Baxi 'System' Boiler - pressure question

[kmillar]

I've just had a completely new system installed. The boiler is a BAXI 'System' 35/60 with pump and expansion vessel built in. We have 6 Radiators plus indirect cyclinder and a 3 way valve.

My question is this. As long as I leave the system running, ie HW and/or CH demand on 'all day' the pressure sits between 1 and 1.5 bar all week, or for as long as you keep it all running.

If I let the system go off, ie do not programme CH or HW to come on at all (say over night), and allow everything to cool right down the pressure drops right down to almost 0 by the morning.

I have been all over the system and cannot find any leaks and it only takes a small amount of water through the filling loop to bring the pressure back up.

My question is this. Why does the pressure only drop when the system is allowed to cool right down? and could it be related to this... In the commissioning stage of the install instructions for the boiler it says to open the screw on the automatic air valve in the boiler. It does not mention closing it again. I have checked and the screw is definately loose (open). Is this correct? Or should it be closed once the system is filled and bled?

[tarquinlinbin]

Its not critical but your sealed system should be about 1 bar or so when cold and between 1 and 2 when up to full temp. If there are no leaks present then the key might be that its a new system. When new, the system has been filled with oxygenated fresh water,,obviously there is some settlement and so the pressure drops back a bit,,top it up when cold to read 1 to 1.3 bar. Observe it over a few weeks and ensure that the cold pressure remains the same. If not,you probably have a very small leak somewhere.

By the way,the pressure varies with temp simple because hot water expands and occupies a greater volume. As yours is a sealed system,it has nowhere to expand to apart from into. The expansion vessel takes up most of the expansion but some pressurisation does occur....

joe

[kmillar]

Thank you for the reply. BTW, do you know wether the screw on the "auto air vent" in the boiler should be open or closed?

[Set Square]

1 - 1.5 bar is a bit low for a hot system. If you use the filling loop to charge it up to 1 bar *cold*, does it *still* only get up to 1.5 hot and then go back to zero when it cools?

If so - and possibly indicated by the fact that it only takes a small amount of water to re-charge it - it sounds to me as if you could have a problem with the pressure vessel. If this isn't working properly, it can't absorb the expansion when the water gets hot - so pressure builds up (for a short time which you might miss) to over 3 bar - whereupon the pressure relief valve opens and lets some water out. You can test this by tying a plastic bag round the end of the discharge pipe (which should go out through a wall into fresh air) and seeing whether any water collects in the bag. If it does, come back - and we'll tell you what to do next.

I would close the auto air valve once the system is fully bled. I doubt whether this is the source of your problem unless there is any evidence of

*water* having come out through it.

[kmillar]

I found the problem.

I had a leak which only leaked when the system was cold. I've reapired the leak and the problem has gone away.

-K

[Andrew Gabriel]

This is quite common, except I suspect it's more a case of it only showing when system is cold -- when system is hot, it dries up as fast as it leaks.

[kmillar]

Not quite, because the water pressure never dropped while the system was hot - it could run for days (and nights obviously) with no trouble. Only when I let the system go cold did the leak appear.

As it happens, some parts of the install are using PE-X plastic pipe, and I think they are moving when heating/cooling and had highlighted a poor joint when they cooled, and moved again.