Extra Expansion Vessel - What don't I need??

7 Apr 2007
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United Kingdom
Hi - Made the fatal mistake of bleeding air out of a couple of radiators and now my sealed cental heating system fails to start every morning. Brrr.

I have refilled the system to just over 1 bar, and it works fine all day, but by the next morning the pressure has dropped to 0.8 bar and the boiler locks out. No obvious leaks anywhere.

What I *think* is happening:

The system volume is quite large (old victorian house with 4 floors and approx 25 rads). When it gets hot the pressure gets up to nearly 3 bar and tapping the pressure relief valve causes a pressure dump. I think this is happening during the day, then overnight when the system cools the pressure drops below 1 bar and the boiler locks out.

Before I drained the radiators of air, the pressures went from about 1 bar cold to about 1.5 hot. Haven't needed to top up the system for quite a while.

It could be due to a leak only revealed by the higher pressures in the system recently... but I have no sign of it.

I figured the radiators full of air were probably working as an expansion vessel, helping to limit the pressure rise in the system. After I bled them, no air to compress therefore higher pressures and pressure relief valve dumping. So... should I just fit an extra expansion vessel (8L or so). The boiler's expansion vessel is 7L.

If I do fit another expansion vessel, does it need a filling loop if it is not the primary method of filling the system? How do you get the new vessel to fill with water from the system initially? Is there a bleed valve or something?

I am quite up to the job of fitting a new vessel, but have never had to use one before.


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Have you made sure that the existing EV is correctly charged?
7 Ltr is quite small for your described system. Once you have established that the existing EV is ok, the fitting one is simple. Fit it as close to the boiler on the return and all you need is a tee plus the other fittings supplied with the fitting kit. No bleeding of the EV is required.
Well, the existing EV is not exactly easy to get to (the boiler instructions for access include the line 'Remove boiler from wall..!'

But, I can confirm that water comes out of the Schrader valve when depressed - therefore knackered EV?

Would a larger external EV be a reasonable alternative to replacing the boilers small one - I think it probably needs an external one anyways.


Pity you didn't mention the water from the schraeder valve in the first post.
You can safely leave the old EV in place as long as there are no leaks from it and fit a new EV externally. You may need 12/14 Ltrs.
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I hadn't checked it then! :D

Thanks for your help, I'm off shopping for a new EV!
Until you get your new EV if you drain a radiator so tat it is only 1/3 water and 2/3 air a reasonable size one that doesn't mind having air in, then that will work as expansion vessel for as long as you need or want.

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