You didn't mention what brand or model of bath faucet you have, but I'm
guessing it's a pressure balanced single handle, try changing the cartridge,
or removing the pressure balancing spool if it's a separate unit. and
replacing the spool.
I am in Australia. I have a gravity fed hot water system in my roof. 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. What other
info do you need?
It could be a washer that has come loose inside the valve on the hot side,
getting stuck on the seat until a fluctuation in the water pressure
dislodges it, which would happen when you turn on the lav faucet.. Here in
the States, there is an old valve made by American Standard called Aqua-seal
that does this sort of thing occasionally.
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