The written version
MVHR: swap the air, keep the heat
These labs are a visualisation aid to help you picture how a house works. They are not technical advice: the models and figures are simplified, so please don't rely on them for design, specification or buying decisions. Ask a qualified designer, engineer or installer about your own home.
A well-sealed house still has to breathe. Mechanical ventilation with heat recovery (MVHR) takes stale, damp air out of the kitchen and bathroom, brings fresh air into the living room and bedrooms, and passes the heat from one airstream to the other on the way.
How MVHR works, step by step
Build tight
Good new homes are airtight. A continuous airtight layer wraps the heated part of the house: a membrane, a plaster coat or boards with taped joints. It stops draughts and heat loss, and it also traps the CO₂ and moisture people make. Switch the ventilation off in a tight house and CO₂ passes 1,000 ppm within about 3 hours, humidity climbs and the windows start to fog.
Fresh in, stale out
Supply valves bring fresh air into the living room and bedrooms. Extract valves pull stale, damp air from the kitchen, bathroom, utility and WC. In between, the air drifts through the hall and landing and under the internal doors, which each need a gap of about 10 mm.
The heat exchanger
Inside the unit, outgoing stale air and incoming fresh air pass each other between thin plates, flowing in opposite directions. Heat crosses the plates and the airstreams never mix. On a 0°C evening, with the house at 21°C and 120 m³/h moving, fresh air reaches the rooms at about 19°C. That keeps around 760 W of heat in the house for 25 W of fan power, about 90% of the heat the stale air would otherwise carry away. Approved Document L asks for a heat recovery efficiency of at least 73%.
Life happens
A shower puts a few hundred grams of water into the air. A humidity sensor boosts the fans and the steam leaves through the bathroom ceiling valve before it can settle as condensation. Cooking works the same way at the kitchen valve, and because air keeps moving from the living room towards the kitchen, smells stay where they started.
Summer bypass and cooling
After a hot day the house can sit at 25°C. On a cooler night the summer bypass sends fresh air around the heat exchanger, so it comes in cool and flushes the heat out: around 360 W of free cooling at 16°C outside. On a hot day the core works the other way and takes some heat out of the incoming air. MVHR is not air conditioning; shading and night cooling do most of the work.
Running costs, noise and filters
The fans run day and night on about 25 W, roughly 220 kWh a year, or about £60 at the October 2026 price cap. They should be quiet: Approved Document F's guidance is no more than 30 dB in bedrooms and living rooms at the normal running rate. Change the filters every 3 to 6 months, or as the unit's maker advises. You can still open a window whenever you like.
How airtight a house needs to be for MVHR
Heat recovery only works on air that goes through the unit. Air leaking out through gaps in the walls and roof takes its heat with it, so MVHR earns its keep in a tight house: around 3 m³/(h·m²) at 50 Pa or better. The model assumes a very tight house, close to Passivhaus, that leaks about 10 m³/h against 120 m³/h through the unit, so almost all the outgoing air passes the heat exchanger. Approved Document F calls a home highly airtight when its design air permeability is below 5 m³/(h·m²) at 50 Pa or its tested result is below 3. Its guidance for those homes is continuous mechanical ventilation, either MVHR or continuous extract, and it keeps trickle vents with intermittent fans for leakier homes.
The parts of an MVHR system
In the house: valves and air paths
- Airtight layer
A continuous membrane, plaster coat or board with taped joints, wrapped around the whole heated volume. It stops draughts, so heat and moisture only leave where the ventilation system decides.
MVHR pays off in a tight house, around 3 m³/(h·m²) at 50 Pa or better. Passivhaus asks for 0.6 air changes an hour at 50 Pa.
- Living room and bedrooms
Habitable rooms get fresh air. It enters at close to room temperature, low and slow, so it doesn’t feel like a draught.
- Supply valve
Fresh, filtered, pre-warmed air arrives in the living room and bedrooms through adjustable valves. They are set during commissioning to give each room its share.
Part F (2021): a 3-bed home needs at least 31 l/s of whole-house ventilation (about 112 m³/h). Noise limit in bedrooms and living rooms: 30 dB.
- Hall, stairs and landing
Air passes through circulation spaces on its way from the supply rooms to the wet rooms. They usually get no valves of their own.
The air path runs one way through the house: bedrooms first, bathroom last.
- Door undercut
With every door shut, air still has to travel from bedroom to landing to bathroom. A gap under each internal door lets it through. Fit carpets and door seals with that gap in mind.
Part F asks for 7,600 mm² of gap per door: about 10 mm under a standard 760 mm door.
- Kitchen and bathroom
Wet rooms are where air leaves the house. Steam and smells are taken out at the source and don’t spread to the rest of the house.
- Extract valve
Stale air is taken from the kitchen, bathroom, utility and WC, where moisture and smells are made. Nothing needs a separate extractor fan.
Part F (2021) continuous extract rates: kitchen 13 l/s, bathroom 8 l/s, utility room 8 l/s.
- Windows
You can still open them whenever you like. With MVHR you don’t have to, so in winter the heat stays in and the street noise stays out.
Fog on the glass is a sign of damp air.
In the loft: unit, ducts and terminals
- MVHR unit
Two fans, two filters and a heat exchanger in one insulated box. It runs all day, every day, at a low and steady speed, and boosts when the kitchen or bathroom gets steamy.
Best placed inside the warm, airtight envelope (here, a warm loft under an insulated roof) so the ducts stay warm.
- Supply and extract ducts
Round semi-rigid ducts run from the unit to each room valve, usually in floor voids and service zones.
Short, smooth runs keep fan power and noise down. Ducts outside the warm envelope need thick insulation.
- Outside-air ducts
The two ducts between the unit and the roof carry cold air. They are insulated and vapour-sealed, or warm moist air would condense on them and drip.
Keep these runs short: put the unit close to an outside wall or roof.
- Intake and exhaust terminals
Fresh air comes in through one, stale air leaves through the other. They are kept well apart so the stale air isn’t sucked straight back in.
Follow the manufacturer’s separation distance; a few metres is common.
Inside the MVHR unit
- Counterflow heat exchanger
Dozens of thin plates make alternate channels. Stale air runs one way, fresh air the other, a plate’s thickness apart. Heat passes through the plates; the two airstreams never mix.
Running the streams in opposite directions lets the fresh air leave almost as warm as the stale air arriving.
- Supply filter
Cleans outdoor air before it reaches the rooms. A fine filter (ISO ePM1, the old F7) catches pollen and fine dust, which hay-fever sufferers notice.
Change every 3–6 months, or as the maker advises. A clogged filter makes the fans work harder and louder.
- Extract filter
A coarser filter (ISO Coarse, the old G4) on the stale-air side. Its job is to keep kitchen grease and fluff off the heat exchanger plates.
Cheap, quick to swap, and the main maintenance task on an MVHR system.
- Fans
Two EC fans, one per airstream, kept in balance so the same volume goes out as comes in. Balanced flow keeps the house at neutral pressure and the heat exchanger at its best.
A good unit uses well under 1 W per l/s moved.
- Summer bypass
A flap that lets fresh air go around the heat exchanger. On a summer night, cool outside air comes straight in instead of being warmed back up by the stale air leaving the house.
On Auto it opens when the house is warm and outside is cooler. In winter it should stay shut, or you throw the heat away.
- Condensate drain
In cold weather the outgoing air cools below its dew point inside the core and drops water, like breath on a cold window. A tray and trapped drain carry it away.
A loft unit needs a drain with a fall and a trap.
- Frost protection
In hard frost the condensate on the exhaust side can freeze and block the core. A small electric pre-heater warms the incoming air to just below zero, or the unit rebalances its fans.
In this model it comes on below −3°C outside.
The model is a 3-bed, two-storey house of about 100 m² with a very airtight envelope, close to Passivhaus, and a counterflow heat exchanger. Its sums are simple but use real units, so the numbers are the right size rather than exact for any one home.
Questions people ask
How does MVHR work?
MVHR swaps stale air for fresh air through a heat exchanger, so the warmth in the air leaving the house heats the air coming in. Fans pull damp air out of the kitchen, bathroom and WC and push filtered outdoor air into the living room and bedrooms. The two airstreams pass each other between thin plates and never mix. In the lab's model, at 0°C outside and 21°C inside, fresh air reaches the rooms at about 19°C and around 760 W of heat stays in the house.
Is MVHR worth it in a new build?
MVHR makes most sense in a new build that is airtight, around 3 m³/(h·m²) at 50 Pa or better, because most of the air leaving the house then passes the heat exchanger. In a leakier house more of the heat escapes through gaps instead. In the lab's model the fans use about 25 W, roughly 220 kWh a year, and on a 0°C evening keep about 30 W of heat in the house for every watt they use. The rooms also get filtered air without draughts.
Does MVHR cool the house in summer?
Only a little: MVHR has no refrigeration, so it can only cool the house with outdoor air cooler than the rooms. On a cool night the summer bypass lets fresh air skip the heat exchanger and flush out the day's heat: about 360 W in the lab's model at 25°C inside and 16°C outside. On a hot day the heat exchanger cools the incoming air a little. Approved Document L asks for a summer bypass on every MVHR unit. Shading and opening windows at night do most of the cooling.
Do you need trickle vents with MVHR?
No: Approved Document F says background ventilators, which include trickle vents, should not be installed with MVHR, to avoid unintended air paths. Air coming through a trickle vent misses the heat exchanger, so its heat is lost. Fresh air arrives through the supply valves instead, and the windows still open for a quick airing. The 2021 edition and the 2026 edition, which applies from 24 March 2027, give the same advice in paragraph 1.72, so windows for an MVHR house are ordered without trickle vents.
How often should MVHR filters be changed?
Typically every 3 to 6 months, according to the Passivhaus Trust, which also says to check the manufacturer's advice for your unit. Filters clog faster near busy roads, during building work nearby and in the pollen season. The supply filter is a fine ISO ePM1 grade that catches pollen and fine dust; the extract filter is a coarser grade that keeps kitchen grease off the heat exchanger. When a filter clogs, the fans have to push harder, which uses more electricity and makes more noise.
Does an airtight house need MVHR?
Not necessarily: an airtight house needs ventilation that runs all the time, and MVHR is one of the two systems Approved Document F covers for it. The other is continuous mechanical extract ventilation (MEV), which pulls stale air out of the wet rooms and lets fresh air in through trickle vents, so it recovers no heat. Trickle vents with intermittent fans are only for leakier homes. Without ventilation a tight house gets stuffy quickly: in the lab's model, with the fans off, CO₂ passes 1,000 ppm within about 3 hours.
