dMEV stands for decentralised mechanical extract ventilation. A dMEV fan is installed in a kitchen or bathroom like an ordinary extractor, but instead of only running while the light is on, it runs continuously at a very low, quiet trickle rate and ramps up automatically when it detects humidity or occupancy.
The advantage over an old intermittent fan is that moisture from showering, cooking and — importantly — drying laundry is removed as it is produced, including overnight, rather than being left to spread through the home and condense on cold surfaces. dMEV is a strong baseline upgrade for a property where whole-house supply ventilation such as MVHR is not practical.
Like all extract ventilation, dMEV needs a route for replacement air to enter the property — trickle vents, undercut internal doors, or airbricks. It only ventilates the rooms it serves, so bedrooms with condensation on external walls may still need PIV or improved background ventilation.