MEV, dMEV, MVHR and PIV, explained

What is MEV (mechanical extract ventilation)?

MEV, or mechanical extract ventilation, is a whole-house system with a single central fan, usually in a loft or cupboard, that continuously extracts stale, damp air from kitchens, bathrooms and utility rooms through ducting. Fresh air enters passively through background ventilators such as trickle vents. It has no heat recovery and suits reasonably airtight homes.

MEV is one of the continuous systems recognised in Approved Document F. It runs at a low background rate all the time and boosts when a wet room needs more extraction. Because the incoming air is passive, through vents rather than fans, MEV is simpler and cheaper than MVHR, but it does not recover any of the heat leaving with the extracted air.

Sources: GOV.UK

What is dMEV (decentralised mechanical extract)?

dMEV, or decentralised mechanical extract ventilation, does the same job as MEV but with a separate small continuous-running fan in each wet room instead of one central unit and ducting. Each fan runs quietly at a low background rate and boosts when needed. It is simpler to install, especially in flats and retrofits, and also has no heat recovery.

Because dMEV avoids long duct runs, it is often the easiest mechanical system to fit into an existing home or a flat. The trade-off is a fan in each wet room rather than one central unit, and, like MEV, no heat recovery. Fresh air still enters passively through background ventilators in the habitable rooms.

Sources: GOV.UK

What is MVHR (mechanical ventilation with heat recovery)?

MVHR, or mechanical ventilation with heat recovery, is a balanced system that both extracts stale air and supplies fresh air through separate ducts, passing the two air streams through a heat exchanger so most of the outgoing heat is recovered. It is the only one of the four systems with heat recovery, and it suits airtight new builds and deep retrofits.

MVHR is the most capable of the four systems and the only one that both supplies and extracts air mechanically. That balance, plus the heat exchanger, is what lets an airtight home stay ventilated in winter without cold draughts or the full heat loss of an open window. It is also the most expensive and the most involved to install, because it needs full supply and extract ducting. See our full guide to what MVHR is.

Sources: GOV.UK

What is PIV (positive input ventilation)?

PIV, or positive input ventilation, is a single unit, usually in the loft, that gently pushes filtered fresh air into the home, creating a slight positive pressure that drives stale, damp air out through gaps in the building. It does not extract from wet rooms and has no heat recovery, though some units add a small heater. It is popular for tackling condensation in existing homes.

PIV works with the leakiness of an older home rather than against it: it relies on air being able to escape through gaps, so it is not suited to a very airtight house where there is nowhere for the stale air to go. It is low-cost and simple to fit, which is why it is often recommended as a damp and condensation fix, but it is a different approach from the extract and balanced systems above.

What is the difference between PIV and MVHR?

PIV and MVHR are often confused but are very different. MVHR is a balanced, ducted system that extracts and supplies air and recovers heat, and is best for airtight homes. PIV is a single loft unit that only pushes air in, has no heat recovery, and relies on a leaky building to let stale air out. PIV cannot meet the same standards as MVHR in an airtight home.

This is the single most common mistake in consumer ventilation content, so it is worth being clear. If a home is airtight enough to need controlled ventilation, PIV is usually not the answer, because there is nowhere for the stale air to escape and no heat is recovered. MVHR, or a continuous extract system, is the appropriate route. PIV earns its place in older, leakier homes with a condensation problem.

Which ventilation system do I need?

It depends mostly on how airtight your home is. Very airtight new builds and deep retrofits usually need MVHR to ventilate well without losing heat. Reasonably airtight homes can use MEV or dMEV. PIV is a low-cost option for existing, leakier homes, often to tackle condensation, but it is not a heat-recovery system. Our system selector tool walks through your property.

Cost, disruption and the age of the home all matter too. In a new build, ducting for MVHR can be designed in cheaply, so the heat recovery is usually worth it. In an existing home, retrofitting full MVHR ducting is disruptive, so dMEV or PIV may be more realistic. The right answer follows from your airtightness, your budget and whether you are building or improving.