No CH but HW or HW+CH okay

My central heating is playing up. Help, its starting to get cold! Can anyone point me in the right direction? Its a pumped system with 3-way valve. CH and HW are operated individually by timer. CH has room thermostat and HW has cylinder stat.
Hot water alone is fine. Central heating together with hot water is fine.
Central heating alone provides no life from the boiler whatsoever. If I'm desperate for heat then I can turn on CH and HW on the programmer and then turn the cylinder stat right up (cylinder usually up to temp, so CH ceases along with HW), but its beginning to get to be a bit of a pain. The programmer and room stat seem to send the right signals to the 3-way valve (CH indicator on it lights up), but the boiler doesn't fire (no "request for heat" indicator on boiler). I'm certainly no expert in all things plumbing (novice is probably a more accurate description), but I'm thinking that its the 3-way valve causing the problem. Does the valve normally send a signal to the boiler, or is it part of the circuit sending the signal to the boiler? If this is the case, is it likely to be a wiring or mechanical defect? The valve seems to operate correctly in either HW only or CH+HW positions - ie there was no CH in the summer, HW was on. CH+HW operates fine, but cylinder gets up to temp much faster than the rest of the house!
Any other theories, or simple checks I can carry out in order to narrow down the area of the fault?
Thoughts and advice gratefully received! *shiver*
Thanks
Adam
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The most likely cause is a duff microswitch inside the actuator on the 3-port valve. When the valve moves to the heating-only position, a switch is supposed to make which switches on the boiler and pump. If it doesn't . . . no heating!
The simples solution is to replace the actuator complete. No need to touch the water side, but make a careful note of where each of the 4 wires to the actuator is connected before disconnecting them.
[If you're a glutton for punishment, you may be able to take the actuator to bits and replace some of the internal components without replacing the whole thing - but it is a bit fiddly!]
The actuator sits on top of the 3-port valve, and rotates its shaft into one of 3 positions. Most are held on by 2 long screws, and are easy to remove. You have to ensure that the flat on the D-shaped hole in the actuator is lined up with that on the shaft when replacing it.
HTH, Roger
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On Mon, 29 Sep 2003 23:01:34 +0100, "Roger Mills"

I would concur. However the live feed that this switches is either from a HW-off terminal on the time-switch (if it will switch HW and CH independently), or from the HW-satisfied terminal on the HW stat. Since many people never set their time-switches to HW-off/CH-on, it would be worth checking the HW stat.
If there is access to the 3-way valve wiring, then according to:
http://content.honeywell.com/uk/homes/systems.htm
the grey wire is the feed, and the orange one the output from the microswitch (Y-plan).
--
John
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wrote:

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worth
Yes, I agree. I had assumed that it used to work ok, but that may not be true - in which case it may not have been wired correctly.
Perhaps the OP would like to comment on whether it has *ever* worked in a way which allows the radiators to get/stay hot once the hot water demand has been satisfied.
Roger
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wrote:

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Thanks for the replies. Yes, it did work as it should, but over a period of 18 months got more and more temperamental. Sometimes it would work after turning the room stat down and then back up again, but has since last winter got worse and worse, so the microswitch slowly failing does seem to be a likely cause, if a microswitch can wear out slowly. Out of interest, how long should these things usually last for? The house is two years old! Fortunately I informed the builder of other faults (including the 3 way valve) well within the first year, however, strangely, nobody has turned up to fix ANY of them. Letter writing time I think.......
My only other concern with my current problem is that the casing of the 3-way valve is more often than not, hot to the touch, and the CH indicator stays lit even when the CH should be off (also some "mechanical" noise every so often, repeating). Should I worry about the house burning down, or am I being paranoid. What sort of energy am I wasting with the valve in this state. (wasting may be the wrong word, as it is helping to keep the house a tiny bit warmer).
Adam
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would
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Sorry for replying to my own reply, but thinking about getting the 3-way valve to "switch off", as it was apparently stuck in the CH position, even with CH and HW off, I've been to have a play. I got it to "switch off" (ie no lights) by switching the HW back on and turning the cylinder stat up (HW indicator lit). Turned stat back down and switched HW off on programmer (lights on valve went off). Does this make sense, or does it complicate things?
Adam (now slightly less worried about very hot bit of plastic in airing cupboard)
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Under some circumstances, the valve remains powered even when heating and hot water are off.
You can *sometimes* reset them when they get confused by removing *all* power to the heating system (i.e. switch the whole lot off at the fused spur - or whatever feeds the heating system) for a few seconds, and start again. If the switch is near to the valve, you will hear the valve going back to its spring-return position when you do this. [Simply switching heating and water off at the programmer or thermostat is *not* the same thing].
HTH, Roger
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