Heat recovery efficiency, explained

What is heat recovery efficiency?

Heat recovery efficiency is the proportion of the heat in the air being extracted from a home that the unit transfers to the fresh air coming in. An efficiency of 90 percent means nine tenths of that heat is recovered rather than lost. It is measured under standard test conditions, so figures from different units can be compared fairly.

The heat exchanger inside the unit is what does this. It passes the two air streams close together without mixing them, so warmth moves from the outgoing air to the incoming air. A higher efficiency exchanger recovers more of that heat. The tested figure is what to compare, because manufacturers can quote it under different conditions if you are not careful.

Sources: Passivhaus Institut

What efficiency do MVHR units achieve?

Economy units are typically around 70 percent, mid-range units in the low to mid 80s, and premium Passivhaus-certified units from about 89 percent into the mid 90s. The Passivhaus Institut component database lists tested figures for certified units, which is the most reliable place to compare, because it uses one consistent test method.

Because the test method matters, comparing a marketing figure from one brand with a certified figure from another is not fair. The PHI database puts units on the same footing. As a rule of thumb, treat anything above about 85 percent as good and above 90 percent as excellent, and check the source of the number before trusting it.

Sources: Passivhaus Institut

How much does efficiency affect my heating?

The higher the efficiency, the less heat you lose through ventilation, so the less your heating has to make up. Moving from a 70 percent unit to a 90 percent unit noticeably reduces ventilation heat loss, though the exact saving depends on how airtight and how warm the home is. In a very airtight home the difference is worth having; in a leaky one it matters less.

Ventilation heat loss is only part of a home total heat loss, alongside the walls, roof, floor and windows. So a better exchanger helps most where the fabric is already good and airtight, which is exactly the kind of home MVHR suits. In a draughty home, uncontrolled leakage swamps the difference between a good and a great unit.

Is a higher efficiency unit always worth it?

Usually in an airtight home, but not always. A higher efficiency unit costs more, and the extra recovery only pays back where the home is airtight enough to hold the heat in the first place. Specific fan power, noise and filter quality matter too. Efficiency is the headline figure but not the only one worth weighing.

A unit with slightly lower heat recovery but much lower fan power can cost less to run overall, because the fans run continuously while the heat recovery benefit depends on the weather. For most airtight homes a high-efficiency unit is the right call, but choose on the whole picture, not the single biggest number.

Why might a unit not reach its quoted efficiency?

A unit only delivers its rated efficiency if it is well installed and commissioned. Poor duct design, crushed or overlong ducting, or a system that was never balanced can pull the real performance well below the datasheet. Always check the tested figure, from the PHI database where possible, and make sure the system is commissioned properly.

The rating is measured on a test rig, not in a badly installed house. This is why commissioning matters: it confirms the real, measured airflow, and a system moving the wrong volumes will not recover heat as intended. A great unit fitted poorly performs worse than an average unit fitted well.