Expansion Vessel Questions

[Topslice]

Just some questions re Expansion Vessels:

Q1 when system boiler water is cold, how full should the Expansion vessel be

Q2 18L Expansion vessel - does this mean the whole volume of the vessel is 18L

 

[Deleted member 120897]

What do you mean “how full” - Air or water? An 18L in my understanding is it will take the expansion of 18L.

 

[Topslice]

how full of water
so an 18L EV can take 18L of water meaning that all the air is compressed into where?

 

[Deleted member 120897]

The water compresses against the bladder full of air and retracts once cooled.

 

[Topslice]

say you pressurize the EV to 3bar to match the hot water cylinder tank ?
when the water is cold how much water would be in the EV in the white potable one

What should the pressure to set for the Red EV for the radiators

 

[John.g]

Consider the HW (white) EV first. A typical example would be a 250L HW cylinder with a 20L EV.
If the EV is prcharged with air to 3bar and mounted and then the mains PRV is set to and maintains 3bar pressure then there is no water in the EV, if the 250L of water is heated to 60C (from say 15C) then the water will expand by 4.05L and will (have to) enter the EV, you will now have 15.95L air at one side of the diaphragm and 4.05L of water (in the EV) on the other side but the force of the expanding water will have increased the EV (+ cylinder) pressure to 4.02bar.,

Consider the Rad, CH system. If you had a 100L system volume with a 18L EV, you generally would precharge the EV to 1.0bar, you would then open a filling valve to a pressure of 1.5 bar, this gives you 3.6L of (reserve) water + 14.4L of air in the EV, when the water heats to say 60C, 1.62L of expanded water must enter the EV so you now have , 3.6+1.62), 5.22L of water and 12,78L of air in the EV bur the EV pressure is now 1.82bar. the reason that you precharge to 1.0bar and fill to 1.5bar is that you have 3.6L of reserve water to make up for any tiny leaks or collapsing air bubbles and don't have to top up the system more frequently, one to 2 years could be normal, ai 1bar there is still enough pressure to feed upstairs rads etc.