Hot water flow problem


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 cock 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.
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Not knowing Jack about Australian water systems, I am willing to bet that you have something stuck in the hot water valve on your tub. When you turn on the hot to the vanity, it probably dislodges this small piece of "whatever" in the bath valve and allows water to flow for a while.
Have you tried your normal startup (running the vanity first) BUT just before you turn off the tub valve, turn the vanity back on. Then shut off the vanity and see if the tub works the next time you turn it on (without turning on the vanity).
Let us know what happens.
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Agreed, probably some debris somewhere as if it were piped wrong, it never would have worked and you say it once did.
Try flushing the system by removing all the valve stems and allowing water to spill out through those openings. Obviously temporarily turn off the main supply and outlet from the tank before removing the valves.
If your roof tank is open and vented to the atmosphere, it is not unreasonable that debris has fallen into the water and gotten into the pipes.
Worse comes to worse, connect a hose from another pressurized source to the tub faucet and force water or air into the faucet and up into the roof tank (blow bubbles)

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Hi

low pressure in the gravity system might be a contributing factor.
You could test this by changing the bath valve washer, or swapping the valve with (say) the bath cold valve, or the vanity or shower valve, if possible. Another way to test it would be to turn off your hot water, dump the header tank and refill it with cold water, then see if the vanity trick still works.
I think you can rule out anything strange about the piping arrangement, it sounds perfectly normal to me.
Cheers Richard
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in a gravity-fed system, think of when you hold your thumb over the end of a straw in a restaurant. In a gravity fed system, your water source has to be vented.
When you open your lavatory faucet, you are providing a vent.
This is a very unusual system. I would want to know more about the overall layout (including pictures if you have them)
It sounds like your home is being fed water through a cistern in your attic and using a pump-switch like what is used in a pressure-washing facility.
So how is the water getting to your cistern? Does the city provide water or are you using well water?
Either way, if you have electricity where you are (I assume you do since you're on the internet), if you use a submersible pump in conjunction with your gravity-fed cistern...along with a pressure tank, you can charge your entire system with 60-psi and this problem you're having will be gone forever.
Send us some pics and I'm sure the guys here can take a look and tell you how to modify your current gravity-system by "cheating" and using a hybrid well-pump configuration.
As for the comment on having something blocking flow in your valve...I am doubtful of that. However, if you want to test that theory, disconnect your showerhead and screw a hose adapter where the shower head is now. Then turn the water in your house completely off. Now drag your neighbors hose over..and thru the window and connect your neighbors hose to the adapter on where you used to have a shower head. Turn the water on at your neighbors house. This will completely backwash your system and whatever is jammed inside your valve will get flushed out (though this isn't the problem..you're more than welcome to try).
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