Plumbers!- Expansion tank pressure?

[Matrixx]

I have a larger than average central heating system, someone has fitted a white expansion tank on the heating circuit ( which is ok for water and central heating) but potable water is usually set at 3.5 bar, where as a a central heating tanks would be 1-1.5bar, I have read that in some cases where the systems is going up three floors or has long pipe runs the pressure might be increased to compensate. but i assume it should set to where the running pressure is , which usually is around 1.5 bar.

I have water coming out of the pressure relief valve , (about a pint per run) on the central heating circuit when running , the pressure is below 1 bar cold and close to 3 bar running, the relief valve it set at 2.5 bar.

I am assuming the tank needs to be drained and pumped up to 1.5 bar ( even though it shows 3.5 bar on the label), and see if it holds pressure.

its not a standard boiler. Am I correct?

 

[Nick DV]

When everything is cold, what pressure do you have in the expansion tank? Are you checking the pressure on the system gauge or on the expansion vessel? You can check it with a digital pressure gauge. If it's over pressurising it will obviously cause the PRV to open, the trouble is, if any crud from the CH system gets into the valve, it may not seal shut again.

 

[Matrixx]

Just checked The gauge shows just over 1.5 bar ( its been off for 1.5h hours), but the expansion tank is at 2.9Bar measured at the Schrader valve. so I guess the tank pressure must be to high and not having the correct reaction.

 

[Nick DV]

The expansion tank should be just over 10 psi, yours it around 45!!! When the system pressurises as it gets hot, the bladder in the expansion tank gets 'squeezed' to allow the pressure to stay at a safe level in the sealed system, so if the pressure in the bladder is too high, it won't function correctly if at all.
When you put the pressure gauge on the Schrader valve, did you notice if any water came out?

 

[msh10]

Mine did similar, had to drain the system, leave valves open and then pressurise the expansion tanks (one in back of boiler and the separate tank), the refill system and check pressure on boiler.

 

[Matrixx]

No Water, and its holding pressure, so I am assuming its Ok. I will drain down the boiler and adjust the pressure with no water in the expansion tank.

10 PSI is 0.68 BAr , I have 3 floors and 27 radiators in the system would say 1.5 bar be OK or lower is better?

 

[Seadog]

The expansion vessel needs to be of sufficient capacity to cope with the system volume. If you have too great a pressure at cold it'll never cope with the expansion once hot.

 

[Nick DV]

The expansion vessel needs to be of sufficient capacity to cope with the system volume. If you have too great a pressure at cold it'll never cope with the expansion once hot.

All being well, when the system was installed the correct calculations were done for the expansion vessel!

 

[gaz1]

Just checked The gauge shows just over 1.5 bar ( its been off for 1.5h hours), but the expansion tank is at 2.9Bar measured at the Schrader valve. so I guess the tank pressure must be to high and not having the correct reaction.

theres something wrong if your hitting that pressure

you need to lower your pressure to 1.2 yes you can higher it between 1 and 1.5 bar but the norm is 1.2

if you are still hitting 2.9 then theres faults somewhere

badly reading pressure guage
a weep on the infeed need to disconnect the infeed
faulty pressure tank

 

[Matrixx]

All being well, when the system was installed the correct calculations were done for the expansion vessel!

Well, the local plumber came round when I moved in a couple of years ago and said he had never seen anything like it, it also has auto top up system added to inject water to keep it at 1Bar, and low loss headers, shunt pumps etc.

I will drop it to 1.2 when I get chance and see what happens.

 

[waddycall]

When you set the expansion vessel pressure (it should be same as heating system cold fill pressure), make sure the pipework connecting it to the heating system is unpressurized. You don’t need to drain it completely - just let the pressure out.