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Author Topic: Air getting into Pressurised system  (Read 1121 times)
Alaric
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« on: July 07, 2020, 08:11:54 PM »

Hi All,
First post so please be gentle.

I've got a 30 evacuated tube Baxi Solar Flow system installed about 2007, it's apparently designed to stagnate & temperatures have got up to 210 C + (I've seen 210C reported by the system so it is possible it gets higher). The base pressure in the system (according to the Baxi docs) should be about 2 bar, which it is, the pressure in the expansion tank has been checked & appears to be correct.
 The peak pressure in the system appears to be about 3.5 bar (when the system has stagnated)

As long as the system doesn't stagnate all is good. However once the system has stagnated, air starts to get into the system (I cna hear it circulating), the cold pressure doesn't appear to drop significantly for a considerable period & then suddenly drops to almost 0.

If pressure slowly dropped & then air got in once the system had dropped to almost 0 then I could understand but when the system is at 2 bar how is air getting in?

Not sure if it has any significance but I have found some leaks at the unions to the array on the roof (which incidentally are a strange setup - a couple of "o" rings around the Flexible Stainless pipe which then slides into a brass fitting & is all held in place by a split washer around the pipe & nut), the o rings had failed so have been replaced with some high temperature ones which should be good for over 250C.

Has anyone got any thoughts?

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Baxi SolarFlo 30 Evacuated Tube Solar Thermal
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nowty
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« Reply #1 on: July 07, 2020, 08:34:20 PM »


If pressure slowly dropped & then air got in once the system had dropped to almost 0 then I could understand but when the system is at 2 bar how is air getting in?


Air cant get in as such, but when the system is first filled, some air is dissolved in the water and can come out of solution. Plus other gas is also created by corrosion of the internals. Same thing happens in central heating systems.
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Philip R
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« Reply #2 on: July 07, 2020, 08:48:14 PM »

The membrane in the expansion vessel is probably perished or worse, Suggest you look at the expansion vessel.
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Alaric
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« Reply #3 on: July 07, 2020, 08:53:37 PM »


If pressure slowly dropped & then air got in once the system had dropped to almost 0 then I could understand but when the system is at 2 bar how is air getting in?


Air cant get in as such, but when the system is first filled, some air is dissolved in the water and can come out of solution. Plus other gas is also created by corrosion of the internals. Same thing happens in central heating systems.

Thankyou, I had thought about dissolved air but had discounted it as a) the problem only seems to occur when the system has stagnated and b) the volume that I have to vent to get rid of it seems (to me) to be far more than I would expect to have been dissolved.

The working fluid is a Glycol/water/inhibitor mix so hopefully corrosion shouldn't be a significant problem. Out of curiosity how much dissolved air would be expected? (total fluid volume is approximately 20L including a 12L protection vessel)

Expansion vessel is new & not showing any signs of fluid coming out of the valve on the bottom (previous one died a couple of years ago but at that point the protection vessel wasn't in the circuit so it had got hot) is there any sensible way to test an expansion vessel (apart from these checks)?
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oliver90owner
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« Reply #4 on: July 07, 2020, 08:59:32 PM »

the cold pressure doesn't appear to drop significantly for a considerable period & then suddenly drops to almost 0.

Maybe check your pressure gauge is clear and working properly as a first check?
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rogeriko
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« Reply #5 on: July 07, 2020, 10:17:35 PM »

The expansion vessel is overcharged with air so it is not doing anything. When the water pressure drops to zero check the air pressure with a digital tyre gauge and let the air down to 0.2 of a bar LESS than the water pressure you fill the system to. Most expansion vessels are precharged to 2.5 bar which is way to high but it is easy to let some air out. You must have some fluid in the expansion vessel when the system is cold. Thats why you set the air pressure lower.
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Alaric
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« Reply #6 on: July 14, 2020, 02:01:49 PM »

Hi Rogeriko,

I've checked the expansion vessel pressure & played around with it (the vessel) somewhat it *appears* to be correct & working. (I can isolate the expansion vessel, but still be able to pump fluid in to it, which behaves as expected - increase in fluid pressure = increase in Expansion vessel air pressure, reduce fluid pressure, expansion vessel pressure drops)

Having had a dig around the forums & found someone who had a similar problem which turned out to be the Expansion vessel connector (which I don't have) I'm wondering if there is something in the safety group / pump which is blocking the return of fluid as the system returns from stagnation for a period (whilst hot?) & then the array side of the system is then pulling a small amount of vacuum (which given the pressure gauge is mounted on the safety group the gauge doesn't register the drop in pressure) allowing air in somewhere & then as cooling continues the blockage opens allowing fluid back in. Trouble is I can't see the pipework being that complicated.
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