Problems with boiler pressure

[nazn]

I have a Ravenheat combi boiler which is now losing pressure after use everyday. The pressure is not lost instantly, but the day after the pressure has dropped to zero.

Filling the system takes no time to reach 1.5 bar. I have checked all the rads for any air in the system and found that all were filled to the top.

This leads me to believe that the leak is not in the pipework but with the boiler, any idea's what could be causing this recurrent problem??

Any help/advice greatly appreciated.

Thanks Naz

[chris French]

In message , nazn writes

It sounds like there might be problem with the pressure vessel/ diaphragm. The pressure vessel is hard to access, being behind the boiler (least ways mine was)

I believe the normal solution is fit a new pressure vessel elsewhere on the system.

[Set Square]

Sounds like you've got a problem with the pressure vessel. This may be a bomb-shaped thing located somewhere on the pipework, or it may be inside the boiler casing - in which case is it more likely to be pancake shaped. Either way, it should have a Schrader valve (like a car tyre valve) for pressurising it with air. With the water system unpressurised, the air pressure in the expansion vessel should be about 0.7 bar (10 PSI). Measure this with a car tyre pressure gauge, and top up if necessary. If water comes out of the valve, the internal diaphragm is shot - and you'll need to replace the vessel.

When the vessel has insufficient air in it, it cannot provide a spring to cater for expansion. So you pressurise the water system cold, the water expands when the system gets hot, and gets expelled through the safety valve rather than going into the expansion vessel. Then when it cools, the pressure drops. I suspect that this is what you are seeing.

[<me9]

Has the preload air leaked out of the expansion vessel? See the sealed heating system FAQ.

[<me9]

More normal to check the pressure vessel preload pressure, and if necessary topping it up to ~0.7 bar with a car foot pump.

[John Rumm]

What the others said! ;-)

If you need a quick fix then deliberately allow some air into one of your radiators - that will then give the system some expansion room. You don't need a huge amount since you are only talking about a few litres of expansion for every 100 litres of system water capacity.

[nazn]

Thanks for all the replies,

The boiler has a tyre valve type unit on the top of the boiler, I pressed the valve in using a small screwdriver and had hot water come out of it (the boiler was running at the time). When you refer to the pressure vessel, is this the big round (15" inch or so) pancake shape unit that sits at the back of the unit? if so this sounds like a major prob! If the diaphram is a seperate unit I would be interested to hear where this is located or what it looks like. On the other hand time to call in a corgi!

Naz

[Set Square]

Yes, that's the pressure vessel. the diaphragm is inside and separates - rather *should* separate - the water from the air. Clearly, yours is shot since water is getting to where only air should be.

You can't replace the diaphragm - you have to replace the whole pressure vessel. As someone else noted recently - either in this or a similar thread - you can simply abandon the pressure vessel inside the boiler and install another one elsewhere in the system if that's easier. [Make sure the valve cap is firmly replaced on the abandoned one]. It's not really a Corgi job per se - just straightforward plumbing - since it's connected to the water pipes - not the gas pipes.

[chris French]

In message , nazn writes

If it's the same as my Ravenheat (in our old house) this is automatic air release valve IIRC (to stop air collecting at the top of the boiler

- like bleeding a rad) nothing to do with the pressure vessel.

Yep that's it.

The diaphragm is in the pressure vessel - basically across the middle it has air on one side, and the system water on the other, this is pressurized (by letting in mains water normally) to then pressurize the system. If this leaks then of course you can't pressurize the system.

however, as others have said you should potentially be able to re-pressurize this, though I don't know how accessible it is, you boiler manual should give instructions on this if possible.

[John Rumm]

Have a look at part number 11351 on:

can find any suitable place on the system to install one of these - probably simpler than going through the hassle of getting the correct replacement part for the boiler and fitting it.

[nazn]

Thank you John for the information you have provided, I have looked up the part and am seriously considering this option, however before I go down this road, I would like to attempt to let air in one of the radiators, as a temporary solution. Any idea's how one would go around doing this?

As for the expansion vessel install would I need to isolate the existing unit? If this is the case this sounds tricky

[chris French]

In message , nazn writes

Drain down the system a bit, letting air into the rad. Refill but don't bleed one rad (or not fully anyway).

Don't imagine so.

[John Rumm]

Yup that would work. As would: turn off the valves on both sides of an upstairs rad. Place a tray under it, and open one of the connections between valve and radiator a little to allow water to leak out. Open the vent on the rad to let air in. Once you have a say 5L out do up the tail and the air vent again and reopen the valves.

(make a note of how many turns were required for the lockshield valve when closing so that you can get the same position when opening it again)

[Set Square]

I can see where you're coming from with the bit in brackets, but I'm not sure of the validity of restoring a half-empty radiator to the same balance position as was needed when it was full.

[chris French]

In message , Set Square writes

but you'll need to know anyway once the pressure vessle problem was sorted and things were back to normal.

[Set Square]

That's true! Meanwhile, you'll probably need it wider open.