My rads won't come on except when the timer is set to hot wet? My rads won't come on. The thermostat is set high and the...

My rads won't come on. The thermostat is set high and the times is set to 'heat constant' but the cirulating pump stays off, and the boiler too. However, when I switch the hot wet to constant (with the heating still constant), the boiler comes on and the rads too - is this a failure with the three process valve - should I replace it - can it be serviced?
How do you patch a drive beside tar?
Answers:    yes it is a show disapproval with the three port stopcock,and no it cant be serviced,if this fault have just occured next you need to replace the tap,sometimes these valves are wired wrong which will net it work so that you get hw on its own,but the hw wishes to be on to get heat.if thats the case afterwards a bit of alterations to the wiring between the programmer and the cylinder stat is reqd
Can you give somebody a lift an iron...
Im not a plumber, but I meditate its OK...mine used to do that...you had to own the hot water on for the radiators to work.. you could hold hot water whenever, .............but it be an older system, the combi boiler within my new house is different.