The written version
How MVHR works
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.
The tour, in words
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 swap
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.
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 nights
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.
Living with it
The fans run day and night on about 25 W, roughly 220 kWh a year, and should be quiet: Part F sets 30 dB as the limit in bedrooms and living rooms. Check the filters every few months and replace them at least once a year. You can still open a window whenever you like.
The parts
In the house
- 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
- 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 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.
Check every 3–6 months, replace at least once a year. 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 an airtight envelope 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.
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