Combi Boiler Pressure Loss

[bombardy]

I hope somebody can help us.
Our oil Worcester combination boiler powers an old cottage with 17 radiators.
We have major pressure loss.
One radiator was feed by a pipe within the concrete floor but I have now replumbed that to be above ground. So all pipes are visible.
We were told by the first repairer that it was the expansion tank.
We now have an external to the boiler new expansion tank, a new pump and a new PRV.
The pressure even drops over night when the boiler is off from 1.5 bar to zero.
The second repairer who fitted the above components left me with the task of regular radiator bleeding over the five days following the repair. I bleed them three times a day and top up the pressure five times a day.
In those five days nothing has improved. Five time a day the pressure goes down to zero.
I’m considering that the old expansion vessel inside the boiler, which I am told is OK, should be plumbed out of the loop.
Then install isolation valves on the heating pipes adjacent to the boiler. I could then turn off the boiler, isolate it, pump it to 1.5 bar and see if the pressure still drops.
The other observation is that the pressure doesn't rise above the 1.5 bar when then boiler heats up.
There is no water dripping from the PRV. There is no moisture coming out of the flue vent.
Any advice would be much appreciated

 

[SimonG]

Fit the valves and see if its boiler or system related.

No other pipework in the concrete? Nothing leaking in the boiler?

 

[oz-plumber]

If you're topping up the pressure 5 times a day, you have something major going on.

1/ Isolate the boiler and test.
2/ Isolate the system and test.

Find out which part of the system is leaking.

 

[Dr Doolittle]

you have a substantial leak that as the water comes out, is sucking air in. Start by taking a closer look at the radiators

 

[SJB060685]

Is this a condensing boiler? If so you could have a split in the secondary heat exchanger. This can be confirmed or ruled out by checking the condensate discharge with the boiler off. If its dripping then that's probably your problem, if not then you could rule that out.
Where is the new external expansion tank fitted, on the flow or return? If multiple expansion tanks are used then they should ideally be on a manifold system entering the system in one place keeping only one point of no pressure change in the system.

 

[Dr Doolittle]

Is this a condensing boiler? If so you could have a split in the secondary heat exchanger. This can be confirmed or ruled out by checking the condensate discharge with the boiler off. If its dripping then that's probably your problem, if not then you could rule that out.
Where is the new external expansion tank fitted, on the flow or return? If multiple expansion tanks are used then they should ideally be on a manifold system entering the system in one place keeping only one point of no pressure change in the system.

the secondary HE split would have a very noticeable characteristic LOL. He has a leak...

 

[Timmy D]

Checked the condense when the boiler is off for discharge? Could be a split in main heat exchanger.

Isolate boiler, top pressure up. Put bags over condense and prv.

 

[Dr Doolittle]

had several situations where pressure relief valve has failed and allows a steady stream out and so reduces pressure. If this has been piped into condensate then it would not be visible.