Hot Water Only but Radiators Still Coming On?

[ashburnham]

I have a programmer which has a separate Heating and Hot Water option. All through winter I have just had the Heating option on (Hot Water left in the off position). The pump and boiler both turn on whenever the thermostat goes below the selected temperature, radiators come on and hot water is heated up at same time.

With weather getting better, the thermostat is not going below the set temperature as much and consequently I'm not getting enough hot water.

I have therefore switched the Heating option off and the Hot Water option on. The hot water heats up fine but the radiators are still coming on. The only difference with this Hot Water option is that it seems to override the thermostat (which you would expect) but this means piping hot radiators while heating my water.

I guess I can go round and switch all my radiators off but this is surely defeating the object of having a hot water only option on my programmer?

My system is as follows:

Programmer: Honeywell ST799
Pump: Grundfos Selectric UPS 15-50
Boiler: Glow-Worm Fuelsaver 35F/45F/55F (1988)
Thermostat: Honeywell (standard "twisty/clicky" one)

P.S. This is my first house which I only moved into in September so I have no previous knowledge of heating systems and am by no means a handyman but any help would be appreciated.

 

[weareleeds]

With that blr system must be fully pumped so i would be suspecting diverter valve