You may have a tap that isn't suitable for your hot water system. In
particular, there is likely to be a substantial pressure difference between
hot and cold supplies. Your cold supply is probably at mains pressure (v.
high), whilst the hot supply is tank fed (low pressure). When the two mix in
the chamber, the cold may push the hot back up the pipe. This is more likely
to happen on the shower than the bath because the shower head is high up. If
the header tank is on the loft floor above, this substantially reduces the
hot pressure, making the effect more likely.
If you have a combination gravity cylinder on the same floor, you may even
find that you have no head at all, as the header tank will only be around
1. Replace hot water system with mains pressure.
2. Install low/-ve head shower pump.
3. Learn to enjoy baths.
More info needed before anybody can respond helpfully!
Is this a mixer tap where you switch between bath taps and showerhead
with a lever?
Do you mean that if the hot tap is running on its own, nothing comes
out of the shower head at all, but it does come out of the tap? And
that cold on its own is OK? Would be unusual...
Or is the problem that you just can't get a decent mixture of hot and
cold? Which would imply the cold is at much higher pressure than the
hot and is overwhelming it, and you probably have the wrong type of
What sort of system do you have - what type of boiler is it; and do
you have a hot water cylinder and/or cold water tank in the house?
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