Your Guide to Mechanical Extract Ventilation (MEV)

Your Guide to Mechanical Extract Ventilation (MEV)

By Ruth MacEachern

Product Manager

There are two main kinds of multi room ventilation systems that are installed in homes, and while they are both highly effective at reducing condensation, damp, mould, and improving air quality, the way they work is fundamentally different and they are suited to different uses.

The two main types of whole house ventilation system are:

• Continuous Mechanical Extract Ventilation (MEV)
• Positive Input Ventilation (PIV)

The two types of ventilation system work in opposite ways to each other to achieve the same result.  While positive input ventilation systems work by introducing air into the building through a central unit and then distributing it throughout the building, Continuous Mechanical Extract Ventilation (MEV) works by drawing the air directly out of wet rooms through a central system and expelling it from the building.  The moist, stale air is replaced with fresh air from outside.

Continuous Mechanical Extract Ventilation (MEV) systems are installed in lofts and have ducting running to rooms that are prone to elevated levels of humidity – such as a bathroom or kitchen.  They work in a comparable way to an extractor fan by drawing the air out of the room to create a negative pressure that will draw dry air from elsewhere in the building and preventing the moist air escaping into other rooms where it could cause condensation.

Despite running constantly to circulate air, MEV systems use little energy.  The systems include humidity sensors ant automatically can adjust the amount of airflow automatically to make them more effective while running as efficiency as possible.  Aside from their effectiveness, a major advantage of MEV systems over conventional extractor fans is that they produce a lot less noise because the fan system is mounted outside the room where the ventilation is needed.

How are MEV systems installed

MEV systems are not normally retrofitted into an existing property.  The installation process needs to be conducted by a professional, as the system needs to be fully planned, and holes will be needed to cut into brickwork to accommodate the intake and outflow air vents.  If you are undertaking a major refurbishment, an MEV system might be suitable, otherwise a positive input ventilation system may be more appropriate.

The central unit for the MEV system will be installed inside the loft with all ducting mounted inside walls and in roof spaces. 

Mechanical Ventilation with Heat Recovery

Many MEV systems are installed with heat exchangers that use the heat from extracted air to warm the incoming air.  This helps to reduce your energy bills as less energy is wasted.  In MVHR systems, additional ducting is used to control the inflow of air from outside so that it can be fed through the heat exchanger.  MVHR systems require a property to be almost airtight to ensure that as much heat is recovered as possible.

Find out more

If you are considering a major renovation to your existing property, or in the process of planning a new home, Continuous Mechanical Extract Ventilation should be considered thanks to the benefits to the building fabric and air quality that it offers.  For more information, you can arrange a free survey with one of our local specialists who can provide advice and help you with specifying a suitable system for your needs.