Fitting a new external expansion vessel

7 Jan 2010
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United Kingdom
So, I've now taken the plunge and arranged for my installer to fit a new external expansion vessel in a couple of weeks. I'm supplying the parts as I can get employee discount and I'm in the process of putting a list together, which has caused me to wonder about a couple of things.

How easy is it to recharge an external expansion vessel? I'm assuming the whole CH system needs to be drained and the drain cock left open to enable the vessel to be charged properly. This seems like a lot of work - much harder than recharging the vessel in the boiler, which only requires the boiler to be drained! Is there a valve that can be included in the new vessel pipework to enable it to be isolated and only the vessel pipework be drained? I've read that you shouldn't isolate an external expansion vessel as this can be dangerous as there's nowhere for the pressure to go, although I still have a working EV in my Vaillant boiler, so maybe this isn't such an issue for my system.

I was thinking of getting an expansion vessel that would allow the diaphragm to be replaced should it ever perish, but the ones I've looked at seem to be sealed systems, so this isn't possible. Admittedly, they are cheaper than I thought, but it still feels wrong to have to throw one away, just because the diaphragm fails!

The return pipes are 22mm, so I assume 22mm pipe will be used to connect up to the EV, or does that depend on the make of the EV?

Finally, does anyone have any favorite EV manufacturers?

Thanks in advance for your helpful replies.
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you will not need to drain the system just zero the system pressure and leave a drain point open when you re-pressurise both or either expansion vessels
you will not need to drain the system just zero the system pressure and leave a drain point open when you re-pressurise both or either expansion vessels

Thanks Ian, but surely if I open a drain point to re-pressurise, water will drain from the system!
nope it might but most will be held in the vaccum in the system, you need to leave a drain point open so that the expansion vessel bladder can expand and then be pressurized, then you close the drain point and pressurise the system.
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Okay, I see, so some water will come out, but that will be limited providing no other drain points/radiator bleed valves are open.
Why all the questions?..can you not rely on 'your' installer to get the job done?
Why not pressurise the EV before you instal it, then you don't need to leave a drain open to allow for displaced water.
Why all the questions?..can you not rely on 'your' installer to get the job done?

As much as I am capable of fitting an external expansion vessel myself, I also have some boiler parts to be replaced, so yes "my" installer will be the doing the job for me. That doesn't mean I shouldn't understand the job that needs to be done so I can have an informed discussion with him. When he popped round to determine where the EV could be placed and explained what he would do, he didn't mention anything about isolation valves etc. Given the issues I've had with the internal expansion vessel over the years I don't want to be in a position where I have to call him, or any other installer, to recharge it, or even replace it, when its a job I can do myself, and it seems to me that giving some thought to how the EV will be installed now, could make things easier for me going forward.

Having asked the question, I can see now it isn't strictly necessary to install an isolation valve to enable the vessel to be recharged, however, I think it would still be useful to assist in replacing the vessel, when it fails. Each time the system is drained, it costs me around £30 in inhibitor!
There's no problem with fitting an isolation valve. Simply remove the handle from it once the system is up and running. You could even tie a tag onto the valve stating it must be left open. In effect a "special" tool will be required to operate it (the displaced handle).
Is there such a thing as a 22mm isolation valve with an incorporated drain off cock? I found this, but don't see how the drain bit works as it's just a screw. The advantage of a drain off cock is that you can connect a hose to it.
If I were to turn up @ a job and the customer handed me parts to fit I would politely tell him to fook off.
You obviously haven't any faith in the 'plumber' or you are carrying out this work yourself...I've never heard so much drivel.
Does anyone have experience of the following model of expansion vessel:

Reliance - Aquasystem 18 Litre Heating Expansion Vessel & Bracket XVES100050

I like the idea that the membrane can be replaced and I assume this is done by unscrewing the silver top, although it's not obvious where the valve is located - usually on the opposite side to the pipe fittings? I assume expansion vessels can be fitted either way up, although I've seen some that seem to be "top" connected and floor standing.

Most examples of EV's I've seen on the web seem to be fitted high up the wall, with the pipe connections at the bottom and all the pipework running downwards (this is the case with my potable hot water EV), but I'm aware you can get floor standing EV's, so I guess it isn't a requirement to have them "high up". Is the fitting hight related to having a certain length of pipe running to the EV to allow the water to cool as the membrane may be affected by very high temperatures?

I've also wondered how an EV is vented as surely air will collect in it over time?

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