Vogue C40 - CH temp problem.

[nickburt]

Hi all,
I have Vogue C40 boiler that is around 9 years old.
Sadly lacking in regular servicing !!!
Recently it has been running the CH water temp too high.
We normally have the CH water temp set to 60deg, but in the last couple weeks, it has been heating to 70+ (despite the stat being set at 60) and, therefore increasing the pressure, sometime beyond 3 bar.
The cold pressure is normally at 1 bar, and goes up to 2 ish when the temp gets to 60, but with the temp going above 70 and the pressure beyond 3 bar, it's dumping water through the relief valve and then when cools down the pressure often drops below minimum, resulting in a top being needed.
I've just done a system flush (using a Norstrom Proflush), but this doesn't seem to have helped. Will be reflushing this evening and refillint with inhibitor tomorrow)
I'm suspecting the flow thermistor at the moment because of the temperator overshoot from what it's set to. I've just tried with the CH water temp set to 50 deg and it still went up to over 70.
I also think the system needs a bigger expansion vessel, which I'll be looking into getting fitted very soon.

Any ideas, thoughts etc. greatfully recieved.
TIA.

 

[CountryFan]

Sounds like the expansion vessel needs charging up again. As for the temperature increase, could be a few things, such as the heat exchanger partially blocked, weak pump et al.

 

[nickburt]

Sounds like the expansion vessel needs charging up again. As for the temperature increase, could be a few things, such a small the heat exchanger partially blocked, weak pump et al.

Yup, thanks, expansion vessel pressure was down, so that's now up at 0.75 bar as per the manual.
Just the temp control to sort now.
Just restarted for cold with CH temp set at 50, about 1/2 hr or so running and the temp is up to 62 and the burnder still running.

 

[nickburt]

5 mins later:
CH temp 70 and boiler still on
Pressure is just under 2 bar - so expansion vessel pressure restored seems to have sorted that.

My thoughts now is that since the setpoint is 50 deg and boiler is still heating the water, there must be a fault with the temperatur control? Surely, when the water temp reaches the set point, the burner should shut down?

Edit: another 5mins and temp is at 76.
Max flow temperature is set to 75 deg, yet the boiler still heats the water beyond that.

 

[CountryFan]

Usually it would, so could be out of calibration.

 

[nickburt]

Usually it would, so could be out of calibration.

Interesting thought.
I've just dropped the max flow temperature to 70 from 75 and the boiler let the water temp drop to 62 initially, but is now back up at a steady 70, even though the set point is still set at 50.

 

[nickburt]

Almost an hour later
Temp was still holding at 70
Have now dropped max flow temperature to 60 (which is the temp we normally run it at) and it's stablised at 60 +/- 1.
Set point still at 50.

 

[nickburt]

Next morning:
Pressure OK (1 bar cold, around 2 bar @ 60dg)
Temp at 60
So the only issue to resolve is the dial setting of the setpoint temp not working. Still set at 50, but with max flow temp set to 60, that's what it's running at.