Hi,
When I run hot water the temperature fluctuates from scalding hot to cold. Also frequently cuts out when hot water used without heating. Have renewed DHW thermostat and 3 way valve but still the same. Have also tried flushing with FERNOX which I had done previously with a cheaper version prior to problem becoming more serious. Any ideas please as hard to find an engineer who will work on it.
Cheers

Andy McIntosh, June 2008

sounds like you need to replace the dhw heatexchanger as it has scaled up on the mains water side (hence the fernox not doing anything for you)

also, check your flow rates (with weir gauge & thermostat) to make sure you have adequate water flow at the draw off point

T, June 2008