Central heating - Air not gases replaces water but the pressure doesn’t drop

[Ltnman]

I have changed the title to dismiss gas causing the problem.

I can understand that if there is a weep on a sealed system then the boiler gauge pressure will eventually show a drop. On my combi the pressure gauge doesn’t move and stays constant until I bleed a towel rail. Then the pressure gauge drops.

I have had this issue for years where the top rail of the towel rail goes cold maybe 2 times in the heating season so requires a bleed.

In September I had a new boiler fitted with the heating going on in anger in late October. Now I am having to bleed around once a month with the top two rails being cold before bleeding. The pressure gauge reads one bar cold once bled after a slight top up. Is the air getting in due to the low 1 bar pressure? When installed, the pressure gauge was rereading around 1.2 bar so didn’t need topping up but as the pressure drops the air appears to be entering faster, but that could be my imagination.

Question. Why is my pressure gauge not just dropping until the towel rail is bled.

 

[45yearsagasman]

Air is probably not the problem. Your system is possibly forming gas within. While you bleed the rad, try lighting the "air" coming out. If it ignites, then it's hydrogen.

 

[fixitflav]

Air is probably not the problem. Your system is possibly forming gas within. While you bleed the rad, try lighting the "air" coming out. If it ignites, then it's hydrogen.

And if it is, get some inhibitor in there

 

[bernardgreen]

While you bleed the rad, try lighting the "air" coming out.

That can be a high risk.

When water molecules are electrolysed in both hydrogen and oxygen are created in the perfect explosive ratio.

Igniting this mixture at the bleed tap might result in the flame flashing through the bleed tap and igniting the explosive mixture of gases inside the radiator.

 

[fixitflav]

When water molecules are electrolysed in both hydrogen and oxygen are created in the perfect explosive ratio.

That is true in a school chemistry experiment, but I don't think it holds in a rad. The reaction is water (hydrogen oxide) + iron = iron oxide (black sludge) + hydrogen. I've lit gas from a rad a few times and not had a problem. Might be worth keeping curtains etc at a safe distance!

 

[Ltnman]

I did hold a flame to the radiator while bleeding and nothing happened apart from the flame going out after a few seconds. I also witnessed 2 bottles of inhibitor going in on a 12 rad system. On my old system it was well dosed as I used to do it myself.

 

[45yearsagasman]

That can be a high risk.

When water molecules are electrolysed in both hydrogen and oxygen are created in the perfect explosive ratio.

Igniting this mixture at the bleed tap might result in the flame flashing through the bleed tap and igniting the explosive mixture of gases inside the radiator.

Never heard such a load of twaddle, or any incidence of this ever happening in my many years in the industry.

 

[bernardgreen]

Never heard such a load of twaddle, or any incidence of this ever happening in my many years in the industry.

It is true that I am only aware of one incident ( from a fire brigade officer ) where this happened.

I accept that the explanation (hydrogen oxide) + iron = iron oxide (black sludge) from fixitflav does explain 99.9% of the incident of hydrogen production.