IFLO CH boiler switch (problem setting only heating for frost protection)

[plumb01]

IFLO boiler switch:
I have separate settings for both hot water and heating. However, when I want to set the heating by the thermostat whilst I am away (ie. for frost protection), I seem to have to set both the heating and hot water to "ON" rather than just setting the heating to ON. If I only set the heating to ON, the boiler doesn't kick in until the hot water is also set to ON.

Grateful for any suggestions.

 

[gassafe]

Programmer set to gravity or if it's just started to happen then a faulty control valve in the airing cupboard maybe, such as the 3 port valve