Strange place for an expansion vessel?

8 Sep 2015
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United Kingdom
Its an oil boiler, heating and hot water, hot water is via a copper cylinder in the hot press, it has an external expansion vessel.

You know when you fill a central heating system via the service valve? Between this valve and the boiler is an expansion vessel. I was always under the impression this should be on the heating return side of the boiler.
Anyway, when the system is cold its been reading 1 or less bar via the gauge ontop of the expansion vessel, lately when hot its been climbing to over 3 bar, I only noticed this because water has been dripping from the warning pipe in the garden.

Today, without the heating being on for a few days, so the system was cold the gauge read 3.5 bar, I opened the red valve to let the pressure out and it sounded like air escaping.

I've got a split diaphragm in the expansion vessel haven't I? I just found it odd that it would climb to 3.5 bar even though the system is cold.
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Its not very clear.

Your EXV may need the air pumping up to about 1.0 bar.

But the filling valve may be leaking!

That's what I was thinking Tony, I cant see why the pressure would rise like that in a cold system.
Ill release any pressure and pump up the expansion vessel, would it be prudent to release any air pressure in the vessel first?
I take it the 1 bar will be read from the pressure gauge ontop of the vessel?
No open the system to the air and pump up to an air pressure of 1.0 bar and that is read on your footpump or compressor!

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Ok, so I looked at the bottom of the vessel, dipped the pin in the air valve and nothing happened, no air came out nor did any water.
So I released the pressure in the system then tried a pump on the vessel, it wouldn't let any air in, is this a sign that its full of water or just that the valve is stuck?

Ive tried the central heating and although the stat is calling for heat the boiler wont fire, pressure is at 1 bar.
No open the system to the air and pump up to an air pressure of 1.0 bar and that is read on your footpump or compressor!

Ok, thanks Tony, I only have a bike pump, so its looking like I'll give the land lord a call and hand it over to him to get sorted, thanks again.
Ive found out why its pressurizing, I think.

I tried to heat some water, its an indirect system as far as I can see, I may be wrong, the boiler heats up a copper cylinder, it also has an electric immersion as well.
Anyhow, ran the boiler to heat water and the system is reading 3.5 bar again. So it would seem heating the DHW pressurizes the heating system, any ideas? My kitchen rad was warm also.
The water that heats your rads also heats your cylinder. Your expansion vessel needs attention
So the motorised valve isn't working either? If I try to heat water my rads also get hot thus expanding the water to 3.5 bar?
seperate issues.

The vessel needs to take up expansion whether you are heating hot water or heating
Will do, Ive switched on the central heating, stat switched to 30, hot water off. So the stat was calling for heat but the boiler didn't fire up.
I manually activated the motorised valve via the little paddle lever on the side, it clicked and wirred then the boiler fired up.
So I take it the motorised valve is faulty?

I watched the gauge as the rads got hot and it went from one to two bar gradually but I had to switch it off as the little ones are going to bed and it would only get too warm for them.

Would a faulty motorised valve cause high pressure in the heating side?

I will get some photos.





That photobucket takes forever...
If its a rented property just get the landlord to get it fixed, before you get accused of causing problems by playing with equipment thats not yours.

The 3 port valve may be passing if the rads are heating while the hot water is on.

The Vessel probably just needs repressurised to stop the system pressure rising when hot, you should have had air out the pin if it was ok. Sometimes just need a bit more of a beefy pump to get it started.

If the pressure is rising while the whole system is cold then the filling valves are suspect.
Its a non compliant filling loop so only one valve to try to prevent it leaking.


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