Feed and Expansion and Vent Pipes.

[GraeseNipple]

Just curious. Why do the F & E and vent pipes on an open vented central heating system have to be sited on the flow side of the boiler, before the pump and within 150mm of each other. Thanks

 

[stevetheplumber]

its so the pump has equal effect on both in a negative preasure therefore stopping pump over

 

[GraeseNipple]

Hi STP. Thanks for the reply but can I go a little deeper. You say negative pressure, but isn't the head of water from the F&E tank creating a positive pressure wrt atmosphere. And therefore, why doesn't the pump pull the water from the F&E tank and the vent pipe.

 

[Best]

Hi STP. Thanks for the reply but can I go a little deeper. You say negative pressure, but isn't the head of water from the F&E tank creating a positive pressure wrt atmosphere. And therefore, why doesn't the pump pull the water from the F&E tank and the vent pipe.

I think strange things sometimes do happen with this setup. I have noticed airlock problems in two of these. One was when outside pipes to an oil boiler froze last winter, - pipes air locked in hotpress afterwards. Happened again when owner turned heating off despite being told not to & pipes froze.Obviously prefer sealed heating better & simple for oil or gas, if system decent state.

 

[stevetheplumber]

in theory first time you use pump pulls the level in tank and vent down ballvalve refils it so next time it pulls down the valve will not run and no further water is added
there is always a minimum hieght for the header tank above boiler which means the pump cant continusly draw water in CH pump are actually circulators and dont actuall provide much suction