Leak = air ingress? Not understanding the basics.

[Ltnman]

We had a combi boiler that has now been replaced by a combi new boiler last year. Both have the same symptoms.

The old boiler was set to 1 bar pressure. This new one sits at 1.5 when the CH water is cold. Both maintain pressure until I bleed a towel rail then the pressure drops. Both systems are and were fully dosed with inhibiter. A match test reveals no gases.

Now the question. I can grasp the fact that a leak will drop the pressure but I can’t grasp how a pressurised CH system can leak water over time that is replaced by air of equal pressure.

This has been happening now for 16 years. Do I really have a leak or is some magic occurring?

 

[terryplumb]

Water is a chemical compound of two elements,the gases oxygen and hydrogen.
Within your sealed system , " air" doesn't miraculously enter ,the elements are already there.

 

[Ltnman]

So water within the CH is replaced by a certain amount of air indefinitely of equal pressure? My understanding is that if fresh water is introduced then air from that water will separate into the tops of radiator or towel rails. So I don’t have an issue then?

 

[terryplumb]

Steel panel radiators corrode ,inside. That corrosion is the primary cause of gases to be created through the oxidation process ,and they collect at the top of radiators,commonly referred to as "air" that you bleed out. As gases can be compressed by the system pressure ,there is no discernable drop in pressure. As some of the waters molecular structure has been altered during the formation of gas,when the rads are bled ,system pressure drops and you then top pressure up by introducing more water back into the sealed system.

 

[Ltnman]

Thanks for the expansions but I thought the whole point of adding inhibitor was to prevent the corrosion. My system is fully dossed with X100 and has been tested for its strength.

 

[terryplumb]

It's called inhibitor for a reason,because that's what it does...it inhibits the process, it does not totally prevent it.

 

[cjcoffin]

The old boiler was set to 1 bar pressure. This new one sits at 1.5 when the CH water is cold. Both maintain pressure until I bleed a towel rail then the pressure drops.

When you top up the system the additional water contains dissolved air. When it is heated this comes out of solution and collects at the top of the rads/towel rail. You then bleed this off, the pressure goes down, you add more water....