Most of the time we have a problem with the hot water not being hot (or even warm). Boiler is 5 years old. Has a 12litre Expansion tank. I have water flow of mains turned down to help flow but to no avail. Heating appears to work fine and pressure appears to stay constant. Hot water sometimes doesn't come on at all and then all of a sudden will heat slightly. I have heard putting a larger expansion tank on may help. Any other suggestions / comments welcome.