Y Plan Hot Water Problem

I have recently moved my hot water cylinder, 3 port valve, pump , cold water tank and heating expansion tank into my loft. Everything seems to work fine but when i turn on the hot water only the boiler fires for a few minutes then shuts off on the boiler stat, without any hot water going through the cylinder coil.
I do not understand why it does this because when when i turn on the heating and hot water, i get hot water going through the cylinder coil. The heating works fine both on its own and with the hot water turned on.
For some reason i seem to have poor circulation on the hot water side of my heating system, which causes the water not to circulate from the boiler and cuts it off. I just can't figure out what could be causing this.
I have checked the 3 port valve and pump and they seem to work fine. I have fitted air vents at my high points, so i dont think it is trapped air. I also back fed the system off the mains cold water to try and push out any air pockets and i still had the same problem.
Could it pssibly be that my pump is located in my loft (on the flow) and my boiler is downstairs in the kitchen? About 7 meters between them.
I will be grateful of any advise.
Ant.
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In an earlier contribution to this discussion,

I suspect that it is an electrical - rather than plumbing - problem. Check, double check, and threble check that: * all the wires from the 3-port valve go to the right place * the cyl stat is wired the right way round (i.e. NC and NO not reversed) * the connections to the programmer are correct - including the HW OFF wire
It should correspond more or less with the Y-plan schematic in http://content.honeywell.com/uk/homes/systems.htm - except that your pump may be connected to to the boiler's over-run stat rather than just being wired in parallel with the boiler (in which case, the boiler will need a permanent live in addition to the switched live).
HTH
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Can you check to make sure that the pump runs when the hot water only is selected? A wiring fault could cause the boiler to come on, but not the pump when only the hot water is required.
Christian.
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In an earlier contribution to this discussion,

What sort of wiring fault do you have in mind?
The Y-plan diagram on the Honeywell site shows the boiler and pump wired in parallel. With modern boilers, it would be more usual to connect the pump to the boiler's pump connection so that it can over-run. Either way, you should never get the pump on without the boiler.
I'm inclined to think - as per my earlier post - that the 3-port valve, cylinder stat, or HW connections on the programmer are wired incorrectly.
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I wasn't particularly thinking about it particularly hard!
I agree in this case that the pump not starting is probably not the problem, unless there is a complete pump failure and the layout of the CH circuit is such that reliable convective circulation occurs. Even this possibility seems pretty unlikely.
Christian.
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The pump does run when the hot water is turned on. I just dont understand why it doesnt circulate correctly when hot water only is on. I have checked my wiring and everything seems fine. When the boiler clicks of on its stat the pump continues to run.
Could it possibly be something in the pipe restricting the flow on that side of the system.
Any other suggestions
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As it works with both CH+HW, it probably is either a wiring issue, or a knackered 3 port valve. Basically, one of the following things is probably happening.
1. The 3 port valve is not responding to a HW call for heat, but will go to midposition if both HW+CH are called for.
2. The wiring is such that the 3 port valve is not being asked to go to the HW position.
As the boiler is firing and the pump running, there appears to be no problem in the boiler interlock wiring.
Christian.
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In an earlier contribution to this discussion,

I thought you said that you *do* get hot water when CH + HW are selected - but not when only HW is on. Is this correct? If so, there cannot be a blockage, other than inside the 3-port valve - because that's the only bit of plumbing which moves according to what is selected.
If you *don't* get hot water even then - there *could* be a restriction in the HW circuit. Is there a gate valve in the pipe between the 3-port valve and HW cylinder - or maybe in the return pipe from the cylinder? Such a valve is sometimes fitted to give a measure of balancing between CH and HW when they are both on. If you have one, make sure that it is at least partially open. If it were closed, it would explain your problem.
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