Central Heating Problem

Hi,
I am after a bit of advice about troubleshooting a central heating problem. I am not an expert so some of my terminology may be incorrect.

problem is that when the system is on 'Hot Water Only', 3 of my downstairs radiators get very hot. The motorised valve head unit is clearly showing that it has shut off the radiator circuit. Adjusting the manual bypass valve does not seem to make any difference.
What could my problem be? I thought perhaps it could be that the valve itself is not operating even though the motorised head is, or that the pump maybe running in reverse?
Does anyone have any suggestions for how I might fix it?
Thanks in advance.
AJJ.
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Conventional central heating system with a water cylinder and three port valve in the airing cuboard.
Recently, the radiator in the bathroom has been getting warm when switched to "hot water only". It didn't do this before.
My thoughts are that the three port valve is sticking. Other than this maybe the heat exchanger in the cylinder has failed. Any other suggestions as to the cause?
How to diagnose whether 3 port or cylinder?
sponix
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s--p--o--n--i--x wrote:

The bathroom radiator is often connected to the pipework serving the hot water storage cylinder, so it would get hot when switched to 'hot water only'. You'd have to trace the pipes to confirm this.
The theory is that you may want heating in the bathroom when the rest of the heating system is off and the pipes are more convenient. This wouldn't explain why it wasn't getting hot previously.
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Aidan wrote:

All the ones on our estate are like that. Its quite easy to spot because there is a pipe feeding the bathroom rad Teed off the inlet to the 3-way valve
If its not connected like that then the 3-way valve is maybe not shutting the CH side off properly. Although that should allow other rads to get hot too. If the coil in the cylinder was leaking this would cause the CH header tank to overflow when the water was heated.
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wrote:

Even if the pump is not running?
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s--p--o--n--i--x wrote:

Yes, gravity circulation can cause the upstairs radiators to get hot. Typically, the rads will get warm with the pump off, but will get very hot as soon as the pump comes on. It depends on the vertical heights, the pipe sizes, number of fittings, etc..
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Are you sure it wasn't supposed to work like this before? It is common for the bathroom radiator to be uniquely plumbed to come on in the situation you describe. Perhaps little fingers have been playing with its TRV/handwheel, so its only just come on?
Do other radiators also get warm?
Christian.
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On Mon, 18 Jul 2005 17:44:34 +0100, "Christian McArdle"

I'm told that it never did..

No TRVs on the sysem, only normal valves.
It turns out that one radiator was changed by a plumber recently, so I suppose that he may have re-balanced the system which in turn could cause this effect. I'm inclined to leave it and see what happens, in the abscence of any other problems.
sponix
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AJJ wrote:

Could you let us know if this is a new problem, i.e. has it only recently started occuring or has it always been this way?
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It has been like this for quite a while. I replaced the pump & the valve about a year ago and am fairly sure it started happening at that point. The valve was certainly broken and the pump was so noisy I replaced it at the same time.
AJJ
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Classic symptom of the immersion reurn pipe not being connected to the heating return pipe "close" to the boiler. Oher causes may also be considered.
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I am not sure where the return pipes join but it is certainly not in the vicinity of the hot water tank. I suspect it is close to the boiler but cannot check it without having the room apart. If they join a considerable distance away from the boiler is it possible the problem was worsened by installing a new more powerful pump?
AJJ.
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In an earlier contribution to this discussion,

What you *mustn't* have is some radiators returning one side and some the other side of the hot water return - they must all be on the *same* side - otherwise you can can get reverse circulation through some of the rads.
If this problem started when you changed the 3-port mid-position valve, I would be inclined to suspect that there is some crud in the valve which is stopping it from closing off fully - allowing *some* CH flow in the HW-only position.
--
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There are certainly no radiators between hot water return and boiler.
I will take the valve out and check that it is closing properly. That'll have to wait until I have time to drain the system It is quite possible there is some crud in the valve as when I changed the pump it was full of crud. So much so that I was amazed it was working at all. The aperature size that was left for water to flow through was about 5 mm at best.
Thanks for the advice.
AJJ.
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