Another combi boiler losing pressure

[Granite21]

Hi all,

So I came back from a 2 week holiday last month and my boiler had lost all pressure. It's a Worcester Bosch Greenstar 28i Junior and had no previous issues. Anyway, I refilled the system to 1.5 bar. After a couple of days the system loses pressure to around 0.8 bar but then holds. The heating still works but every time i refill the system the pressure drops back to 0.8 bar. With the heating on I've noticed the pressure rise to near 3 bar! Also I cannot see a leak anywhere, although I've not lifted the floorboards yet.
Can anyone advise what this problem could be?

Thanks.

 

[hertsboilers]

Expansion vessel need re-pressured.

 

[Granite21]

Ok thanks. Any idea why it may have lost pressure in the first place?

 

[hertsboilers]

Ok thanks. Any idea why it may have lost pressure in the first place?

Expansion vessel flat = no ability to absorb the expansion of the water = reach 3 bar when system is hot = discharge to reduce pressure

 

[Granite21]

Ok brilliant thank, time to re-pressure the expansion vessel then.

 

[MANDATE]

The air side of the vessel is similar to car wheel which may leak at the valve.
The water pressure will help the air to escape and moves the position of the diaphragm so the volume of air is reduced.reducing the volume by half before it gets compressed doubles the pressure so it soon reaches 3bar. At 3bar the relief valve opens and discharges the excess water and pressure drops.
Note also once opened the prv seldom closes properly and leaves you with a small leak.
You need to re charge the vessel to about 0.8bar with the vessel empty or open to atmosphere so you get Max volume of air to compress.
Also replace the prv.
With heating on the water pressure will rise about 1bar but should not reach 3bar.
If it does, then the vessel has not been recharged properly.
A blocked tube between vessel and boiler will prevent a vessel working properly

 

[Granite21]

That's a great reply MANDATE. Thank you for taking the time explain that. I shall re charge the vessel tomorrow.
Thanks again for the advice.

 

[mogget]

Don't forget to switch off the boiler and have the heating circuit open to atmosphere when checking the exv pressure - I think there's detailed instructions somewhere in the faq or wiki for doing this.

 

[Granite21]

I'll take a look at that wiki, thanks mogget.

 

[Granite21]

So i located the expansion vessel on the boiler last night and there doesnt appear to be enouth room from the top of the boiler to the ceiling to remove it.
I want to avoid getting the boiler removed to gain access the vessel, as it is behind the boiler located in the wall mount. So as I can see and access the valve on the vessel can I re-charge it in place or do I need to remove it from the boiler?

 

[oldbuffer]

You can recharge it in place, but you must make sure there is no pressure in the system.

No need to drain the whole system, just reduce the pressure to zero (via a drain off point if convenient, else a radiator bleed point), then open a radiator bleed point so the water has somewhere to go as you pump up the expansion vessel. Get someone to help mop up the water as it comes out.

Take the cap of the expansion vessel air connection - its the same as a car tyre. (See note below). Often worth removing and replacing the Schraeder valve insert (same as a car, you can get them from Halfords, but take the old one as there are long and short patterns) - again same as a car wheel.

Attach tyre pump connector and pressurise air side of expansion vessel to about 1.0bar (0.8 to 1.2), replace cap and all should be well.

Note - When you remove the air cap, briefly depress the valve. If water comes out then the expansion vessel itself may have failed allowing water past the rubber diaphragm which separates the water from the air. You'll need an engineer to sort it out if this has happened.

 

[Granite21]

Thanks oldbuffer, very useful information.

 

[Granite21]

I have now re charged the expansion vessel to just under 1bar and refilled the boiler to 1.5bar. I then the heating on and the radiators heated up nicely.

However, the pressure began to rise. It got up to and held at about 2.4 bar, just a the top of the green phone on the gauge. I have a copper pipe and a upvc pipe going through the external wall so I took a look at them outside.

The copper pipe just runs down the outside of the external wall and stops just above ground level. The end has some water on it but I can't see it dripping.

Now the upvc pipe (approx. 20mm diameter) runs down the wall, along the floor to the drain. I can see it dripping, roughly 1 drop every 5-10 seconds.
Any ideas??

Surely the boiler pressure shouldn't rise that much when in use. It is set to max though....

 

[andytw]

If you pressurise the vessel to 1 Bar and fill the system to 1 Bar then you will have the whole of the volume of the expansion vessel for the heated water to expand into. By filling the system to 1.5 Bar then you have already used up quite a lot of the required space.
I think you will have used a third of the volume of the expansion vessel already

andytw

 

[andytw]

The copper pipe is where excess pressure is relieved by the pressure relief valve, PRV. This pipe should not be dripping. As previously posted the PRV is probably now not sealing properly after having being opened by the previous excess pressure.
The plastic pipe is where the boiler discharges condensate from the condensing process of the boiler. This is normal.

andytw