Expansion vessel calculation

[phattanglo]

I've just bought a 12lt expansion vessel to take over from the dead one in a Ravenheat CSI85.
There are only 4 rads and at a cold pressure of 1 bar the gauge shows a rise to 2 bars at full working temp.
Could you please tell me what the correct air pressure needs to be in the vessel to accomodate this range.
I haven't seen a leak from the prv but I'll replace it at the same time just to be sure.
Also what is the equivalent to Bar in Psi as I'll be using a tyre pressure gauge.

 

[ianmcd]

it will be stamped on the new expansion vessel what pressure you need to pressurise it to (if required) you do need to check the pressure but not often you need to pressurise if it is a new one, a lot easier to check/re-pressurise before installing it

 

[phattanglo]

The reason I asked was because it occurred to me that a set pressure in the vessel might render it redundant.
A pressure that was too high might not be compressed very much and be effectively solid and a pressure that was too low would be squashed flat by the cold water pressure and again be effectively solid when the temp rises.
I thought that the vessel pressure would need to be similar to the water pressure give or take a bit.
I've no experience in these matters and I expect I'm miles out but it seemed like something I should check.

 

[45yearsagasman]

1 bar is equal to 14.7Psi

 

[Jackrae]

14.7psi is the equivalent of 1 Atmosphere, measured at sea level

1 bar equates to 14.504 psi

Not a lot of difference perhaps but it's the right answer.

 

[ianmcd]

The reason I asked was because it occurred to me that a set pressure in the vessel might render it redundant.
A pressure that was too high might not be compressed very much and be effectively solid and a pressure that was too low would be squashed flat by the cold water pressure and again be effectively solid when the temp rises.
I thought that the vessel pressure would need to be similar to the water pressure give or take a bit.
I've no experience in these matters and I expect I'm miles out but it seemed like something I should check.

You know more than a lot of heating engineers yes you are correct over presurissing an EV is as bad as an empty one but I see them pressurised to 2 Bar + on regular occasions

 

[picasso]

The expansion vessel is ok if the pressure is between 1 and 2 bar at working temp especially as the prv isin't opening, whats makes you think you need to change it.?

 

[phattanglo]

I happen to have a new one

 

[MANDATE]

You're on the right line of thinking. To have the maximum amount of air to compress, which you need to prevent the pressure from going sky high, the diaphragm needs to be biased toward the system water.
Too low a air pressure and there will be little air to compress and pressure rises well beyond the PRV pressure.
Too high a pressure means water pressure could increase to the PRV pressure before it has any effect on the diaphragm.
So if the 'water' pressure when cold is set the same as the 'air' pressure, then the diaphragm is held in equilibrium.
Of course when the water is heated, diaphragm is moved and air is compressed to a higher pressure, this being indicated by a higher water pressure. The amount of increase is dependent on (a) the volume of water in system, (b) the size of the expansion vessel (c) the 'air' pressure in vessel (d) the actual amount of air subject to compression (e)The temperature rise of the system water.
You're right in thinking the air pressure and water pressure should be similar when in operation.
I'm aware you could set the air to say 2 bar and the water pressure to 1 bar, but the diaphragm would not move until the water pressure had gone beyond 2 bar.

 

[bolshy]

Standard pre-charge is usually 1 bar anyway. Make sure you connect it to the return. Due to the size of your vessel, it will be fine at 1 bar, fill your system to 1.25 - 1.5 bar when completely cold.

 

[phattanglo]

Just to add to my last post.
Due to a lack of space I can't fit the vessel near the boiler.
Would it be possible to fit it in the cellar into the return pipe from the living room rad.

 

[Jackrae]

You can fit it anywhere in the system but bear in mind that a raised elevation, say in the attic of a 3 storey house must account for gravitational head (30 feet of water head equals roughly 1 bar). So if it's in an attic 30 feet above the boiler, 1 bar in the expansion vessel when cold will equate to roughly 2 bar at the boiler when cold.

 

[phattanglo]

I would say it would be approximately 8 to 10ft below the level of the boiler.
so given a working pressure of 1 - 1.5 bar would a charge of 1 bar in the vessel be ok ?

 

[Jackrae]

1 bar coldset should do nicely