Plumbing question - emptied tank, closed gate valve, now no pressure

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We have a water tank in our loft that we turned off the mains supply to it, emptied it out, then when it got basically empty and just trickling through, we turned off the gate valve too. Did the work, opened gate valve, filled tank, all good, until we realised the pressure is none existent now to the cold taps, with the pressure only coming from the water built up in the pipes, and after a few seconds, this goes and you end with a trickle, but say flush the toilet and then turn on the tap, there's nothing coming to the tap.

First thought was gate valve seized up, however it freely moved the entire time, no stiffness, and you can see it raising up and down as you screw it. You also can still stop the water flow when losing the valve, and then open it again, though that's not to say it gets stuck when just a tiny bit open, I assume I'd feel resistance?

An idea is that, when the tank was emptied, could bits of debris from the bottom of the tank, now get sucked into the pipe and cause a blockage, letting only a tiny bit flow through?

If so, my issue is the tank is about 30cm max from the ceiling, so next to no access from above, and the height of it is greater than my arms reach.

If it is this, any ideas how I could unblock it? Thought maybe to push water up the pipe backwards, though no idea how'd I'd do this. There is a mains feed running right near the cold water feed to the taps.

If not this, any ideas what may have caused such a drop in pressure?

Thanks for the help
 
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Does the tank have a byelaw 30 kit fitted?

Got a picture of the gate valve? It's most likely snapped internally. You would need to remove the valve, or the top of the valve to check this. Once that's done, and you still have poor flow, you then need to explore the possibility of an air lock in the system pipework then look at options of removing it.

You can buy a bung kit to help with work on header tanks, Regin sell them.
 
Soldato
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Associate
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A bylaw 30 kit (lid, vent, jacket etc) would mean there isn't going to be a blockage due to debris entering the header tank.

Tanks that supply water pressure through 'gravity' are very easy to airlock once they have been drained/refilled. Just a length of pipe with inadequate 'fall' will cause this. But I would only look into this once that gate valve has been inspected.

Bung kit: https://www.wolseley.co.uk/product/regin-regr05-radiator-valve-change-kit/

Very handy to plug the vent and cold feed to work on plumbing, especially if you have a snapped gate valve that will not allow a header tank to drain ;)
 
Soldato
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I had a similar issue, it was an air lock. Put a wet and dry vacuum on a tap, or force a garden hose up the tap.
 
Soldato
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Bung kit: https://www.wolseley.co.uk/product/regin-regr05-radiator-valve-change-kit/

Very handy to plug the vent and cold feed to work on plumbing, especially if you have a snapped gate valve that will not allow a header tank to drain
Gotcha.

The pipe is actually 28mm, but also, how would I bung it when I can't get access to the entry way of the pipe from inside of the tank?


Tanks that supply water pressure through 'gravity' are very easy to airlock once they have been drained/refilled
In that case, attaching the mains from the tap end and pushing water back, would solve it?


What did “did the work” entail?
Just removing a t off for the cold that wasn't needed. It's not a simpler single pipe that goes like this.

_____ tank (the straight line is a lil decline)
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/ \
Wc - tap
 
Soldato
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Does this tank supply anything other than the toilet and bathroom basin?

Wet Vac and reverse fill are both good ideas if it’s an airlock, with such basic pipe runs I’m thinking this is less likely and the gate valve has failed. Gonna take a while to drain the tank now though.
 
Soldato
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take a while to drain the tank now though.
It already took ages due to its size, so yeah OK not excited to be emptying it again with next to zero coming through.

Gate valve shows it's going up and down, but I guess this could just be the spinning piece and what's inside is loose? What is wierd with that, is that I can still close the gate valve, and then get it to open enough to allow water to pass through, it's just not much
 
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The pipe is actually 28mm, but also, how would I bung it when I can't get access to the entry way of the pipe from inside of the tank?

If you've got 1 ft above the tank, then just reach in, up to your armpit and push the bung into the outlet at the bottom of the tank, those bungs will fit inside a 28mm pipe. But I doubt very much that the outlet flange is any bigger than 3/4.


Gate valve shows it's going up and down, but I guess this could just be the spinning piece and what's inside is loose? What is wierd with that, is that I can still close the gate valve, and then get it to open enough to allow water to pass through, it's just not much

Old valves (depends what you've got, some pictures would help) used to have what's called a 'jumper' inside. This isn't always permanently attached to the handle mechanism, hence they can shut under screwed force, but not always open back up correctly.

Back feeding the hot, or tank fed cold is a good way of clearing air locks. Just be aware that doing so can add pressure to a system that doesn't normally handle mains pressure. You also need to make sure that the vent pipe is installed and correct before adding mains.

If you add mains pressure to back feed against a stuck valve, that is also a combined vent, then you could pop the cylinder.....Then it gets exciting :D playing with over 100l water!

I've not heard anyone before using a wet vac to 'suck' an airlock out. But I suppose it's possible, but as above, the opposite could happen you don't want to collapse the cylinder.

Apparently you can collapse a copper cylinder just by draining it, if you don't have an open vent for the air to enter. The force of the draining water under gravity is enough to buckle the cylinder inwards..apparently.
 
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