Gurgling and air in central heating

[jim843]

Hydrogen build-up is a by product of the rads corroding, I believe it can form from galvanic corrosion as well, however I can't find anything to stare that for sure. Hydrogen Sulphide however is caused by a bacterial problem but is also accompanied by a rotten egg smell, unlike hydrogen which is odourless. If it's air in the system then it must be coming from somewhere.
I take it it was another combi installed?
The sludge/magnetite needs to be removed. Ideally one would have a thorough powerflush, then suitable inhibitor added to the system afterwards, plus a filter on the return going back into the boiler. When the new boiler was installed the engineers should have taken into consideration the quality of the existing system water and pipework for foulness, you probably would have been better off separating the boiler from system side with a plate heat exchanger, this would have prevented any fouled water and debri from entering the boiler.

Yes, previous boiler was a combi. New boiler is also a combi.

The installer added an inhibitor but didn't powerflush or add any cleaner. I remember asking him about cleaning the system and he said he'd do "everything required".

When I cleaned the system (a few months after install) there was quite a bit of black water coming out.

There is a filter on the return side (assuming the return side is correctly plumbed!).

The pipework on the bottom two floors of the property are fairly old as far as I know. From the second floor to third floor it's JG Speedfit.

If I understand I need to:

• see if it's hydrogen in the system
• if it is hydrogen, something is corroding in the system
• if it is corrosion, then I need to powerflush the system and add inhibitor

If it's not hydrogen... argh!

 

[SJB060685]

That was another consideration, did you have any plastic pipework? Which you say you do. If the plastic pipework is non barrier then that could very well be the point of air ingress.

 

[jim843]

That was another consideration, did you have any plastic pipework? Which you say you do. If the plastic pipework is non barrier then that could very well be the point of air ingress.

Just checked and it's Barrier PB 15 x 1.5mm.

This pipework starts below the towel rail and goes upstairs in the wall and services two rads above.

I can isolate the plastic pipework fairly easily (I'll have to drain and cap off the system) to see if this is the cause of air? Not sure why the air would end up back up on the second floor and not on the third?

 

[SJB060685]

If it's barrier pipe then it should not be a means of air ingress. The problem with non barrier pipe is air can pass through it and enter the system and would continue to do so until a pressure equilibrium is met. You could isolate it but I doubt it will reveal anything, might be wrong though.
The reason I asked if it was another combi installed is because they are internally laid out so the pump and vessel locations provide a positive pressure throughout the system and therefore shouldn't be any points where air would be drawn in, rather a leak would be visible if there was a failed connection somewhere.

 

[jim843]

If it's barrier pipe then it should not be a means of air ingress. The problem with non barrier pipe is air can pass through it and enter the system and would continue to do so until a pressure equilibrium is met. You could isolate it but I doubt it will reveal anything, might be wrong though.
The reason I asked if it was another combi installed is because they are internally laid out so the pump and vessel locations provide a positive pressure throughout the system and therefore shouldn't be any points where air would be drawn in, rather a leak would be visible if there was a failed connection somewhere.

I was confused about that, too: but you say the entire system is positive pressure, not negative.

I'd been wondering if some part of the system was negative and somehow pulling in air.

If there was a leak, the pressure would drop, right?

 

[SJB060685]

If there was a weak point in the system at low pressure you would draw in air yes.
The expansion vessel will be positioned before the pump, with the pump pumping away. The point the vessel connects to the system is the neutral point or point of no pressure change. Between the vessel and pump will be a very short section of pipework under negative pressure, the pump will then add it's differential pressure to the discharge side, this will supply a positive pressure throughout the system back to the point of no pressure change at the vessel connection point.

 

[Deleted member 123817]

Pressure won't drop with a leak if the filling loop has a PRV with the isolation valve open, so called auto filling.

 

[SJB060685]

Pressure won't drop with a leak if the filling loop has a PRV with the isolation valve open, so called auto filling.

A crude means of auto top up yes but if there was a leak I'm sure he would of noticed somewhere, that would be a lot of fresh water being added to the system.

 

[jim843]

If there was a weak point in the system at low pressure you would draw in air yes.
The expansion vessel will be positioned before the pump, with the pump pumping away. The point the vessel connects to the system is the neutral point or point of no pressure change. Between the vessel and pump will be a very short section of pipework under negative pressure, the pump will then add it's differential pressure to the discharge side, this will supply a positive pressure throughout the system back to the point of no pressure change at the vessel connection point.

Yeah, so if the system is at less than 1 bar and there's a weak point, air would be pulled in.

A crude means of auto top up yes but if there was a leak I'm sure he would of noticed somewhere, that would be a lot of fresh water being added to the system.

Isolation valve is a manual turn on/off thingy.

As far as I can tell there is no pressure drop based on the gauge.

Conversely, if air is being generated in the system, shouldn't I see a pressure increase on the gauge?

 

[SJB060685]

If you read .5 bar on the pressure gauge at the boiler then the pressure at the highest point of the system will be less, perhaps no water up there at all and yes air can enter.
Would air being admitted into the system cause the pressure to rise? Yes it would.