a control panel and/or sensors.. In an airtight building, an MHVR unit is the single crucial element controlling internal climate.

While the unit is the expensive and technical part of the heat recovery system, it is only a part, and as such it is only as good as the system as a whole. Ducting, filtration, insulation and installation are crucial factors which can make or break the efficiency of an MHVR system. The most efficient unit on the market will perform worse than the least efficient unit if it is poorly installed. The efficiency promised by a particular unit can only be reproduced in the installed environment if it is properly implemented as part of a well-planned system.

First of all, the unit needs to be the right size for your property. There can be no compromise on this. It may be tempting to install a pricier, more efficient unit and save on costs by downsizing: for instance, installing a top-of-the-range unit designed for a three-bedroom house in your four-bedroom new build. However, this decision would cost more than efficiency. The unit would struggle and may even become noisy.

Indeed, you should choose a unit which can ventilate your propertyoperating at around50-60%. Your unit should not be working at 100% all the time.
Choosing the right duct system is also crucial. The important factors are noise transfer, airtightness and diameter. Ducting should be the right size for your unit.