The job in short
The HIU's primary strainer and heat exchanger were blocked, starving the unit of flow and causing hot water problems. After flushing out the debris from the system we got the hot water working again. The customer had been recommended a full HIU replacement elsewhere — we fixed it without any replacements, saving them thousands.
The Problem
We were called out to a flat that had lost hot water through its Heat Interface Unit. The customer had already been advised by another company that the unit was beyond saving and that a complete HIU replacement was the only option — a job that can run into thousands of pounds once the new unit, labour and making-good are accounted for.
Before condemning any unit, we always diagnose the actual fault. A loss of hot water on an HIU is far more often a flow or blockage issue than a failed unit — see our guide on why an HIU loses hot water.
What We Found
On inspection, the HIU's primary strainer — the mesh filter that protects the unit from debris carried in the building's heat-network water — was completely choked. Behind it, the plate heat exchanger was also clogged with the same sludge, so heat simply couldn't transfer across to the domestic hot water side.
This sludge is magnetite and corrosion debris that builds up in the wider heating system over time. When it collects in the strainer and heat exchanger, flow drops and hot water performance falls off a cliff — exactly what this customer was experiencing.
Photos From the Job
What We Did
Rather than rip the unit out, we stripped and cleared the strainer and flushed the debris out of the system, clearing the blockage from both the strainer and the heat exchanger. With the flow restored, we tested the unit through a full hot water draw-off and confirmed temperatures were back to normal.
No replacement parts were needed — the unit itself was perfectly healthy underneath the sludge. The customer kept their existing HIU and got their hot water back the same visit.
The Result
A working HIU, full hot water restored, and a customer who avoided a needless replacement costing thousands. This is exactly why we diagnose before we condemn — a blocked strainer is a clean, quick fix, not a reason to replace a whole unit.
The takeaway
It is vital to have your HIU serviced every year to prevent sludge build-up like this. An annual service clears the strainer, checks the heat exchanger and keeps your hot water and heating running reliably — and it's far cheaper than an emergency callout or a replacement you may not even need.
Find out more about what's involved in a proper annual HIU service, or read about our HIU powerflush service for systems with heavier sludge build-up.
Lost Hot Water? Don't Replace — Diagnose First
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