Direct cylinder replacement help wanted

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I have just helped my son replace his hot water tank, which is direct/vented, and has two immersions on economy 7. Having completed the job successfully, we can't get any water out of the hot taps. He tells me that they already had a problem that the taps ran dry when the hot was used up, but now they are dry even when the tank is hot.
So my questions are, should we have done something to prime or bleed the system, is it that there is insufficient head of water from the loft tank, and what should we do to resolve the problem?
Any help gratefully received.
 
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Airlock . if you have a mixer tap turn hot on cup hand over outlet tightly but with a gap then turn cold on this will force the cold up the hot pipe pushing air out the way . It may take a couple of attempts.

IF no mixer connect hot an cold taps with a hose and do it that way or even one hose between washing machine taps
 

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Check that you did not turn off any valves and forgot to put them back on ;)

the pre-existing problem also suggests that the cold-water tank which supplies the cylinder is too small, or has a clogged ball-valve or some other restriction. A bath takes 25 gallons or so, and you need a tank that can supply a bath without runnning out (as a bath tap drains it faster than the ball-valve can fill it).

If the cold tank is in the loft, have a look at the ball-valve, put a container under it and see how many litres per minute it delivers. it is very easy and cheap to swap in a new one (which will probably also be quieter).

Taking up a new, bigger plastic tank is a bit more of a job.

Edited: It helps if the cold tank is higher, e.g. on a platform in the loft, but even this will not prevent airlocks depending on pipe runs. My tank is about 4 metres above the bath, but it still takes an hour or so to clear the air lock if I have drained it.
 
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Thanks Namsag, we will give that a try although we have tried putting water into the venting pipe in the loft and that does deliver water to the hot taps, so if there is an air lock it would have to be between the tank and the tee piece in the pipe running between the loft and the taps.
 
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if you get an airlock though, and remove it, kit may be persistent. that why pipe runs to bathrooms and to the hot water cylinder should beas straight as possible, and hot water distributing pipe from the tee (to the vent/taps from the cylinder should fall at 90 degrees don and then run straight, not at slanted angles.

i hope you get it sorted out!
 
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The hand over the mixer tap worked a treat, and the float valve in the loft was jammed. Two problems solved. Thanks very much guys.
 
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