Expansion vessel pre charge

[Matt0029]

I have a 35litre central heating expansion vessel that has come with a pre charge of 1.5 bar. Does this sound a little heigh? As the pressure in the system will have to rise above 1.5bar for it or absorb any expansion? Thanks

 

[John.g]

Normally, a E.vessel would be installed pre charged to 1.0 bar and then filled to 1.5 bar, it will then rise to ~ 2 bar with a hot system, no problem. If its pre charged to 1.5 bar and its only filled to 1.5 bar then the final pressure will be practically the same but will have to be topped up more frequently after any air venting etc as there is no reserve. If pre charged to 1.5 bar and filled to 2 bar then the final pressure will be ~ 2.6 bar, still OK but getting a bit near the boiler PRV (safety valve) lifting pressure of 3.0 bar.

Why have you a 35 litre E.vessel for a CH system??, it's normally around 10 to 12 Litre, are you sure its not attached to your unvented HW cylinder? If it is attached to your CH system then the pressure will only rise ~ 0.25bar even with precharge/filling pressures of 1.5/2.0 bar

 

[Matt0029]

Normally, a E.vessel would be installed pre charged to 1.0 bar and then filled to 1.5 bar, it will then rise to ~ 2 bar with a hot system, no problem. If its pre charged to 1.5 bar and its only filled to 1.5 bar then the final pressure will be practically the same but will have to be topped up more frequently after any air venting etc as there is no reserve. If pre charged to 1.5 bar and filled to 2 bar then the final pressure will be ~ 2.6 bar, still OK but getting a bit near the boiler PRV (safety valve) lifting pressure of 3.0 bar.

Why have you a 35 litre E.vessel for a CH system??, it's normally around 10 to 12 Litre, are you sure its not attached to your unvented HW cylinder? If it is attached to your CH system then the pressure will only rise ~ 0.25bar even with precharge/filling pressures of 1.5/2.0 bar

It's a big system commerical 35 rads. Plan on running the system at 1 bar cold. I know most expansion vessels in combis are pre charged to 0.75 bar.

 

[John.g]

If you want to run at 1 bar cold then reduce the pre charge to 1 bar and fill to 1 bar or preferably reduce pre charge to 0.75 bar and fill to 1 bar, even with a system capacity of 500 litres then the hot pressure at 1bar/1bar will only be 1.71 bar and at 0.75bar/1bar, 1.85 bar.

 

[fezster]

If its pre charged to 1.5 bar and its only filled to 1.5 bar then the final pressure will be practically the same but will have to be topped up more frequently after any air venting etc as there is no reserve.

Would you mind elaborating on this?

If your EV is charged to 1 bar and you top up the system to 1.5 bar, you are utilising the expansion capacity of the vessel when cold, reducing the amount of expansion available when hot. Is there some advantage in doing this (which is what I think you were implying)?

 

[John.g]

Yes, the advantage is that you have a reserve volume of water to make up for any air venting, micro bubbles collapsing or any tiny leaks without the system pressure falling below your minimum requirement of 1.0 bar before top up is required, you can also monitor for any leakage as the pressure will fall from 1.5 bar cold to 1.0 bar before exhausting this reserve.
If you take a fairly standard CH system of 75 litres with a 12 litre EV pre charged to 1.0 bar and filled to 1.5 bar then you will have a reserve volume of 2.4 litres and a final hot pressure of 1.92 bar, if both pre charge and filling pressure are 1 bar then no reserve and if any slight leakage/venting etc the cold pressure will start falling immediately, but yes, the reserve does reduce the amount of expansion available as the final pressure with no reserve will be 1.26bar (vs 1.92bar), any pressure up to 2.25/2.5 bar is not a problem for any system, one can always install a slightly bigger EV if any worries re final pressure.

 

[Matt0029]

To adjust I'm I OK just realising the pressure till it gets to 0.75 /1 bar? Or fulling deflating and re charging. Thanks

 

[John.g]

You will have to fully de pressurize on the water side before reducing the air side pre charge pressure.
If you have no special concerns re pipework/rads why not leave it at 1.5 bar and initially fill to 1.5 bar, I wouldn't expect the hot pressure to be >2.2 bar and try that, if you fill to say 1.7 bar would expect ~ 2.3 bar, you may well find those final pressures are lower as I calculated them on a system vol of 400 litres which may be lower.

If you do the above and post the findings I can probably calculate the air side reduction required without draining down the system, just ensure all the system up to normal temperature.

 

[Matt0029]

You will have to fully de pressurize on the water side before reducing the air side pre charge pressure.
If you have no special concerns re pipework/rads why not leave it at 1.5 bar and initially fill to 1.5 bar, I wouldn't expect the hot pressure to be >2.2 bar and try that, if you fill to say 1.7 bar would expect ~ 2.3 bar, you may well find those final pressures are lower as I calculated them on a system vol of 400 litres which may be lower.

If you do the above and post the findings I can probably calculate the air side reduction required without draining down the system, just ensure all the system up to normal temperature.

Thanks its yet to be fit so if I'm going to depressurise now is the time. I want to run to system at no more then one bar cold. Boiler manufactor specifics this. Also is a convert from open vented to sealed so want to keep the pressure as low as I can. Thanks.

 

[Matt0029]

If you want to run at 1 bar cold then reduce the pre charge to 1 bar and fill to 1 bar or preferably reduce pre charge to 0.75 bar and fill to 1 bar, even with a system capacity of 500 litres then the hot pressure at 1bar/1bar will only be 1.71 bar and at 0.75bar/1bar, 1.85 bar.

Can I also ask what would happen if the system was filled to 1 bar cold. And the expansion vessel was left at the pre-charge of 1.5bar? Guessing it wouldn't start using the expansion till it gets to 1.5 bar. So the pressure would rise quiet alot?

 

[ShaunCorbs]

Normally 0.2 bar below operating pressure matt

 

[John.g]

Can I also ask what would happen if the system was filled to 1 bar cold. And the expansion vessel was left at the pre-charge of 1.5bar? Guessing it wouldn't start using the expansion till it gets to 1.5 bar. So the pressure would rise quiet alot?

No, since water is virtually non compressible it would almost immediately rise to 1.5 bar (after heating up a few degrees) and would behave exactly the same as if the pre charge and filling pressures are the same ie 1.5 bar.

If the system capacity/precharge/filling are 400ltrs/1.5bar/1.5 bar the final pressure will be ~ 2.16 bar with a 35 litre EV.

If the system capacity/precharge/filling are 400ltrs/1.5bar/1.75 bar the final pressure will be ~ 2.57 bar with a 35 litre EV.

If the system capacity/precharge/filling are 400ltrs/0.75bar/1.0 bar the final pressure will be ~ 1.63 bar with a 35 litre EV.

 

[Matt0029]

No, since water is virtually non compressible it would almost immediately rise to 1.5 bar (after heating up a few degrees) and would behave exactly the same as if the pre charge and filling pressures are the same ie 1.5 bar.

If the system capacity/precharge/filling are 400ltrs/1.5bar/1.5 bar the final pressure will be ~ 2.16 bar with a 35 litre EV.

If the system capacity/precharge/filling are 400ltrs/1.5bar/1.75 bar the final pressure will be ~ 2.57 bar with a 35 litre EV.

If the system capacity/precharge/filling are 400ltrs/0.75bar/1.0 bar the final pressure will be ~ 1.63 bar with a 35 litre EV.

Thanks I was going to go with one bar system pressure cold. And a pre charge of 1 bar. This should go up to around 1.75bar did you say?

 

[Matt0029]

Normally 0.2 bar below operating pressure matt

Ah so I should drop the pre charge to around 0.8. But then the expansion vessel will use some of its storage even when the systems cold? Interesting thread. Didn't no the ins and out of pre charge pressures etc. Just thought was abit high at 1.5 bar on the vessel bought.

 

[Matt0029]

Yes, the advantage is that you have a reserve volume of water to make up for any air venting, micro bubbles collapsing or any tiny leaks without the system pressure falling below your minimum requirement of 1.0 bar before top up is required, you can also monitor for any leakage as the pressure will fall from 1.5 bar cold to 1.0 bar before exhausting this reserve.
If you take a fairly standard CH system of 75 litres with a 12 litre EV pre charged to 1.0 bar and filled to 1.5 bar then you will have a reserve volume of 2.4 litres and a final hot pressure of 1.92 bar, if both pre charge and filling pressure are 1 bar then no reserve and if any slight leakage/venting etc the cold pressure will start falling immediately, but yes, the reserve does reduce the amount of expansion available as the final pressure with no reserve will be 1.26bar (vs 1.92bar), any pressure up to 2.25/2.5 bar is not a problem for any system, one can always install a slightly bigger EV if any worries re final pressure.

I think that answers my above question.