Sealed system grabbing air

[Matt0029]

Any ideas why a sealed system with a combi could be grabbing air. The upstairs rads have air in and the system reads 3 bar cold. When the air is bleed out the system returns to 1 bar or less. It's an ideal instinct boiler basically a logic.

 

[ShaunCorbs]

You sure it’s not hydrogen ?

 

[Chuck]

Active corrosion. Has there been any recent work on the system, new radiators / pipework, etc.? Check the pH and inhibitor level in the CH water.

 

[Matt0029]

You sure it’s not hydrogen ?

How would I test this?

 

[Matt0029]

The system is around 6 years old new build all in copper. I'm pretty sure it had inhibitor put in it.

 

[Matt0029]

Active corrosion. Has there been any recent work on the system, new radiators / pipework, etc.? Check the pH and inhibitor level in the CH water.

If was active corrosion, it would be hydrogen?

 

[EvilDrPorkChop]

Sure it isn't blowing off and letting water out the system?

 

[Matt0029]

Sure it isn't blowing off and letting water out the system?

It's not no

 

[simonharper1971]

How would I test this?

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

 

[Matt0029]

I could bleed and test with a lighter? I wondered if you'd be anything to do with the expansion vessel. It's happened to quiet a few houses on this new build estate as said around 6 years all

 

[John.g]

For a start, check that you havn't got a auto fill system.

 

[simonharper1971]

I could bleed and test with a lighter? I wondered if you'd be anything to do with the expansion vessel. It's happened to quiet a few houses on this new build estate as said around 6 years all

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

 

[John.g]

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.

 

[Chuck]

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.

 

[Matt0029]

Was thinking of deflating and inflating the expansion vessel