Vaillant boiler will only run on high pressure above 2.7

[Ron6a]

Hi. My Vaillant boiler will only run on high pressure above 2.7bar. Anything below that and the dreaded F75 low pressure warning comes up and I have to put more water in till it gets to 2.8.
The sensor has been changed at least 3 times on this 6 year old boiler. It runs an underfloor heating system and the only metal parts are the boiler itself and 3 circulation pumps. Was power flushed in 2021.
Any thoughts please?

 

[Johntheo5]

It also senses the pressure rise when the circ pump starts, the boiler pressure should rise by ~ 0.25 to 0.5bar when the pump starts, have you observed this?, have you a system boiler with the circ pump internally in the boiler?.

 

[Ron6a]

Yes. It does this as soon as the system fires up.
It is an Eco tech 630 and I 'think' it has a pump internally and 2 Grundfos pumps for the ground and 1st floor pipe runs.

 

[Johntheo5]

If it has a internal pump then should have a internal E.vessel which presumably is connected in to the pump suction side because you are seeing the pressure rise.
What does the pressure rise to with a hot system?, the boiler PRV lifts at 3.0bar.

 

[Ron6a]

It is running now and is a 3.0

 

[Johntheo5]

Do you have to top it up regularly or do you have a auto fill arrangement, if it is lifting the PRV every tme it heats up then all that fresh water won't do the boiler any good, if it is lifting the PRV then you should see some evidence of this, maybe a small pipe sticking out of the wall at the boiler rear.
I wonder is the boiler sensor on the same tapping point as the "pressure gauge", have vaillant suggested anything?, it would be nice to see a schematic of your boiler internally.
When the boiler is running normally and if you very slowly bleed off some water from a rad or where ever to reduce the pressure, does F75 annunciate?.

 

[Ron6a]

I only top it up, via the Magnaclean, when I get the F75 message and this has only fixed it the last 2 times (since Feb this year). Before then was about a year ago when I got Vaillent in and they changed the sensor. Just saying that it gets dirty. The last time I had it all flushed through.
I believe that the pressure slowly drops, as there is a slight seeping from a connection on the hot water cylinder, which, I only see when the boiler fails, does not produce any hot water and so seeping water does not evaporate off of the pipe.
I have been looking at Google stuff and wonder, as I have 2 expansion tanks, could it be that they have lost air pressure and need pumping up again?
No signs of PRV leaking and there is a tundish(?) that goes outside but also shows no signs of water.
No schematic but I can take some photos if it helps?

Thanks for your time.

 

[CBW]

Sounds like a weak/failing pump if 3 sensors have been changed, that or it is the pressure sensor again.

 

[Johntheo5]

There will be 2 E.vessels if your HW cylinder is unvented, (except it contains a air bubble), this is normally white, there will be another one in the boiler which is probably red but these serve 2 different purposes, the white one is to aborb the volume rise when the HW cylinder is reheated and the other one is to take the expansion when the boiler/system volume expands.

The reason I suggested lowering the boiler pressure slowly while the boiler is running is that the pump "kick" is no longer required to keep the boiler firing so reducing the pressure slowly down to say 2.0bar should not trip the boiler?.

You might post a photo of the UFH manifold and pump(s) etc, any rads on the system?.

 

[Ron6a]

 

[Ron6a]

Here are some photos as requested. No rads at all, UFH in polypipe throughout.

 

[Ron6a]

Sounds like a weak/failing pump if 3 sensors have been changed, that or it is the pressure sensor again.

3 pressure sensors over a 6 year period. Always the same fault.

 

[Johntheo5]

3 pressure sensors over a 6 year period. Always the same fault.

Is that the pressure sensor in the photo with a pressure gauge on the end?.

Does replacing the sensor then mean that the cold system pressure can be reduced to a normal 1.5bar or if not,what pressure is then set to?.

The first photo shows a E.vessel on the right, can you see where this is teed into the system?.
Can you also show the UFH manifold pump?

 

[Ron6a]

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.

 

[ianmcd]

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