Thermal wheels are one of the most effective forms of heat recovery available in an air handling unit, and one of the most commonly neglected. A wheel that has stopped turning, or that’s leaking around worn seals, quietly costs a building money for years before anyone investigates.
How a thermal wheel works
A rotary heat exchanger sits across the supply and extract air streams. As the wheel turns slowly through both, the matrix picks up heat from the warmer stream and gives it up to the cooler one. In winter you’re preheating incoming fresh air with heat you’ve already paid for. In summer the process works in reverse.
Efficiencies are high compared with plate exchangers and run-around coils, which is why wheels are specified on large systems where the volumes make recovery worthwhile. Our guide to HVAC energy recovery systems compares the options.
On the bespoke unit we supplied for Heathrow, thermal wheel heat recovery was central to the design, in a building where the ventilation load never lets up.
What goes wrong
The faults we find most often are mechanical rather than exotic:
- Drive belt failure. The wheel stops and nobody notices, because the unit keeps supplying air. Recovery drops to nothing.
- Worn or misadjusted seals. Air leaks between the two streams, which costs recovery and risks cross-contamination.
- Dirty matrix. Years of dust reduce heat transfer and increase pressure drop across both sides.
- Bearing wear. Noise and vibration, eventually seizure.
- Purge sector set up wrongly. The purge exists to limit carryover of extract air into supply. Set up badly, it either doesn’t do its job or wastes recovered heat.
- Controls doing nothing useful. Wheel speed should modulate and bypass should be used in mild weather, otherwise you’re recovering heat you don’t want.
A stopped wheel is a good example of why AHU commissioning and planned maintenance matter. Nothing alarms, and the building just costs more to run.
Refurbish or replace?
Refurbishment tends to make sense when the matrix is intact and the casing is sound. Belts, bearings, seals, drives and controls are all replaceable, and cleaning the matrix restores most of the lost performance. The wheel itself is often the longest lived part of the assembly.
Replacement tends to make sense when the matrix is damaged or corroded, the wheel is badly undersized for the current ventilation rates, or the unit is being substantially refurbished anyway and the economics of doing it all at once work out better.
Where there’s no recovery at all, retrofitting is worth exploring. Our guides to retrofit heat recovery systems and heat recovery ROI and payback periods cover the decision and the numbers.
Hygiene and cross-contamination
Because a wheel physically rotates between the two air streams, a small amount of carryover is inherent. That’s managed with the purge sector and correct pressure relationships across the unit, and it’s why wheels aren’t used in every application. Where cross-contamination cannot be tolerated, a plate exchanger or run-around coil is the safer choice.
The wheel surface also needs keeping clean, which we cover in our guide to AHU hygiene.
What to check on your own units
If you have wheels on site and haven’t looked at them recently, four quick checks are worth doing:
- Is the wheel actually turning?
- Are the seals in contact and in reasonable condition?
- Is the matrix visibly clean?
- Is wheel speed modulating and is the bypass working?
We’ve picked up all four on sites where maintenance was otherwise in good order. Our experience on the four units at Dalston Cross was that performance work often starts with the components everyone assumes are fine.
How we could help
We survey, refurbish and replace heat recovery devices as part of our AHU refurbishment services, and we design recovery into bespoke units on our air handling units page.
Get in touch and we’ll take a look at what you’ve got.
Frequently asked questions
Well maintained wheels achieve high recovery efficiencies compared with other recovery methods, which is why they’re used on large air volumes. Real world performance depends on seals, cleanliness and controls.
Sometimes, if there’s physical space and the supply and extract paths are adjacent. Where they aren’t, a run-around coil system is often the practical alternative.
As part of the unit’s planned maintenance, with belts, bearings, seals and drive checked routinely and the matrix inspected for cleanliness.
