Hot water cylinder Vent pipe

[kevin12345]

Hi everyone,
A quick question I can't seem to get my head around.
I have a gravity feed water system.
I'm putting a whole house pump in.
I have to put a side entry on the hot water tank for hot feed which means I have disconnected the house hot water feed pipe from the vent pipe because I'm now taking my hot water feed from side of tank, I then connected pump hot water outlet back to hot house supply.
This leaves me with the vent pipe on the tank coming out of top of tank and going back to loft cold tank.
My silly question is what would stop the hot water tank filling up and continuing to flow up the vent pipe back to the loft?
Thanks

 

[oldbutnotdead]

Is that the vent for the DHW (so coming off the top of the cylinder) or the primary vent for the boiler?

Either way, water finds its own level so the level in the vent pipe won't go higher than the level in the header tank (which is why you have to have the vent pipe outlet 500 above that level)

 

[kevin12345]

Thanks for reply, it's the vent from top of hot water cylinder

 

[oldbutnotdead]

So all it is doing is controlling pressure variations (mainly due to temperature change) in your DHW. The new pump won't affect it in the slightest (not quite sure why you had to change anything tbh).

On the primary side, a badly designed system can end up pumping over into the header tank....

 

[kevin12345]

Thanks for reply.
The reason I decided to put a pump in was because although it worked the water flow was so low

 

[oldbutnotdead]

Oh hang on. I read your post as converting primary feed from gravity to pumped. Whereas what you're doing is putting a booster pump on the DHW. Doh. The pump will be pumping out of the cylinder (from the side you say) so the vent pipe will behave exactly as it used to.