my shower starts of hot then runs cold with vokera excell combi boiler?

Try googling 'Vokera venturi cycling' or similar combinations. Very often caused by the differential air pressure switch operating intermittently. The primary heat exchanger needs cleaning, and the fan & flue venturi removing for a proper clean of grime (cant do it properly in situ). Since this involves opening the sealed combustion chamber you should not realy attempt this unless you are Corgi registered. If the flame goes from full to cut off to full this is probably your problem, if the flame modulates down, and the temperature guage still reads high, and then cuts out, it may be over temperature in the primary loop caused by a furred up secondary heat exchanger that is your problem. The venturi is the more likely culprit and a cheaper and easier job to fix. If you DIY be very carefull that you get the sealed chamber cover back on properly, mistakes in reassembly could allow (poisonous) combustion products into your living space.

C
August 2007

thanks for that Andy, had been told the heat exchanger could be blocked but needed conformation.

john
August 2007

could be a couple of different issues, depends on the boiler type? If it is a combi then run the tap to see if the flames at the boiler are continually on or if they cycle on and off. If it cycles there could be an issue with schale in the secondary heat exchanger which would need flushing out. Other possibilities are thermistors out of calibration or possibly the shower mixer itself being faulty

Andy
August 2007