Corrosion problem in central heating system with standard plastic piping?

[Ben-gee]

Unlikely to be many oxygen atoms about as it exists as molecules, not atoms.

 

[John.g]

It must gain access one way or the other otherwise plastic pipe makers would hardly spend lots of money developing a barrier type.
As I've said previously though I'm a bit surprised that system pressure don't rise with this and why systems require top up after venting air, assuming no system losses.

 

[Dr Doolittle]

Unlikely to be many oxygen atoms about as it exists as molecules, not atoms.

LOL

 

[Dr Doolittle]

It must gain access one way or the other otherwise plastic pipe makers would hardly spend lots of money developing a barrier type.
As I've said previously though I'm a bit surprised that system pressure don't rise with this and why systems require top up after venting air, assuming no system losses.

how about I suggest that the air is already in the pipe rather than being sucked in.

 

[EvilDrPorkChop]

It must gain access one way or the other otherwise plastic pipe makers would hardly spend lots of money developing a barrier type.
As I've said previously though I'm a bit surprised that system pressure don't rise with this and why systems require top up after venting air, assuming no system losses.

What I usually find is that the system doesn't loose pressure, more than it gains Air instead. I don't know the ins and outs but it must be reacting with something in the Air gained with the water in the system to replace the water with Air instead?

For instance the Radiators fill up with air, but the pressure will maintain at 1.5bar if there's no automatic air vent. If there is then the pressure drops, as the air is let out and you need to refill the system.

There's an estate that I work on where the cheaper houses were OV and the more expensive ones got Sealed systems with Unvented cylinders. The developer 20 years ago used Speedfit fittings with Non Barrier Polypipe pipe. It's the Sealed systems that i've had issues with, not the OV ones so far. Many have had the original OV systems taken out and had Combis in so it'll be interesting to see what happens long term.

It's a problem i've really only myself come across in the last couple of years, and the sweet spot seems to be systems that are between 15 & 20 years old.

 

[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?.

 

[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.