Baxi WM 38/3 RS

I have a problem with my central heating boiler, a Baxi WM 38/3 RS. After the burners have been lit for 20 minutes they turn off and the pilot light is out. I can immediately re-light the pilot but if I start the boiler again, the pilot goes out and the burners do not start. If I light the pilot and leave it for 10-15 minutes, the burners will fire up again but only for the 20 minute period. Looking at the fault guide it say that a cause of nuisance shut downs is a dirty or loose thermocouple connection; defective thermocouple or a defective power unit. I'm unwilling to do any diy on a gas appliance so have spoken to a plumber (BUT I have checked that the thermocouple is not loose). Over the 'phone he has suggested the boiler is near its end of life (its 12 years old) and that the fault is probably with the gas value which will cost 150+ and labour cost to replace. Is this reasonable in the opinion of the members of this group?
Thanks for any contributions.
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On Thu, 20 Nov 2003 19:14:20 +0000, Stephen Clark wrote:

I'm not familair with this model but I strongly suspect that there may be an overheating fault. Which might simply be that the over-heat thermostat has become faulty and open at too low a temp.
From you description it sounds like a boiler with a permanent pilot.
The gas valve is probably the _least_ likely part to fail, the valve is also likely to cost sub 100 including VAT (although Baxi are quite expensive for spares). HTH
--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
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I'd replace the thermocouple first and then see if it fixes it. A thermocouple is about 3 quid and is usually fitted very easily. I don't specifically about the Baxi. It is fail safe. If you don't fit it right, the boiler won't light.
I wouldn't replace the gas valve on it. It'd be like putting a new engine into an 'A' reg car.
Christian.
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A gas valve for this boiler cost me 70 three months ago.
Mark
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But the OP will need someone to fit it. This may cost a lot of money.
Christian.
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Hi Stephen,
I've got a Baxi WM381. I'd definitely go for the thermocouple first, universal thermocouple kit will do the job. Over the 20 odd years service this boilers given I've needed to replace the thermocouple three times, and the symptom you describe is typical. The gas valve solenoid failed once and had to buy the complete valve assembly, about 75 + vat three years ago, this gave a different fault to what you are describing, the pilot light was ok but the main burner didn't light. The WM381 has no overheat. The internal circuit is shown in the manual and consists only of boiler thermostat and gas valve.
Replace the thermocouple first, it's a straight forward job and you won't be disturbing any gas joints.
Good luck
Bill
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value he say that it is "constantly" clicking making him think the value is faulty. Anyway he did not do anything because its not really something he likes doing (he didn't charge me for this visit!) and recommends I call around plumbers in the Yellow Pages for someone who would have the spare and replace the value (today's plumber ws recommended by a friend). BUT first I will follow the suggestion of members on the group to replace the thermocouple. It looks easy - just:
1) unscrew the current one at both ends; 2) insert the new one; 3) tighted the bolts.
Is there a guide on what I might need to look out for? For instance is it just sufficient to ensure the boiler is "switched-off" (ie no electricity) or do I need to do more (no gas).
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Don't go to a plumber!!!!! Go to a heating engineer. No wonder he didn't have a clue. If the company say heating and plumbing avoid them. Go for heating only.
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On Sat, 22 Nov 2003 17:55:44 +0000, Stephen Clark wrote:

likely given these symptoms is a loose connection on the electrics to the gas vavle. Also the themrostat contacts can get 'jumpy' with age and that could easily cause the clicking. These contacts are essentially a microswitch with all the usual problem that such switches sometimes have.
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corroded. I visited a DIY store and got a Universal Thermocouple. It does not exactly match the existing one for length etc so I'm no sure which adaptors to use to fix it correctly. Since it's a fail safe component - I imagine that I can't do any harm by trying various combinations.
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On Sun, 23 Nov 2003 13:55:22 +0000, Stephen Clark wrote:

The hot end it is only necessary to make sure it is held securely in place, with about the last 10mm in the flame. The connection to the gas valve is usually quite fiddly but is likely to be only one of a couple of the nuts.
--
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the value WAS fiddly, it took 20 minutes. So far the burners are quite stable. Hopefully all is OK. Thanks to this group for giving the the courage to replace the thermocouple - you've saved me a packet. Perhaps if you ever have a meeting in Leeds I'll buy you all a drink? (o:
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Stephen Clark Wrote:

I have a similar problem, but one that follows a strange series o event before xmas:
1) begining of december, Heating goes down. I investigate and discove an apparently faulty motor in my Y valve. While looking at the valve, notice that the controlling circuit looks like it has burnt at som point in its life... I replace the motor, test the circuit, which stil seems to work, but I can't fire the boiler with the room thermostat. look at the thermostat to find a short in the live wire. The thermosta is too old to see if that short has caused the circuit to arc, bu replacing the thermostat fixes the fault.
2) Everything work fine for 2 weeks, until I go away for xmas. I com back to find the circuit breaker for the CH circuit down! I stupidl just try to switch it on again (it was after all midnight after a hour journey!) and it trips again, with a big falsh (that woke me up!) In the morning, I open the Y-valve control box again, look at th controlling circuit, clean it up a bit, check the connections, pu everything back on, and switch the circuit back on. Surprise, tha seems to work...
3) A day later, boiler has been running continuously and all I have ar lukewarm radiators and water :-( The pipes coming from my boiler are no very warm either. I borrow an electric radiator and put my roo thermostat to the minimum, so that whatever little heat I get goes t the water circuit.
4) Boilers goes down overnight... I start it again.
5) Room is getting cold, I put the thermost to max again: boiler goe down! I start it again, but it will only work in the water position.. I leave the thermostat to the minimum position.
6) Boilers goes down overnight... I switch the pilot light back on, bu any attempt to draw heat for the water switches everything off!
So here the question: am I being very unlucky, or is there possibility for one problem to cause the other ones? I will have a go at changing the thermocouple, but I can't see how thi is linked to the other problems. I have talked to a CH engineer t replace the Y valve with a new one (with removable head), but I don' see how that would fix the boiler problem...
Any help appreciated.
Cheers
PS: the boiler is a Baxi WM 38/3 RS, the Y-valve a landis & staef MAV-32
-- Franck
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