Corrosion is the quiet killer of air handling units. Coils and fans get replaced as a matter of course, but once the casing and frame start going, the decisions get harder, because the structure is what everything else is fixed to.
This is what we look for, what we can put right, and when we tell clients the unit has had its day.
Where corrosion starts
In our experience it nearly always begins in the same places:
- Around the cooling coil and drain pan, where the unit is wet by design. Blocked drains and standing water accelerate everything.
- At the base frame, particularly where units sit on flat roofs or in plant rooms that flood occasionally.
- At panel joints and fixings, where the coating is broken and moisture gets behind it.
- On external units, where weathering and poor sealing let water track inside.
- Around humidifiers and any other component that introduces moisture.
Older galvanised and painted mild steel units suffer most. Once the protective layer is breached, corrosion spreads under the surface faster than it appears on it.
Why it matters beyond appearance
A rusty panel looks bad, but the real problems are functional:
- Air leakage. Corroded seams and panel edges leak, so fan energy is wasted pushing air that never reaches the building.
- Insulation failure. Once water gets into the panel, insulation becomes waterlogged, loses its thermal value and becomes a hygiene problem.
- Structural weakness. The frame carries the coils, fans and access doors. Weak frames mean doors that don’t seal and components that move.
- Hygiene. Corroded surfaces can’t be cleaned properly, which matters a great deal in healthcare and food environments. See our guide to AHU hygiene.
What can be repaired
Plenty, if it’s caught early enough:
- Localised panel replacement. Individual panels can often be swapped without touching the rest of the unit.
- Base frame repair or replacement. Where the frame has gone at the base but the structure above is sound, sections can be cut out and replaced.
- Drain pan replacement. Stainless replacement pans are a common upgrade on units where the original has corroded through, and they solve the problem permanently rather than delaying it.
- Re-sealing and re-insulating. Joints resealed, damaged insulation replaced, access doors rehung so they seal properly.
- Protective treatment. Surfaces prepared and coated where the metal is sound.
This work usually runs alongside other refurbishment. Our guide to what’s involved in an AHU refurbishment sets out the wider process.
When it’s past economic repair
We’ll tell you when repair stops making sense. The usual signs are:
- Corrosion through the main structural frame rather than just panels
- Widespread panel failure rather than isolated areas
- Wet insulation across much of the unit
- A unit that’s also undersized or poorly matched to current requirements
At that point you’re better investing in replacement, which is a chance to get a unit sized for what the building does now. Our guides to AHU refurbishment vs replacement and signs your AHU needs replacing go into the decision.
Where access is the obstacle, and it often is on older plant rooms, flat-pack construction solves it. We’ve built units in situ on plenty of sites where nothing else would fit through the door, including the retail complex in Epsom. Our flat-pack AHU guide explains how it works.
Preventing the next round
Corrosion is largely a moisture management problem. Keeping drains clear and traps correctly configured, fixing leaks quickly, keeping panels sealed and treating small breaches early all extend a unit’s life considerably. It’s routine work that pays for itself, which is the argument for a proper maintenance contract.
How we could help
We survey, repair and rebuild air handling units, and we manufacture replacements where repair no longer stacks up. Visit our AHU refurbishment services page or get in touch for a site survey.
Frequently asked questions
Often, yes. Sections of base frame can be replaced on site where the rest of the structure is sound, without removing the unit.
Treating and coating sound metal is worthwhile as part of wider works. On its own, over corroded metal, it hides the problem rather than fixing it.
It varies with environment and maintenance. Units kept dry inside and well maintained last far longer than those with persistent drainage problems or water ingress.
