Booked as a Complete service on a loft apartment's Caleffi heat interface unit. The water that came out was dark and there was no inhibitor protecting the system. The customer was told, agreed, and it was dosed before we left. Five stars on Google the same day.
A customer in a loft apartment in Birmingham's Jewellery Quarter booked our Complete HIU service on a Caleffi heat interface unit. The Complete package includes an inhibitor check, and that is where this one turned: the water drained from the system was dark brown and there was no corrosion inhibitor in it at all. We showed the customer, explained what that does to a plate heat exchanger and a pump over time, and with their agreement dosed the system with inhibitor before we left, on the same visit. The rest of the service went as booked: strainers cleaned, valves serviced, pressure and expansion vessel checked, temperatures checked at the outlets, and a written report. The customer left a five-star Google review that afternoon.
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An online enquiry from a loft apartment in the Jewellery Quarter, where most of the converted buildings run on communal heating with a heat interface unit in each flat. The customer wanted a proper annual service rather than a look-and-leave, so we booked the Complete package: everything in the Essential service plus the secondary strainer, valve servicing, an inhibitor and expansion vessel check, and performance fine-tuning.
The Caleffi unit itself was in reasonable order. The problem was in the water. When we drained a sample from the secondary side it came out dark brown, the second photo, and the inhibitor test showed nothing in the system. A HIU has its own small sealed circuit on the apartment side, with a pump, plate heat exchangers and control valves, and untreated water in that circuit does the same thing it does in a house: rust forms, magnetite sludge settles in the narrow channels of the heat exchangers, and the pump and valves wear early.
We showed the customer the sample and explained it plainly. There was no pressure to spend more on the day, but the honest advice was that dosing the system now was cheaper than a heat exchanger later, and we could do it there and then. They agreed.
Both strainers cleaned, the control valves serviced, the pump and expansion vessel checked, the system pressure set, and inhibitor added to the correct dose for the circuit. Temperatures were checked at the hot tap and on the heating side, the unit was run through both modes, and the customer got a written report of what was done and what was found.
"Highly recommend. Sayeed has been fantastic since I first enquired right up until completion of the service, with great communication every step of the way. Will definitely be using Synergy Gas & Plumbing for my annual HIU service from now on."
Lia P. · Google review, the day of the service
HIUs are often installed and then forgotten, because there is no gas appliance in the flat and no annual gas safety check to prompt anyone. The unit still has moving parts and a water circuit that needs protecting. Our city-centre flow switch repair and the jobs on our HIU service page show what happens when a unit is left: no hot water, usually at the worst time. An annual service that actually checks the water is the cheap version of that story.
Our HIU services are £95 for Essential, £115 for Complete and £170 for Premium, and the Complete package is the one we recommend for most flats because it includes the inhibitor check that found this. Emergency HIU repairs are £225 including up to three hours of labour. Full details on the HIU service and repair page, or read what a heat interface unit actually is.
NoneYes. The apartment side of a HIU is a small sealed heating circuit with a pump, plate heat exchangers and control valves, and it corrodes like any other if the water is untreated. Inhibitor should be checked at every service and topped up or re-dosed when it is low or absent.
Essential covers the core checks: visual inspection, strainer clean, pressure top-up, venting, temperature checks and a written report. Complete adds the secondary strainer, valve servicing, an inhibitor and expansion vessel check, minor leak repairs and performance tuning. The inhibitor check is why we recommend Complete for most flats, and it is what found the problem on this job.
Rust. With no inhibitor in the circuit, the steel in the system corrodes and the water carries the result around as fine black or brown sludge. Left alone it settles in the plate heat exchangers and the pump. Dosing the system stops the corrosion; a badly sludged system may also need cleaning first.
Yes, along with Hiper, Meibes, Danfoss, Evinox, Altecnic and most other makes found in Birmingham's apartment developments. Tell us the make when you book and we will bring the right strainer seals and test kit.
Job attended and photographed by our engineers, September 2026. Written up by Saeed Iqbal, the engineer who leads Synergy Gas & Plumbing (Gas Safe registered 594385). More recent jobs in the area are on our Birmingham page, and every job we write up is on the case studies index.
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Pick a slot online insteadThe diverter valve was packed with sludge and came off the van on the first visit. The plate heat exchanger behind it was blocked too, so it was ordered and fitted on a return visit. Landlord and tenant both sorted.
Read the case studyBurner off, heat exchanger brushed clean, gaskets replaced. Then cleaner circulated through the radiators for an hour, the system drained, and corrosion inhibitor added.
Read the case studyRust at the back of the heat exchanger and a carbon monoxide reading on the analyser, in a house with a young family. We capped the boiler off, told the tenant and landlord, and replaced the cracked flue collector and gaskets with the landlord's permission.
Read the case studyA Complete service is £115 and includes the water check that found this. Book online or ring the office.
