Worcester 35DCi RDF Combi - no constant hot water

Worcester 35DCi RDF Combi is the description off the front of the boiler - it went in in the summer of 2001. It doesn't say 11 on the lid and so I think it is the first boiler. It is not a greenstar either. When a hot water tap is in full flow the water runs very hot, then runs cool, then very hot alternatly. Whilst it is doing this the HW demand light and the burner light remain on constantly. If the demand HW temparature control knob is adjusted to the lowest or the highest the temparature of the hot water does not change nor do the symptoms. If anything when the hot water is turned up full the flow turns cool more quickly - difficult to be sure this is the case though. From reading other posts there isn't 1 that exactly matches but similiar posts would suggest I need a new DHW Sensor? Switching off the boiler and removing the lid I can see the sensor attached to the pipe by a simple screw and the manual suggests I need some paste to attach it, the sensor only costs £10, is a boiler engineer required to fit it? Or is it more likley to be a different problem? The wife is driving me crazy - the plumbers I have spoken to want £50 to look at it which I don't mind if they fix it at the same time - so any help would be much appreciated. Thanks

theres a good chance its the thermistor. worcester thermistors usually come with a small packet of heatsink paste anyway.

turn off the electrics to the boiler at the wall or consumer unit and give it a go.

No change annoyingly - so if I had thought the CH wasn't working so well this winter but had put it down to the cold - but now am thinking the CH maybe doing the same thing that the HW is doing and so is not as hot overall - does this mean there is something more fundemental wrong? the CH and HW have different sensors so it must be a fault further back in the circiut?
Any help gratefully recieved.

remind me which boiler you have. two pumps or one?

the boiler may take a reading from the primary t/m during HW use. i would need to check that though.

One pump version believe from what I have read on other posts

Sound like a blocked plate heat exchanger to me!

mrgas said:

Sound like a blocked plate heat exchanger to me!

Click to expand...

Agree, I had a 24 cdi with same symptoms , changed the DHW hex and it was working perfectly.

Thanks - I'll try that next. I assume this is a job for a boiler engineer? Anyone got any idea how long it should take? Can it be unblocked or to replace?