Vented, combination oil boiler/open fire multifuel boiler heating system. Air lock ?

[Manxie0]

Hello there. . . I have a vented heating system, on an oil boiler. Also connected to the system is an open multifuel fire boiler which can be used with the oil boiler or on its own. Both will heat the radiators and the hot water. . .
I recently drained down the system to change a radiator (a different size, and had to change the pipework etc). After refilling the system, using the oil boiler, the heating works fine, but the hot water gets warm but is nowhere near the temperature it should be. If I use the fire, the hot water is fine, although when the water gets hot I can hear a rushing noise in the riser pipe from fire boiler.
This same thing happened some time ago when I did similar work on the system. On this occasion I lit the fire, the water got to near boiling and it I think blew out the airlock as I heard a noise in the pipework that indicated this, and all was well again. I am aware that I should not really do this, so my question is, how can I get rid of what I think is an airlock in the system in a more system friendly way, than boiling the water in the system is please ?
All the taps in my house are the mixer type with one handle to operate each. . .
Thanks alot. . .