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Home ventilation

dMEV continuous-running extract fans

Quiet low-energy fans that run all the time and boost automatically when humidity rises.

  • From £345 inc VAT per fan
  • Free survey on the UK mainland
  • Airflow tested and recorded
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Slim continuous extract fan on a bathroom wall

A dMEV (decentralised mechanical extract ventilation) fan is a low-energy extract fan that runs continuously at a quiet trickle speed and boosts automatically when it senses humidity or a light switch. Because it never fully switches off, moisture is removed steadily instead of building up between showers.

How it works

dMEV fans replace ordinary intermittent fans in bathrooms, kitchens, utility rooms and WCs. On trickle speed they are barely audible and use very little power; on boost they reach the higher continuous-extract rates in Approved Document F (in England, at least 13 litres a second for a kitchen, 8 for a utility room or bathroom and 6 for a WC). Landlords and housing associations often choose them because tenants cannot switch them off and forget them.

Under Approved Document F, continuous mechanical extract systems need commissioning: the airflow on both trickle and boost is measured and recorded. We do that as part of the job, so you have a record for building control, a landlord file or a future buyer.

Options we fit

Bathroom and en-suite

Through-wall or ceiling fans with humidity-controlled boost.

Kitchen

Higher-rated continuous fans where a ducted hood is not practical.

Whole-home dMEV upgrade

Every wet room converted at once, with trickle vents checked in the habitable rooms.

Commissioned and recorded

Trickle and boost rates measured and written on a commissioning sheet.

Signs you need this

  • Occupants switch fans off because of noise
  • Mould keeps returning in a bathroom with a working fan
  • You manage rental homes and want ventilation that cannot be switched off
  • You are refurbishing several wet rooms at once
  • An energy retrofit has made the house more airtight
  • Building control has asked for continuous extract

Guide prices

Continuous extract fans (dMEV) guide prices
WorkGuide price
Continuous-running extract fan (dMEV) with humidity boost, flow tested (per fan)£345–£595 inc VAT
Airflow testing and commissioning sheet for extract fans (up to 6 fans) (per home)£145–£295 inc VAT

Guide prices using West Midlands (our baseline) rates. Home prices include VAT at 20%; business prices exclude VAT. A guide price is not a quote: we confirm a fixed price in writing first. London is about 20% higher and the North East about 8% lower.

What happens, step by step

  1. Survey

    We look at each wet room, the existing ducts and the trickle vents in the habitable rooms.

  2. Specify

    Fans chosen and set for each room size, with humidity or switch boost.

  3. Fit

    Existing fans removed, new fans fitted to existing or new ducts, wiring made safe.

  4. Commission

    Trickle and boost airflow measured in each room with a calibrated hood.

  5. Record

    Commissioning sheet and user instructions left with you.

The rules that apply

England

In England, ventilation is covered by Part F of the Building Regulations and its guidance, Approved Document F (2021 edition). For homes, it sets minimum extract rates such as 15 litres a second for a bathroom and 30 or 60 for a kitchen, and expects mechanical systems to be airflow-tested.

Wales

Wales has its own Part F ventilation guidance, published by the Welsh Government, and work is checked by building control. We design to the Welsh rates and commission mechanical systems so the results can be recorded.

Scotland

In Scotland, ventilation is covered by the Building (Scotland) Regulations and section 3 of the Technical Handbooks, and most building work needs a building warrant first. We work to the Scottish ventilation standards and record the airflow achieved.

Northern Ireland

In Northern Ireland, ventilation is covered by the Building Regulations (Northern Ireland) and Technical Booklet K. We design to the Northern Ireland guidance and measure the airflow achieved.

Questions people ask

How much does a continuous extract fan cost?

A dMEV fan usually costs £345 to £595 including VAT per room, supplied, fitted and commissioned. Converting several rooms in one visit brings the cost per fan down.

Is it expensive to run a fan all the time?

No. dMEV fans use very little electricity on trickle speed, typically a few watts, so running costs are low compared with the condensation and mould they prevent.

Are continuous fans noisy?

A correctly sized dMEV fan is very quiet on trickle; most people only notice it on boost. Noise usually means the wrong fan or a poor duct.

What is the difference between dMEV and MEV?

dMEV uses one small fan in each wet room. MEV uses one central fan, usually in the loft, ducted to all the wet rooms. Both run continuously.

Do I still need trickle vents?

Yes. With continuous extract, Approved Document F still expects background ventilators in habitable rooms so replacement air can get in.

Can dMEV fans be fitted where a fan already exists?

Usually yes. Most fit the existing hole or duct, which keeps the cost and mess down.

Related services

Modern bathroom with walk-in shower and wall extractor fan

Extractor fan installation

Bathroom, kitchen and utility fans sized to the Part F rates, ducted outside and tested.

From £175 inc VAT per fan

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