What is MVHR?

Mechanical Ventilation with Heat Recovery - how airtight homes breathe without losing the heating bill

MVHR - Mechanical Ventilation with Heat Recovery - is a whole-dwelling ventilation system that extracts stale air from bathrooms and kitchens while recovering up to 90% of its heat to warm incoming fresh air from outside. As UK new-build homes get more airtight under Part L and the Future Homes Standard, MVHR is increasingly the only sensible way to manage indoor air quality without paying twice for your heating.

How MVHR works

An MVHR unit sits in the loft, plant room or service cupboard, with two fans and a heat-exchange core:

  1. Stale warm air is pulled from wet rooms (bathrooms, kitchens, utility rooms) through one set of ducts
  2. Fresh cold air is drawn in from outside through a separate duct
  3. Both airstreams pass through the heat-exchange core, which transfers heat from the outgoing to the incoming air without mixing the two
  4. Pre-warmed fresh air is delivered to the habitable rooms (living room, bedrooms) through a separate set of supply ducts
  5. Stale air leaves the building, now cooled to close to outside temperature

A modern unit typically recovers 85-95% of the heat that would otherwise have been lost to ventilation. In airtight homes, that's a significant slice of the annual heating bill.

Why MVHR matters under Part L

Trickle vents and passive stack ventilation - which work fine in leakier homes - don't deliver reliable air change rates when the fabric is tight. You end up with either condensation and poor air quality (if the natural ventilation doesn't work) or excess heat loss (if it works too well on cold or windy days).

Under Part L 2021 and increasingly under the Future Homes Standard, new dwellings are reaching airtightness levels where natural ventilation alone isn't enough. The Building Regulations set minimum ventilation rates for habitable rooms and minimum extract rates for wet rooms, and those rates have to be proved at handover. MVHR delivers both reliably and recovers much of the heat lost in doing so.

The FHS connection - airtightness drives ventilation strategy

The Future Homes Standard pushes airtightness below 5 m³/(h·m²) at 50 Pa for most new builds, with some developers targeting Passivhaus-adjacent figures around 1.0-1.5. At that level, mechanical ventilation isn't optional - it's the only way to meet Part F (ventilation) requirements.

MVHR also tilts the SAP calculation. Because heat is recovered rather than lost, the as-built energy performance improves, which feeds the EPC rating.

Installation evidence for Appendix B

As with other regulated services, an MVHR installation needs photographic evidence under Appendix B. Things an OCDEA will typically look for:

  • The MVHR unit itself, clearly showing make, model and serial number
  • Duct runs where they pass through thermal elements
  • Insulation around any ductwork outside the thermal envelope
  • External terminations and weatherproofing
  • Commissioning certificate showing balanced supply and extract rates per room

Buildsnapper has an Appendix B section for heating, hot water and ventilation plant. Capture the MVHR unit, duct runs, external terminations and commissioning paperwork in one place. GPS and timestamp on every photo. See how it works.

Commissioning requirements

MVHR has to be commissioned - balanced to deliver the correct airflow to each room as designed - by a competent installer. Out-of-the-box default rates aren't acceptable. Commissioning certificates from schemes like BPEC or NICEIC sit alongside the Appendix B photos and contribute to the as-built SAP.

Incorrectly commissioned MVHR is one of the bigger contributors to the Performance Gap: a unit set to generic factory rates can under-ventilate wet rooms (leading to condensation) or over-ventilate the rest of the house (losing heat unnecessarily).

Common MVHR installation mistakes

  • Uninsulated ductwork in cold loft spaces. Any duct outside the thermal envelope has to be insulated to avoid condensation.
  • Flexible ducting instead of rigid. Flexi-duct creates pressure drops and noise. Rigid ducting with gentle bends is the industry best practice.
  • Long duct runs. The further the air has to travel, the harder the unit works. Careful placement of the MVHR in the centre of the dwelling reduces runs.
  • Missing commissioning paperwork. An installed unit without the commissioning certificate is effectively non-compliant.
  • Neglected maintenance. Filters need changing every 6-12 months or the unit loses efficiency. Relevant for post-handover, but worth flagging to occupants.

Frequently asked questions

What does MVHR stand for?

Mechanical Ventilation with Heat Recovery. It's a whole-house ventilation system that extracts stale air while recovering up to 90% of its heat to warm the incoming fresh air.

Is MVHR mandatory under Part L?

Not strictly mandatory - but at the airtightness levels required by Part L 2021 and the Future Homes Standard, it's usually the only practical way to meet Part F ventilation requirements. Positive-input ventilation or continuous mechanical extract ventilation are other options that may suit smaller or lower-airtightness dwellings.

How much does MVHR cost to install?

Typical costs for a new-build three-bedroom home run from £3,000 to £8,000 including the unit, ductwork, and commissioning, depending on layout and product tier. Retrofitting into an existing dwelling costs substantially more.

Does MVHR work in hot weather?

Yes. Better units include summer bypass modes that route incoming air around the heat-exchange core when heat recovery isn't wanted, so the home still gets filtered fresh air without being warmed on a hot day.

How often does MVHR need maintenance?

Filters should be inspected every 6 months and replaced every 6-12 months, depending on use and location. The heat-exchange core typically needs cleaning every 3-5 years. Commissioning should be re-checked if airflow rates change or rooms start experiencing condensation or stale air.

Do I need evidence of MVHR installation for Part L?

Yes. Photographic evidence under Appendix B should cover the MVHR unit, duct runs through thermal elements, external terminations and weatherproofing, and the commissioning certificate showing balanced airflow rates. The OCDEA will reflect the installation in the as-built SAP.

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