Unusual Air Lock Solution

John.G

Neighbour asked me yesterday to have a look at a air lock problem he had with his upstairs cold water supply to his toilet cistern and the cold tap on his mixer tap, both upstairs. His CWST in the attic had run dry about 2 weeks ago, the hot supply was perfect as was the cold supply to his (pumped) Triton 90 SR shower.
The CWST has two outlets, one directly to his HW cylinder, the other to the toilet cistern and the mixing tap, the shower 1/2" supply is teed off this outlet just outside the CWST.
I disconnected both cold water supplies to the cistern and the mixer but this didn't clear the air so I re connected the cold mixer suppy and blocked the mixer outlet with my thumb and gradually moved the mixer from fully hot to sort of mid position until I felt the cold supply getting hot, eventually, after approx. 10 minutes everything returned to normal. What surprised me is that the hot supply which is fed from the CWST (and the HW cylinder) is at exactly the same head as the cold water supply was able to displace the air or whatever, I thought I might have to rig up the mains supply to clear the air lock.

Sleeper12

There might have been a high point in the cold supply that the hot doesn't have.

A handy trick is to join two or three shower hoses with 310s & use the pressure from the pumped electric shower to force water back up the cold tap. Usually works & is less trouble that using garden hose from kitchen mains to bathroom.

John.G

Yesterday, flow had slowed to a trickle from cold side of mixer valve with no make up to toilet cistern, CWST has been full since last incident. Did same knack as previously, water flow back to normal after a few minutes. I also connected in the mains via the shower 1/2" T off isolation valve to backflush the 3/4" outlet, all clear, CWST started overflowing rapidly, when I get time I will drain down CWST and fit a 3/4" gate valve on the outlet, I can then apply the mains pressure to supply full pressure to the mixer (cold side) and see if it clears any debris/air from system.
Will also look at implementing Sleeper12's solution as back flush is best.

Wearb

Man who lives nearby had a filling loop fitted under sink, joining hot and cold. He just uses this to clear air locks.
I didn't think to ask him if it was just an airlock at the sink hot supply.