Corrosion problem in central heating system with standard plastic piping?

[Dr Doolittle]

Dr Doolittle, Oxygen, Nitrogen and Hydrogen exist in the atmosphere as diatomic molecules, NOT single atoms. The diatomic compounds have two atoms e.g. O2, N2 and H2.

Whether a material allows molecules to pass through it depends on the shape and size of the molecule. Diatomic molecules are ‘I’ shaped whereas water molecules are ‘V’ shaped due to having 3 atoms.

The ‘I’ shaped diatomic molecules can align themselves with tiny holes in the plastic and pass through easily whereas the ‘V’ shaped water molecules get stuck. That’s why some pipes/materials are water tight but not air tight.

Finally, nitrogen molecules are larger than oxygen molecules, so oxygen will pass more more easily through a solid material. It’s why some garages fill tyres with pure nitrogen instead of regular air.

so when you google and don't have the education to actually understrand the cut and paste, its just sounds stupid. V shaped molecules ? align in tiny holes? wow, you really should read it before you post it. Are these "tiny holes" the space between the electrons LOL

 

[Dan1990]

I don’t need to google what is quite basic chemistry. I’m a scientist who visited this forum as a DIYer, but when I saw your daft mocking comments I had to reply.

 

[SimonG]

I don’t need to google what is quite basic chemistry. I’m a scientist who visited this forum as a DIYer, but when I saw your daft mocking comments I had to reply.

Well played sir 👍👍👍👍

 

[SJB060685]

Funnily enough, I've had a theory that it doesn't affect OV systems the same as the trapped air can escape through the vent. My theory is that it only does damage when the air gets trapped in a sealed system and doesn't get out.

Might be a load of rubbish like but all the problems i've come across have always been sealed.

Your theory is a sound one mate. OV systems can readily expel dissolved oxygen bought out of solution, or trapped air and micro bubbles. Its a different story with sealed as you say because these gases are harder to expel and without the right means can stay in the system to accelerate the corrosion process.
All systems when filled will contain a certain amount and after around 60-90 days (if I remember correctly) most of it has escaped but like I said sealed systems can and quite often do cause problems.

 

[John.g]

Dr Doolittle, Oxygen, Nitrogen and Hydrogen exist in the atmosphere as diatomic molecules, NOT single atoms. The diatomic compounds have two atoms e.g. O2, N2 and H2.

Whether a material allows molecules to pass through it depends on the shape and size of the molecule. Diatomic molecules are ‘I’ shaped whereas water molecules are ‘V’ shaped due to having 3 atoms.

The ‘I’ shaped diatomic molecules can align themselves with tiny holes in the plastic and pass through easily whereas the ‘V’ shaped water molecules get stuck. That’s why some pipes/materials are water tight but not air tight.

Finally, nitrogen molecules are larger than oxygen molecules, so oxygen will pass more more easily through a solid material. It’s why some garages fill tyres with pure nitrogen instead of regular air.

If non barrier pipe is used in a sealed system then obviously the system water will then absorb oxygen as distinct from "air", does this still mean that eventually this oxygen, once the water is saturated, will appear in the rads and require venting like air?.

 

[Chuck]

What tends to happen is that the tallest upstairs radiator has to be bled two or three times during the first couple of months after draining & refilling, and then no more air collects. Once that process has finished, the boiler pressure is steady - i.e. the system doesn't need re-pressurising,

This observation can be explained by noting that the water used to refill the system contains dissolved air. IME, two or three months is a typical period for a newly filled system to 'settle down'. Repeatedly draining and refilling the system is simply providing it with the oxygen corrosion needs.

Inhibitor doesn't stop all corrosion BTW, it just slows it down significantly.

One possibility, which is a bit of a stretch, is that if you have two metal systems at diferent potentials connected by a section of plastic pipe you have the possibilty of electrolytic corrosion. This might happen, for example, if the earth bonding had been done without considering this issue.

If you are really worried there is something wrong, wait until the system has 'settled down' then get a sample of the heating water lab-tested. This will cost £50-£100 for a one-off test and will tell you exactly what chemistry is going on in your system.

 

[SJB060685]

If non barrier pipe is used in a sealed system then obviously the system water will then absorb oxygen as distinct from "air", does this still mean that eventually this oxygen, once the water is saturated, will appear in the rads and require venting like air?.

Any oxygen that passes through non barrier pipe will do so by diffusion and would stop when its partial pressure equals that of the atmosphere. This oxygen would be dissolved oxygen and to erase dissolved oxygen from a system you need to either use a vacuum degasser, although they affective they also cost. The other option is through heat bringing it out of solution but this being a sealed system can linger internally to cause corrosion if proper means of removal are employed. Open vented systems can readily expel air, micro bubbles and dissolved gases through the vent.
I'm not a chemistry expert but I believe what ive just said is correct.

 

[EvilDrPorkChop]

If non barrier pipe is used in a sealed system then obviously the system water will then absorb oxygen as distinct from "air", does this still mean that eventually this oxygen, once the water is saturated, will appear in the rads and require venting like air?.

That’s what my experience is, yes.