worcester bosch CDi 28 Hot water overheat

[bigrad]

When I run the hot water at differing rates and at differing settings, the water is "ketteling" in the boiler, then the system shuts down. Logic tells the water in the boiler to hot water heat exchanger loop in over heating. Is this a simple fix of replacing the hot water limit switch visible behind the hot water heat exchanger. The system does shut down so Im guessing the main boiler limit stat is working.

 

[dcawkwell]

What does it do on central heating with just one radiator running?

 

[bigrad]

Cant tell, with just one rad operating the valves at siezed

 

[dcawkwell]

Cant tell, with just one rad operating the valves at siezed

If they are siezed it tells me all I need to know about the state of the pipe work and radiators. Flush out and new radiator valves before going any further.

 

[bigrad]

of the 7 radiators, it only needs two to be siezed and I cant opperated with just one rad on. Two siezed radiator valves doesnt tell you anything about the pipe work or radiators.
I get the one radiator test, to see if the same thing happens with CH loop only. BUT
I have now changed both c.h. limit stat and dhw stat . £40 and an hour to fit. seems okay keep you posted. Cheers

 

[bigrad]

Not much changed. With the hot water set at II and the tap half open the boiler behaves. Turn the tap full on and the boiler overheats AAAaaaaaaaaargh. I cant understand how, with more cold water going through the heat exchanger, it overheats. Is there a flow sensor that increases the gas as the water flow increases??