Energy recovery is one of the most effective ways to reduce HVAC running costs. The principle is straightforward — instead of exhausting conditioned air and heating or cooling fresh air from scratch, you transfer energy between the two streams. The savings are immediate and, depending on the system, substantial.
We design and manufacture air handling units with integrated energy recovery for clients across the UK. Whether it’s a new installation or a retrofit to an existing system, we specify the right technology for the application — because the wrong type of recovery system can create more problems than it solves.
Why Energy Recovery Matters
A typical commercial building exhausts air at around 21-22°C in winter. Outside air coming in might be 2-5°C. Without energy recovery, your heating system has to bridge that entire gap — and it does so constantly, every hour the building is occupied.
The numbers add up fast. A medium-sized office AHU handling 5m³/s of air, running 10 hours a day through a UK heating season, can waste tens of thousands of kilowatt hours annually in rejected heat. At current energy prices, that’s real money leaving through your exhaust duct.
Energy recovery captures 50-85% of that waste depending on the technology used. Our Colmore Row project in Birmingham demonstrated this clearly — upgrading the energy recovery and fan assemblies on an existing office block AHU delivered measurable savings that the client saw reflected in their utility bills immediately.
Types of Energy Recovery
Plate Heat Exchangers
Plate exchangers pass supply and extract air through a matrix of thin plates. Heat transfers through the plates without the air streams mixing. No moving parts, no cross-contamination risk, minimal maintenance.
Typical recovery efficiency sits between 60-80%. We specify plate exchangers where cross-contamination must be eliminated — laboratories, healthcare facilities, and any application where extract air contains contaminants that can’t be allowed into the supply stream.
The main limitation is that supply and extract ducts must pass through the same unit. In new installations this is straightforward. In retrofits, it can require ductwork modifications that aren’t always practical.
Thermal Wheels
Rotating thermal wheels achieve the highest efficiencies — up to 85% in some configurations. A slowly rotating wheel absorbs heat from the exhaust air and releases it into the incoming supply air. Some wheels also transfer moisture, which can be beneficial for humidity control.
The trade-off is a small amount of cross-contamination (typically 1-3%) as the wheel rotates between air streams. This rules them out for applications with contaminated exhaust, but for standard commercial ventilation they’re an excellent choice.
Thermal wheels do have moving parts — the wheel, drive motor, and seals — which means they need regular maintenance. We factor this into our maintenance schedules for any installation using wheel-based recovery.
Run-Around Coils
Two coils connected by a pumped water circuit — one in the extract duct, one in the supply duct. Heat transfers from extract air to the water loop, then from the water loop to the supply air.
Efficiencies are lower than plates or wheels (typically 40-55%), but run-around systems have a unique advantage: the supply and extract ducts don’t need to be anywhere near each other. We’ve installed run-around recovery systems where the supply and extract AHUs are on different floors of the building — something neither plates nor wheels can achieve.
This makes run-around coils the go-to technology for retrofit projects where modifying existing ductwork to bring both air streams together isn’t feasible.
Heat Pipes
Heat pipes use a sealed refrigerant circuit to transfer energy passively between air streams. No pumps, no moving parts, virtually zero maintenance. Efficiencies of 40-60% are typical.
Heat pipes suit specific applications where simplicity and reliability are priorities. They’re less common in commercial HVAC but worth considering for industrial applications where maintenance access is limited or operating conditions are harsh.
Choosing the Right System
The decision isn’t just about efficiency numbers. We consider the full picture before recommending a technology.
Application requirements come first. Is cross-contamination acceptable? What are the humidity requirements? What are the operating hours? A laboratory needs a different solution to an office block.
Physical constraints often dictate the options. Available plant room space, existing ductwork layout, and access for maintenance all influence the practical choices. The most efficient technology on paper is worthless if it doesn’t fit the building.
Whole-life cost matters more than purchase price. A thermal wheel delivers higher efficiency than a plate exchanger but costs more to maintain. Over a 20-year equipment life, the total cost picture may favour either option depending on operating hours and energy costs.
Redundancy and resilience are critical for some applications. Plate exchangers and heat pipes have no moving parts to fail. Wheels and run-around pumps introduce mechanical components that could interrupt recovery if they malfunction.
Summer Operation
Energy recovery isn’t just for winter. In summer, the same systems work in reverse — the cooler exhaust air from an air-conditioned building can pre-cool the warmer incoming supply air, reducing cooling loads.
Most systems include bypass arrangements for mid-season conditions when recovery isn’t beneficial. Our control strategies automatically engage or bypass recovery based on actual conditions, ensuring the system only operates when it delivers genuine savings.
Integration With Building Management
Modern energy recovery systems integrate with BMS platforms to provide monitoring, control, and performance data. We design our AHU control systems to report recovery efficiency in real-time, allowing facilities teams to verify that systems are performing as designed and identify any degradation early.
This data also supports compliance reporting for energy performance regulations and carbon reduction commitments.
Our Approach
At i-Flow Technologies, we don’t default to a single recovery technology. We assess each project on its merits — the building, the application, the constraints, and the client’s priorities — and recommend the solution that delivers the best balance of performance, practicality, and value.
Whether you’re specifying a new bespoke AHU with integrated recovery, looking to retrofit recovery to existing equipment, or want advice on improving the efficiency of your current systems, get in touch and we’ll talk through your options.
