Corrosion problem in central heating system with standard plastic piping?

[stevecase51]

Five years ago I had a new central heating system installed. The pipework for two of the radiators was done in plastic, running under the floorboards. Although the system was dosed with plenty of Sentinel X100 inhibitor, after about 18 months the water suddenly started to go brown, and I had to drain the system, flush and refill it, dosing with Sentinel again. The same thing seems to happen every 18 months. The only explanation I can think of is that the plastic pipe used by the installer may not have had an oxygen barrier. The system is operating at about 70 degrees and my theory is that the rate of oxygen intake through the walls of the plastic pipe is such that the Sentinel becomes depleted much more quickly than it would normally. Has anybody else encountered this problem? Also can somebody tell me if it is possible to tell the difference between barrier and non-barrier pipe just by looking at what is printed on the pipe?

 

[ShaunCorbs]

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

 

[stevecase51]

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

When you say it's normal, do you mean that having to add inhibitor very regularly is normal with plastic pipes?

 

[ShaunCorbs]

When you say it's normal, do you mean that having to add inhibitor very regularly is normal with plastic pipes?

yep or discoloured water normally around 2 years they need attention

 

[stevecase51]

yep or discoloured water normally around 2 years they need attention

Thanks - so that seems one good reason for sticking to copper?

 

[ShaunCorbs]

Thanks - so that seems one good reason for sticking to copper?

yep or mlcp if you need flexibility (with brass fittings)

 

[EvilDrPorkChop]

It's unusual for Plastic pipe so recent not to be barrier. Have you checked the printed text on the pipework?

I've had a few issues with systems that were done in plastic 15/20+ years ago when the initial pipework wasn't barrier, but nearly all pipework is now from the past 10+ years.

Is it possible Air is getting in somewhere?

 

[stevecase51]

It's unusual for Plastic pipe so recent not to be barrier. Have you checked the printed text on the pipework?

I've had a few issues with systems that were done in plastic 15/20+ years ago when the initial pipework wasn't barrier, but nearly all pipework is now from the past 10+ years.

Is it possible Air is getting in somewhere?

Thanks for that. I plan to check what's printed on the pipe as soon as I can find time to lift the floorboards. I can't think where else air could be getting in. Also I don't have air collecting in the radiators apart from during the first few weeks after draining and re-filling the system.

 

[EvilDrPorkChop]

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.

 

[John.g]

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.

 

[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

 

[Dr Doolittle]

air will penetrate cells, with water it won’t

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

 

[ShaunCorbs]

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

it’s true so not silly its why they installed the plastic barrier layer as they found it out and why when you find a non barrier heating system installed its slugged up and normally needs a repipe

Standard Plastic Pipe non barrier allows a very small amount of oxygen (not air) to enter the system

Also this is why mlcp is 100% air and water tight non permeable due to the alu layer

 

[Dr Doolittle]

it’s true so not silly its why they installed the plastic barrier layer as they found it out and why when you find a non barrier heating system installed its slugged up and normally needs a repipe

Standard Plastic Pipe non barrier allows a very small amount of oxygen (not air) to enter the system

Also this is why mlcp is 100% air and water tight non permeable due to the alu layer

I can't comment further as I think that you actually believe this so its no longer a joke

 

[ShaunCorbs]

I can't comment further as I think that you actually believe this so its no longer a joke

explain your theory then

 

[Dr Doolittle]

explain your theory then

I don't have any theories about how oxygen atoms pass specifically through plastic heating pipe but nitrogen/hydrogen etc don't.

 

[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.