My hot water system has developed a strange fault. We have a pressurised system with no cold water tank, all taps fed mains cold water. When using the hot water taps in the upstairs rooms we get a few seconds of hot water and then it runs to a tepid temperature. Meanwhile the taps downstrairs run hot with no problems. The boiler is obviously heating the water and it is getting to the upstairs taps as they do run hit for a few seconds but why are they not staying hot.
Any help greatly appreciated, please do not subject me to another cold shower :)