Expansion Tank - Fluctuating pressure

Postby venom78 » Sun Jan 25, 2009 12:32 pm


I have noticed that when the heating (thermostat) is turned up to 25 the pressure gauge just above the expension tank goes up from between 1.0-1.5 psi to over 2 and sometimes 3 !!

At one point, when the pressure was high just over 3psi, water overflowed out of the expansion tank. When the thermostat is set at around 22 the pressure goes down and remains at around 1psi and sometimes slightly lower.

My questions are:

Should the pressure remain constant between 1.0-1.5psi regardless of whether the heating is turned up or down?

Does the expansion tank need replacing or topping up? I heard in some cases they need air pumped into them?

Finally, if it is the expansion tank, then how much are they?

Thank you :)
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Simply Build It

Postby DEEARR2 » Sun Jan 25, 2009 8:36 pm

Sounds very much as though the expnssion vessel requires attention. There is a air filled bag inside the vessel that absorbs the system pressure as the heated water expands. It may be the air pressure in the bag is to low and will require to be topped up to the required pressure. Or it may be the bag is burst and a new vessel will be required. Any way the repair should be carried out by a registered service engineer.

Without knowing the make of the boiler, pressure vessels vary in cost but I would think in region of £50-£80 plus any labour charge.
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Postby venom78 » Tue Jan 27, 2009 10:38 pm

Thank you for your help.
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Joined: Sun Jan 25, 2009 12:25 pm

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