What's involved in wiring an MVHR unit electrical supply

Helpful video reference. We use Nuaire's official installation and configuration video "NUAIRE MVHR INSTALLATION & CONFIGURATION VIDEO" as the video reference here. Nuaire is a Cardiff-based UK manufacturer and one of the most commonly specified MVHR brands in domestic new builds and deep retrofits. The video covers unit positioning, duct connections, electrical connections and commissioning in a real installation.

1. Understand what you are connecting

A domestic MVHR unit has two types of electrical connection: a mains power supply (230 V) and a low-voltage controls circuit for the boost switch, humidity sensors and any remote controller or timer. Mixing the two up, or using mains-rated cable where controls cable is specified, is a common mistake.

The mains supply powers the two fans inside the unit (supply and extract). The controls circuit allows the fans to ramp up to boost speed when, for example, someone switches on the shower. The two systems are electrically isolated inside the unit but physically share the same enclosure.

2. Read the wiring diagram before ordering anything

Find the wiring diagram in the installation manual - usually a fold-out sheet near the back. It will show:

• Which terminal block takes the mains supply (live, neutral, earth).
• Which terminals are for the boost switch input.
• Which terminals are for any humidity sensor.
• Whether the boost switch uses a volt-free contact (no mains at the switch) or a live signal from the unit (which governs the cable type you use).

If you are fitting a Nuaire MRXBOX, a Zehnder ComfoAir, a Titon HRV, or a Blauberg unit, each has its own terminal arrangement. The principle is the same, but the terminal labels differ. Never guess terminal positions on any MVHR unit.

3. Choose the mains power supply route

Most domestic MVHR units draw between 50 W and 300 W depending on size and speed setting. This is well within the capacity of a 5 A fused connection unit (FCU) on a spur from an existing circuit.

The practical options, in order of preference:

• Dedicated FCU from a local lighting or ring circuit: the simplest route if there is a convenient nearby circuit in the loft or roof space. The FCU provides local isolation and a 3 A or 5 A fuse appropriate to the unit's maximum current draw.
• Dedicated spur from the consumer unit: preferable if the loft has no existing circuits, or if you are installing a high-capacity unit. An RCBO at the consumer unit covers both overcurrent and 30 mA RCD protection. This is notifiable under Part P.

Run 1.5 mm² twin-and-earth from the FCU or spur to the unit's mains terminals. Keep the mains cable physically separate from the controls cable where possible.

4. Run the boost switch cable

The boost switch goes in the bathroom, en-suite, or kitchen - wherever the occupant wants to trigger higher ventilation speed. It is typically a standard single-pole rocker switch in the same style as the rest of the room, but it connects to the MVHR unit rather than any lighting.

Cable for the boost switch depends on the unit's controls specification:

• If the boost input uses a volt-free contact (the most common arrangement on modern units), use 0.5 mm² or 0.75 mm² two-core shielded cable, or plain twin bell wire. There is no mains voltage at the switch position.
• If the unit supplies a low voltage (typically 12 V or 24 V) to the switch, the same light-gauge cable is appropriate. The unit's manual will say the maximum cable length for reliable operation - usually 50 to 100 metres.
• If the boost switch is being wired to operate from the bathroom light (switching on boost when the light comes on), the connection is different again: some units sense the presence of a mains voltage from the bathroom circuit. In this case, use 1 mm² or 1.5 mm² T&E and treat it as a mains cable - not a controls cable.

In bathrooms, the boost switch must be outside Zone 1 (inside the bath or shower tray). A standard switch plate on the wall outside the bath area is fine. If the switch is a pull cord, the cord itself must not be reachable from the bath or shower tray.

5. Connect the humidity sensor if fitted

Some MVHR units include or accept an optional humidity sensor. This allows the unit to increase ventilation rate automatically when humidity rises above a set threshold - useful for properties where the occupants forget to use the boost switch.

Humidity sensor connections vary by manufacturer but typically use the same light-gauge controls cable as the boost switch. Observe polarity if the sensor has a positive and negative terminal. Route the cable away from heat sources, which can affect the sensor reading.

6. Commission the unit properly

Once all connections are made and checked, restore power and run through each operating mode. On minimum (trickle) ventilation, the fans should be almost inaudible from the living areas. Trigger the boost switch and confirm the fan speed increases. If the unit has a summer bypass damper (it opens in warm weather to provide free cooling without heat recovery), check the damper opens and closes correctly.

An MVHR system only works as intended when the supply and extract airflows are balanced. Most units have a commissioning mode where you can adjust the fan speed independently for supply and extract. Match the measured airflows to the design figures in the ventilation specification. If you do not have design figures, the Domestic Ventilation Compliance Guide gives minimum extract rates for kitchens and bathrooms as a starting point.

When to call us

The mains supply connection from an existing local circuit is within the scope of a minor works certificate and does not need a Part P registration. The moment you need a new dedicated circuit from the consumer unit, it becomes notifiable, and you need a registered electrician. Richard covers Sandwich and east Kent and can handle the electrical side of an MVHR installation from FCU to boost switch to Part P notification.