do i need an open vent?

Sounds like you have two problems.
First your header tank feeding the hot water cylinder should not be running out under normal conditions. Get a bigger one or double up using a second one.
Second, the air which is entering the system (because the header tank runs empty) should be escaping naturally up the expansion pipe which should be tee'd off the HW cylinder top outlet (supply) pipe. Sounds like you don't have one, which is dangerous.
For 'open vent' read 'expansion pipe' and the answer to your question is YES.

Cledlyn
August 2008

add open vent as this will help prevent air locks, upgrade to bigger tank

andy n
August 2008