MVHR vs MEV vs PIV: Which Whole-House Ventilation System Do You Actually Need?

Introduction

The alphabet soup of ventilation acronyms, MVHR, MEV, PIV or dMEV, can be genuinely confusing, even for experienced builders. They all have different approaches to providing whole-house mechanical ventilation and each has its place. Choosing the wrong one is an expensive mistake.

This guide explains clearly how each system works and which type of home or project it suits best.

What is MVHR?

Mechanical Ventilation with Heat Recovery (MVHR) is a balanced ventilation system that simultaneously extracts stale, moist air from wet rooms (kitchens, bathrooms and utility rooms) and supplies fresh, filtered air to habitable rooms (bedrooms, living rooms, studies). The key feature of this is a heat exchanger at the heart of the unit. As the warm outgoing air passes through the exchanger, it transfers up to 95% of its heat to the incoming cold fresh air, without the two airstreams mixing. The result is continuous fresh air throughout the home with minimal heat loss.

When is MVHR appropriate?

MVHR works best in well-insulated, airtight new-build homes, typically those built to Part L Building Regulations or better, with an air permeability of 5m³/(h·m²) @50Pa or lower. MVHR can work in providing excellent ventilation when refurbishing leaky older buildings although the heat recovery benefit is likely to be greatly reduced and the ducting installation to all living/wet rooms can pose problems .

What is MEV?

Mechanical Extract Ventilation (MEV) is a simpler system that only extracts air. A central fan unit extracts stale air from wet rooms continuously at a low background rate, with higher boost rates triggered by humidity sensors or manual boost switches.

Fresh air enters the building via trickle vents in windows and purpose-provided ventilators. There is no heat recovery.  All the extracted air (and its heat) is lost to outside. MEV systems are cheaper to install than MVHR and are less sensitive to airtightness levels, making them a practical option for new builds that do not achieve the airtightness needed to make MVHR worthwhile, or for homes where a full MVHR duct run is not feasible.

What is PIV?

Positive Input Ventilation (PIV) is the simplest and cheapest system of the three and works best when located centrally in the house. A single fan unit, typically installed in the loft, draws in fresh outside air, passes it through a basic filter and pushes it into the house through a ceiling diffuser, usually in a central hallway.

The gentle positive pressure created throughout the house pushes stale, moist air out through natural leakage points - gaps around windows, doors or construction junctions. There is no heat recovery and no mechanical extract.

PIV is most commonly used as a retrofit solution for existing homes with condensation and mould problems. It is not appropriate for new builds as it does not meet Part F requirements on its own and provides no control over where air goes.

Side-by-side comparison

Which system is right for my project?

Choose MVHR  if...

• • You are building a new, well-insulated and airtight home
  • Your target air permeability is 5m³/(h·m²) @50Pa or better
  • You want the best indoor air quality with filtered fresh air to every room
  • You want to minimise ventilation heat loss
  • You are building to Passivhaus or near-Passivhaus standards

Choose MEV if...

• • You are building new but your airtightness will not reach MVHR-suitable levels
  • You want a simpler, lower-cost system that still meets Part F
  • The building layout does not suit a full MVHR duct installation

Choose PIV if...

• • You have an existing home with persistent condensation and mould problems
  • You want the lowest-cost mechanical ventilation solution
  • You understand that PIV alone will not meet Part F for new builds

If you're not sure which system suits your project, please get in touch with Epicair's UK-based design team who can advise you on the right solution for your specific building.

Call us on 01892 600121 or email:  mvhr@epicair.co.uk and we will be delighted to discuss the options available for you

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