If your hot water tap hisses and bubbles then stops altogether, you can be confident that you have an airlock.
Removing an airlock is straight forward and all you need is a bit of old hose pipe and a couple of hose clips. Attach the hose between a tap that is supplied direct from the mains (usually a cold water tap) and the affected tap, then secure them with the hose clips.
Turn on the affected tap first and then the tap supplied direct from the mains and leave them both for around 30 seconds. When you close the taps, ensure that you close the main supplied tap first.
Now check to see if the airlock has cleared, if it has not then simply repeat the process until it is. It may take a few goes to clear it.
You may try connecting pillar taps by cross connecting with a piece of hose, with mixer taps you may be able to put your hand over the spout.
To remove airlocks from the system requires one end of the hose to be connected to a cold water tap fed with higher pressure than that supply with airlock. If you have check valves on any supply or think you may have – do not attempt this as it could jam the check valves. And won’t work anyway.
If you succeed in clearing the air lock, you must ensure you sanitise the pipe runs as cross connecting water supply pipe may have dislodged a lot of biofilm and crud from the pipe wall. A sanitiser can be added to the tank and purchased from plumbers merchants.
If you suffer from recurring airlocks, the likely cause is that the cold water storage tank is not filling up as quickly as it is being drained. This can be easily checked. In addition it could be poorly fitted pipe with high spots.
Watch the tank whilst someone else fills the bath. If the tank empties before the bath fills, then this will be drawing air into the system.
A likely cause is a stopcock not open enough or a faulty float valve on the tank.