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View the thread, titled "Sealed system grabbing air" which is posted in Air Sourced Heat Pumps Advice Forum on UK Plumbers Forums.
You sure it’s not hydrogen ?
Sure it isn't blowing off and letting water out the system?
id go for galvanic corrosion . It was common years ago to test for hydrogen gassing by holding a lighter against a venting radiator bleed nipple and see if you get a small flame. Make sure the curtains aren't anywhere near
If you can prove to have to have hydrogen in your system, you know its galvanic corrosion. Add some more inhibitor.
Incidently, I had a long chat with Hepworth technical (Hep2O) years ago about the virtues of barrier vs non barrier pipe (in relation to oxygen crossing it). And yes non barrier pipe does let a lot more oxygen across compared with barrier but its still nominal compared with the oxygen that gets into a system via radiator valve glands- apparently its the top crossing point by a country mile for oxygen ingress. Those cheap unglanded lockshields are a death sentence for your system. Volume house contractors often use cheapo valves . Id also be looking at that.
As a side issue ,after 6 years it would be normal to have to replace the expansion vessel in your boiler . They're only 10+ litres and subsequently have to work really hard in an 8+ radiator system . Its also a lot warmer in the boiler casing compared to an external one (and plumbers tend to oversize external ones because theyre relatively cheap ) and heat reduces the the life of the butyl rubber dramatically. External ones are barely warm to the touch during use
Strange though that its not lifting the PRV as the pressure is 3bar when cold which should mean that it had to be higher when the systen is/was hot?
Apparently non barrier piping can still absorb oxygen even in a pressurized system but one would think that rad valve glands would not inhale air except that some are under negative pressure.
I have a > 30 year old rad in a converted attic to bedroom with a open vented system (combined cold feed and vent), this rad allways runs with slight negative pressure but have never had any problems, I also think that at least some of the piping is non barrier.
Apparently non barrier piping can still absorb oxygen even in a pressurized system but one would think that rad valve glands would not inhale air except that some are under negative pressure.
I have a > 30 year old rad in a converted attic to bedroom with a open vented system (combined cold feed and vent), this rad allways runs with slight negative pressure but have never had any problems, I also think that at least some of the piping is non barrier.
If the expansion vessel is correctly charged and working okay then you'd need a lot of corrosion to evolve enough hydrogen to raise the pressure of the cold system to 3bar. It would take ages to bleed. (I suppose it's possible but, unless the system is an unusual combination of metals and a sacrificial anode needs replacing, it doesn't seem likely.)
I suspect you have a failed/failing expansion vessel. If this is the case, even a small amount of corrosion will jack up the cold-system pressure dramatically.
So, I'd investigate the EV first, also check the pH and inhibitor level in the system water. If the EV has been faulty, the PRV in the boiler will have been operating. The system seems to sit at 3bar when cold so it's probably reseating okay but I'd want to keep an eye on the outlet and pressure in normal operation to make sure it isn't passing.
I suspect you have a failed/failing expansion vessel. If this is the case, even a small amount of corrosion will jack up the cold-system pressure dramatically.
So, I'd investigate the EV first, also check the pH and inhibitor level in the system water. If the EV has been faulty, the PRV in the boiler will have been operating. The system seems to sit at 3bar when cold so it's probably reseating okay but I'd want to keep an eye on the outlet and pressure in normal operation to make sure it isn't passing.
Time to flush the system
I would add cleaner but also have a look at any towel rails sometimes they can cause bad corrosion if there cheap
Have you physically checked the pressure in the system ? it could just be a faulty gauge or partial blockage in the tube , I'd suggest and good hot flush then a chemical clean and mains water flush check then expansion vessel is charged to 0.7 -1 bar add a couple of litres of quality inhibitor before filling slowly and avoid trapping air in the system even fit manual air vents on high spots if necessary. Kop
Hhmm was there a smell when vented did water come out? test the water there's many Kits available and we should really be doing it . I would say it needs a clean and possibly a new vessel you may possibly get away with not doing it ? , but if you're gonna be there a day why not do it and avoid another trip later your customer only has the one bill then and just gives a bit more peace of mind you've done the job best you can. Kop
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