Five years in, the burner came off and the heat exchanger was cleaned. The customer also asked for the system itself to be looked after: cleaner through the radiators for an hour, drained, and inhibitor added.
A Baxi 800 we have serviced since it was installed reached its fifth year, so this visit was a full strip-down service: burner and fan assembly off, heat exchanger and burner brushed clean, gaskets and seals replaced, combustion checked on the analyser. The customer also asked us to cleanse the central heating system. We ran Fernox F3 cleaner through the radiators for an hour, drained the system down, refilled it and added Fernox F1 corrosion inhibitor. Looking after the boiler is only half the job. The water going round the radiators is what wears a boiler out, and industry estimates put central heating sludge behind 60% to 80% of annual boiler breakdowns in the UK.
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This Baxi 800 has had a service every year since it went in. Most of those have been basic or intermediate services, which check combustion, safety devices, seals and the flue without taking the burner off. At five years we recommend the burner comes off, so this year was the full strip-down: fan and burner assembly removed, the heat exchanger brushed out, the burner cleaned, new gaskets and seals, and everything checked on the flue gas analyser afterwards.
The first photo shows the heat exchanger being brushed. The ring of deposit around the chamber is what five years of normal use looks like on a boiler that has been well looked after. It is not a fault, but it is the reason a full strip-down exists: left for another five years it becomes the fouled block that pushes combustion readings up and shortens the life of the boiler.
The customer also asked us to clean the central heating system while we were there. Servicing the boiler keeps the boiler right. It does nothing for the water going round the radiators, and that water is what wears a boiler out. Over time it picks up rust from the inside of steel radiators and forms a black sludge, magnetite, which settles in radiators, pipework, the pump and the boiler's own heat exchanger.
We added Fernox F3 cleaner to the system and ran it with the heating on for an hour, which loosens the sludge and scale so it can be moved. Then we drained the system down, refilled it, and added Fernox F1 protector, a corrosion inhibitor that stops the rust from starting again. That is the order that matters: clean first, then protect. Inhibitor added to a dirty system just protects the sludge.
Industry estimates generally put central heating sludge behind 60% to 80% of annual boiler breakdowns in the UK. The burnt-out pump in our Walsall overheat lockout case study is a typical example: the pump did not fail on its own, the sludge choked it. Cold patches at the bottom of radiators, a boiler that kettles and bangs, and a magnetic filter that fills between services are all the same warning.
A chemical cleanse like this one is the light-touch option and suits a system that has been reasonably maintained. For a system that is badly sludged, with radiators cold at the bottom and a filter that fills within weeks, a power flush is the proper fix. We will tell you which you need rather than sell you the bigger job.
A full strip-down service is £185, including the gaskets and seals and an inhibitor top-up. A basic service is £95 and an intermediate service £125. A system cleanse with cleaner, drain-down and fresh inhibitor is quoted alongside the service depending on the size of the system. Details of what each service level includes are on our boiler servicing page.
Every five to eight years for most boilers, with a basic or intermediate service in between. Every year for models whose heat exchangers foul quickly, Ideal and Vokera among them. This Baxi 800 had its first strip-down at year five, which is where we would put it for a well-maintained boiler.
A cleanse adds a chemical cleaner to the system, runs it for an hour or so, then drains, refills and adds inhibitor. It suits a system in reasonable condition. A power flush connects a pump to the system and forces water through each radiator at high flow to shift heavy sludge. It is the right choice when radiators are cold at the bottom or a filter fills quickly, and it costs from £550 for up to ten radiators.
The cleaner loosens and removes sludge and scale. The inhibitor stops fresh corrosion from starting inside the radiators and pipework, so the system stays clean. Inhibitor should be topped up at every service and replaced whenever the system is drained, which is why it is included in our strip-down service.
Yes. Industry estimates generally put sludge behind 60% to 80% of annual boiler breakdowns in the UK. Sludge blocks the pump, jams diverter valves, restricts the heat exchanger and causes overheating and lockouts. Most of the pump and valve failures we replace have sludge behind them.
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 on what each service level includes is on our boiler servicing page, and every job we write up is on the case studies index.
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Pick a slot online insteadA routine Complete service that found dark system water and no inhibitor. The customer was shown the water, agreed to the fix, and the system was dosed before we left. A five-star review followed the same day.
Read the case studyThe 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 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 studyAsk for a system cleanse with your next service. Clean first, protect second, and the boiler lasts longer for it.
