means overheated max temp exceeded so what is the remedy thanks in advance

John B, February 2011

Thanks Carefree for your professional and informative advice

John B, February 2011

The only side that gets blocked is the CH side, it'll be blocked with sludge/shale from the system, you can change the DHE and it will relieve the problem, but only temporarily, it will soon start to refill with the same system debris.
A full system clean is the proper answer. However, you could try a cheaper option of cleaning the DHE with a system clean acid, and fitting a sludge/debris filter such as a magnaclean or spirotech filter to the CH return. This would collect debris and sludge before it reaches the critical parts of the boiler.

Carefree, February 2011

Thanks for the info Carefree you say domestic H/Ex choked which side ? presume its not the draw off side so i could get my engineer to system clean the boiler

John B, February 2011

F20 occurs when the water is not clearing the main heat exchanger correctly. This usually occurs when running HW. It points to a badly choked domestic heat exchanger, the temprature rises rapidly when running HW (can reach 95+ degrees although flame shut down occurs around 84).
If it happens in CH mode (doubtful but possible).
Check the pump is operating (remove the pump centre screw and ensure the impellor is spinning).
Most likely cause DHE, possible secondary cause pump.

Carefree, February 2011