There is no shortage of material explaining how heat recovery works. The theory is straightforward and the manufacturers’ figures are easy to find.
What’s harder to come by is an honest account of what happens when you install it in a real building — where the plant room is the wrong shape, the client can’t shut down, and the efficiency quoted on the datasheet meets British weather and a filter nobody has changed for two years.
This is a look at what we’ve learned doing it, rather than another explanation of the principle. If you want the fundamentals first, our complete guide to heat recovery ventilation covers those.
Lesson one: the technology should follow the building
The three main options behave very differently in practice, and the decision is usually made by the building rather than by the numbers.
Thermal wheels offer the highest recovery efficiency of the three and can transfer moisture as well as heat, which matters more than people expect in winter. The catch is that they need depth, they need the supply and extract to be adjacent, and there’s a small degree of transfer between the two air streams — which rules them out where cross-contamination is a concern.
We specified a thermal wheel on a project at Heathrow, where the recovery efficiency justified the space it took up and the airflow volumes made the arithmetic work comfortably.
Plate heat exchangers have no moving parts, need no maintenance beyond keeping them clean, and completely separate the two air streams. Efficiency is lower than a wheel, but on a building where reliability and zero cross-contamination matter more than the last few percent, they’re often the better answer.
Run-around coils are the least efficient of the three by a distance. But they’re the only option when supply and extract are in different parts of the building, and in a retrofit that constraint decides it. We use them more often than the efficiency figures alone would suggest, because they’re frequently the only thing that fits.
Lesson two: the savings are real, but they arrive slowly
Heat recovery is a long payback rather than a quick win, and it’s better to say so at the outset than to have the conversation eighteen months later.
On our project at Colmore Row, the energy savings from the recovery installation were substantial enough to change the building’s running costs meaningfully.
The pattern we see is consistent. Heat recovery pays back well on buildings that run long hours, in climates with a real heating season, where the difference between inside and outside temperature is large for a good part of the year. It pays back poorly on buildings that are occupied intermittently, or where the ventilation runs at low volumes.
That’s not a reason to avoid it. It’s a reason to model it properly for the specific building rather than applying a rule of thumb. Our article on heat recovery ROI and payback periods goes into how that calculation works.
Lesson three: quoted efficiency and delivered efficiency are different things
Every heat recovery device has a headline efficiency figure. Very few of them achieve it in service, and the gap is almost always down to the same handful of causes.
Dirty filters. As filters load, airflow drops, and the recovery device is no longer working at the volumes it was selected for. This is the most common cause of underperformance we find, and the easiest to fix.
Bypass dampers left in the wrong position. Almost all recovery systems have a summer bypass. When the actuator fails or the controls are wrong, the bypass can sit open through winter, and the recovery does nothing at all. It’s a fault that produces no alarm and no obvious symptom — the building still gets ventilation, it just costs far more to heat.
Fouled surfaces. A plate exchanger or a wheel matrix that has never been cleaned loses efficiency steadily. Nobody notices, because the decline is gradual.
Controls that were never properly commissioned. The most efficient recovery device in the world achieves nothing if the strategy driving it is wrong. We’ve found systems where the recovery was effectively disabled by a control setting made during commissioning and never revisited.
All four of these are maintenance and commissioning issues rather than design ones. Which is the real lesson: heat recovery is not something you install and forget. It needs to be in the maintenance regime, and it needs checking.
Lesson four: retrofit is usually possible, and usually harder than it looks
Adding recovery to an existing system is one of the most common requests we get, and one of the most variable in difficulty.
The question is rarely whether it can be done. It’s whether there’s space, whether the existing fans can handle the additional resistance, and whether the plant room can be accessed to get the equipment in. That last one catches more schemes than the first two combined.
Where the answer is no — and sometimes it is — a run-around coil arrangement or a partial solution may still deliver worthwhile savings. We’d rather propose something modest that works than something ideal that can’t be installed. Our guide to retrofit heat recovery systems covers the assessment in more detail.
What we’d tell a specifier
Model the actual building, not a generic one. Occupancy hours and climate matter more than the device’s headline figure.
Choose the technology for the constraints you have, not the efficiency you’d like. A run-around coil that fits beats a thermal wheel that doesn’t.
Budget for commissioning properly. More recovery systems underperform through poor commissioning than through poor selection.
Put it in the maintenance schedule. Filters, bypass dampers and heat exchange surfaces all need attention, and a recovery system that isn’t maintained quietly stops recovering.
Be honest about payback. A realistic five-year figure that turns out to be right builds more trust than an optimistic three-year one that doesn’t.
If you’re considering heat recovery on a new project or looking at retrofitting it to existing plant, we’re happy to look at the specifics. Have a look at our projects to see the kind of work we do, or get in touch and we’ll give you an honest view of what’s achievable.
