New homes are built to be far more airtight than older ones. That keeps heat in, but it also means fresh air no longer leaks in through gaps around windows, floors and doors. Ventilation has to be designed in instead, and in many new-builds and larger extensions that means mechanical ventilation with heat recovery, usually shortened to MVHR.
Because MVHR relies on ducts running to almost every room, it has to be planned alongside the layout, the lighting and the ceilings. This guide explains how it works and where each part goes, so you can plan for it before the plasterboard goes up.
How MVHR Works
An MVHR system runs continuously. It extracts stale, humid air from the rooms that produce moisture and supplies fresh air to the rooms you live and sleep in.
- Air is extracted from the kitchen, bathrooms, toilets and utility room.
- Fresh air is supplied to the living room, dining room and bedrooms.
- Both air streams pass through a heat exchanger in the central unit, where heat from the outgoing air warms the incoming fresh air.
- Filters in the unit catch pollutants such as pollen and dust before the fresh air reaches your rooms.
Many systems can also bypass the heat exchanger in warm weather, so the fresh air coming in stays cool rather than being warmed.
Why New-Builds Often Need It
Ventilation in England is covered by Part F of the Building Regulations, and its guidance is set out in Approved Document F. It says natural ventilation is not suitable for highly airtight homes: those with a design air permeability below 5 m³/(h·m²) at 50Pa, or an as-built result below 3 m³/(h·m²). Every new home has an air pressure test, so the result is measured, not assumed.
Continuous mechanical extract ventilation and MVHR can be used in any home. One practical difference for interiors is that a home with MVHR does not need background ventilators, the trickle vents usually fitted in window frames.
What Changes If You Are Building an Extension
An extension brings its own ventilation requirements. Under Approved Document F, a new habitable room needs a source of ventilation, such as a background ventilator or an extension of an existing ducted system. A new kitchen, bathroom or utility room needs extract ventilation.
If your house already has MVHR, extending the system into the new rooms is often the neatest answer, but it has to be planned while the structure is still open. Larger energy-efficiency renovations can also bring MVHR into play: where several major measures are added, the guidance says a continuous mechanical extract or MVHR system should be installed.
Where Everything Goes
- The unit. The central unit is often located in the loft. Wherever it goes, Approved Document F asks for reasonable access for maintenance such as replacing filters, so avoid a spot that can only be reached by moving stored boxes or crawling across joists.
- The ductwork. Ducts run from the unit to each room, typically through ceiling voids. Keeping the runs inside the insulated part of the house avoids having to insulate them. The guidance favours rigid ducting, with flexible sections kept to short end connections of no more than 1.5 metres, so allow enough depth in ceilings and boxing for rigid runs.
- The terminals. Each room gets a supply or extract terminal in the ceiling or wall. Plan their positions with your lighting layout so they sit neatly between downlights rather than wherever the duct happens to finish.
- The drain. Many MVHR units need a condensate drain, depending on the model, which affects where the unit can sit in relation to your plumbing. Check the manufacturer’s instructions for the unit you choose.
The earlier an installer sees the drawings, the more choice you have over where all of this goes. If you are planning a new-build or extension in the capital, a company that handles MVHR installation in London, such as JP Air Conditioning, can plan the unit position and vent placement as part of the project rather than fitting around finished ceilings.
Commissioning and Handover
A system that has been fitted is not finished until it has been commissioned. Approved Document F says the airflow must be measured at each terminal with a calibrated instrument and the system balanced to the design rates. The results go on a commissioning sheet, which is sent to building control within 5 days for a new home, or within 30 days for other work.
At handover you should receive clear, non-technical instructions on operating and maintaining the system, plus a copy of the completed commissioning sheet. A new home also comes with a Home User Guide that includes a section on ventilation. Keep all three, because you will need them when the system is serviced.
Living With MVHR
MVHR needs regular maintenance so the fans keep running correctly and the filters stay clean. Note when the filters were last changed, and do not switch the system off to save energy, because it is designed to run continuously.
Planned at the design stage, MVHR is barely noticed once the home is finished: a few discreet ceiling terminals, a unit tucked away in the loft, and fresh, filtered air in every room.
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