Heat recovery ventilation and the retrofit it rarely justifies

Warming the air on its way in

A ventilation system with heat recovery extracts air from the wet rooms of a house and supplies fresh air to the living rooms and bedrooms, passing both streams through a heat exchanger on the way. The two flows don’t mix. They exchange heat across a large area of thin material, so the incoming air arrives close to room temperature having started outside.

The physics is thoroughly sound and the effectiveness of a good exchanger is high. It solves a real conflict, which is that a well-sealed house needs continuous fresh air and that fresh air arrives cold.

It is also the measure most often recommended to houses that cannot benefit from it.

Recovery only counts on the air that goes through it

The system can only recover heat from air it actually handles. Any air entering the building through gaps in the fabric — around service penetrations, under skirtings, through a chimney, past a poorly sealed loft hatch — bypasses the exchanger completely and carries its heating cost with it.

In a house with substantial uncontrolled leakage, that bypass flow can exceed the designed ventilation rate, which means the machine is recovering heat from a minority of the air exchange while consuming electricity continuously to do it. The headline effectiveness of the exchanger becomes largely irrelevant.

This is why heat recovery is a standard feature of airtight new construction and a difficult proposition in an ordinary older house. The prerequisite is not insulation. It is airtightness, and that is a much harder retrofit than most people expect.

Ducting is the part that decides whether it works

The performance of an installed system depends far more on its ductwork than on the unit at the centre of it. Runs need to be short, smooth, adequately sized, sealed at every joint and insulated wherever they pass through unheated space, and every bend and every restriction costs airflow that the fan has to make up with more power.

Retrofitting that into a finished house is the actual project. Ducts have to reach every habitable room and every wet room, and in a building without service voids that means lifting floors, boxing in corners, or losing a cupboard to a riser. It is intrusive work and it is where the cost lands.

Insulated duct through a cold loft deserves particular attention, since a supply duct losing heat there gives back exactly what the exchanger recovered. It is a common defect and it is invisible once the hatch is closed.

Commissioning and filters decide whether it keeps working

A ventilation system has to be balanced, which means adjusting the flow at each terminal so that supply and extract match the design and match each other. An unbalanced system either pressurises the house or depressurises it, and a depressurised house draws air in through exactly the gaps you were trying to control.

Filters then need changing on a schedule, and a neglected filter progressively restricts flow until the system is providing less ventilation than the building requires while the fans work harder. This is the commonest reason an installed system underperforms, and it is entirely a maintenance failure rather than a design one.

A household unwilling to change filters a few times a year should not install one. That isn’t a criticism; it’s a compatibility question.

The simpler options that suit most existing houses

Continuous low-rate extract from the wet rooms, running quietly all the time and boosting when needed, addresses moisture at its source and costs a fraction of a full system. It recovers no heat, and in a leaky house the heat it loses is a small addition to what was leaving anyway.

Single-room heat recovery units, fitted through an external wall in a bathroom or kitchen, are a middle option that recovers some heat without any ducting at all. They are less effective than a whole-house system and vastly easier to install, which for many houses is the better trade.

And where a house is genuinely draughty, sealing it is the measure that comes first. Ventilation equipment installed before that is solving a problem the building does not yet have.

When it is clearly the right answer

A new building constructed to a demanding airtightness standard needs mechanical ventilation, and heat recovery is the obvious way to provide it. A deep retrofit that has taken airtightness seriously, with a measured result to show for it, is in the same position.

Anything short of that should be assessed rather than assumed. The question to ask is what the air leakage of the building is, and if nobody can answer it, the case for a whole-house system has not been made. That is an unfashionable conclusion for an attractive piece of equipment, and it is the right one more often than not.