Worcester Bosch 350 Combi DHW boiler shutdown

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17 Apr 2009
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Location
Sussex
Country
United Kingdom
My Worcester Bosch Boiler shuts down on overtemp when the DHW is required. The Boiler works fine on CH. When the hot water tap is opened the boilers starts fine and then boiling inside the boiler can be heard after approx 5-10seconds. The boiler then shuts down (thermo stat trip on boiler).

I have changed/cheched the below:

DHW temp sensor and thermostat.
Airvent near gas/water heat exchanger
Removed water/water heat exchanger - visually checked no signs of scale. I did blow through the EX and seemed all clear.
Checked DHW pump runs
Swaped over DHW and CH pump (including wiring)
Placed the DHW sensor in a bath of very hot water and checked the boiler shuts down when DW is up to temp.
I have checked the voltage on the gas modulating valve. This seems to stay high approx 18V for the DHW, but the boiler overheats before this modulates the gas.

The only way to get DHW is to run the CH upto temp and then select HW. this seems to work.

Any suggestions on what to try next - please help!!!

Dave H.
 
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The system has two seperate pump for the CH and DHW, no diverter valve is fitted. The only item I have not inspected is the one way valves fitted to each pump. However these has small test plungers that you can press - seem OK. I will remove the water/water heat exchanger again today and see if I can flush it out evan though it looks OK.
 
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its fairly comman for the pipe that runs from Water pump to the DHW HEX to get blocked up

its pressed into the infrastructure of the boiler and is impossible to get to, if you have some circulation I suppose it is possible to flush it out but I have never had any success

but then I am shyte at flushing

:)
 

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