If your existing air handling units are exhausting warm air straight to atmosphere in winter, you’re effectively heating the outdoors. Retrofitting heat recovery captures that waste energy and uses it to pre-warm incoming fresh air — reducing heating costs significantly without replacing the entire unit.
We’ve retrofitted heat recovery systems across commercial and industrial sites, and the energy savings are consistently substantial. Our Colmore Row project in Birmingham is a good example — upgrading the AHUs with improved heat recovery and fan assemblies delivered measurable energy savings that the client noticed immediately in their utility bills.
How Heat Recovery Works
The principle is simple. Your AHU extracts warm, stale air from the building and draws in cold fresh air from outside. Without heat recovery, these two air streams never interact — the extracted heat is wasted and the incoming air needs heating from scratch.
Heat recovery systems transfer energy from the outgoing exhaust air to the incoming supply air. Depending on the technology, recovery efficiencies of 50-80% are achievable. That means if your exhaust air is 21°C and outside air is 5°C, the incoming air could be pre-warmed to 13-18°C before it even reaches your heating coil.
The heating coil then only needs to raise the temperature by a few degrees instead of sixteen. The energy saving is obvious and immediate.
Retrofit Options
Not every building or AHU is suitable for every type of heat recovery. The right choice depends on existing ductwork configuration, available space, contamination risk, and budget.
Run-Around Coils are the most versatile retrofit option. Two coils connected by a pumped water circuit — one in the extract duct, one in the supply duct — transfer heat between air streams that don’t need to be adjacent. Efficiencies of 40-55% are typical.
This is often the only practical option for retrofit because supply and extract ducts rarely run side by side in existing buildings. The coils can be installed wherever there’s space in each duct run, with pipework connecting them. We’ve installed run-around systems in buildings where the supply and extract AHUs are on different floors — something plate exchangers or wheels can’t do.
Plate Heat Exchangers offer higher efficiency (60-80%) but require supply and extract air streams to pass through a single unit. For retrofit, this typically means modifying ductwork to bring both streams together — which may or may not be practical depending on plant room layout.
Plate exchangers have no moving parts, which keeps maintenance simple. Cross-contamination between air streams is eliminated by the sealed plate construction, making them suitable for applications where extract air contamination is a concern.
Thermal Wheels achieve the highest efficiencies (up to 85%) but again require supply and extract ducts to be adjacent. The rotating wheel transfers both heat and moisture between air streams. Wheels work well in new installations but can be challenging to retrofit due to space requirements.
There’s also a small amount of cross-contamination inherent in thermal wheel operation — typically 1-3% of extract air transfers to supply. This rules them out for applications with contaminated extract air (kitchens, laboratories, process extraction) but is acceptable for most office and commercial applications.
Is Retrofit Worth It?
The financial case depends on three things: how many hours the AHU runs, how much heating energy it consumes, and how much the retrofit costs.
For a typical commercial AHU running 10-12 hours a day through a UK heating season, run-around coil retrofit typically pays back within 2-4 years. Higher-efficiency plate exchangers may pay back faster despite higher installation cost because the energy savings are greater.
Systems running 24/7 — data centres, hospitals, manufacturing facilities — see the fastest payback because they’re recovering energy around the clock.
We always model the specific energy savings before recommending retrofit. Generic estimates are meaningless — the actual saving depends on your airflow rates, operating hours, temperature requirements, and existing system efficiency. Our engineers calculate projected savings based on your actual operating data so you can make an informed decision.
What’s Involved
Retrofitting heat recovery is more complex than bolting on a new component. It requires careful engineering to integrate with the existing system without compromising performance.
Survey: We assess the existing AHU, ductwork layout, and plant room space to determine which heat recovery technology is feasible.
Design: The heat recovery system is designed to match the existing airflow rates and pressure drops. Getting this wrong can starve the system of air or overload the fans.
Installation: Coils or exchangers are installed, pipework connected (for run-around systems), controls integrated with the existing BMS, and the system commissioned.
Commissioning: We verify that the heat recovery system is performing as designed and that the overall AHU system hasn’t been compromised by the additional pressure drop.
The work can often be completed during a planned shutdown or over a weekend, minimising disruption to building operations.
When Retrofit Isn’t the Right Answer
Sometimes full replacement makes more sense than retrofit. If the existing AHU is approaching end of life, investing in heat recovery retrofit may not be justified — better to specify a new unit with integrated heat recovery from the outset.
Similarly, if the plant room can’t accommodate the additional equipment, or if the existing system has fundamental efficiency problems beyond heat recovery, a wider energy saving upgrade may deliver better value.
We’ll always give you an honest assessment. If retrofit doesn’t make financial sense for your situation, we’ll tell you.
Talk to Us
i-Flow Technologies designs, manufactures, and retrofits energy-efficient air handling solutions across the UK. If you’re interested in exploring heat recovery retrofit for your existing AHUs, get in touch and we’ll arrange a survey to assess your options.
