Biasi Combi Domestic Hot Water Not Regulating

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We have a Biasi Prisma 280SE combi boiler fitted. When the hot tap is opened fully (really good flow) the boiler maintains a temp of 60 C but when just the shower is used (big rain can head with mixer on a cool as it will go but still runs very warm) the temp rises to 90 C before it cuts out. I assume this is the high safety setting. The temperature doesn't respond to the position of the settings on the control dial and will do as described no matter what the position of the dial. I have swapped out the main PCB for a known good one and fitted a new thermister on the DHC but the problem persist. The central heating appears to work as it should.

Any advice greatly appreciated.

John
 
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If the fault only exists at the shower, then that's where I'd be looking for faults.
Shower thermostat maybe?
 
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Thanks for the reply. I only used the shower as an example, the same effect can be had by throttling the hot tap in. The boiler just isn't regulating as it should and if the flow is too low then the water just get hotter until the high high cut out is reached.
 
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Did you check the thermistor on the boiler flow (primary circuit) as this water circulates through the PHE and heats the DHW.
Maybe just put the CH on for 30 min or so and turn down the CH temperature control and see does this control to its setpoint and the boiler should modulate down.
 
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I switched the heating on and it works normally, all radiators toasty and boiler cutting in and out as it should. No change with the DHC it still gets hotter and hotter, could this point to the thermistor on the primary circuit?
 
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I changed the thermistor but no change. I would appreciate any more suggestions please as the wife is getting fed up of hot then cold showers.
 
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I switched the heating on and it works normally, all radiators toasty and boiler cutting in and out as it should. No change with the DHC it still gets hotter and hotter, could this point to the thermistor on the primary circuit?
yes if the primary thermistor is faulty the boiler will cycle on and off using the hi-limit stat just change it
 
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I changed the thermistor but no change. I would appreciate any more suggestions please as the wife is getting fed up of hot then cold showers.

Assuming it's a modulating gas boiler shouldn't it modulate down first before cycling which it will then do when it's minimum output is greater than the rad demand and the flow temp reaches its SP+5C, it should modulate on DHW as well if DHW SP is changed or if flowrate is reduced.
 
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Yes it should modulate but even though it seems to control the heating OK the burner is either on or of, no half measures. The DHW doesn't respond at all to the temperature controller, it either holds the temperature around 60 C with a tap on full bore or , if the tap is throttled in, continues to rise to around 90C then cuts out until it cools off. This is with both thermistors replaced.
 
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Something on the control side so?, if not modulating in either CH or DHW.
 
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I'm guessing the gas valve either isn't responding to the signal or the actual controller is faulty, but since the problem persists even with a new PCB then maybe it's the valve. I'll get the multi meter out tomorrow.
 

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