The tenant had no heating and a boiler locking out on overheat. The pump had burnt out, and the sludge that killed it was the real story.
A tenant in a landlord's property in Walsall reported the boiler locking out on overheat. An overheat lockout means the boiler is making heat that is not being carried away, and the usual reason is that the water has stopped moving. We found the system pump had burnt out. We carry pumps as van stock, so it was replaced on the same visit and the heating brought back. The pump had failed because the system was full of sludge, and we said so: we recommended a system flush, and advised the landlord that this repair was avoidable if the system had been regularly maintained.
It is often the pump, and often the sludge behind it. Leave a number and we will ring you back — usually within the hour during office hours.
The tenant of a Walsall rental property had lost their heating. The boiler kept going into overheat lockout: it would fire, the temperature would climb too fast, and the safety thermostat would shut it down. An overheat lockout is the boiler protecting itself from heat it cannot get rid of, and nine times out of ten the reason the heat is not going anywhere is that the water has stopped moving round the system.
The system pump, mounted on the pipework next to the unvented cylinder, had burnt out. With it off and turned over the reason was plain: the inlet and impeller housing were choked with black sludge. Sludge is the iron oxide that builds up in a heating system that has never been flushed or dosed with inhibitor. It settles in radiators and pipework, it gets drawn through the pump, and eventually the pump is trying to move something closer to mud than water. The motor overheats and fails, the circulation stops, and the boiler locks out.
A circulating pump is one of the parts we carry as van stock, because it fails often enough to be worth the space. The old pump came off, the new one went on, the pipework was reconnected and the system was refilled, vented and run up. Heating back on for the tenant, one visit.
Fitting the pump fixed the symptom. It did not fix the cause, and we do not pretend otherwise. We recommended a system flush to the landlord, and we told them plainly that this repair was avoidable: a system that is maintained, with a magnetic filter fitted and the inhibitor topped up at each annual service, does not produce sludge like this, and does not eat pumps.
For a landlord the arithmetic is simple. A power flush is £550 for up to ten radiators and a magnetic filter is £320 fitted. A burnt-out pump is a callout, a part and a tenant without heating. Left alone, the sludge that took the pump goes for the boiler's heat exchanger next, and that is a far bigger bill. Our landlord page covers how we bundle servicing with the annual gas safety certificate so this kind of thing gets caught early.
A boiler repair callout is £99, covering attendance, diagnosis and the first hour, with parts charged on top and quoted before we fit them. A pump is a modest part and we carry them. The flush and filter prices are on our power flushing page, and if you are not sure whether your system needs one, read do I need a power flush first.
The boiler is producing heat faster than the system can carry it away, so the water inside it gets too hot and the overheat thermostat shuts it down. The usual causes are a failed pump, a blockage, closed valves or air in the system. Sludge is behind a large share of them, because it blocks the flow and kills pumps.
Sludge is heavy iron oxide from corrosion inside the system. The pump draws it through and it packs into the impeller housing. The motor then has to work far harder to turn, it overheats, and eventually it fails. Once the pump stops the boiler cannot shift its heat and locks out.
If the old pump was full of sludge, yes. A new pump in a dirty system will fail the same way. A power flush clears the sludge from the radiators and pipework, a magnetic filter catches what is left, and inhibitor stops it forming again. Without that, you are buying pumps.
Annual servicing that includes an inhibitor check, a magnetic filter on the system, and a flush when the water comes out black rather than after the pump has died. We bundle a boiler service with the landlord gas safety certificate so the system gets looked at every year in one visit.
Job attended and photographed by our engineers in Walsall, April 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 Walsall page, and every job we write up is on the case studies index.
Four taps for a fixed figure.
What do you need?
What sort of job is it?
Here is the price range
Call us — do not wait on a form
We answer enquiries the same working day. If it cannot wait, ring 07413 385767.
Pick a slot online insteadA previous engineer had reported a leak on the safety check. We found it at the condensate collector, whose seals had perished. Everything came out of the combustion box to replace it, and the boiler went back together burning cleanly.
Read the case studySeveral engineers and a shower pump had not fixed it. The gate valve on the vented cylinder had snapped inside, turning freely but barely open. A full-bore lever valve in its place, and the tenants had proper hot water pressure for the first time in over a year.
Read the case studyThe old cylinder was leaking through the ceiling below. We quoted a replacement, the customer accepted, and a new 150-litre stainless unvented cylinder went in to current standards with the pipework insulated. G3 work, done by G3 engineers.
Read the case studyService and CP12 in one visit, magnetic filter £320, power flush £550. Call or WhatsApp us.
