cylinder blockage???

[plumb_know]

just looking to pick all your brains again on a job i recently attended for the insurance company i work for. Job background: customer wanted to replace downstairs basin taps and did this him self to save money (which has not happened), he isolated water via turning off mains internal stopcock and running taps to drain CWSC. he then refilled cistern but now has no water from hot taps. on arriving i suspected air lock so tried clearing using aqua vac and then tried pumping mains supply through hot water pipes, i managed to get water out of every hot tap doing this but when i switched mains off, water worked for about 20 seconds then slowed to a trickle and stopped.
The customer said he did not touch any other valves (but he did only admit at replacing basin taps himself after me being there for a hour, after suggesting original plumber should be called back) and i did check gate vales were open. there was one gate valve that just spinned but customer said he had not touched this.
The CWSC in loft was clean and did not have any loose lagging on it, plus outlets on CWSC are small holed ones with small lip inside so don't think anything has gone down supply to hot water cylinder. I suspect there is a blockage on the hot pipe work somewhere and was suspecting could it be in the cylinder from scale or scale has got in hot water feed pipe?
The cylinder is 19 years old and i did shake it to see if i could hear scale but i couldn't, i am just wondering if anyone on here agrees with my assessment or has any other ideas what could be happening??

sorry for the essay but i just wanted to be as detailed as i could.

cheers everyone

 

[lame plumber]

big air lock or sheared gate valve, check what the gate valve feeds then go for blockage

 

[vern]

there was one gate valve that just spinned but customer said he had not touched this.

cheers everyone

Lmao......the problem is above, I wish I had a quid for the line "I never touched it"

 

[plumb_know]

i don't think it can be a big air lock as had mains water from every tap which would of pushed all air out of pipes. i did think that about the gate valve, i just wanted to see if everyone agreed with my suggestion that it must be a blockage somewhere in the hot water system (either said gate valve or scale in or from cylinder) and i haven't overlooked any other possibilities?

 

[lame plumber]

as stated, twirly gate valve first