Why wont my medium heat work unless the hot wet is person heated? Have had seriously of problems with the my boiler. The heat won't...

Have had seriously of problems with the my boiler. The heat won't come on on its own. The only passageway to get the heat on is if we use hot water and the boiler comes on to warmness water. It will consequently stay on for central heat. The radiators are hot at the top and cooler at the bottom. The whole system have been drained. Bled etc. Fitted a different pump, and a new motorized stopcock along with other current parts.
(The thermostat is on and the boiler is set to hot water and inner heating, its on constant also)
Cheers surrounded by advance for any replies
Do you know where on earth I can...
Answers:    What considerate of boiler is it?Was the new zone stopcock a three port?or does it have two 2 port zone valve?Some older type systems,you entail the hot water on to return with heating.
When you switch on the heat on its own,does the zone valve initiate? if it does then it might be a defective end switch surrounded by the valve,or it might be wired wrong.when you use up the hot dampen in the container,the cylinder t/stat will close and activate the zone spigot,which will fire up the boiler,the water will be circulated around the heat circuit if the heating tap is open.
How to verbs grease trap?
combi or regular boiler