Multivent Systems

How a Multivent MEV system works

The unit sits centrally, usually in the loft or a cupboard, with a duct running from each wet room to its own spigot. It runs all the time at a low trickle rate, which keeps moisture from building up, and switches to a higher boost rate when a light is switched on or, on humidity models, when the air in a room gets damp. Fresh replacement air comes in through trickle vents in the windows of the living rooms and bedrooms. Because there is one fan instead of several, there is only one motor to power, one unit to service and far less noise in the rooms themselves.

Frequently asked questions

What is a Multivent extractor fan?

Multivent is Vent-Axia's name for its central mechanical extract ventilation (MEV) units, and it has become the everyday term for any multi vent extractor fan of this type. Instead of a fan in every room, one unit with several extract spigots is ducted to each wet room and runs continuously at a low trickle speed, boosting when a room is in use or humidity rises. The stale air is discharged through a single duct to outside. Greenwood's equivalent is the CMEV central ventilation unit.

MEV, MVHR or PIV: what is the difference?

MEV extracts only. It pulls moist air out of the wet rooms and replacement air comes in through trickle vents in the windows, so there is no supply ducting and no heat recovery. MVHR extracts and supplies through two duct networks and recovers heat from the outgoing air, which makes it the better choice for airtight new builds but a much bigger installation. PIV pushes filtered air in from the loft rather than extracting. For a house with trickle vents where you want every wet room covered by one quiet unit, MEV is usually the simplest and cheapest compliant option. See our whole house MVHR and PIV units for the alternatives.

Does a central MEV unit meet Building Regulations Part F?

Yes. Continuous mechanical extract is one of the ventilation systems recognised in Approved Document F, often referred to as System 3. For a continuous system, Approved Document F sets minimum high extract rates of 13 l/s for a kitchen, 8 l/s for a bathroom or utility room and 6 l/s for a WC, plus a whole dwelling rate based on the number of bedrooms. The Vent-Axia Multivent and Sentinel units are listed in the SAP Product Characteristics Database, and the Greenwood CMEV.4eHT has a Specific Fan Power of 0.16 W/l/s, which helps with Part L and SAP calculations.

How many rooms can one Multivent serve?

The Vent-Axia MVDC-MS and MVDC-MSH have three 125mm extract spigots, and the Greenwood CMEV.4E and CMEV.4eHT have five. A typical three or four bedroom house with a kitchen, bathroom, en suite and utility room sits comfortably on one unit. Extra rooms can be picked up with branch fittings as long as the total extract rate stays within the unit's capacity. For larger homes the Sentinel Multivent Plus HX extracts up to 159 l/s. Send us the room list and we will check it.

Vent-Axia Multivent or Greenwood CMEV?

Both are proven whole-house MEV units with low energy DC motors. The Vent-Axia Lo-Carbon Multivent is the long-established choice, with three spigots and simple switched-live boost and purge control, and the Sentinel models add finer demand control and, on the Plus HX, app and wireless control. The Greenwood CMEV.4E is very compact, has five spigots and is a drop-in replacement for the older Greenwood Centair and CML units. The CMEV.4eHT adds humidity and timer control and a 0 to 10 V input for BMS. If you are replacing an existing unit, matching the brand usually means the ducting lines up without changes.

Do I need a model with a humidity sensor?

It is worth it in most homes. A basic unit runs at its set trickle speed and only boosts when a switched live, usually a light switch, tells it to. A humidity model such as the MVDC-MSH, the Sentinel Multivent Humidity, the Sentinel Multivent Plus HX or the Greenwood CMEV.4eHT boosts automatically when a shower or cooking raises humidity, so condensation is cleared even when nobody remembers to switch anything on. Use the Humidity Control button below to see those models.

What ducting does a central extract unit need?

Use 125mm (5 inch) rigid ducting for the main runs, because smooth solid duct has far less resistance than flexible hose and keeps the unit quiet and efficient. Keep flexible duct to short final connections only. Insulate any duct running through a cold loft so warm, moist air does not condense inside it and run back into the unit, and fit a single discharge duct to a roof or wall terminal. Each room needs an extract valve or grille at the ceiling so the airflow can be balanced during commissioning.

How much does a Multivent cost to run?

Very little, because it runs on a DC motor at low speed most of the time. Vent-Axia puts the running cost of the Lo-Carbon Sentinel Multivent at around £15 a year, and the Greenwood CMEV.4eHT and Sentinel Multivent Plus HX have a Specific Fan Power as low as 0.16 W/l/s. A single central unit also uses less energy than several individual extractor fans running separately.

Can I replace an old Greenwood Centair or Vent-Axia unit?

Yes. The Greenwood CMEV.4E is designed as a drop-in replacement for the Greenwood Centair and CML, so it fits the existing duct layout. Older Vent-Axia Multivent units can be replaced with the current Lo-Carbon MVDC-MS or MVDC-MSH. Send us the model on the label of the existing unit and we will confirm the right replacement.

Do you supply MEV units for trade and housing projects?

Yes. We supply electricians, M&E contractors, developers and housing providers, and hold stock across the Vent-Axia and Greenwood ranges for next working day delivery. Send a schedule or plot list and we will price the units, ducting and valves together. Call 0161 399 3825 or email hello@efans.co.uk.