I must say I am no expert at all, my late Dad did a lot of DIY, I am doing what I can from what he taught and learning now he has gone.
Have, I think we always had issues with air in radiators, anyway a few weeks ago the CH pump packed up after making a lot of noise over a long time but basically the system would only run for about 3 minutes then turn off and hence struggle to heat up. I have swapped the pump and now both CH and hot water are fine, but there is always air in the upstairs radiators, it gathers in I think the first one after the pump.
When the pump runs you can hear bubbles in the pipe near it and around the radiators. When the pump stops there is occasional bubbling (I think air settling) in rads and to the fill pipe in the F/E tank.
The pump when on max and set to CH and hot water makes more 'air' type noise than on its lowest setting. And there is less air in the rads when on its lowest setting.
I checked the F/E tank and could feel air being sucked down the vent. Also, air bubbles up the fill pipe occasionally.
System is: Gravity system, not pressurised. Baxi solo boiler Grundfos Alpha 2 pump Motorised valve 8 radiators - 3 big ones down stairs, the rest singles upstairs - with TRV's
Water comes from boiler in kitchen to airing cupboard where the pump pumps it downwards immediately into a motorised valve to hot water tank or rads or both. Vent pipe and fill are located in the airing cupboard.
System can be easily drained due to custom drain point fitted and is annually, water is clear and has inhibitor applied.
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