vaillant turbomax tripping out

[gbrown58]

Hi, my vaillant turbomax VUW242E is tripping out via the boiler overheat stat. some times twice a day it might then go a week without tripping out. the boiler is about 12 years old I have changed the overheat stat for a new one and the NTC temperature sensor. However, the problem remains.
Bit mystified on this. Any ideas?
Also how do you check to see if the overheat stat has gone?
many thanks

 

[Gas2Air]

You will stay mystified.

 

[gbrown58]

You will stay mystified.

sorry but what do you mean?

 

[adlplumbing]

is their water in the radiators

and when heating is fully on
are the bottom rads getting warm

 

[gbrown58]

is their water in the radiators

and when heating is fully on
are the bottom rads getting warm

all radiators full of water and working fine. The same problem happened a few years ago I had several engineers out who fit various components however the problem reoccurred. It was suggested that the boiler needs replacing as the fault was not cured. An engineer suggested to me that it could be the main heat ex changer needed replacing or the pump was not working correctly. In other words he told me that it was to do with poor circulation in the boiler causing it to overheat. Heat ex changer was cleaned and also was the pump. Problem cured for a short time only to return.

 

[adlplumbing]

quite possible but have checked the bottom of the rads

 

[gbrown58]

quite possible but have checked the bottom of the rads

yes as said all radiators are working fine getting hot top and bottom

 

[adlplumbing]

then check what the return temp is then using a thermonitor

 

[gbrown58]

then check what the return temp is then using a thermonitor

when it trips out it has a temp of 47.5 degrees cel
and a flow temp of 59.7 degrees