c/h failed; cold weather coming; help!

I hope someone can help me out here!
My heating (baxi back boiler) was working this morning, but isn't now.
But the hot water's ok so boiler, thermostat, pilot, pump etc are working?
The room stat, (Honeywell T40) has mains across live and earth, but not live and return.
The terminals 1 & 3 are open circuit when thermo open, shorted when closed, so the electromechanical bit looks ok.
However on the rudimentary circuit diagram on the thermostat case there appears to be a 3A? fuse, and I can't see this. Can I get the back of the thermostat off, it looks a bit like having to break in?
When I turn the heating on at the controller, there's a healthy relay clonk, similar to the functioning hot water circuit, but no action.
Any diagnoses, please? I'm thinking about the fuse in the room stat mainly, (please let it be!) or replacing the room stat, (no trouble either), or is there some other likely cause?
mike r
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mike ring wrote in message ...

Mike, you conclude the pump is working because you have hot water. Are you sure your HW is pumped? Many back boiler systems use gravity feed HW and pumped heating. The room thermostat then switches the pump on and off. Set your room stat to max and see if the pump runs. If not see if it has power to it. There is possibly a capacitor alongside the pump. These can fail, pumps do too but less often in my experience. You can buy motor run capacitors from CPC or RS - get a similar value typically 4 to 8 microfarads or marked uF
It is 22 years since I had a Baxi but I think the fuse was located behind a wooden flap below the gas fire.
Your system might have a motorised valve that gives priority to hot water - this could have stuck or the motor failed. A proper plumbers merchant or heating suppliers will have replacements - not likely to find these at the sheds.
HTH
Bob
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In an earlier contribution to this discussion,

Are you sure that it's not a gravity hot water and pumped central heating system? If so, the likely culprit it the pump - which only runs when the programmer and room stat tell it to. The boiler will still run and heat the hot water by gravity (if it *is* a gravity system) even with a failed pump.
If, on the other hand, it is a fully pumped system, there must be a 3-port diverter valve (Y-plan) or two 2-port zone valves somewhere (S-plan). In which case, the pump is probably ok (since hot water works) and the diverter valve or CH zone valve (as appropriate) become the next suspects.
--
Cheers,
Set Square
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On Sun, 25 Jan 2004 21:21:19 +0000, Set Square wrote:

The 3A fuse in the circuit diag is not a fuse but a symbol for the heating apparatus (max 3A). The designation is often 3(1)A meaning the contact in the themrostat can switch a 3A resistive load (e.g. lighting) or a 1A inductive load (like pumps, boilers and motorized valves).
If you have a motorized valve failure you shoudl be able to move it into the manual position to get _some_ heating until you can fix it later.
Generally the only work around for a pump failure is a new one.
--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
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Thanks Ed, that is what it says on the box

That seems to be helping, the rads are slowly warming up, so I've cranked up the pump a bit.

If you've got me past this upcoming wintry bit (if we can believe forecasters, I'll light a candle or something for you
mike r

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Thanks for the answers to date.
It is fully pumped with a motorised valve, and the hot water is fully working.
There is a diverter valve - if this has failed it's almost the worst scenario as the system would have to be drained to replace it, I wonder if there's a way to check it first?
Does it interrupt the electrics to the pump - I'm a bit confused about dependencies, would the valve failing stop the pump?
The stat appears to have no return ciruit, but it's hardwired int a big juntion box, so I'll have to try to work it out - or change bits till it's fixed, but I didn't want to drain it in a bitter cold spell (sigh)
mike r
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In an earlier contribution to this discussion,

The motorised valve is in 2 parts - the wet bit (tee piece with paddle inside) which directs the water to the HW, CH (or both together) circuits - and the electrical actuator, which moves the paddle of the first bit.
It's most likely that it's the actuator which has failed. This can be changed *without* draining down - it simply unbolts from the top of the valve. The actuator is quite complicated - with a couple of micro-switches inside which control the boiler and pump when it reaches certain positions.
If you remove the actuator, you can check whether the spindle of the valve is free. This should turn - either with finger and thumb or with pliers without using excessive force - through 90 degrees from the HW to the CH position - with the BOTH position being at 45 degrees. As Ed has said, you can probably get the radiators hot by manually moving it to the mid or CH position.
When you come to replace the actuator, make a careful note of the wiring before disconnecting it - it's *very* important that all 6 wires on the new one are connected up in exactly the same way.
There's just one other thing to try before ripping it apart. Just occasionally these actuators get confused and can be reset by removing all power to the heating system for a few seconds, and allowing the return spring to drive them to the home position. [Turning HW and CH off at the programmer won't do, because the actuator will still be powered - it *has* to be at the isolator switch for the heating]. If you do this, it just *might* work when you switch back on - but I'm not making any promises!
[It might help your understanding of the system if you study the Y-plan schematic and wiring diagram at http://content.honeywell.com/uk/homes/systems.htm You'll see that the room stat is just a switch in the wire which goes from the CH output of the programmer to the 3-port valve actuator.]
Good luck!
--
Cheers,
Set Square
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On Sun, 25 Jan 2004 23:35:03 +0000, Set Square wrote:

A quick test to see if the valve is a removeable version is to see whether it is held by 4 screws to the wet part. If it is then you can't change the valve with out draining down (or at least bunging the header tank). If the head is held by just 2 screws to the wet part you should be OK to remove it.
However you will probably find that it's the motor which has failed these are held by one small screw. How easy they are to change will depend on how accessible the valve is. The motors are 8 from s/fix part #19276
Motors can be changed even when the valve is the older 4 screw type.
HTH
--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
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Yes, the valve appears to be functional manually, but it's all fitted facing away from me in a corner, I'll have a go at it tomorrow.

No luck, but thanks for the tip, I think it must be b*****d, there appears to be no path to neutral through it

Thanks for the reference, that's a nice clear diagram

Ta
mike r
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I've not seen a thermostat with a fuse in it - what you're seeing may be a resistor which accelerates the action. If this is in use, there will be a neutral as well as live and return. But watch out for the earth wire being used for this - I've seen this several times by plumbers too mean to use 3 core and earth.
If you've got volts coming into the thermostat, then there should be volts going out when it's made.
--
*Beauty is in the eye of the beer holder *

Dave Plowman snipped-for-privacy@argonet.co.uk London SW 12
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shorting. but wrt Earth the 240v appears on the return; looks like it's got nowhere to go
Cheers
mike r
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You'd expect to see as near as dammit the same volts both in and out of any switch - if it didn't, it would get hot and burn out.
--
*Gargling is a good way to see if your throat leaks.

Dave Plowman snipped-for-privacy@argonet.co.uk London SW 12
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the confusing representation on the box that I'd took to be a fuse which I couldn't find. I hadn't thoroughly examined the symptoms, and had to be redirected to what was actually occurring by answers to my post.
Even Homer nods, and if I had a brain I'd be dangerous
mike r
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