Air in system? Puzzling!

[Phlize]

We had a new combi boiler fitted last year and I find that no matter how often I bleed the system it needs bleeding again in a few days. Actually I haven't bled it for a few weeks now as the (oil fired Worcester Bosch) boiler is outside & I was worried about diluting the antifreeze too much by re-pressurising. So I now have a half cold bathroom ladder rad (the highest one) where the air collects.
The question is, if the system is pressurised to 1 1/2 bar, how can air get in but no water get out? My logic being that if water was leaking somewhere then the pressure would fall - which it doesn't, unless I bleed the system. Surely water would leak out under pressure until the pressure was zero. Or----- is it possible for air to be sucked in on the return side but the 'hole' to be so small water doesn't come out - even when under pressure?? But if air was sucked in wouldn't the pressure increase as the volume of air & water has increased?? All seems a bit weird to me!

 

[wards]

dog leg.

 

[stuartmac]

Hi - I am not qual on oil but the principles of water in the system are the same, Had an issue of constantly bleeding air in a customers system (worcester Bosch) a couple of weeks ago, it can be caused by a number of things,

1. Lack of inhibiter in the system when commisioned
2. not flushed properly so sludge causing a parcial blockage
3. Rad fitted so that the air plug on rad not at highest point could be on any of the rads leaving air in the rad not matter how many times you bleed you wont get the air out.
4. leak in system somewhere could be off weeping rad valves
5. the system layout as mentioned earlier or a fault in the system as mentioned can cause it
6 corrosion in the system.

In the case i dealt with, drained down system repaired two weeping rads, added inhibiter and commsioned bleeding downstairs first then upstairs constantley bleeding out any air. Remember Combi is a closed system so only way air can get in is by one of the above. Good luck.

 

[dontknowitall]

Thank you for those. Didn't know about 1, 2 or 6 and wouldn't have thought that 1 and 2 could have had that affect.

 

[kay-jay]

Thank you for those. Didn't know about 1, 2 or 6 and wouldn't have thought that 1 and 2 could have had that affect.

i also have doubts about 1 and 6 but 2 definately can have this effect. essentially what happens is that the sludge prevents the circulation upon system filling, and gives a false reading on the pressure gauge. and when you have gone home and the heating is turned on some of the sludge moves allowing the previously blocked water and air pocket to circulate with the effect that the gauge drops and rads need bleeding.

KJ

 

[Phlize]

dog leg.

Could you elaborate please??

 

[TGor]

Can also suck in air through the pump gland .. chec for leaks when the pump is off.

 

[fuzzy]

could be gas as a bi product of corrosion