Boiler fails randomly after expansion vessel replaced

[martind123]

Hi,
I am out of ideas what may be wrong with our system.

Some background: We had some pressure loss, 0.1 bar per day, on our Vaillant ecotec plus Boiler. No obvious signs of leakage across the house, no building work, radiator valves not leaking. Heating engineer diagnosed the expansion vessel and PRV needed replacing and this was done thereafter by them.
However, our problems have gotten worse since doing this. Previously, the pressure was falling in a predictable way and i could top up once a week or so. Now, we get random total failure of the system was a F.74 error. These are based on random complete loss of pressure: In the evening, pressure may be 1.5bar and then as the system cools down the pressure drops to 0.5bar at night, sometimes within just a few hours. The system then gives a fault.
I can then revive the system by opening the filling loop. I need very little top up, the pressure jumps up quite quickly. I also noticed that it seems as if there is air in the system when topping up that little bit. The boiler is in the loft, almost at the highest point if the system
I had two visits from the company that replaced expansion vessel and PRV and they insist everything is fine and that there must be an actual leak somewhere. They can't explain why the system keeps on failing so suddenly now.

I suspect the expansion vessel is defective or wasn't installed properly.

Help please!

 

[Agile]

The expansion vessel EXV needs to be filled with air at an appropriate pressure.

The internal communicating pipe to the EXV has to be clear of debris.

The way you are measuring the pressure has to give a correct reading.

Several things can be involved there.

May need a competent engineer!

Tony

 

[Steelmasons]

Are you reading the system pressure via the digital gauge or the mechanical gauge?

 

[martind123]

I have a vrc 470f, so reading from there as well as the manual gauge at the front. When it fails the pressure shows 0.8 or lower on the remote and at the lowest end at the mechanical gauge at the front. Both jump back to normal when I top up a little. Normal seems to be 1.5/1.6 going to 1.9 when firing and pump running. I don't get higher pressure readings and it usually is quite stable at the above readings.
It is almost like a cliff edge where the pressure collapses sometimes. This only happens when the system cools down.

 

[Agile]

Increasing from 1.6 bar to 1.9 bar would seem to indicate that the EXV and communication pipe are both working correctly.

Can you be more informative about the mechanical gauge which you say is at "the lowest end" ?

Be aware that both digital and mechanical displays can be inaccurate and also display hysteresis as well.

Tony

 

[martind123]

What I meant with "mechanic" is the analog gauge at the front of the boiler. But I guess this is no different from the other readings - all readings are probably coming from the same sensor!
So I think you are telling me that the pressure reader is failing. Which would make sense as there is no evidence of lots of water disappearing and topping up requires very little to get it working again. Why would this start to happen after the EXV was replaced? The problem happens when the boiler isn't working and cools down - and the boiler is in the loft which isn't insulated. Just debris or a loose contact as a result of previous repairs? Or air in the system which shows up around the reader and obscures the reading (I hear lots of air bubble noises when restarting the boiler)?
Is there a way to fix it? Or further testing? Or just replacing it?

Thanks so much...