System drops to 1 bar ish..

[Mojocvh]

..pressure and hot water flows from boiler overflow when system pressure reinstated to mains water pressure say 2.4 bar.

System consists of Ideal classic boiler [gas] heating hot tank upstairs [hot water ok] and central heating rads.

Noticed that highest rad needs bleeding regularly as well. [when bled water discoloured]

It seems that the overflow happens when the system pressure reaches 3.2 bar [I've just done this using the heating circuit] so prv is working correctly [YES ?] the reason ??

I think this is linked with the run down of system pressure to 1 bar ish and once "topped up" with mains water pressure the fun begins BUT WHY?

HELP anyone got some ideas??


Mo.

edited twice to try to make more sense sorry

 

[MANDATE]

Well Pressure Relief Valves are normally set to 3bar.
System pressure is usually set to 1bar cold.
System pressure usually rises by 1bar when heating on and returns to 1bar when cooled.
The expansion vessel when empty of water and pre charged on the 'air' side of diaphragm should be 0.8bar.
My thoughts are that the expansion vessel has lost some of its initial air pressure, meaning it then contains some water, meaning there is less air to get compressed when heating is on. This results in a higher than normal pressure which opens the PRV.
The PRV once opened does not always close properly so allows a small leakage with small pressure when cold and a larger leakage when heating on with larger pressure.
It would appear the PRV may need replacing, but no point in doing that until the expansion vessel has been checked and pressure adjusted.
It could be the diaphragm is 'holed' and there is no air to compress, in which case the excess water, due to expansion, is forced straight out through the PRV

 

[Mojocvh]

Thanks Mandate.

Maybe I'm chasing shadows?

Mo.

 

[MrNev]

If the system pressure water is set at 1 bar with the system cold and then rises to 3 bar when the system gets hot before discharging through the PRV it usually just means your Expansion Vessel is flat, it may just need re-inflating.

 

[Mojocvh]

OK so the system has been sat all night and has cooled pressure is showing 0.8 bar.

Quote
"The expansion vessel when empty of water and pre charged on the 'air' side of diaphragm should be 0.8bar."

1. Hmmm <scratchchin>Expansion vessle related

OR

2. system has been High and Hot after excess topping up post rad bleed it has bled off excess and after cooling is showing 0.8 bar because of that.

thanks

Time for a service methinks.

Mo. [edited again to clarify thx.]

 

[MrNev]

The pressure you can see in the pressure gauge is the pressure of the water on the central heating system.

The expansion vessel pressure is some thing different, you can't find out what that is without draining the water pressure out of the system & attaching a gauge to the expansion vessel.