Discuss Expansion vessel pre charge in the USA area at PlumbersForums.net
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.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
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.
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.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.
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?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.
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