Since I partially drained and re-filled my heating system (to add anti corrosion additive) I've had intermittent problems with my hot water. The system is pump driven heating and hot water using a 3 way valve - looks pretty standard. When the heating/water is switched on automatically (i.e. by the timer) it works fine. When we try to manually switch on the heating/water then the heating seems to override the hot water. With both switched on manually the heating has heated the house but the water is stone cold. Manually switching off the heating and almost instantly the hot water started to heat (I could feel this from the pipes). Just to confirm I switched the heating back on and the hot water stopped heating. Any ideas would be welcome.