EMERGING TECHNOLOGY

MVHR Electrical Installation: Ventilation Wiring Guide for UK Electricians

Mechanical Ventilation with Heat Recovery is becoming standard in energy-efficient homes. This guide covers the dedicated circuit, boost switch wiring, summer bypass, condensate pump, duct heater, Building Regs Part F compliance, and commissioning.

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14 min readUpdated 2026-06-10Andrew Moore, Founder of Elec-Mate

Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.

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Key Takeaways

  • 1MVHR (Mechanical Ventilation with Heat Recovery) units require a dedicated fused spur or MCB circuit from the consumer unit — typically a 3A or 5A fused connection unit for units drawing 50 to 200W.
  • 2Boost switch wiring connects a momentary or latched switch (typically in the kitchen and bathroom) to the MVHR controller, triggering high-speed extraction during cooking or bathing.
  • 3A summer bypass control allows the MVHR to bring in fresh air without passing through the heat exchanger when the outdoor temperature is higher than desired — controlled by an automatic damper motor wired to the MVHR controller.
  • 4Condensate drain or pump is required because the heat exchanger produces condensate. If gravity drainage is not possible, a condensate pump needs a power supply.
  • 5Building Regulations Approved Document F (Ventilation) sets the minimum ventilation rates that the MVHR system must achieve, and the system must be commissioned to demonstrate compliance.
01 · Emerging Technology

MVHR Electrical Installation: What Electricians Need to Know

Mechanical Ventilation with Heat Recovery (MVHR) is becoming standard in new-build homes and energy-efficient retrofits. As buildings become better insulated and more airtight, natural ventilation through gaps and openings is no longer sufficient — and opening windows defeats the purpose of insulation. MVHR provides controlled ventilation while recovering up to 90% of the heat that would otherwise be lost.

An MVHR system consists of a central unit (typically installed in a utility room, loft, or cupboard) connected to a network of ducts that supply fresh air to habitable rooms and extract stale air from wet rooms. The heart of the system is the heat exchanger, which transfers heat from the outgoing air to the incoming air without the two airstreams mixing.

The electrical scope includes the dedicated power supply, boost switch wiring, summer bypass control, condensate pump circuit (if needed), duct heater circuit (if fitted), and commissioning. This guide covers each element in detail.

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02 · Emerging Technology

Dedicated Circuit Requirements

The MVHR unit requires a dedicated electrical supply. Although the power consumption is low (50 to 300W), a dedicated circuit ensures the unit is not affected by other loads tripping and provides a clear means of isolation.

  • Supply arrangement — a switched fused connection unit (FCU) with a 3A or 5A fuse is the standard arrangement. The FCU provides local isolation and overcurrent protection. Alternatively, a dedicated 6A MCB at the consumer unit with a double-pole switch adjacent to the unit.
  • Cable — 1.5mm twin and earth is adequate for the low power draw. Route the cable to the MVHR unit location, terminating at the FCU or double-pole switch. The unit is then connected via a flex outlet plate or direct connection.
  • Location — the MVHR unit must be accessible for filter changes (every 3 to 6 months) and servicing. Common locations include utility rooms, large cupboards, and loft spaces. If in the loft, ensure the circuit and isolator are accessible without specialist equipment.
03 · Emerging Technology

Boost Switch Wiring

Boost switches allow the occupant to temporarily increase the MVHR fan speed for rapid air extraction — typically during cooking, bathing, or when humidity is high.

  • Switch type — momentary push-button (press once to activate boost, it runs for a timed period then returns to normal) or latched rocker switch (on/off manual control). Some systems accept both types — check the MVHR controller input specification.
  • Wiring — most MVHR systems use a low-voltage signal input for the boost switch — typically a volt-free contact (two wires from the switch to the controller terminal strip). Some systems use a 230V switched live input. Run a 2-core (or 3-core if 230V) cable from each boost switch location to the MVHR controller.
  • Locations — install boost switches in the kitchen, bathroom, and any other room with an extract terminal. Position at standard light switch height (1.0 to 1.2m). Some systems use a dedicated boost switch plate; others can use a standard retractive light switch.
  • Humidity sensors — as an alternative (or addition) to manual boost switches, humidity sensors can trigger boost automatically. These are wired to the MVHR controller and powered from the controller or a separate supply.
04 · Emerging Technology

Summer Bypass Controls

The summer bypass is a critical feature that prevents the MVHR from heating incoming air when heat recovery is not wanted. In summer, fresh air is routed around the heat exchanger to provide ventilation cooling.

  • Automatic bypass — a motorised damper inside the MVHR unit diverts the incoming air around the heat exchanger. The damper motor is controlled by the MVHR controller based on indoor and outdoor temperature sensors. The wiring is internal to the unit in many cases, but some systems require an external temperature sensor to be wired to the controller.
  • Manual override — some systems include a manual bypass override switch on the controller or a remote switch. This allows the occupant to force bypass mode regardless of temperature conditions.
  • Temperature sensors — if the MVHR requires external temperature sensors for bypass control, these are typically NTC thermistors on 2-core cable, mounted at the supply air intake and in a habitable room. Run the sensor cables back to the MVHR controller.
05 · Emerging Technology

Condensate Pump Circuit

The heat exchanger in an MVHR unit extracts moisture from the outgoing air, producing condensate that must be drained. The volume is modest — typically 1 to 3 litres per day in a domestic system — but it must be managed.

  • Gravity drainage — the preferred option. A 22mm overflow pipe from the MVHR condensate outlet to a nearby waste pipe or external drain. No electrical work needed for the drain itself.
  • Condensate pump — needed when the MVHR is below the nearest drain point or in a loft space. A small pump (10 to 30W) lifts the condensate to a waste pipe. The pump requires a power supply — typically a 3-pin plug to a nearby socket or a fused spur. The pump must remain energised whenever the MVHR is running.
  • Overflow alarm — some condensate pumps include an overflow sensor that can signal the MVHR controller to shut down if the pump fails or the drain blocks. Wire the alarm output to the MVHR controller fault input if available.

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06 · Emerging Technology

Duct Heater Circuit

A post-heater (duct heater) is an electric heating element installed in the supply air duct after the MVHR unit. It provides additional heating to the incoming air during very cold weather and is most commonly found in Passivhaus and ultra-low energy buildings where the MVHR supply air is the primary heating delivery mechanism.

  • Power rating — domestic duct heaters are typically 1 to 3kW. A 2kW heater draws approximately 8.7A at 230V. This requires its own dedicated circuit — a 10A or 16A MCB with 1.5mm or 2.5mm cable.
  • Safety interlocks — the duct heater must be interlocked with the MVHR fan so it cannot operate without airflow (which would cause overheating). This interlock is typically managed by the MVHR controller, which provides a control signal or relay contact to enable the heater only when the fan is running.
  • Thermal cut-out — the duct heater must have a manual-reset thermal cut-out to prevent overheating if the airflow drops below the safe level. This is usually built into the heater element but must be verified during commissioning.
  • Duct fire rating — where a duct heater is installed, the ducting must be suitable for the temperatures involved. Metallic ductwork is required in the immediate vicinity of the heater element. Fire dampers may be needed in duct penetrations through fire compartment walls.
07 · Emerging Technology

Building Regulations Approved Document F

Building Regulations Approved Document F (Ventilation) sets the minimum requirements for ventilation in dwellings. MVHR is one of the approved ventilation strategies (System 4 in the Approved Document). The key requirements are:

  • Minimum extract rates — kitchen: 13 litres per second (l/s) at boost, bathroom: 8 l/s, utility room: 8 l/s, WC: 6 l/s. These are the minimum rates that the MVHR must achieve at boost speed.
  • Whole-house ventilation rate — the continuous background ventilation rate (trickle speed) must provide at least 0.3 l/s per m2 of internal floor area. For a 100m2 dwelling, this is 30 l/s continuous.
  • Specific Fan Power (SFP) — the MVHR must achieve a Specific Fan Power of no more than 1.5 W/(l/s) for the whole system. This limits the total electrical power consumption relative to the airflow delivered.
  • Commissioning — the system must be commissioned and the results recorded. The commissioning data is submitted to Building Control as evidence of compliance.

The electrical installation is notifiable under Part P of the Building Regulations (new circuit from the consumer unit). Notify through your competent person scheme.

08 · Emerging Technology

Commissioning and Testing

Commissioning an MVHR system involves both electrical testing and airflow verification.

  • Electrical testing — continuity of protective conductors, insulation resistance, polarity, earth fault loop impedance, and RCD operation on the MVHR circuit (and duct heater circuit if fitted). Issue an EIC or Minor Works Certificate.
  • Functional testing — verify the MVHR operates on all speed settings (trickle, normal, boost). Test each boost switch. Verify the summer bypass activates and deactivates at the correct temperature thresholds. Test the condensate pump (if fitted). Test the duct heater interlock and thermal cut-out (if fitted).
  • Airflow commissioning — measure the airflow at each supply and extract terminal using a balometer or anemometer. Adjust duct dampers and fan speed to achieve the design flow rates per Approved Document F. Record the results on the commissioning sheet.
  • Documentation — the commissioning record, EIC, and MVHR system specifications are submitted to Building Control as part of the completion certificate. Provide the homeowner with the commissioning data, filter replacement schedule, and operating instructions.
09 · Emerging Technology

For Electricians: The MVHR Opportunity

MVHR installation is growing rapidly as Building Regulations tighten and more homes are built (or retrofitted) to high insulation standards. The electrical scope is typically worth £300 to £800 per installation, with more complex systems (duct heaters, multiple zones, humidity sensors) commanding higher prices.

Building relationships with ventilation installers and new-build developers provides a steady stream of MVHR electrical work. As an electrician, your value is in getting the wiring right first time — incorrect boost switch wiring or missing condensate pump circuits are common snags that delay handover.

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