Hot water runs out for hours

[vicars]

Hi All, I have an open-vented Hot Water Cylinder (common sort with boiler and copper storage tank) and sometimes I can run a bath and then the water slows down to a trickle and stops for ages. I have been told by a relative I should replace the cylinder or boiler but is there another cause? I have drained the radiators twice in the past and I am thinking maybe it has to do with air pockets but I have bled ebery radiator several times and I am sure this is not it althouth the pipes bang when the boiler comes on in the morning. Any suggestions? Many thanks in advance and sorry if this a question asked 200 times before.

[buildersmate]

This sounds to me as if there is a partial restriction in the cold water feed from the tank in the loft space and the copper cylinder. Alternatively the cold water tank is not large and is being emptied by the cold and hot water feeds into the bath at a faster rate than the replenishment system (via the ball valve) can replace it. This probably means that the rate of refill from the ball-valve is too slow.

Tell us please, is it BOTH taps that stop flowing, or just the hot?

[vicars]

Hi, thx for reply - it is just the hot tap that stops - cold taps run fine. In the loft there is a large cold water storage tank and a small tank which I think is for hot water overflow but I could be mistaken. I have shut both off myself at some time and restarted and also replaced ball valve on the cold water tank but I believe it is working OK.

[gucciman]

not quite sure why your bleeding the radiators when they're nothing to do with the hot water taps but have to agree with builders mate entirely.
basically, the water is leaving the copper storeage cylinder quicker than the water header tank can refill it and you are airlocking the system.
same thing happens in the old system ive inherited and to get the water flowing again i run a hosepipe from the cold tap to the hot tap, open the hot tap up and then blast cold water up the hot pipework to backfill the system and clear the airlock.

[scorpious321]

i agree with gucciman but that is only a temporarily quick fix it will happen a gain and again ,you need to find out what size the cold feed it might be 22 copper feed and might need 28 mm cold feed.
Or the restriction it could be scaled up inside and blocking the cold feed inlet to the cylinder,so when you draw of hot water to the eg to the bath the cold feed is restricted and you are emptying the tank quicker than you are filling it ,which will pull in air and your bath will trickle out and cough and splatter

[markja]

on my hot tank;
the lagging that was covering the cold tank in the loft had fell in; and after being pulled out i didn't notice a portion had stayed in the tank. ( it was fluffy lagging ) this unnoticed fluff had been drawn into the outlet from the tank that feeds the hot cylinder and then wedged a little way down the pipe. this stopped the cylinder from filling at a decent rate and gave the symptoms you describe.
if it is a straight pipe from the loft to the cylinder gently feed a wire or simillar down the pipe to see if there is an obstruction. do this ever so gently so you don't knock a blockage down into the cylinder.
if there is a restriction take the pipe off the cylinder and then push it through and out so it's not just moved lower down the system. you may have to push it up rather than down.
and before doing anything see if there is a tap that isolates the feed to the cylinder, if it's in and airing cupboard it may have been knocked/turned and you didn't notice. ( granny, eggs, sucking. spring to mind! )

[vicars]

Thanks everyone - I will investigate why the feed is too slow. It is strange though as it takes AGES for the hot water to come back again, as though there is a real problem for the water to get through. Maybe it is badly furred or something or there is a blockage - I will try to see what it is anyway. Thx again.