Hi,I wonder if you can help.. when both hot water and heating is on I'll get a tank of hot water... sometimes, but when its used it won't allways re heat,boiler is a new condenser and seems to ruin OK,coul it be the "honeywell" switch over valves as system (other than boiler is old,Thanks.

Are you in the UK and is your storage tank in the attic or air cupboard?