depends - moved into previous house where thermostat on cylinder and on
boiler were set way too high, so much hot water passed through the open
vent
that the cold cistern was definitely a hot water cistern ;-))

I had that when the thermostat on the hot water tank failed so it stayed on
and heated the cold water tank. It was discovered by somebody going for a
number 2 and getting a bit more than they expected!

Rob.