Replacing an Expansion Tank (Pressure

[John McClane]

Hi all,
apologies if this post is confusing but I really do need your help.

The 18ltr expansion tank in my loft is faulty, its losing pressure quickly and leaking water from the schrader valve.I have now bought a new 18ltr one which is factory pre-charged to 1.5 bar.
There is pressure gauge above the old tank connected to the pressure release valve which shows the maximum pressure of 4 BAR and has a red indicator arrow set at 1.BAR.(Pic attached)

I have read on the www that the new tank should be pressurised (which I can do with a pump)to the same as that of the house water pressure which I have tested with a meter via a tap at the rear outside patio.The housewater pressure shows 4.5 BAR .

When I have connected the new tank and I re-pressure the water using the loop system to 4 Bar will this be too much pressure for the system ?.

The instructions with the new tank state ' the pre-charge should never exceed 50% of the maximum pressure (4 BAR) which if I am correct is 2 BAR .

Could someone please clarify what the pressures should be ?

Thank all.

 

[Gasguru]

You are getting confused with the domestic water system (hot & cold taps) and the heating system pressures.
It is completely normal to have a little water from the schrader valve given it's at the base of the vessel....even 1/2 a cup could be expected.

Given the vessel is at the highest point in the system then set the vessel to around 0.8 bar and then presurise the system to 1 bar.

 

[John McClane]

Hi,
thanks for your quick reply.I now understand about the domestic and heating system pressures.
Will have another look at the EV pressures and reconfigure to see what happens.

 

[John McClane]

Hi,
I checked the EV earlier and found that the Schraeder valve is defiantly leaking air (not water).
Rather than replace with the whole EV I am going to replace the inner valve with a new one.

After I depressurise the EV and fit the new valve do I then re-pressurise the EV to 0.8bar presumably against the water in the tank or does the water have to be evacuated from the tank before hand after which I the refill with the loop system ?.

If this is correct how do I drain the water from the EV ?

Thanks all.

 

[Gasguru]

Yes you can just replace the valve core.
Whilst you check the vessel the system pressure must be at zero.
As you pump up the vessel the system pressure may rise but keep draining water down to get the pressure back to zero and then check the pressure.
I suggest you bleed all the radiators and then drop the pressure to zero using a bleed point....a bathroom/kitchen radiator with tile flooring is less risky.

 

[John McClane]

Many thanks Gasguru ,the penny has finally dropped...as they say "its easy when you know how"