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My Baxi combi boiler is losing pressure overnight when not

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My Baxi combi boiler is losing pressure overnight when not on, the next morning I re pressure up to 1.5 bar with system off. When system re starts it quickly rises to 3.0 bar & shuts down again. at this point I have to let water out of the system to get the bar down to 1.5 again, just to have heating & hot water through the evening. This has been going on for 4 days & I have got BG homecare coming in 4 days but I'm worried about continued use until then. I believe a 1.5 bar pressure loss is equal to losing 5 litres of water. i have no signs of a leak [ especially upstairs, shower, PRV etc] so I'm hoping it's a boiler problem that can be fixed. Can you advise me what to do until BG get here.
Hi, are you sure you havnt got a dripping safety valve. You've checked the copper pipe outside on the wall where the boiler is?
Customer: replied 2 years ago.
Outside I have the flu, letting steam out, a capped copper pipe & a plastic pipe below that running at an angle down to a drain approx 10 feet away. Looking at the boiler the copper pipe is on the right & plastic is coming from the left. I did want to do the plastic bag over the end to see but no access to end of pipes. If you can confirm the plastic one is from the safety valve I will try to bet access.
Hi , copper one is the safety prv. The plastic pipe will be the condensate pipe going to drain.Its the copper one to check
Customer: replied 2 years ago.
As I said, it's been capped. How do I check for water coming out?
It shouldn't be capped. If that's the prv. Is it capped with a copper soldered cap or a compression (nut)fitting?
Customer: replied 2 years ago.
Can you tell me why pressure is rising to 3.0 bar when system is on, I can understand why it might drop.
Inside your boiler is a metal vessel with a balloon inside, called the expansion vessel.
When the boiler fires and heats the water, the water expands and the expanded water pushes into this vessel. If the balloon inside goes flat or splits, it fills with water during filling the system. Then when the boiler is in the heated water has nowhere to go so pushes the pressure above 3 bar and dumps out the safety valve
Customer: replied 2 years ago.
Not a nut, it's round like a large nail cap & almost flush to wall. I expected to find something like an overflow pipe.
Carrying on from before;. This then starts a circle of topping up the pressure, dumping,etcIts called a mushroom fitting.it can be pushed out from inside and then pulled back to be soldered. Please read what I wrote about the expansion vessel
Customer: replied 2 years ago.
Can't dump water out if it's capped so where does it go?
Good question. One way to find out would be to fill boiler to 2,turn heating on and when it few he's 3 bar and dumps ,try and find out where its going
Customer: replied 2 years ago.
Only the plastic pipe to the drain surely, where else can it go? No internal leaks from boiler. Can't go back into system.
Am safe to continue in this filling / dumping circle until Monday?
Yes, although a lot of boiler engineers dontvknow why it happens or how to solve it. Simply pumping it back up to 1.5 bar with the pressure at zero.You could look under the boiler and see where the copper safety valve runs to. If not , all the best
Customer: replied 2 years ago.
The capped copper pipe is part of the re presurising system, joining the right hand wheel from the right.
You are looking on the outside?
Customer: replied 2 years ago.
Looking directly at the bottom of the boiler from under my stairs.
You know the safety prv dumps outside
Customer: replied 2 years ago.
Just trying to let you know where it runs to when it comes back to the boiler. Does that help ruling it in or out as the safety prv.
Can we talk on the phone , this is getting a bit confusing?
Customer: replied 2 years ago.
send me your number
Tried to do it for free. It the system would only do minimum 5 dollars.
I can't put a number on here , it gets blocked.. never mind.
Plumberpro and other Appliance Specialists are ready to help you
Customer: replied 2 years ago.
Thanks for your help, I'll give you a good rating.
Ok, thanks . The safety has a red cap on it usually. You could trace it from the boiler to where it ends up