Hot water flow slows to nothing - help

I hope someone can help with this since it is something I haven't come across before but suspect it could be a common problem.
A hot water only system is using one of those cylinders that have the cold feed tank in-built into the top of it. The whole device seems to be split about one third cold feed to two thirds hot cylinder (with immersion). I am sure you will all know the name for these things so hope I can be enlightened!
The problem is that the water runs fine to fill a bath and the pressure seems ok but after a couple of inches of water is run, the flow just slows to a dribble. Is this normal with this type of setup? I can see that as the water is run from the header tank then the pressure will be less, but not nothing at all, surely.
Could there be some sort of air lock in it, since I drained the header to do some pipe work. Of course, the finger is now firmly being pointed at me that my replacing a section of 22mm with 22mm is what has caused the problem. I suspect it could be the design of the thing and that when all the hot water is run off then the header is somehow giving a *selective* fill to the cylinder below. Am I correct?
Thank you all as always for all advice. Rob
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If you have replace pipe somewhere then it appears it is airlocked. Also check that the ballcock is not blocked and fresh water is filling the cold tank. The are called combination tanks, or some call them Fortics, as Fortic were one of the first to make them.
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[snip]

I cannot see that it can be airlocked since the flow rate is great for the first couple of inches.
But I agree that it is consistent with the cold tank not filling quickly enough to give a head to push the hot water out the cylinder below. I shall check to make sure it is filling back up quickly enough. Could be the ball valve. Although the combination tank has been fitted for a year or so, they have never run a full bath of water before, using a shower instead. So it could be that the cold 15mm supply pipe is blocked somewhere.
Thanks again. Rob
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It would appear that you are emptying the tank faster than the mains is filling it.
Take a look at the ball valve in the header tank. THere's a fair bet that if you have been doing some plumbing that a piece of crud has found its way to the jet of the valve. Either that or the flow is inadequate from the mains for some reason such as the stop valve being turned down.
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[snip]

most likely and will be easy to check.
There is no stop valve fitted so there is another job to be done.
Cheers Rob
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For water to flow out, you need air to flow in, so check that the expansion pipe hasn't been blocked and that the cold tank isn't choked with gunge in the pipework between the two tanks.
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