before i ask for help here's a brief breakdown of my system:
I have two hot water cylinders in the back attic. The previous owners of the house added the second cylinder to supply the hot water for an upstairs bathroom. the hot water is sent upstairs via a pump.
when we moved in I put the hot water and heating on to check it and it seemed fine. that said i just ran every tap to see there was hot water - i didn't run a full bath.
However, i ran the upstairs bath the other week and whislt the water was initially warm, after a couple of minutes worth the water went cold.
I have since tried leaving the hot water on all day, incase not pre-heating the water for long enough was the problem but still the hot water from my second cylinder is warm for a few litres then runs stone cold! I
the rest of the house is fine though - so it seems that the problem is with the second cylinder.
the pervious owner when we viewed the house said something about turning the bleed valve on the cylinder (i think anti-clockwise) when we were not using the upstairs bathroom and this would let air into the cylinder and save on heating bills by not heating the second cylinder.
I never tried this but i wonder if the previous owner did this prior to moving out. I'm not sure what to do to reverse this anyway.