Combining cold feed and vent to prevent pumping over

[Johntheo5]

The pipe in the middle of the cistern above is the ballcock supply and the cistern level won't be remotely near that even after expansion of say 2.5L, my 30L F&E level will rise by 2.5L/inch so would expect yours to be similar except its tiny and would expect the level to rise by 20 to 30mm max.
So you just tee in the cold feed outlet from the cistern to the existing vent which now becomes the cold feed as well and stick a manual air vent on the cut off old cold reed.

 

[Johntheo5]

Thinking about this set up again, I wonder if there's a simpler solution:-
The main problem to resolve is air getting into the system, which is being drawn in through the vent pipe. We know that (I think only when the boiler is starting up) water is being forced up the cold feed into the F&E tank and because that amount of water exceeds the capacity of the vent pipe, when the system stops and cools, it draws all the water in the vent pipe plus some air into the system.

Why can't I simply extend the vent pipe so it is submerged in the tank? That would still allow it to vent and at the same time result in water being drawn back instead of air.

I would love to hear any comments on this - might even give it a try tomorrow with some push fittings to see if it works.

No, the expansion is "99%" accomodated in the F&E cistern, (which is why its called just that) so as stated above, the level in both the cistern AND the vent will rise by ~ 25mm say 1 inch (u tube effect) but the level in the vent alone if used solely as the expansion would rise by almost 8 meters even if it was high enough. Also a vent pipe must never be immersed in water, remember your's is a open vent system, so forget it and just do it as above.

 

[Miseryguts]

Okay, thanks; so that idea was a brain fart!

 

[danfre]

Reposition vent and inlet before the pump.
Looks like a s plan so you probably need a bypass

 

[Miseryguts]

Yes it's an S plan and there is already a gate valve controlled bypass next to the HW cylinder. Post #19 of my previous thread has a photo of the cylinder.

 

[Johntheo5]

Here is a nice photo that someone posted on here of a VCP system (Vent, Cold Feed, Pump), the pump is pumping downwards and the installer just swung the pump head through 180 Degs.

 

[fixitflav]

Here is a nice photo that someone posted on here of a VCP system (Vent, Cold Feed, Pump), the pump is pumping downwards and the installer just swung the pump head through 180 Degs.

Personally I wouldn't do it quite like that for 3 reasons
1. I don't like the vent straight across the top of a tee, as it gives the water a head start towards the vent, increasing the risk of over-pumping. Better to come off the top of a horizontal section.
2. The cold feed should be up and under, to reduce convection up the pipe warming the F/E tank, encouraging algae growth.
3. Better to pump upwards unless it can't be avoided.