Can't drain off hot water cylinder if header tank empty

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I'm trying to install a replacement shower pump. To do so, because there are no isolation valves on the inlet pipes, I believe I have to a) empty the cold water tank in the loft, and b) empty the hot water cylinder. I've drained off the loft tank OK, and have tied up the ball valves so it doesn't refill. Now, however, I can't drain off the hot water - the hot water taps run for a very short while then stop. I was puzzled about this, so I untied the ball valves in the loft tank to allow water into the header tank, and sure enough the hot water started flowing again. Why is this? Air lock?

So, how do I achieve what I need to achieve, namely to shut off both hot and cold water feeds to the shower pump?
 
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unless you are going to do some work on the cylinder itself .you don't need to drain it.just empty the cold water loft tank.
 
I think I do need to, because there is no way of shutting off the hot water feed to the shower pump otherwise. If I were to detach the hot water inlet pipe from the shower pump, I'm pretty sure hot water would pour out of it.
 
hot water comes out of the top of the cylinder ,cold goes in at the bottom, from the loft tank. nowt going in means nowt comes out. so empty loft tank and isolate it. turn all hot taps on to drain down. this will leave a small amount of water in the pipes to the pump ,so be prepared to contain it. you will also have some in the cold pipework. if you have a bath that is fed cold from loft tank turn bath taps on to aid draining cold pipework.
 
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As I said, the hot water stops running as soon as the loft tank is empty. From the little bit of research I just did, this is apparently how it's supposed to be - it's a safety feature. I got this from http://www.diyplumbing.co.uk/drain-hot-water-cylinder/:

"This is because of how a gravity-fed system works. It is gravity acting on the water in the cold water storage cistern which effectively pushes hot water out of the top of the cylinder and off to the taps. Isolating the hot water cylinder from its supply prevents this from happening, but that’s all.

This arrangement is actually a safety precaution: it means that even if the supply to the cold water storage cistern is interrupted and the cistern runs dry, the cylinder will still be full of water and the immersion heater does not attempt to heat an empty cylinder, which could be dangerous."

That does suggest that in the absence of any cold water in the loft tank, the hot water supply to the shower pump might also only run for a short while before drying up, but when I loosened off the connection a short while ago, a lot of water did come out rather quickly!
 
told you it would !! its what is left in pipework from cylinder and tank to pump.
 
My apologies, it seems that the hot water flow to the shower pump has in fact stopped (by virtue of half a bucket or so spewing out a short while ago, probably).
 
would be a good idea to fit full bore lever operated isolation valves to both hot and cold supply's to pump ,whilst its all drained out.that would make life a lot easier in future.especially if when you fit new pump if you hit a snag you can just isolate it and do what ever you need to do.
 
Fortunately there are isolation valves on the flexible connector pipes for the new pump.
 
Thanks for the tip. Fortunately they were already in the 'on' position, so nothing needed to be done (yet).
 
While your at it... add a full-bore lever valve to the cold feed to the cylinder, whilst the cwsc is empty... BUT... make sure you cut into the cold feed ABOVE the top of the cylinder... or you'll get wet!
 
they will because his pump is shyte , but people don't listen

No, I do listen. I read 50 reviews of the pump on the Screwfix website, the great majority of which were very positive, and I read your 'review', where you kept repeating that the pump was 'shyte', without further explanation. So on balance I thought the pump was probably a good buy, especially as I also kept seeing favourable ratings for Salamander pumps (maybe not quite as good as Stuart Turner ones but a lot better than e.g., Wickes ones).
 

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