Vaillant Sine 18 Overheat reset

[halfbake]

I have an old Vaillant Sine18 T3 which was working fine until last week when for no apparent reason the boiler failed, no hot water or central heating but the pilot was on. Having trawled through these forums I discovered that the overheat reset had tripped (thanks to weargas reply to Beavis similar problem on this forum). Once the little button on RH the side of the PCB housing box had been reset the boiler fired up and both DHW and CH worked ok.

I was able to run a bath ok but when I turned the central heating on later in the day the boiler failed again and I once again had to reset the overheat switch.

I have now noticed that with either the central heating on or running hot water that the temperature gauge on the boiler above the pressure gauge never moves (remains on lowest mark) and I now suspect this may be related to the overheating problem.

Any suggestions would be much appreciated.

 

[compheat]

The overheat problem could be down to a few problems, the first one being a blockage in the main heat exchanger the second slow pump due to sludge in system. This boiler is very good but also getting old. try putting some sentinal x400 in system to loosen any sludge and do a mains flush.

 

[kevplumb]

I have now noticed that with either the central heating on or running hot water that the temperature gauge on the boiler above the pressure gauge never moves (remains on lowest mark) and I now suspect this may be related to the overheating problem.

moves from where ?? reading ?

 

[Agile]

Do you have the pressure at about one point two

There may be air in the system or dirt in the heat exchanger

Tony

 

[halfbake]

Thanks for the advice I will try the flush as soon as I can get the x400.

I can see the temp gauge thing was a red herring now as I traced the cable and it appears the gauge is a standalone thermometer and is not connected to the pcb in any way so I guess it is just faulty but not part of the problem(I put the sensor end in a cup of boiling water and it did not move of the mark).

The pressure was low and I bled the system including the bleed valve on the boiler itself (under the cover) and set the pressure to 1.1 cold but when I put the CH on the pressure eventually (after about 5mins) started to go past the red mark and eventually started pushing water out of the pressure relief valve before I noticed it and managed to shut the heating down, I guess this was at about 3bar. I must admit I did not notice this before but assume this is why the overheat switch tripped in the first place.

I have dropped the pressure down to about .4Bar cold and it still goes over the red mark when hot so this seems to confirm your blockage theory or dodgy pump.

I can run DHW all day no problem but it appears to be CH thats the problem.

Also just remembered that in the summer the first radiator was getting hot when DHW was used even with the CH turned off at the timer and room thermometer. In fact I bought the wax capsule thinking it may be that but never got around to fitting it, do you think this may be related or a secondary problem to the high pressure/overheat cutout.

Thanks for your help so far, much appreciated
halfbake

 

[kevplumb]

expansion vessel needs recharging

see FAQ at the top of the forum

 

[halfbake]

You guys are so bloody good!

Kevplumb your were spot on, I now have a working system thanks to you and everyone else here who helped me fix this problem.

Went out today and bought a £5 foot pump from Argos, pumped the expansion vessel up to 1bar (with the system drained down) refilled to 1.1 bar, job done. Max pressure sits at around 1.4 when hot and can run CH all day with no overheat cut out now.

Once again thank you so much for your help and expert advice, much appreciated.

 

[kevplumb]

thank you for letting us know you got sorted