Extract-only or heat recovery: how to actually decide

The question comes up on every renovation project, and it is almost always framed the wrong way: “which is better?” The useful answer lies elsewhere — which system suits your building, your works and what you are prepared to maintain.

Here are the criteria that genuinely tip the balance, without the sales pitch.

What each system does

Extract-only, or system C, mechanically extracts air from wet rooms — kitchen, bathroom, WC. Fresh air enters passively through vents in the window frames or the façade of living spaces and crosses the home towards the extract points.

Heat recovery, or system D, uses two networks: one supplying filtered fresh air to living spaces, one extracting stale air from wet rooms. Between them, an exchanger transfers heat from the outgoing air to the incoming air without mixing the streams.

The comparison, criterion by criterion

The decisive condition: the building's air-tightness

This is the point sales arguments happily leave out. Heat recovery reclaims warmth from air passing through its exchanger. Air entering through the building's leaks — cracks, cable penetrations, loft hatches, old windows — never passes through that exchanger.

In a leaky building, a significant share of the renewed air therefore bypasses the system. You pay for high-performance equipment that treats only a fraction of the air, and the expected gain fails to materialise. It is the main cause of disappointment after installation.

When extract-only is the right choice

• Light renovation, with no way to run two duct networks
• An apartment with no dropped ceiling or service shaft available
• A building that will remain leaky, where air-tightness will not be addressed
• A constrained budget, with priority given to insulation or new windows
• A small home, where the number of valves stays limited
• An occupant who prefers a system with simple, infrequent maintenance

When heat recovery fully justifies itself

• New build, or heavy renovation including air-tightness work
• A well-insulated building, where ventilation heat loss becomes significant
• A polluted outdoor setting: main road, nearby pollution source, seasonal pollen
• An occupant with respiratory sensitivity, justifying filtration of incoming air
• Bedrooms exposed to outdoor noise, where façade vents are best avoided
• A project where space for both networks was planned from the design stage

What heat recovery demands in return

An MVHR system is not equipment you fit and forget. It has two filter sets to replace, an exchanger to clean, a condensate drain to watch and balancing to check periodically.

Neglected, it becomes frankly worse than a maintained extract-only system: saturated filters, collapsed airflow, a fouled exchanger and rising electricity use. That is a legitimate selection criterion — if you know maintenance will not be kept up, a simpler system that tolerates neglect is the better call.