airlock in hot water pipe run

[ewan]

hi guys
went to a job where no water coming from any of the hot . sorted it by forcing cold water into hot water pipes via the kitchen mixer tap. I had same problem in same flat about a year ago. so its not a major problem but i would like to know whats causing it. tenant is adamant that hot tank had not been drained down for any reason. its a cylinder with a header tank and only feeds a kitchen and bathroom. the hot flow at the kitchen is very poor but very good in bathroom. No obvious leaks anywhere. Could it just be a crap kitchen mixer?
Land lord probably wont want to do anything else as I have "fixed it"but I like to know what causes problems like this
cheers ewan

 

[SimonJohns]

Usually mains interruption

Also I've had one where excessive use and undersized pipe word caused it. But I doubt very much if this would happen in a domestic property

 

[zeebeedee]

Is the header tank part of the cylinder? If so is the cylinder filling up quick enough, ie is the bath filling faster than the header tank and sucking in air that way? Also seen these poorly designed where the draw off is to close to the top causing the same problem.

 

[stevetheplumber]

as said its the cylinder emptying the header tank usually its the ballvalve thats got slower due to debris in the orifice change the ballvalve