Pump straining

[AnonymousImport]

Hi,
I have an indirect gas heating and hot water system with a cistern in the roof etc. Hot water seems fine, but when I turn on the central heating, the pump periodically seems to be straining and slows down considerably. At this time the vibration increases and sometimes other load noises can be heard in the pipes as well as a bubbling noise sounding from the pipes near the pump. This happens for a couple of minutes, then it works smoothly for a while before being noisy and strained again etc. Any ideas?
Thanks,
Alex

 

[Hamiltonandy]

Sounds like you have got 'air' in the system. Normally caused by lack of correct inhibitor. After putting in bleed radiators.