Do I need MVHR or will extract fans do?

What this usually means

Extract fans and MVHR solve the ventilation problem in different ways. Extract ventilation — intermittent fans or continuous low-rate units (dMEV/MEV) — removes moist, stale air from the wettest rooms (kitchen, bathroom), and fresh air is drawn in elsewhere through trickle vents and leakage. MVHR (mechanical ventilation with heat recovery) is a balanced system: it extracts stale air from wet rooms and simultaneously supplies filtered fresh air to living rooms and bedrooms, passing the two airstreams through a heat exchanger so most of the heat in the outgoing air is recovered.

The decision hinges on airtightness. Extract-only ventilation relies on fresh air finding its own way in through vents and gaps, which works when the home has enough background ventilation and some leakage. MVHR controls both supply and extract, which is what makes it efficient — but it only achieves that efficiency in a reasonably airtight home, because in a leaky one the controlled airflow is overwhelmed by uncontrolled leakage and the heat recovery benefit is lost. So the building, not preference, sets the sensible choice.

This is why the question cannot be answered in the abstract. A draughtier home with good extract and background ventilation may be perfectly healthy without MVHR. A tight, modern or deeply retrofitted home, where there is little accidental leakage, benefits greatly from MVHR's continuous filtered supply and heat recovery. Measuring the home's airtightness and ventilation needs is what turns a generic debate into a clear recommendation.

Common causes

The ventilation options compared

Signs and symptoms

The performance of any ventilation system depends on whether it controls the airflow or merely adds to uncontrolled leakage. Extract ventilation removes air from wet rooms and lets replacement air enter wherever it can; it controls extraction but not supply. In a home with some leakage and good trickle vents, that is enough to keep moisture and stuffiness in check. MVHR controls both supply and extract, which lets it filter the incoming air and, crucially, recover heat from the outgoing air — but only if the home is airtight enough that most of the air movement actually goes through the system rather than through gaps.

Heat recovery is the reason airtightness matters so much for MVHR. The heat exchanger transfers most of the warmth from the outgoing stale air to the incoming fresh air, so ventilation no longer means throwing heat away. In a leaky home, however, large volumes of air bypass the unit through gaps, carrying their heat straight outside; the recovery benefit collapses and the system cannot balance. This is why MVHR belongs with good airtightness — the two are designed as a pair, summarised again by 'build tight, ventilate right'.

For extract-only ventilation, the building physics is simpler but the trade-off is heat: every cubic metre of warm, moist air extracted is replaced by cold outdoor air that must be heated. That is acceptable in a leakier home that was losing heat anyway, and it keeps the system cheap and robust. The engineering judgement, therefore, is to match the ventilation strategy to the home's airtightness and needs: continuous extract and background ventilation for leakier homes, balanced MVHR for airtight ones — with the airtightness measured so the decision rests on the actual building rather than assumption.

How to choose the right ventilation system

Let the building decide. Measure airtightness and ventilation needs first, then specify the system that matches — neither under-ventilating a tight home nor over-engineering a leaky one.

1. 01

   Measure airtightness

   A blower door test establishes how tight the home is, which is the single biggest factor in whether MVHR will perform.

2. 02

   Assess the ventilation need

   Log carbon dioxide and humidity and review occupancy and moisture load to define how much ventilation is required and where.

3. 03

   For leakier homes: continuous extract + background

   Specify good continuous or intermittent extract at the wet rooms plus working trickle vents — cost-effective and adequate where there is some leakage.

4. 04

   For airtight homes: balanced MVHR

   Specify MVHR to supply filtered fresh air and recover heat, designing duct routes and commissioning it properly.

5. 05

   Match ventilation to airtightness as a pair

   Ensure the chosen system and the home's airtightness are designed together, so ventilation is effective and efficient.

6. 06

   Commission and verify

   Whatever the system, commission it to the right flow rates and verify with carbon dioxide and humidity that it keeps the air healthy.

How to prevent it coming back

• Decide ventilation strategy alongside any airtightness work, not after it.
• Don't fit MVHR to a leaky home expecting heat-recovery benefits — improve airtightness first.
• Keep extract fans and trickle vents working if relying on extract-only ventilation.
• Commission mechanical ventilation to the correct flow rates and maintain filters.
• Verify the chosen system with carbon dioxide and humidity readings.