Corrosion problem in central heating system with standard plastic piping?

[John.g]

I see where you are coming from, there probably is dissolved oxygen in the water to begin with and over time will slowly rise to the highest point so when it's vented the pressure must fall?. I think deaerated water is recommended when filling solar thermal systems to avoid air eventually gathering up in the roof tubes/panels which are then difficult to access for venting.
I had years of experience with what were known as heating Deaerators where the HP boiler feed water was scrubbed (mixed) with low pressure steam to get rid of any air, this was then vented through a automatic air vent but if this got blocked the deaerator temperature would start falling even though the deaerator was kept at 1 bar pressure corresponding to a (steam) temperature of 120.4C, we flagged a alarm when the water temperature fell to 115C.

 

[Dan1990]

what is water made from? Hydrogen and oxygen. Can you see how silly your argument just became?

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.

 

[Ben-gee]

The real reason some garages fill with nitrogen is to get more money from you!
The n2/o2 size difference is very small and in domestic tyres in real world situation the benefits are negligible.

 

[John.g]

Expansion vessels are also pre charged with nitrogen but for many many years aircraft tyres are charged with nitrogen too.

 

[EvilDrPorkChop]

The real reason some garages fill with nitrogen is to get more money from you!
The n2/o2 size difference is very small and in domestic tyres in real world situation the benefits are negligible.

Tbf when i've had nitrogen in my tyres in the past they last much longer at the correct pressures than they do with just Air.

Also once the Nitrogen has been depleted in a Expansion Vessel and replaced with Air you find that you have to recharge them much more frequently - from factory in Combis they can last about 5 years with nitrogen, afterwards they only generally last 2 with air in.

 

[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?.