Various pressure questions (with a subject line!)

[AWJS01]

I fixed my problem with the leaking filling loop, so we're making progress!

With the initial system pressure set to 1.5 bar, I turned the heating on. The system pressure rose to approaching 3.0 bar after half an hour or so, so I turned it off and let the pressure drop again.

What I've read on this forum would suggest that the expansion vessel may be either shot, or in need of repressurising. Before I go down this route, is there any other possible reason for the pressure to rise in this way (I now know that the filling loop problem has been fixed)?

A couple of follow up questions...

I read a post which suggested that letting air INTO the system would help provide room for expansion as the system heats up? Can anyone expand (no pun intended!) on this? I've been bleeding enthusiastically since the system was drained, and I'm now thinking I've made the problem worse by doing so...

Finally (and I'm sorry to go on). I've been redecorating and I'm still one radiator short at the moment. I would have thought that if I still need to set the system pressure to 1.5 bar (regardless of how many radiators) there are in the system. Is that correct?

I've got a Sime 90 combi boiler by the way. If anyone knows how to find a valve to pressurise the expansion vessel I might consider doing it, but I'm inclined to get someone in at the moment!

Thanks very much!

Alan.

 

[breezer]

repressureising the expansion vessel can be done with a foot or bicycle pump

 

[ollski]

1.5 bar cold could rise to about 3 bar hot although I would say that is a bit high to begin with. 1 bar cold rising to about 2bar hot is more about the mark.
You introduce air into the system and you will rust your rads away and knacker your pump impeller!
cold system pressure as stated in the manufacturer's instructions will be the same whatever the system size

 

[MANDATE]

The only air you want in the system is in the expansion vessel.
When installed new they are already pressurised to somewhere around 0.5 bar, this means the diaphragm is already flexed towards the water side.
When the water side is filled and pressurised to say 1.5 bar the diaphragm is flexed back towards the air side.
Now if you have a external leak the water is being pushed out by the air pressure on the diaphragm.
If the diaphragm splits, water will pass through it to the air side of the expansion vessel. You will see advice to depress the shrieder valve to test for water.
Now when the water expands due to heat it pushs against the diaphragm and compresses the already compressed air further, hence the increase in pressure.
you should be able to find the valve on the expansion vessel, it's like a car tyre valve. so I'm told.
Of course to verify the pressure is correct you should have no pressure on the water side and beside a cycle pump you'll need a pressure gauge.

 

[oilman]

I read a post which suggested that letting air INTO the system would help provide room for expansion as the system heats up?

That was an emergency job over Christmas, not intended for a permanent solution, but to relieve the strain on the system components. The air wound be in an upstairs radiator, and it's debatable whether it would cause any more corrosion than the continual introduction of fresh water. It wouldn't necessarily get into the pump, hence the number of posts which ask why radiators are hot at the bottom and cold at the top.

As for the pressure rise from 1.5 to 3 bar, you will not have read any other cause than a failed expansion vessel, that's because there isn't one except if you have a grossly undersized vessel. Bigger expansion space equals smaller pressure rise.

When installed new they are already pressurised to somewhere around 0.5 bar,.....

All the ones I've bought have been at 1 bar.