MVHR vs PIV — Comparing Ventilation Strategies

MVHR vs PIV — which ventilation strategy is right for your home?

Choosing the wrong ventilation system is one of the most expensive — and most common — retrofit mistakes. MVHR, PIV, dMEV and intermittent extract are not interchangeable; they suit different building types, airtightness levels and occupant needs. This article walks through the decision framework we use.

Why ventilation choice matters

Every home needs continuous fresh air. Without it: indoor CO₂ rises (cognitive performance drops), humidity climbs (condensation and mould), VOCs and PM2.5 accumulate (health impact). Opening windows works in theory but fails in practice — it's intermittent, it throws away heating energy and most occupants do it far too little in winter.

The four mainstream strategies all solve this differently. The right choice depends on your airtightness, layout, occupants and budget.

MVHR — Mechanical Ventilation with Heat Recovery

A central unit with two ducted networks — one extracting moist, stale air from kitchens and bathrooms; the other supplying filtered fresh air to bedrooms and living rooms. The two airstreams cross in a heat exchanger; 80–93% of the heat from the outgoing air is transferred to the incoming air without the airstreams mixing.

PIV — Positive Input Ventilation

A single fan in the loft pushes filtered outside air into the central stairwell, slightly pressurising the home and forcing stale air out through trickle vents, extract points and the envelope itself. No heat recovery — incoming air is at outdoor temperature.

dMEV — Decentralised Mechanical Extract Ventilation

Continuous low-rate extract fans in wet rooms (kitchen, bathrooms), with fresh air drawn in passively through trickle vents elsewhere. A pragmatic middle-ground for moderately airtight homes.

Intermittent extract (the worst option)

The default in most existing UK homes — a noisy fan in the bathroom that runs while you shower. Almost completely useless: it only operates 5% of the time, throws away heat, and does nothing about CO₂, low-level humidity or VOCs the rest of the day. We never recommend it for a serious retrofit.

The RetrofitIQ decision framework

Blower door test first — establish actual airtightness in ACH₅₀.
If ≤3 ACH₅₀ and ductwork is feasible → MVHR is almost always the right answer.
If 3–7 ACH₅₀ → dMEV is the pragmatic choice; consider tightening up to enable MVHR.
If ≥7 ACH₅₀ and the building can't realistically be made airtight → PIV is the most cost-effective whole-house solution.
Verify after install with measured CO₂, humidity and a re-test where applicable.

Common questions from clients.

Can I retrofit MVHR into an existing leaky house?

You can install the unit, but it won't deliver its rated heat recovery until the envelope is tightened. We always pair MVHR with airtightness remediation — sealing service penetrations, intermediate floors, lofts and intermediate party walls.

Will MVHR make my home dry / stuffy?

Done properly it controls humidity to 40–60% RH (the building-physics sweet spot) and delivers continuous filtered fresh air. Stuffiness is usually a sign of a poorly-commissioned system or undersized fan.

Is PIV good enough on its own?

PIV does a single job well — it whole-house ventilates a leaky home and combats condensation. If your priorities are filtered fresh air to bedrooms, heat recovery or asthma management, PIV alone is not enough.