Underfloor heating with MVHR ventilation

MVHR stands for mechanical ventilation with heat recovery. A central unit extracts stale, moist air from kitchens, bathrooms and utility rooms, supplies filtered fresh air to living rooms and bedrooms, and passes the two airstreams through a heat exchanger so the incoming air picks up warmth from the outgoing air. It runs all the time.

Underfloor heating (UFH) suits the same kind of home: well insulated, airtight and often heated by a heat pump. This guide explains what each system does, why you usually still need heating with MVHR, what to plan in the floor, and how to commission and control both.

Do I still need heating if I have MVHR?

Yes, in almost every UK home. MVHR cuts the heat you lose through ventilation, but it does not add heat of its own. A typical unit recovers most of the heat from the air it extracts, and the rest of the home's heat loss through walls, floors, roof and windows still has to be made up.

The exception is a certified Passivhaus. The Passivhaus standard limits the heating load to 10W/m², low enough that the home can be heated by warming the fresh supply air alone. Most new UK homes are built to Building Regulations, not Passivhaus, and need a heating system sized from a room-by-room heat loss calculation.

Our guide to the UFH heat loss calculation shows how ventilation loss is included. With MVHR, the designer uses the reduced ventilation loss, which makes the UFH design smaller.

When does a new home need MVHR?

MVHR is one of the ventilation systems Approved Document F gives guidance for, and it can be used in a home of any airtightness. The more airtight the home, the more likely it is to need mechanical ventilation.

• Highly airtight homes. Approved Document F treats a home as highly airtight when its design air permeability is below 5m³/(h·m²) at 50Pa, or its tested result is below 3m³/(h·m²). Natural ventilation is not suitable for these homes without expert design advice.
• MVHR requirements. BEAMA's summary of Part F lists a minimum heat recovery efficiency of 73%, a summer bypass, a variable speed controller and ducted supply to every habitable room.
• No trickle vents. Background ventilators must not be fitted with MVHR.

Part F sets the minimum continuous ventilation rate by the number of bedrooms, with a separate floor area check that can raise it in larger homes. These are the bedroom-based rates for MVHR.

Bedrooms	Continuous supply and extract
1	19 l/s
2	25 l/s
3	31 l/s
4	37 l/s
5	43 l/s

Our guide to UFH, Part L and the Future Homes Standard covers the energy rules that push new homes towards this airtight, low-temperature approach.

How do underfloor heating and MVHR work together?

MVHR keeps the air fresh and limits ventilation heat loss. UFH heats the fabric of the room from the floor. Because MVHR cuts the heat loss, the UFH can run at a lower flow temperature, which suits a heat pump.

• Low flow temperatures. Approved Document L requires a new or fully replaced wet heating system to run at a maximum flow temperature of 55°C or lower. A low-loss home often runs well below that.
• Steady running. Both systems suit long, steady running. A deep screed floor with MVHR gives very stable room temperatures.
• Ventilation heat. In Passivhaus terms, ventilation without heat recovery would lose about 24kWh/m² a year, against a whole-home heating limit of 15kWh/m². Heat recovery is what makes very low heating demand possible.

Our guide to underfloor heating with a heat pump explains how to set flow temperatures and pipe spacing for a low-temperature system.

What do I need to plan in the floor?

Plan the MVHR ducts and the UFH pipes together, before either is installed. Both want the same floor and ceiling zones, and one cannot cross the other after the screed is poured.

1. Agree the duct routes. Decide which ducts run in ceiling voids, which in floor voids and where they drop through floors. Keep UFH loops and manifold tails clear of these routes.
2. Use rigid ducts. Part F guidance says use rigid ductwork where possible, with flexible duct only for end connections, no longer than 1.5m and pulled taut.
3. Set the door undercuts. Part F expects a 10mm gap under internal doors, measured above the final floor covering. Set it after you know the full UFH build-up and finish, or the gap will close up.
4. Keep valves off the thermostat. Do not put a supply air valve right above a UFH room thermostat or air sensor, or the fresh air can skew its reading.
5. Leave access. The MVHR unit needs access for filter changes, and the UFH manifold needs access for balancing and actuators. Do not box either in.

For build-up depths that affect door heights, see our guide to UFH floor build-up heights.

Can MVHR cause overheating in summer?

It can if the summer bypass is not working, because the heat exchanger keeps warming the incoming air. Part F requires MVHR to have a summer bypass, which sends air past the heat exchanger when the house is warm enough.

Passivhaus allows indoor temperatures above 25°C for no more than 10% of the time. In summer, switch the UFH off or into frost protection, and check the MVHR bypass opens automatically. Some units need a manual prompt. If you want to cool the floor with a reversible heat pump, see our guide to underfloor cooling.

How should I control underfloor heating with MVHR?

Give each room its own UFH thermostat and leave the MVHR to run on its own controls. Approved Document L expects thermostatic room controls in each room for a new heating system, or zones where two similar rooms share one.

• Thermostat position. Mount it on an inside wall away from supply valves, sun and appliances. Our guide on where to put a UFH thermostat has the details.
• MVHR boost. Use boost for cooking and showers. It does not need to link to the heating.
• Setbacks. In a steady low-loss home, small night setbacks work better than turning the heating off.

What about commissioning and maintenance?

Both systems must be commissioned before handover, and both need simple yearly care.

• MVHR commissioning. Part F makes commissioning a legal requirement. Airflow is measured at each terminal with a calibrated hood, balanced to the design rates and recorded on the Part F commissioning sheet. For a new dwelling, the sheet goes to building control within 5 days.
• Noise. Good practice is no more than 30dB in bedrooms and living rooms under normal running.
• Filters. Change MVHR filters when the unit asks, often every six months. One council's Passivhaus guide suggests the end of summer, when filters are most likely to be clogged.
• UFH. Check manifold pressure, flow meters and actuators each year. Our new build UFH sequence guide shows where UFH fits in the build programme.

Frequently asked questions

Can MVHR heat a house on its own?

Only in a very low-energy home such as a Passivhaus, where the heating load is no more than 10W/m². In most homes MVHR reduces heat loss but a heating system is still needed.

Does MVHR make underfloor heating cheaper to run?

It reduces the heat lost through ventilation compared with an extract-only system, so the UFH has less heat to supply. The saving depends on the home, the airtightness and the unit's heat recovery.

Can MVHR ducts go in the same floor as UFH pipes?

They can share a floor zone if the routes are planned first. Keep ducts and pipes apart, and never drill or fix through a screed or deck with UFH pipe in it.

How big a gap do I need under internal doors with MVHR?

Part F expects a 10mm undercut on a 760mm door, measured above the final floor covering. Set it after the UFH and finish are down.

Should the MVHR be switched off in summer?

No. Keep it running for fresh air. The summer bypass stops it warming the incoming air when the house is already warm.

Do I need a heat pump for UFH with MVHR?

No, UFH can run from a boiler. A heat pump is a common choice in airtight homes, because the low heat loss lets both the UFH and heat pump run at low temperatures.

Sources: BEAMA guidance on Part F for dwellings in England, LABC: air permeability testing and ventilation strategy, Approved Document L Volume 1, Encyclopedia: Passivhaus criteria, Norwich City Council Passivhaus occupancy leaflet