Additional Expansion Vessel

[Axel]

I need to fit an external expansion vessel to Worcester Bosch R30 HE Combi. The MI suggests this should go on the CH return side of the boiler, but in this instance it would be a hell of a lot easier to put it on the flow. I note from the schematic that the pump and 3 way valve are on the return side...If I put the new expansion vessel on the flow side, if and when all the TRV's (and not one rad is without one) are closed, there is still an uninterrupted path from the heat exchanger to the expansion vessel. If I was to use the return, then the 3 way valve would disconnect the vessel when in DHW mode. So why should it go on the return side? is it to do with the extra pressure from the heat and flow direction of the boiler?

 

[twgas]

Why do you think a vessel is required?

 

[doitall]

Manufacturer says.

The water capacity of the system is shown in table 12,
page 27. If a greater capacity is required then an additional
expansion vessel must be fitted into the system
return as close to the appliance as possible. The system
pressure can be set up to a maximum of 1.5 bar with 1
bar being the normal setting.

 

[Axel]

Why do you think a vessel is required?

Actually, the built in one has a punctured membrane...I could fit a new one for 90 quid cost of part but the access is bad - some idiot built a cupboard around it and left minus 1mm clearance on one side. This has also limited the access to the CH return pipe. The flow, it seems, has the fill on it (against MI) and would be very easy to connect to.

 

[rojaca]

The relationship between the pump and the expansion vessel is what determines where on the system the expansion vessel should be situated. Ideally it should be on the inlet side of the pump so that it is under negative pressure. Fitting it on the return side of the system also ensures that the temperature of the water is lower which will prolongue the life of the membrane.

 

[Axel]

Thanks, rojaca, that seems to make sense. My main concern is that the water in the system is allowed to expand without increasing in presssure, under all conditions of the boiler. I have to choose between a 90 quid original vessel and a 20 quid screwfix external one, which would be fitted on the flow side, but i suppose it's ok as long as it does the job....?

 

[rojaca]

There is certainly nothing wrong with fitting an external vessel. As far as fitting it on the flow side after the heat exchanger is concerned I would recommend against this- especially if the reason not to fit it opn the return is that it is a little more awkward.

However many combi boilers expansion tank connections are made onto the flow pipe after the primary heat exchanger, that said your installation should comply with BS 4814 and BS 7074 and BS EN 1288. I don't have access anymore to these but hopefully someone may be able to quote any relevant passages.

I'm sure that the position of the expansion vessel has much the same effect as the position of the cold feed pipe in an open vented system in that it determines the the neutral point in a sealed system. This ensures that the majority of the system is under positive pressure when the E.V is situated on the suction side of the pump, which is preferable. (Again if anyone wishes to comment on this please feel free. Was digging something from the far corners of brain which I may have actually just made up)

-Sorry about length of post
-Must learn to use bullet points