NO HOT WATER?

I have a back boiler which runs central heating ok but gravity fed water doesnt work.The flow pipe to water cylinder marginally warms near tank.The system was partly drained to fit rad and this problem has persisted since I think its an air lock in the coil inside the water tank should i fit a bleed valve to the top of flow pipe ,or undo union nuts connecting pipes to cylinder to allow air to escape.Thanks