Circulation pump - bleeding air and now stuck?

[gravelld]

I have a circulation pump on my DHW return which I use in five or 10 minute spells just to circulate the water so that we can heat the entire HWC from solar PV via  the top-ish mounted immersion coil. Otherwise, only the top third or so gets heated before the thermostat kicks in.

 

The pump is installed on the return to the HWC. Vertically, it is in the loft, roughly in line with the bottom of the cold water supply tank which feeds the HWC. The HWC is in the bathroom below. After running through the pump, the water goes back downstairs into the bathroom and into the bottom of the HWC.

 

Before the water hits the pump, the feed from the HWC goes into the loft, across the loft, down into the first floor to service three bathrooms, then down into the ground floor to service a kitchen. Then it returns to the above mentioned pump.

 

I end up having to bleed air from it every few weeks. I know when I need to do it because it begins to make this squeaky sound:

 

 

I bleed the air from it for a few seconds (actually, sometimes I stop and start the pump to try to eek out as much air as possible), and it works again.

 

How do I avoid having to do this so often? It's a bit of a pain to have to go into the loft so often.

 

Now though there's another problem. The pump sounds like it's stuck. It just makes a mid-pitch buzzing noise and the pump doesn't turn on. I couldn't capture this in a video, for some reason.

 

It did this once before and I gave it a tap with a light hammer on the bleed valve. That's not working now. Is it clogged up, and if so is it repairable?