Worcester Bosch 28i Junior losing pressure

31 Dec 2010
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United Kingdom
Hi. I have a Worcester Bosch 28i Junior boiler which is 4 years old. Last year, after the cold snap, when the outside temperature began to increase and the boiler wasn't on as much the pressure began to rise to 3 bar when boiler was operating and discharged water via outside prv overflow. I repressurised expansion vessel and everything returned to normal.
This year, after the recent cold snap, the boiler pressure has began to drop gradually towards 0 bar everytime the boiler is off. I refill it via the filling loop before turning it on and it maintains pressure at 1.5 bar when operating. As soon as it is off the pressure begins to drop again gradually. There are no leaks evident anywhere and no water is being discharged via the prv. I have checked the expansion vessel pressure which is reading 8.5psi. Any qualified opinions as to what could be wrong?
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you've covered all the bases, so you need to go back and find the leak in the pipework, you say every time it goes off, how long is this spell and how quickly does it drop?

do you have pipework under a suspended floor where water could drain away unnoticed for instance?

as you're well aware, that's a good few litres of water you're replacing each time............
8.5 PSI is only just over .5bar.To precharge the EV you must do it while the system is depressurised and there is an opening to atmosphere.Only then can you pump and measure the EV precharge.I would precharge to .8bar as a minimum,but follow MI's.
It drops about half a bar of pressure every two hours or so when the boiler is not operating so yeah quite a lot of water appears to have been lost each time. Definately no leaks at radiators and no leaks in upstairs pipework as I'm sure my ceilings would have collapsed by now! I have suspended floors downstairs but if I had a leaky pipe under these would the leak not be worse when the heating is on and the system under pressure? However, with no water escaping via the prv overflow, which I know for sure as I have a balloon over it, where could the lost water be going?
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copper expands when heated, it also moves. my money is on a leak under the floor.

if you can get your head under the floor, you will likely be able to hear that sort of water leak dripping when you are in the general area.

i'd check around the upstands to the rads first and progress from there, as you say, if it were in the first floor then you would know by now.

nice idea with the ballon by the way.

the only other thought would be a leak of heating system water into the dhw side. you can check this by shutting off the cold supply to the boiler and opening a hot tap, if you get more flow than you would expect, that might be it. that would mean a leaky heat exchanger.
Mostly all of my system is plastic piping except the pipes directly to the boiler itself. Don't know if that affects your copper pipe expanding theory. I'm going under the floors to check for leaks tomorrow and will report back my findings. Is there any way to pressurise the boiler then isolate it from the main heating system to see if the boiler retains or loses pressure? Apparently we are due another severe cold snap soon so anxious to resolve this quickly! Thanks for your advice so far...
yeah, does a bit.

for the purpose of seeing if the central heating water HE is leaking over to the DHW, all you can do is isolate the 15mm cold mains in and the 15mm DHW pipe as it leaves the boiler at the first available. that will contain any leak between inlet and outlet. i'm sure the family can do without hot water for a night.

go for it :)
Well checked under floors today and no leaks at all from pipework or radiators. Pressurised expansion vessel again just to be sure it's not this but system still losing pressure, about half a bar every 2hrs. Don't really know what else to do as isolating things etc is beyond my DIY abilities as I don't know what pipes to isolate, where valves are etc and don't want to break things further. I think I'll just need to monitor things and keep topping up the system to keep it operational meantime as I don't fancy a big call out/repair bill the week after Christmas! Thanks for your help and advice so far...
there is a 15mm copper pipe going into the boiler with a tap on it - turn that off and you have isolated teh mains from the boiler. the you can make sure there are no dripping hot taps in the house and leave them all closed.

if you have no pressure drop after 2 hours, then it's fairly certain it's a cross over of heat side water to the mains. of course, if somone opens a tap then you won't know :)
I can identify all the relevant pipes from the diagram in the manual but I don't know how to isolate them. There are no obvious valves on any of them.
i'm sure there's a valve built into the mains in - the last pipe on the right is the heating return, then the one to it's left is the mains. you might need a small adjustable if it's a square nut jobbie.

can't picture it, but it must have one.
The only thing on the pipe is a slotted screwhead directly under the pipe at the coupling where the pipe enters the boiler. Every pipe on the boiler has the same thing at this point. Could that be the valve? Thanks..
Another point of note is that this afternoon after checking for leaks under the floor I drained down the system and refilled it to 1.5 bar. For four hours the heating was not on but hot water had been demanded several times with showers etc. During this time there was no pressure drop. It seems therefore that there is only pressure drop after the heating has been on for a period of time then turned off. Do you think this indicates anything?
screw slot = ball-o-fix style valve, 90 deg turn and it is off.

when heated the pressure in the system goes up - if there is a leak/cross over issue, then it is likely that increased pressure could reveal it. what is you mains pressure usually like? when you top off via the filling loop is what you repressurise the system to the limit of the mains pressure?
you see where i'm going with that.

when all is said and done, make sure some inhibitor goes back in the system.....
Ok so no pressure drop unless heating has been on, even although pressure is maintained when heating is on, it would seem, without fiddling with any valves, thatbthere is a crossover/leak from heating side of things into the mains. Does that indicate a heat exchanger problem like you suggested earlier then? Ultimately this will need a professional to fix I think.....

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