Pressure drop in Vaillant ecoFit Pure 825- 835

[sofie]

Hello all, my Vaillant ecoFit Pure 825- 835 combi boiler has been running fine from 2019 till early 2024. We have had a mains water pipe leak due to which it had been off for a few months.
The leak was fixed and we switched the boiler on but now its pressure constantly drops. It is still under warranty and the engineer checked it out. Said no leak and replaced vessel, replaced LWPS?, AAV, PRV.
And 3 parts fitted.
It showed error F75 and then F22
Dropped from 2.1 to 0.3 overnight. It does come back on if I repressurise and works till the heating is on. Soon after it is switched off it loses pressure gradually.
What could possibly be the problem? Could the pressure switch be defective?
The radiators were bled recently during the annual service. Could this contribute to the problem?

 

[muggles]

You either have a leak on your system or an internal leak in the boiler's main heat exchanger. It can be tested by turning the boiler off and using the isolation valves to shut it off from the system. If the boiler pressure drops with the valves closed, it's a leak in the boiler. If it stays steady then it's a leak on the system.

You're a little confused about your model number, incidentally. Ecofit Pure are either 25, 30, or 35. 825 and 835 are ecoTEC Plus models. And it's only one of them, it can't be both an 825 and an 835, that's two different boilers

 

[sofie]

You either have a leak on your system or an internal leak in the boiler's main heat exchanger. It can be tested by turning the boiler off and using the isolation valves to shut it off from the system. If the boiler pressure drops with the valves closed, it's a leak in the boiler. If it stays steady then it's a leak on the system.

You're a little confused about your model number, incidentally. Ecofit Pure are either 25, 30, or 35. 825 and 835 are ecoTEC Plus models. And it's only one of them, it can't be both an 825 and an 835, that's two different boilers

Oh. Thank you. I am not very savvy with equipment. I listed the information from the manual regarding the model number.

Could you guide me as to which one is the isolation valve please?

 

[terryplumb]

Does pressure rise to around 3 bar when central heating is in use ?

 

[sofie]

Does pressure rise to around 3 bar when central heating is in use ?

No. It rises by about 0.1 so if it is set to 1.5 it will rise to 1.6. Never goes high or even low when the heating is in use. But when NOT in use the pressure drops.

 

[terryplumb]

No. It rises by about 0.1 so if it is set to 1.5 it will rise to 1.6. Never goes high or even low when the heating is in use. But when NOT in use the pressure drops.

You told us in your first post pressure was 2.1 ??
Check the pressure relief pipe ( usually an open ended copper pipe externally) to see if water is dripping from it. If it isn't ,it's as muggles said above there is a leak.

 

[sofie]

You told us in your first post pressure was 2.1 ??
Check the pressure relief pipe ( usually an open ended copper pipe externally) to see if water is dripping from it. If it isn't ,it's as muggles said above there is a leak.

Yes. I have checked for any leaks from the pipes. The pipes connecting the radiators are all above the floor so checked those as well. No leaks or presence of water.
Is 2.1 too much? It was between the limit so I thought it was OK.

 

[terryplumb]

With heating ,on pressure at 2.1 bar is ok. The pressure when system is cold around 1 to 1:5 bar is usual. Are you certain the external pressure relief pipe is dry and shows no sign of water ever having come from it ?

 

[dilalio]

Yes. I have checked for any leaks from the pipes

Think you misunderstood.

The boiler has a pipe that goes to outside and is an open ended pipe... This is your discharge pipe and allows the system to "dump" water to outside in the event of going above @3bar in system pressure.

Look for a pipe that comes out the wall and bends round or down to point back at wall or down it.

 

[Harry Bloomfield]

Look for a pipe that comes out the wall and bends round or down to point back at wall or down it.

To check if it is passing any water, take a sandwich bag, and an elastic band - put the bag over the end of the pipe, held by the elastic band. If there is any water lost, the bag will show it.

 

[dilalio]

To check if it is passing any water, take a sandwich bag, and an elastic band - put the bag over the end of the pipe, held by the elastic band. If there is any water lost, the bag will show it.

No need.
If it's been passing, it'll be wet inside it... pinky finger inside will show it.

 

[Harry Bloomfield]

No need.
If it's been passing, it'll be wet inside it... pinky finger inside will show it.

The sandwich bag test, is unmistakeable, even in wet weather.

 

[dilalio]

The sandwich bag test, is unmistakeable, even in wet weather.

And how many times have you had to do it?
I have to diagnose it when there, not wait 3 days!

If pipe is wet inside, it's been running.
Plenty other ways to tell but beyond most diyers.

 

[Harry Bloomfield]

And how many times have had to do it?
I have to diagnose it when there, not wait 3 days!

The OP is onsite, and not you, so not an issue. Even, popping into a site briefly, the bag method makes it obvious, between brief visits, that there is loss via the valve.

 

[dilalio]

The OP is onsite, and not you, so not an issue. Even, popping into a site briefly, the bag method makes it obvious, between brief visits, that there is loss via the valve.

It doesn't.
You a plumber? Don't recall you are/were!