high pressure

[pauloc]

hello guys . went around to boiler. worcester combi 25 cdi greenstar. noticed pressure was at 2.5 bar. assuming installer had overcharged it went to let some water out of rad and all that came out was air, lots of it. anyway i then noticed the gauge dropped down to 1 bar. just want to know can a system which hasnt been aired cause rise in system pressure gauge and if so can someone explain? many thanks

 

[Andy165]

If its recently been drained down and refilled then any air present can be dissapated to a couple of radiators. As you said, by venting it off you have lowered the pressure to 1 bar. If the system has not been worked on, then it is possible the expansion vessel has lost its charge and the boiler will continue to rise in pressure until it unseats the prv and you will see it dripping / discharging the water at about 3.5 bar. The expansion vessel will need recharging to its correct pressure or replacing if damaged e.g. punctured diaphragm

 

[simond]

The water in a heating system will expand by around 4% when hot.

This causes the rise in pressure. Some air in a radiator will not make a discernable difference other than the actual water volume in the system will be slightly less.

I hope you are not a plumber.