In most commercial buildings, heating, ventilation and cooling account for somewhere around 40–50% of the entire electricity bill — and the air handling units are usually the single largest part of that. The frustrating bit is that a lot of that spend is avoidable. An AHU that’s ten or fifteen years old will often be running at full tilt around the clock to do a job that needs a fraction of the energy, simply because the fan, the controls or the scheduling have never been touched since the day it was commissioned.
The good news is that you very rarely need a full replacement to fix it. Most of the savings sit in components and controls that can be upgraded in isolation, often over a weekend, with the unit otherwise left exactly as it is.
Here’s where the energy actually goes, and what’s worth doing about it — roughly in order of payback.
Start with the fan motor — it’s almost always the prize
The fan is the part that runs continuously, so it’s where the money is. The big divide is between older belt-driven AC fans and modern EC (electronically commutated) or direct-drive fans.
Belt-driven fans lose energy through the belt itself, run at a fixed speed whether the building needs it or not, and need ongoing belt maintenance. Swapping them for EC plug fans with variable speed control is the upgrade that moves the needle most. In real installations, an EC fan retrofit can cut a unit’s annual fan electricity by somewhere in the region of 30–60%, and in well-matched cases the saving climbs higher still. Birmingham Airport, to take a documented example, cut the energy use of a terminal AHU by more than half by replacing the fan assembly with a grid of high-efficiency EC plug fans.
EC fans are also “soft” upgrades — they’re compatible with most existing units, so the retrofit is usually straightforward and low-disruption.
A quick worked example. Say a unit’s supply fan draws 7.5 kW and runs 4,000 hours a year. That’s 30,000 kWh, or roughly £7,500 a year at around 25p/kWh — for one fan. A 40% reduction from an EC retrofit is about £3,000 a year back. On a retrofit costing a few thousand pounds, you’re looking at a payback inside two to three years, and a quieter, lower-maintenance unit afterwards.
We did exactly this on a retail scheme in Epsom, where replacing the original belt-driven fans with EC plug fans cut the units’ fan energy substantially and quietened them at the same time.
Add variable speed control so it stops running flat-out
A fixed-speed fan delivers the same airflow at 7am on a packed Monday and at 2am in an empty building. Variable speed drives (VSDs, sometimes called VFDs) let the fan ramp up and down to match actual demand, which is where a lot of the waste hides. Typical payback on a VSD retrofit sits around 1.5 to 3 years for fan applications, which is why it’s usually one of the first things recommended when a building is being brought up to standard. If you’re fitting EC fans, the variable speed control is effectively built in.
Recover the heat you’re currently throwing away
Every time an AHU exhausts stale air and pulls in fresh, it’s dumping conditioned air — air you’ve already paid to heat or cool — straight outside. A heat recovery device (a thermal wheel or plate heat exchanger) captures a large slice of that energy and uses it to pre-condition the incoming air. Good systems recover anywhere from 40% up to 80% of the thermal energy, which directly reduces the load on your heating and cooling coils. On units that don’t have it, retrofitting heat recovery is one of the bigger structural wins available.
Then clear the cheap wins — they cost almost nothing
Before any capital spend, there’s a tier of fixes that are close to free and often overlooked:
Filters and pressure drop. A clogged or wrong-spec filter forces the fan to work harder to push the same volume of air, quietly adding to the bill every hour the unit runs. Staying on top of filter changes is the simplest efficiency measure there is.
Scheduling and controls. A surprising number of units run full ventilation overnight, at weekends, or in unoccupied zones. Tightening the run schedule to actual occupancy is pure saving with zero hardware.
Free cooling / economiser. When it’s cool enough outside, the unit can use outside air to cool the building instead of running the cooling coil. If the economiser dampers are seized, miscalibrated or disabled, you’re paying to cool air you could have cooled for nothing.
Coil cleaning. Biofilm and dirt build up on coils and act as an insulator, reducing heat exchange and increasing the pressure the fan has to work against. A dirty coil is a double hit — worse performance and higher energy use.
When does a refurb beat a full replacement?
A new AHU is a significant capital project, and often it isn’t necessary. If the casing and coils are sound and it’s the fan, motor and controls that are dragging efficiency down, a targeted refurbishment — EC fans, variable speed control, recalibrated controls, maybe heat recovery — can deliver most of the energy benefit of a new unit at a fraction of the cost and disruption.
Replacement makes more sense when the unit is at genuine end of life: corroded casing, failed coils, parts no longer available, or a unit so badly oversized for the current building use that no amount of control tuning will fix it. The honest answer usually comes from measuring the unit’s actual consumption first, then comparing the refurb and replacement numbers side by side — not from assuming the worst.
There’s a compliance angle too
This isn’t only about the bill any more. Tightening minimum energy efficiency standards mean poorly performing buildings are becoming harder to let and harder to insure, and EPC ratings are increasingly tied to a building’s mechanical plant. Keeping your AHUs efficient and properly maintained — with a documented service history against a recognised standard like SFG20 — protects the building’s rating and its value, not just its running costs.
Where to start
The single most useful thing you can do is measure. A short survey that logs the unit’s actual power draw and airflow over about a week tells you exactly where the energy is going and which upgrade earns its keep — rather than spending money on a hunch.
At i-Flow Technologies we survey, maintain, refurbish and replace air handling units across the UK, and we’ll always tell you straight whether the answer is a £3,000 fan retrofit or a new unit. If your AHUs are getting on, or your energy bills have crept up and you’re not sure why, get in touch and we’ll take a proper look.
