Valiant boiler pressure problem......

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Hi,
Bit of a long one....... Around 10 months ago we came home to our valiant combi boiler having a puddle of water underneath and it was leaking from the casing. Pressure had dropped but not to 0. british gas homecare agreement so called them out and they said a seal had gone?? , replaced and on their way.
The boiler was at 1.2 bar, gradually over the next week or so it dropped to 0.8 bar, more water underneath. Another British gas visit..... No evidence of leak in boiler. We had a plastic bag tied over the copper pressure pipe outside and that was dry. He fiddled, pumped up the EV and added some leak seal and said monitor for now and don't worry about a 0.1 pressure drop a week as its at worst it's dropping into the space under the house or evaporating!!
Since then the boiler has been serviced.... All fine. Heating and hot water work perfect BUT.... Still getting a pressure loss of aboit 0.1-0.2 bar a week. No leak from pressure pipe..... Bag been over it. All radiators and external pipes seem fine bar one radiator has a very rusty blanking nut but paper tied round and no obvious leak.
Not happy so.... British gas came out again last week. Noted that the pressure rise when boiler activated was above acceptable, went from 1.0 cold to 2.2- 2.3 hot and quickly, but no discharge from prv but he said despite this he is going to change the EV as this was and excessive rise.
Pressure cold was set at 1.2 and seemed to hold for a week but has now started dropping again by the same rate, 0.1-0.2 bar a week. Would the new EV maybe need re preaassuring this soon after being fitted? Aaagggghh!!!!
I know we may be looking at a rip up the floors and expose pipe work as next step but does anyone else have any other poss theories before we get to this?? Poss pin holed heat exchanger?? But condensate pipe goes into drain pipe but not obv dripping when boiler is off??? British gas coming out again on Monday and I just don't have any hope with them, they have been useless. We do have trace and access cover with them but no one seems to want to take ownership of the problem!!
 
Forgot to add......regardless if cold starting pressure falls from 1.3 to 0.8bar, the pressure always reaches 1.8 when heating is on and stays constant, not sure if this is relevant information!
 
If there is zero visible leaks within the appliance and nothing going out the PRV, the simple way to test would be to isolate the flow & return pipework at the boiler for a few weeks and see if the pressure in the appliance holds steady.

It's something you'd wan't to wait for the better weather for as it means no heating during that time. Assuming its a combi, you could still get hot water.

If it holds pressure ok, then it rules the boiler out completely

it is possible for a leak in the main heat exchanger to go down the condensate pipe, however not particularly common and over the timescales your talking about id have money on it not being that.

Its not what your paying for, but I'd have blue roll tied around every flow & return pipe to each radiator and check them a few times over the day, blue roll goes obviously dark when wet and a tiny drip from a radiator valve can easily cause this, sometimes they only drip on the cool down so can seem fine checking the system cold, but after running heating and then letting it cool a little weep can be seen for a short time.

But I absolutely would not accept an 0.1 bar drop in pressure each week as being ok.
 
Forgot to add......regardless if cold starting pressure falls from 1.3 to 0.8bar, the pressure always reaches 1.8 when heating is on and stays constant, not sure if this is relevant information!
If your reading this off the digital display thats quite normal, the electronic pressure sensors will show different pressures depending on whether the pump is running or not.
 
No I understand the rise is normal, what I didn't know is why is still rising to 1.8 regardless of the number it started at and if that's relevant to my issue.....if that makes sense?!
 
If you had water under the boiler first time, then I’d be looking at the auto air vent or diverter valve. Could be anywhere on the system. Yes 0.1 - 0.2 isn’t much, but if it carries on then likely to have pressure loss altogether.
 
No I understand the rise is normal, what I didn't know is why is still rising to 1.8 regardless of the number it started at and if that's relevant to my issue.....if that makes sense?!
Does this happen on its own or with heating on?
 
So the pattern goes..... Pressure 1. 2, heating on and rises to 1.8 over the week as the heating cools when it goes off the pressure drops to about 1.1 ( gradual 0.1 bar decrease over the week) but everytime the heating is on it still reaches 1.8 and stays stable. So for instance the past 3 weeks it started at 1.2 and now is 1.0 but still rises to 1.8 when on and stays stable. I understand the physics of heat expanding m cooling etc and the affect on pressure. I'm just confused as to why there isn't a constant increase, so for instance if the increase in running the boiler was 0.5 bar, then when it starts at 1.2 it should go to 1.7 but if it starts at 1.0 it should go to 1. 5?
 
If you had water under the boiler first time, then I’d be looking at the auto air vent or diverter valve. Could be anywhere on the system. Yes 0.1 - 0.2 isn’t much, but if it carries on then likely to have pressure loss altogether.
Sorry..... What is the auto vent or diverted valve??
 

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