Baxi Boiler shutting down

I've got a problem with my Baxi WM51/3RS central heating boiler. Everything seems to work fine but after around 5-10 minutes the boiler stops burning and the whole system goes cold. Turning the controls off and on after a couple of hours will cause it to burn again for another 5 minutes, but it then shuts down again - otherwise the boiler won't burn again. Pilot light stays lit.

From what I can tell, the pump, valve, themostat and controls all seem to be working normally. When the boiler is actually firing, the radiators will start to get hot and the pipe into the hot water tank also gets hot, which suggests everything is ok.

My first guess was the boiler thermostat but replacing this hasn't made any difference.

Second guess was a blockage or air lock causing the heated water not to be leaving the boiler (thus triggering the stat), so I drained the whole system (boiler included) and refilled it. Same problem.

Not sure how old the boiler is but I'm fairly sure its over 10 years (as Baxi couldn't find it on their list and hence refused to send an engineer out!). I'm beginning to accept I may just need to buy a new boiler but don't want to do this and then discover the problem was elsewhere!

System is a straight forward setup, conventional boiler, single motorised valve, digital room thermostat and digital controls. Pump, valve, thermostat and controls are all less than 2 years old and everything was working fine up until this week.

Going to try bypass the room thermostat tonight and see if that makes any difference (but I'm no electrician so may not have much success).

I'd be grateful for any ideas.

Cheers, Steve

Reply to
steve
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Tell us why you think the pump is OK. It might have a detached impellor if it is not just plain stuck.

Reply to
Ed Sirett

It sounds as if you have an over-heat stat on the boiler which is tripping - and that this can be reset by turning the temperature knob right down and up again.

The most common reason for boilers overheating is insufficient water flow due to a faulty pump or air lock. Are you *sure* your pump is running? Can you remove the end cap and see the shaft going round? This would definitely be my first port of call! [You might be getting *some* flow without the pump running due to natural convection - causing the rads to start to warm up - but not enough to stop the boiler from overheating.]

Reply to
Set Square

Set Square/Ed,

Thanks for your replies.

I've tried turning the temperature knob right down and up again but the boiler still doesn't fire up.

As for the pump, I was assuming the pump was working because when the boiler is running, all the radiators get hot (even the ones furthest away start to get warm). However, I've now removed the bleed cap and I can see the shaft moving round, though I guess it could still be a detached impellor. I could turn the water flow off around the pump and take it apart to actually check it's working correctly.

Not sure whether this is normal but when I first opened the bleed valve it seemed like air was being sucked into the system. After a few seconds though water started to drip out slowly.

Tonight I've ruled out the controls by bypassing the room thermostat and hard wiring the main switch.

Also, when I first tried the boiler tonight I set the boiler stat to it's lowest setting to try avoid it over-heating. It cut off after the usual 5 minutes or so. I then reset the controls and turned the stat up to max and the boiler still didn't fire up. Also, you can hear the stat click and it's not even as far as the first setting, which makes me think it's not actually overheating - or the stat still isn't working - maybe I was sold a faulty one?!

So if the pump is working what else could be causing the lack of flow? Could it be a seriously corroded heat exchanger. I'm assuming there's no air lock or blockage as I've done a full drain and the entire system is now refilled - is that a safe assumption?.

If the flow is ok, could it be a faulty over-heat stat? Unfortunately I don't have a manual for the boiler so can't find out how you'd go about replacing it. I've checked Baxi's web site and they don't have manuals for their older boilers. Would this even be a serviceable component?

Thanks again, Steve

Reply to
steve

What sort of control valve does your system have. Is it a 3-port mid-position valve to direct the water from the boiler either to the HW circuit or the CH circuit or both together? Is there also a thermostat on the hot water cylinder? What is that set at?

When the boiler fails to fire, is the pump still running? What do you actually have to do to get the boiler to fire again? [You mentioned turning the controls off and on. *Which* controls?] Does the boiler have a reset button which has to be pressed in to make it work again?

Reply to
Set Square

is it a powered flue model?

steve

Reply to
r.p.mcmurphy

Set Square,

Yes, it's a 3-port mid-position valve.

I'm not sure about the hot water cylinder thermostat. There's a wire that runs to a plastic housing which is fixed to the bottom of the cylinder but I've not taken it apart and can't find any setting switch. I'll try taking that apart tonight and see if there's some way I can bypass it.

Yes, when the boiler shuts down, the pump continues to run.

The boiler will only fire again after waiting for about an hour and then switching the controls off and back on. By 'controls' I mean the Timer switch that controls when the ch/hw comes on (this is seperate to the boiler - a Lifestyle LP522). By switching I mean setting both the hw and ch switches to 'off' and then back to 'on'. There's no reset button on the boiler that needs pressing, just a recycle of the controls.

From what I can tell the boiler won't fire unless the controls are recycled but I'd have to sit and watch it for a couple of hours to be

100% sure - looks like I'm in for thrilling evening!

Cheers, Steve

Reply to
steve

Ok, we've got to try to isolate this problem a bit! Can you get at the boiler wiring? The boiler should have a switched live connection - which comes from the progammer, stats, mid-position valve etc. to tell it when to fire. When it fails to fire at the right time, is this connection live? If it *is*, it indicates a problem within the boiler. If it *isn't*, it indicates a problem in the external controls - for which the actuator on the mid-position valve would be my prime suspect.

What happens if you cycle the controls *without* waiting for an hour?

Reply to
Set Square

Set Square,

I got to the main wiring terminal and that just connects another wire which runs to the gas valve. I checked at both points and there's definitely a live connection when the boiler should be firing.

This also confirms what I tried last night which was to bypass the room stat and timer.

I guess it doesn't rule out any internal wiring within the gas valve unit though? I could replace this but I've never suspected the gas valve as the boiler is capable of firing up.

Just cycling the controls whenever the boiler has shut down has no affect other than it switching the pump on and off respectively. Also, in all the time I've spent messing around I've never seen the boiler come on without a cycle of the controls.

I tried a few things to get the boiler to fire and your original suggestion of turning the boiler thermostat down and up seemed to have some effect. After I'd waited for everything to cool down, the usual cycle of the controls caused the boiler to fire up. I immediately turned the controls off and the boiler correctly shut down. I then turned the controls back on but the boiler didn't fire. When I turned the stat down and back up and cycled the controls the boiler fired again (albeit only for about 2 seconds). I've tried to repeat this though without any success, but I'll keep trying.

Does this imply a dodgy over-heat stat or are we back to the original idea of it being a circulation problem?

If it's the over-heat stat, where is this likely to be located and is it serviceable?

The pipe from the boiler is getting hot when the boiler runs which implies there is flow from the boiler but maybe it's not sufficient like you suggested. As I said in the original post though, when the boiler does run for its longest time, all the radiators start to get hot implying there isn't any blockage.

Once again I'd be really grateful if you could offer anymore good ideas.

Cheers, Steve.

Reply to
steve

Looks like we have some sort of boiler problem.

I'm not at all familiar with this model however it would be very helpful to know its general design. a) Very low tech, permanent pilot light no electronics, possibly a 230V-24V transformer for the gas valve. Over-heat sensor (if present) will simply be in series with the thermocouple. Boiler does not have a permanent live supply.

b) Electronics present: over heat done by a seperate sensor requiring a power down and up to reset. Might or might not have a permanent pilot.

Sounds like type 'a' can't give you the problems whilst type 'b' could. In which case the over heat sensor (or its wiring?) is too enthusiastic. Does the boiler have any warning lights?

Reply to
Ed Sirett

Bit of a problem, this one!

Like you, I can't find any reference to this model on Baxi's website in order to be able to download a manual.

My Baxi Solo boiler (which is now about 10 years old) has a diagnostic chart printed on the back of the part of the casing which has to be unclipped to get at the controls. Does yours have anything like that?

From the symptoms which you now describe, it is beginning to sound like a problem with the main gas valve - probably its solenoid. If the pilot light is on, and the gas valve is is being told to open but fails to do so, it suggests something in that area. Does your boiler have a PCB? If so, that could just be faulty - possibly with a dry joint somewhere. It seems possible that when everything is cold it works ok, but as soon as it warms up (and I'm talking here of the electrics warming up rather than the water system - but they go hand in hand) either something loses contact on the PCB - if there is one - or the solenoid no longer has sufficient urge to be able to keep the gas valve open. When everything cools down again, it's ok. This doesn't really explain the effect of turning the controls off and on again - but that could be resetting something.

Hopefully someone who remembers that boiler (Geoff?) will come forward with some ideas based on knowledge rather than conjecture!

Reply to
Set Square

steve used his keyboard to write :

As you close the valves near the pump with it running, you should hear the change in the noise. It would saving you actually removing the pump to check the impellor.

Reply to
Harry Bloomfield

Ed,

It's definitely in the 'a' category. Permanent pilot, no electronics, no controls other than the stat and the pilot ignitor. All the wiring and controls are seperate - junction box in the airing cupboard linking the valve, hot water cylinder stat etc... and a seperate timer/programmer control unit.

Coming into the boiler is just one live supply which is then directly and only linked to another wire going into the gas valve unit. Boiler themostat is a seperate box which seems to be wired directly into the gas valve unit (and obviously the sensor going into the heat exchanger).

There's nothing else of note in the boiler, no PCB or warning lights. Seems everything is contained within the gas valve unit.

Cheers, Steve

Reply to
steve

Set Square,

There is a diagnostic chart on the back of the casing but it's not particularly useful. There's no symptom that describes my problem and the closest (pilot on but boiler not firing) suggests it's either the boiler thermostat, closed governor (?), external controls or... doh, can't remember - in work at the moment - I'll post back tonight if there's anything else on there.

I could replace the gas valve unit and rule that out - found a place

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selling the unit for £30+VAT which is siginficantly cheaper than a new boiler! The unit is pretty much the main part of the boiler so there's a good chance it would solve the problem, assuming it's not a problem with the water flow.

Cheers, Steve

Reply to
steve

Hi,

Try finding the overheat thermostat and temporarily bypass it to see if it runs longer, - but switch off if the water in the boiler starts kettling! You'll need to find out if it's normally open or closed and bypass it accordingly.

If it still cuts out then the problem is elsewhere, possibly another sensor or loose connection.

cheers, Pete.

Reply to
Pete C

It sounds from your other post in response to Ed's that the boiler is pretty basic - with no fancy electronics. It should thus be easy to diagnose - because there are very few things to go wrong.

I'm pretty sure now that you *don't* have an over-heat stat - and that lack of flow won't make it trip - so the problem has to be within the boiler itself.

It only has a few components which - I believe - work as follows: [I am sure that you - or others - will correct me if there are any glaring errors in this description/analysis]

The gas valve control is actually 2 separate valves to control both the pilot light and the main burner. In order to light the pilot light, you have to hold in a button on the gas valve to over-ride the flame protection device, and press another button to generate a spark. Once the pilot lights, it heats up a thermocouple which causes the pilot valve to remain open - enabling the over-ride button to be released.

The boiler thermostat is an electrical switch which opens and closes depending on whether the boiler output water temperature is above or below the set temperature. The required temperature is set by turning a knob, and the actual temperature is measured by a bulb, on the end of a capillary tube, which is inserted into a housing on the heat exchanger. [This contains a liquid whose pressure varies with temperature, and the electrical switch is actually operated by a pressure capsule - and turning the knob biases the operating point].

The main gas valve is operated by a mains-voltage solenoid which is energised when fed by mains current- coming from the programmer/room stat etc. and wired via the boiler thermostat. It has an interlock which prevents it from firing unless the pilot is lit - as detected by the thermocouple. _____

If the above is correct, and if the boiler fails to fire when fed with a demand signal, one or more of the following must be happening:

  • The boiler thermostat could be faulty. [This could be eliminated by temporarily bypassing it]
  • The solenoid could be faulty [Seems the most likely to me - works for a bit and then its windings get warm and go high resistance or open circuit. Works again when it cools down]
  • The interlock is faulty [Seems unlikely, since the pilot stays on ok]

If it fires when the stat is bypassed, but not otherwise, replace the stat. If bypassing the stat doesn't do the trick, I think I'd take a flier and replace the gas valve.

Reply to
Set Square

In message , Set Square writes

Hmm, p pilot, no pcb

no experience, no idea.

The obvious possibilities have already been mentioned

Reply to
raden

I'm pretty sure it hasn't got one - older boilers didn't. [See my post of

8.14pm]
Reply to
Set Square

Hi,

Another thing, check there is enough water in it. Sometimes if there's not enough the level can drop slightly while the boiler runs and make it cut out.

cheers, Pete.

Reply to
Pete C

Have you put a probe/multi-meter or neon tester on the gas valve? This will split the problem between the gas valve or the thermostat. My gut feeling is that it will be the thermostat because you find that you need to turn it down and up to get a result.

The themrostat is like less than £20. If you replace the gas valve make sure you know what you are doing.

Reply to
Ed Sirett

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