Overheating/cut out on Worcester Bosch H/flow 400 Combi

23 Apr 2004
Reaction score
United Kingdom
Hello all

Over the past 2 weeks my boiler has started to overheat and cut out. It runs perfectly for 20mins approx from a cold start up (H/W or/both CH) then makes a 'back flushing' noise, vibrates then cuts out. I then wait for about 5mins or so before pressing the safety temp limiter reset button and the boiler starts again but cuts out after a few mins.
I have switched off the CH and ran DHW seperately and repeated vice versa but the problem persists.

Checks I have done:
1.There is no air in the system (bled the pump and rads) and no leaks
2. Pressure is at 1 bar@cold but flutuates to 2 bar when the noise starts before dropping down to 1bar once it cuts out.

Any ideas?

Many thanks

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Hve you tested ut in CH only with the hot water stat at off and the timer off at hot water.How old is your 400 ?
Hello COG

The Combi is 4yrs old.
I have ran the CH separately, with the DHW on the OFF switch and its thermostat in the off position as well, but the problem still persists.
Also ran the same with the DHW with CH OFF and its thermostat in the off position with the same results.

What part of the country are you in? Get a heating engineer who can diagnose (no plumber and no heating engineer that will fault find by part changing) the defect. To be able to do this, one has to have e general idea of how the boiler works.

Your boiler is not switching off when target temperature has been reached. There could ba any number of things that could cause this to happen. Only a good heating engineer will cunningly seek out the defective component to make your boiler well again.
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You have a restriction somewhere in the boiler if its not a hard water area then the first place to look would be the filter fitted inside the top manifold, instructions on how to remave it are in the fitting instructions book. If no instructions in your book then your boiler does not have a filter , Worcester started fiting filters about 4 years ago so you could be lucky. It takes about 10 mins to check if you are mamiliar with the boiler and about 45 mins if not as its a bit of a sod to get at.
I am living in the Cheshire area and, according to a goverment water website, it is a slight to moderate hard water area. I have stripped and cleaned the water filter as you suggested but the problem still persists.

Would dosing my CH system with descaler have a good effect on this?
Also, a plumbing friend reckons it could be a control panel problem but I would rather find out what the cheaper possibilities for the problem were.

Guirrez, you are clutching straws. Despite a perfectly clear post you do not want to take the logical route but a poke and hope to get the boiler repaired. You dose the boiler with chemicals (which should already have been the case) and the boiler still misbehaves. What do you do then?

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