Problem with my central heating?

My boiler (gas) is working to heat up my hot water (not brilliantly but it's working) but I can't get the heating to work, I've tried changing the pressure, the valve starts to move when I switch it on but the boiler doesn't fire up even though it does with the hot water any ideas? What pressure should the valve read? (it's 'Honeywell' for the room unit and a Baxi boiler)