Vaillant boiler will only run on high pressure above 2.7

[fixitflav]

I only top it up, via the Magnaclean,

How do you top up the pressure via the Magnaclean?

 

[Ron6a]

In your second pic it looks like the auto air vent has been leaking, has this been repaired ?

Yes, it was leaking. The little black valve on top was tightened down but it is a new one, as are the 2 others and it does not do it now.

 

[Johntheo5]

No, the pessure gauge shown is not near the pressure sensor, which when they changed them before, I think were to the left side. The HWF pump seems to be inside, next to the gauge just mentioned.
I would need to dismantle the shelf above the E.vessel to get a photo.

There seems to be something that looks like another white E.vesseL on top of that shelf?,
That red E.vessel should be teed into the UFH manifold return, if not, it may effect the boiler pump pressure rise and be part of the problem.

I asked above, when a new sensor is installed can or was the boiler pressure reduced?.

 

[Ron6a]

How do you top up the pressure via the Magnaclean?

Sorry I meant reducing pressure. Top up is by a feed in via flexible hose and 2 taps.

 

[Ron6a]

There seems to be something that looks like another white E.vesseL on top of that shelf?,
That red E.vessel should be teed into the UFH manifold return, if not, it may effect the boiler pump pressure rise and be part of the problem.

I asked above, when a new sensor is installed can or was the boiler pressure reduced?.

the white e.vessel is on top of the hot water cylinder (pic.4). What is above the red one is a plastic food tub.
The system works very well normally. It is only when I get this problem, that things go astray.
Yes the pressure is released, via the Magnaclean.

 

[Johntheo5]

I am interpretting that "the pressure is released via the Magnaclean" to mean that you can reduce the pressure to say 1.5bar after sensor renewal, then wonder if the boiler E.vessel charge/precharge is checked as well and pumped up, if it is then gradually looses air through its schrader valve then maybe the other E.vessel is adversely affecting the pump "kick".
I also wonder why a second E.vessel is required as 9 UFH, 90MX16 mm loops + boiler contents shouldn't be more than 100 litres. A 8L E.vessel with precharge and filling pressures of 1.0bar/1.5bar will only rise to 2.0bar assuming 45C mean UFH temperature. Has the external E.vessel precharge pressure ever been checked?.

 

[Ron6a]

I am interpretting that "the pressure is released via the Magnaclean" to mean that you can reduce the pressure to say 1.5bar after sensor renewal, then wonder if the boiler E.vessel charge/precharge is checked as well and pumped up, if it is then gradually looses air through its schrader valve then maybe the other E.vessel is adversely affecting the pump "kick".
I also wonder why a second E.vessel is required as 9 UFH, 90MX16 mm loops + boiler contents shouldn't be more than 100 litres. A 8L E.vessel with precharge and filling pressures of 1.0bar/1.5bar will only rise to 2.0bar assuming 45C mean UFH temperature. Has the external E.vessel precharge pressure ever been checked?.

Yes, by taking the draining nut out of the magaclean, I can reduce the pressure. Both E.vessels were checked the last time the sensor went.