I posted this problem ages ago without getting any real solution, so I am trying again.
Can someone offer an explanation for this problem?
The hot tap over my bath does not work unless I first run the tap over the vanity nearby. It will run normally almost as soon as the vanity tap is turned on. If I turn on the hot tap at the bath, no water comes out. If I then go and turn on the hot tap on the vanity, water comes out of the vanity tap and then the water comes out of the bath hot tap. If I turn off the vanity hot tap, the bath hot tap functions properly. I have to repeat the process the next time I want to use the bath hot tap. The vanity tap need not be left running. Once the bath tap has started working it works fine until next time I need it.
I am in Australia. I have a gravity fed hot water system in my roof. This is a very simple set up. The HWS has an inlet from the mains. There is a ball c*ck control on the inlet to the HWS. It is gravity feed from the HWS down a pipe to the bath. The pipe runs directly to the bathroom from the HWS. Both bath and vanity are on the same line of piping. Each has a separate hot and cold tap. The bath has separate hot and cold taps but a single outlet. The pipe then continues on to the vanity and further on to a shower.
The pressure is not great but ok. The vanity tap is about two feet higher than the bath tap. When they both flow, they are of similar pressure. When these were first installed, both taps had equal flow. This change occurred much later.
Is there an easy solution?
Any help will be appreciated.