Corrosion problem in central heating system with standard plastic piping?

[stevecase51]

If it was drawing in oxygen through the pipes i'd expect you to be constantly getting air in the system at the highest point. From my experience of none barrier systems this is what usually happens, or the Auto Airvent lets air out and the system is having to be repressurised regularly.

Your easiest way would be take a valve off and take an insert out to see if there's a barrier.

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, I wondered if oxygen entering via the plastic pipes is being removed by the inhibitor, thus gradually lowering the concentration of the inhibitor.

I will bear in mind what you said about taking a valve off,

 

[stevecase51]

Assuming its a sealed system It might also be worth checking out to see if the system filling valve is set to autofill, this will hide any small leaks but the introduced fresh water will rapidly absorb any inhibitor and lead to corrosion.

The filling valves are both closed so I don't think fresh water is entering that way.

 

[John.g]

All my upstairs system was installed with "plastic" piping > 25/30 years ago, have no idea if it was barrier type, doubt it, but my system was/is perfectly clear even though open vented and only gets the odd drop of inhibitor, still have three rads over 40 years old.

 

[stevecase51]

All my upstairs system was installed with "plastic" piping > 25/30 years ago, have no idea if it was barrier type, doubt it, but my system was/is perfectly clear even though open vented and only gets the odd drop of inhibitor, still have three rads over 40 years old.

That's interesting. Perhaps what's happening in my system is nothing to do with plastic pipes.

 

[Scott_D]

Could be cheap rads / towel rads rusting
Do you have a magnetic filter fitted?

 

[EvilDrPorkChop]

All my upstairs system was installed with "plastic" piping > 25/30 years ago, have no idea if it was barrier type, doubt it, but my system was/is perfectly clear even though open vented and only gets the odd drop of inhibitor, still have three rads over 40 years old.

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.

 

[Dr Doolittle]

Tbh that’s normal for plastic piping as the fittings arnt barrier and the pipe isn’t 100% air tight

that must mean they are not 100% water tight then? Did you think that through?

 

[Dr Doolittle]

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, I wondered if oxygen entering via the plastic pipes is being removed by the inhibitor, thus gradually lowering the concentration of the inhibitor.

I will bear in mind what you said about taking a valve off,

oxygen entering the pipes? just oxygen? do you have some special membrane set-up that only allows oxygen molecules through? How does it filter the C02, Nitrogen etc etc? that must be amazing technology. Is it alien?

 

[John.g]

I'm not in the trade but have helped out a few neighbors around me here who all have the same OV systems as my own, combined vent & cold feed, they also don't seem to have any problems with sludge/corrosion and I would say that in the majority have never seen a drop of inhibitor in their lives.

Also one would think that in a sealed system that the system pressure should rise with air ingress and should return to normal after any air venting but it seems that they need topping up after the air is released?.

 

[ShaunCorbs]

that must mean they are not 100% water tight then? Did you think that through?

air will penetrate cells, with water it won’t