converting from open to sealed sytem


5 Apr 2010
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United Kingdom
Assuming the boiler is capable, how easy is it to convert from an open to a sealed system (s plan)?

For example, could the expansion vessle be cited in the same location as the F&E tank (in our case, in the loft 1m above the boiler)?

Using a kit like this: Vessel Control Kit

Would it be as simple as removing the olf F&E tank and connecting the old vent pipe to the new expansion vessel with the filling loop connected to the old cistern mains supply?

Have I missed anything/are there any pitfalls?

In terms of vessel sizing I was thinking of a 24l as we have an 18kw boiler and amedium sized house with 10 (largish) rads.


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Do you have a fully pumped system?

You will need a filling loop, pressure gauge and PRV with discharge pipe... if you are in any doubt speak to your boiler manufacturer.

Personally I wouldn't put anything in the loft if you can avoid it. Can you fit the EV in an airing cupboard [wall mounted]?
It's a fully pumped system with a glowworm energy saver 60. The manual claims that it is sealed system compatible.

Good tip on the airing cupboard. The only issue in the airing cupboard is where to route the discharge pipe to (as the cupboard is in the middle of the house).

Does the size of vessel I suggested sound okay?


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The PRV needn't go in the airing cupboard... where is the boiler? A PRV could go on the pipework near the boiler, which is hopefully on an outside wall. Filling loop and gauge where ever is convenient.
take also into account that your radiator valves may need replacing to cope with the additional pressure and that any weak spots in the system will be found pretty quickly.
energysaver is suitable for sealed systems. As mentioned before, PRV on boiler pipework, EV in airing cupboard, 24l is overkill. 12l will be sufficient. Make sure to put auto air vents on high points in system.
Thanks for all the useful advice. Much appreciated.

We replaced all rads, TRVs and pipework this year so I think the system should be okay.


energy saver.. id change the boiler, hate um! :LOL: could buy a system boiler with the ev incorporated into it? if you do convert to a sealed sys make sure the prv is fitted close to the boiler with no valves in-between
Aye, new boiler is on the cards for next year. Was planning to stick with a regulare boiler though (WB 18ri or Viessmann compact) as I like the relative simplicity.

I thought I'd maybe try the closed system mods, get it all working and then get an RGI in to swap the boiler over.


First question is why?

If you like simple a ballvalve and tank is simple, much simplet than a PRV, filling loop, gauge and vessel. Lots cheaper too.
Our house is a little one storey cottage. One half of the roof space is empty and contains all the usual plumbing (cold tank and F&E tank). The other half of the roof space has been converted into a study area but because it is at the same height (or actually slightly above) as the F&E tank, we can't put a radiator up there and it is baltic in winter.

There is no practical way to raise the F&E tank much higher than it is so I though I'd convert to a sealed syatem so I could run a radiator up to the study.


Sealed systems are better - less issues with air, heat up quicker, easier to work on without draining down. It is best to put the expansion vessel as close to the boiler as possible, following the boiler manufacturer's instructions. No valves of any kind in line with it. Filling loop should be located close enough to PRV with pressure guage on it so you can read it while filling. PRV needs an outside wall visible from outside the house.
Our system currently looks like this.

View media item 28257
The only problem in following the guidlines that you mention would be installing the EV near the boiler. The only place it would easily fit would be the airing cupboard whihc is three meters away. That said, it would be on a direct 22mm pipe back to the outlet of the boiler with no valves or other junctions in between. My guess is that that would probably be okay?



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