Hot Water Confusion

purpleistturtle

My hot water tap in my bath only has always been a bit lacklustre in the pressure department. We have a 1980s gloworm boiler and a hot water tank in the bathroom, which is upstairs. Sometimes the water slows to a trickle, sometimes it's more of a babbling brook but never better than that. It's perfectly decent pressure in the kitchen and the bathroom sink. Until about 8 weeks ago that is. I turned the hot tap on in the kitchen and promptly forgot about it. I don't know why it how. Literally hours later, at least 2 or 3, it suddenly started spewing water out at a rate of knots. I had been in the kitchen a few times in between and it was not running at all. I can only theorise that the tank had emptied so it stopped running for a while. The water after it started running again was, obviously, cold. It was also clear. The same night I came up to run a bath and instead of the trickle, the hot tap spluttered a lot, then let out yellowish water for 20 seconds, then started running faster than I've ever seen a tap run. It went from taking 45 minutes to draw a decent bath to under 5 minutes. And it stayed like that until today. Now suddenly, without any warning or slowing, it is back to a dribble. 

Has anyone got any idea what has happened and how to fix it without leaving my tap running for hours?

Thanks 

nofoollikeold

1. Probably an airlock, which cleared itself when the tank ran dry.  They can be a bit of a nightmare to find.  Usually caused by pipes not having a gentle fall from tank to outlet, perhaps diverted up over an obstacle then down again.
2, Ways to clear an airlock:
2a. Force water from an outside hosepipe up through an open hot or cold tap, depending which is airlocked.  Don't do it for more than a few seconds or it can cause the cold water storage cistern  in the loft to overflow.  Can be messy as the mains pressure water tends to spray everywhere.
2b. If you have a mixer tap in the kitchen with mains pressure cold, hold you hand over the spout, and open both hot and cold.  Should force water up the hot side, so essentially the same as 2a.  (And the same warning).
2c. A wet and dry vacuum cleaner in wet mode.  Seal the nozzle to the offending tap (rag or your hand), vacuum on, tap on.