Mechanical Ventilation efficiency is not only about Heat Recovery

May 5, 2020

In the UK current Building Regulations prescribes an air-tightness level of 5m3/m2/hr@50pa. If we achieve that as a minimum and also wish to preserve the fabric of the building and a healthy living environment for the occupants of the building then we need to introduce controlled ventilation.

The efficiency of the Mechanical Ventilation systems depends on a number of things: Firstly there is the recovery of the heat from the stale air that is being exhausted from the property. The heat exchange efficiency on Mechanical Ventilation Heat Recovery (MVHR) systems ranges from around 70% to over 95% and this is the figure that is usually quoted in most advertising and sales pitches to prove system performance, but there are many other factors that can, and do, influence the efficiency of a mechanical ventilation system.

Many modern mechanical ventilation systems use low energy, high efficiency direct current fans with backward curved centrifugal type impellers and any resistance to air movement will increase the energy consumption of the fans.

Ducts should ideally always be constructed from rigid, or semi rigid, materials if possible. Any flexible ductwork should only be used to connect a rigid duct to either a room terminal or the ventilation unit to assist with misalignment and acoustic separation. It should be kept to a maximum of 300mm and the duct should also always be installed and fixed at 90% of its maximum length. Any ducts that are connected to the outside of the property, as well as any that go through a cold space such as an un-insulated loft, should be insulated to avoid condensation and potential damage to the fan and filters. Filters and air inlet and discharge grilles should also be periodically checked for build-up of dirt.

On the supply side of the fan it is best to consider the layout of the property and design the ductwork to have the minimum length and number of bends and also use the bore that is a minimum of at least the same size as the fan spigot and is also appropriate for the air flow rate required for that part of the dwelling.

Extract air should always be from the areas of high humidity such as bathrooms and kitchen / utility. It is also important to make sure that replacement air can enter the room. For extract only ventilators this may be an appropriately sized trickle vent in the window, air-brick in the wall or an undercut on the door. MVHR systems, however, rely on fabric air-tightness and a balance of air movement between inlet and extract terminals so a pathway is required for air to pass between rooms, usually in the form of an appropriate undercut on the door of around 9mm on an 838mm door.

In any ventilation system, if the free movement of air is restricted, either on the extract or supply side of the system, the fan will use more power and the system will be a lot less efficient.