caravan?

Sounds very much like an airlock to me.
Sound like introducing air but it worked on my Carver.
Ensuer water is cold in heater in case. Turn on the tap then either mouth over the spout and blow hard or fix tube to the spout do the same it sometime clears out the water and air trap.water flow will blow out any air you put in. My Carver water heater inlet has a none return valve on it and is teed off the cold feed, if yours is also fed from single cold supply it may be its stuck.

Bob
September 2009