Air getting in to sealed system ... !!

[Ponx]

I am collecting air in one downstairs radiator which requires bleeding once per week and a small amount on the auto bleed valve on the hot water side of the system. However, there is no pressure drop until I bleed the rad which then drops about 0.1 bar. The system was flushed couple of months ago when we had a new cylinder fitted and refilled with 2 litres of sentinel. I have checked the strength. There was no evidence of sludge etc after being there for 12 years. I have checked everywhere for leaks... however, if there were leaks I would expect the pressure to drop on its own which it doesn't. I don’t see how air can be get in to a system pressurised at 1.5 bar unless the air pressure is at a higher pressure itself.. which makes me suspect the pressure vessel (which was also replaced at the same time as I had a new one spare.) so it that likely that a new Zilmet pressure vessel could be faulty ? Any other ideas please ?

The schematic is below

 

[Last Plumber]

Looking at the drawing, the large majority of the system is under negative pressure.

Air can be drawn in under those circumstances. I personally would alter that first, in order to rule it out as the possible cause.

With it being flushed out and treated, it is unlikely to be corrosion although you can't rule that out.

 

[Gasmk1]

i would install a air separator and magnetic filter and look for where air can be entrained

 

[Ponx]

It’s been fine for 12 years.. never had to bleed a radiator... if air was getting in on the suction side of the flow, why would pressure drop when you bleed radiator. ? Surely if air was getting in water would be getting out when the pump stops ?? But the pressure doesn’t drop at all until the radiator is bled...

 

[Gasmk1]

when you blled a radiator your are in essence letting pressure out of the system so you need to retop up the lost pressure

 

[Last Plumber]

It’s been fine for 12 years.. never had to bleed a radiator... if air was getting in on the suction side of the flow, why would pressure drop when you bleed radiator. ? Surely if air was getting in water would be getting out when the pump stops ?? But the pressure doesn’t drop at all until the radiator is bled...

It is not necessarily true that if air gets in, water will get out.

If you had it flushed a few months ago, was it powerflushed?
If so, they will have opened and closed rad valves, maybe one or two of the packing's are leaking?
Maybe they are all leaking a bit because they are 12 years old. Some mechanical joints can begin to weep over time, especially things like packing glands.

If the water is clean and treated/dosed correctly, then it is unlikely to be corrosion. If however, it is corroding it could be a build up of Hydrogen gas and not air as you think.

As I said earlier, you really need rule out the pipe work configuration and then you 'know' it isn't that which is allowing air to be drawn in. If that solves it then all's well. If it doesn't you can move onto the next possibility.

 

[EvilDrPorkChop]

Have you got any Auto Airvents?

 

[Ponx]

i would install a air separator and magnetic filter and look for where air can be entrained

It’s got an air separator on the flow side of the hot water cylinder.. should I get it moved to be on the main heating circuit...

 

[Best]

Check any auto vents and close off the wee manual plug in them to see if that helps.
The pipes to coil at unvented cylinder might have an auto vent.
If you want to test for leaks, put the system when it is cold up to the maximum ‘normal’ pressure and leave for a few hours while checking all auto vents, valve glands and compression fittings for any sign of slight weeps

 

[Ponx]

Thanks for this. The bit I don’t get is that if there is a leak and air is getting in then if air is replacing the water you would expect to see a pressure drop as air in compressible but water isn’t. But I don’t get any pressure drop at all - only when I bleed the air out.

 

[Best]

Thanks for this. The bit I don’t get is that if there is a leak and air is getting in then if air is replacing the water you would expect to see a pressure drop as air in compressible but water isn’t. But I don’t get any pressure drop at all - only when I bleed the air out.

If air is drawn in to a sealed system the pressure wouldn’t drop unless water is lost.
Auto air vents in my opinion are problematic as risk leaking or drawing air in. I use manual vents where it isn’t essential to have auto vents

 

[Ponx]

The small screw on the auto air vent is screwed in tight. This is what happens. Pressure when system cold 1.5 bar. No air in radiator. Run it for a week. Pressure when cold still 1.5 bar. Bleed radiator, drops immediately to 1.3 Doesn’t make sense to me....

 

[Gasmk1]

if it doesnt make sense may be time to phone your freindly heating engineer

 

[Julian]

Thanks for this. The bit I don’t get is that if there is a leak and air is getting in then if air is replacing the water you would expect to see a pressure drop as air in compressible but water isn’t. But I don’t get any pressure drop at all - only when I bleed the air out.

This is not correct. The pressure in a close vessel is the same all through out regardless of whats inside or how many kinds of gas or fluid is in there. And by the way when air is heated it expands many folds that of water so it means it can add pressure rather than subtract to it.
Someone mentioned above "Majority of system is under negative pressure"If you can fill that system with positive pressure the leak should reveal itself. There should be water inside the the pipes of course.

 

[Ponx]

I am not measuring pressure in the vessel .. I am measuring it in the system ie waterside...

 

[SJB060685]

Just because your system is pressurised to positive 1 bar it doesn't mean air will not be sucked in. Any portion of that system subjected to negative pressure can and will suck air in, be it a leaky packing gland on a TRV or a compression fitting etc. As Julian said if all this system was subject to a positive pressure with pump running the leak would reveal itself. As you said above if there was a leak and air getting in this will not reduce pressure, it will most likely add.

 

[Julian]

I am not measuring pressure in the vessel .. I am measuring it in the system ie waterside...

Yes that's what I mean a close system not vessel. Sorry for the bad English, Anyways the pressure in a close system is not determined by air or any kind of fluid. but rather by the pressurizing device. ie, pump boiler pressure regulator take your pick.