Ventilation and Heat Pumps — Requirements Explained

Ventilation requirements when you fit a heat pump.

Getting a home ready for a heat pump usually means insulating and air-sealing — which makes the home tighter. Once you do that, controlled ventilation is no longer optional: it is what stops the tighter home from trapping moisture and growing mould. 'Build tight, ventilate right' is the rule that keeps a heat-pump retrofit healthy.

Why ventilation is part of heat pump readiness

A heat pump runs best in a low-heat-loss, reasonably airtight home. But as you reduce the air leakage, the accidental ventilation that used to control humidity disappears. The moisture from cooking, washing, drying clothes and simply breathing has to go somewhere. Without a designed replacement, indoor humidity rises, surfaces fall below the dew point, and condensation and mould follow — often blamed unfairly on the new heat pump or insulation.

What the ventilation needs to achieve

Remove moisture at the source (kitchens, bathrooms, utility rooms) before it spreads
Provide a continuous supply of fresh air for healthy indoor air quality
Control humidity to keep relative humidity in the healthy 40–60% band
Do all of this without throwing away the heat the heat pump has provided

The main ventilation options, and what suits which home

There is no one-size-fits-all answer — the right strategy depends on how airtight the home becomes and the moisture load.

Continuous mechanical extract (dMEV/MEV): quiet, continuous extract from wet rooms, suitable for moderately tight homes — simple and effective
Mechanical ventilation with heat recovery (MVHR): supplies filtered fresh air and recovers up to ~90% of the heat from the extract air, ideal for airtight homes
Positive input ventilation (PIV): a lower-cost option that dilutes humidity, useful in some properties but without heat recovery
Trickle vents and background ventilation: a minimum baseline, rarely sufficient on their own in a tightened home

Why commissioning matters

A ventilation system only delivers its rated airflows if it is installed and commissioned correctly — ducts sized and sealed, runs kept short and smooth, and flow rates measured and set on handover. Poorly commissioned MVHR is a common cause of disappointing performance and noise. Whatever system is chosen, it must be measured on completion, not just switched on.

How RetrofitIQ specifies ventilation for a heat-pump home

Assess the current ventilation and the measured airtightness from the blower door test
Match the ventilation strategy (dMEV, continuous extract or MVHR) to the achieved tightness and moisture load
Size airflows to the occupancy and the wet rooms
Coordinate the ventilation with the fabric and heating works so the air barrier and ducting are continuous
Verify humidity and measured airflows after installation

Common questions from clients.

Does a heat pump itself need extra ventilation?

The heat pump unit does not. It is the readiness works — insulation and air-sealing — that tighten the home and create the ventilation requirement. The fabric, heating and ventilation must all be planned together as one system.

Is MVHR necessary for a heat pump?

Not always. In a very airtight home MVHR is ideal because it recovers heat. In a moderately tight home, continuous mechanical extract may be enough. We match the strategy to your measured airtightness rather than defaulting to the most expensive option.

Will ventilation waste the heat from my heat pump?

Continuous extract loses some warm air, but the amount is controlled and small compared with uncontrolled draughts. MVHR recovers most of the heat. Either way, controlled ventilation loses far less heat than the accidental leakage it replaces.

I already get condensation — will a heat pump make it worse?

Not if ventilation is addressed as part of the works. In fact, resolving the ventilation and warming the surfaces usually eliminates the condensation you already have. We diagnose the moisture problem and design it out alongside the readiness works.

Do trickle vents count as enough ventilation?

Rarely, once a home is tightened. Trickle vents provide background ventilation but cannot reliably control the moisture load on their own. A continuous extract or MVHR system gives dependable control.