Filling expansion vessel with water

[dazlewis1970]

Hello

I wonder if anyone can answer a plumbing question. I just changed the potab le water expansion tank for my unvented water system. All seems to be worki ng fine, but I am wondering if I have a missed a step.

I have a zilmet 24l tank pre-charged to 50 psi sitting at the highest point in the system. Obviously the tank was full of air when I first connect i t up and I am wondering if  I need to bleed the air from the water side o f the membrane to fill it up with water? There seems to be no way to do it, and no suggestion it should be done, so I will presumably just contract le gionaire's shortly?

Any comments appreciated!

Thanks Darren

[Rick Hughes]

I have 4 Zilmet expansion systems on my install.

The Zilmet 24Litre on Boiler feed loop has just failed (water coming out of air charge valve) ... so have to obtain replacement next week and fit.

According to my install instructions - these units are charged with air when 'not connected' to a pressurised water system ... you would (or at least should) drain water if you want to check the air pre-charge.

In my case you then fill the pipework and charge up the system to 1 bar static charge.

[dennis]

The cylinder should be filled with air on the otherside of the membrane. It will partly fill with water once there is some pressure in the system. It should never fill compltetly with water, its bust if it does.

[Rick Hughes]

forgot to add if this is a closed loop ...which is sounds like, then there will be an airvent on it somewhere (top of your tank ?) may be an autovent type. Or a bleed screw to let air out.

It won't be on the expansion vessel.

[dazlewis1970]

Hi, yes I realise the membrane separates the water side from the air side, but my question is how should I get rid of the air remaining on the water s ide from when it is first installed. As it stands there will be air on both sides of the membrane. This means there is an air space in contact with th e hot water. I guess like an old style non-membrane expansion vessel. I jus t wondered if anyone thought this was an issue, and if so, what can I do as there is no means for bleedingg the water side of the membrane.

Cheers

[RJS]

able water expansion tank for my unvented water system. All seems to be wor king fine, but I am wondering if I have a missed a step.

nt in the system. Obviously the tank was full of air when I first connect it up and I am wondering if  I need to bleed the air from the water side of the membrane to fill it up with water? There seems to be no way to do i t, and no suggestion it should be done, so I will presumably just contract legionaire's shortly?

Sorry if this a stupid question Darren: is it essential that the expansion vessel is at the highest point of the system? In my pressurised system - admitedly only for CH - the pressure vessel is not at the highest point in the system and any air is bled from the towel rads and attic rad. So if yo u are using the PV to pressurise a water system, shouldn't there be an air bleed point higher than the PV?

Richard

[dazlewis1970]

I'm not sure if it needs to go at the highest point, that's just where mine was when I moved in - sitting on top of water tank on the upstairs level. I can't think of any technical reason why it needs to be there, but someone else might know otherwise. My question would apply wherever it is located (unless it was placed upside down lower in the system so the water would dr ain into the vessel (or at least the part where there is meant to be water! ).

Darren

[dennis]

The membrane will stop that. There isn't any air in the vessel on the water side when its working. The air pressure will keep the membrane filling the whole of the vessel until the water gets to at least the precharge pressure.

[Rick Hughes]

There has to be an air bleed somewhere ... as I mentioned on my install the air bleed is an 'auto vent' on top of Thermal store ... not the highest part of pipework but it works as air does end up there.

Follow the pipe run you will (or should) find either an auto air vent or a manual bleed screw.

[dazlewis1970]

Hi

I cannot see any bleed at all on the hot potable circuit. There is an auto bleeder on the central heating circuit (I had to replace that too this week end), but seemingly not one on the domestic water. Even of there were, my q uestion is about bleeding air from the expansion vessel (the water side of the mebrane) because the expansion vessel is in effect on a spu at the high est point, I cannot see how bleeding from lower down can hel anyway.

I suspect over time the water will dissolve and come out that way.

Darrem

[Onetap]

able water expansion tank for my unvented water system. All seems to be wor king fine, but I am wondering if I have a missed a step.

nt in the system. Obviously the tank was full of air when I first connect it up and I am wondering if  I need to bleed the air from the water side of the membrane to fill it up with water? There seems to be no way to do i t, and no suggestion it should be done, so I will presumably just contract legionaire's shortly?

I understood the question. I wouldn't bother. The air (on the water side of the membrane) will just act as an additional expansion device, similar to the air trapped in a pressure gauge syphon tu be.

The oxygen will dissolve and get carried out by the water. The remaining nitrogen will gradually get entrained if/when the water spee d exceeds 2 ft/s or so. It doesn't do any harm and I can't see any need to remove it. There's lots of dissolved oxygen in the mains water anyway. Some will come out of solution when the pressure drops, after the PRV set and refill the p ipes anyway.

[Mathew Newton]

Your premise regarding the initial state is incorrect. If pre-charged, whic h is the state that all expansion vessels are usually supplied in, there is very little air on the water side of the membrane hence they don't need to be bled.

Take a look at the following cut-away:

Of course, there can be no pressures involved in such a situation however i t might help visualise how the diaphragm sits when pre-charged i.e. it does n't sit midway down the vessel as you might otherwise think (if it did it'd have to stretch to accommodate the necessary movement which would do nothi ng for longevity).

The following diagram shows the subsequent accommodation of expansion:

png

Mathew

[smackay75]

Hi Mathew, I know that this is from an old post but I was just wondering the same question about possible air trapped on the water side of an expansion vessel. Your answer is brilliant and the addition of the pictures makes it all make sense so thanks for your time. Steve

[harry]

There is no air on the water side. When you buy the tank the diaphragm is fully deformed against the connection. It only moves away when the water pressure exceeds the air pressure.

[Chris Hogg]

Would it actually matter if there was? That air would still be compressible in the same way as the air on the other side of the diaphragm, and slowly over time it would dissolve in the water it was in contact with and disappear.