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Andrew Smith
Andrew Smith, HVAC Technician
Category: HVAC
Satisfied Customers: 3612
Experience:  Heating and Controls engineer
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Pilot light wont stay on, Baxi WM 40/4 RS

Resolved Question:

Pilot light won't stay on, Baxi WM 40/4 RS, we can't get it to fire up either. The theromocupple has been replaced recently and has been working fine up to wddnesday evening, we've not been able to fet it started since then. 

Submitted: 2 years ago.
Category: HVAC
Expert:  Andrew Smith replied 2 years ago.

XXXXX XXXXX :

The boiler will not work unless the the thermocouple can hold the pilot light on after you let go

XXXXX XXXXX :

Can you ensure the pilot light is big enough to envelope the thermocouple tip

XXXXX XXXXX :

You can test the output of the thermocouple with a multimeter - about 30mV

XXXXX XXXXX :

If this is all correct and the gas valve will still not hold the pilot on then it it will be the gas valve at fault

Customer:

I'm not sure we have a multimetre, but the thermocouple is new, it was replaced about 6 weeks ago.

XXXXX XXXXX :

You need to ensure the pilot light is big enough and covers the end of the thermocouple - can you see it?

Customer:

Yes, the pilot light looks normal, the is no problem getting it lit, but it doesn't stay lit.

XXXXX XXXXX :

Does the end of the thermocouple sit in the flame?

Customer:

Yes, I can see the thermocouple and theres a white coloured pipe going in next to it too i can see.

XXXXX XXXXX :

Is the tip of the thermocouple glowing red?

Customer:

No.

XXXXX XXXXX :

Is the end of the thermocouple in the flame?

Customer:

Yes.

XXXXX XXXXX :

But no red hot?

Customer:

Not red hot.

XXXXX XXXXX :

If it is sat in the flame then it should be producing a voltage.

XXXXX XXXXX :

Where it connects to the gas valve, does it screw in the front or back?

Customer:

Both thermocouple screws are at the front.

Customer:

Got to do the school back in 10 minutes.

XXXXX XXXXX :

No problem

XXXXX XXXXX :

If the thermocouple screws in the front of the gas valve then it may screw into a "shoe" that has a couple of wires from/to it?

XXXXX XXXXX :

There could be an overheat thermostat in the circuit that may be faulty

XXXXX XXXXX :

Did the engineer that replaced the thermocouple say anything?

Customer:

Back.

XXXXX XXXXX :

Ok

XXXXX XXXXX :

just have a look at what I have put and see if it makes sense?

XXXXX XXXXX :

Can you attach a photo of where the thermocouple attaches to the gas valve?

Customer:

boiler front
Full Size Image


 

Customer:

It just connects with a bolt top and bottom, no wires.

XXXXX XXXXX :

I have the GC no. now, so I will just dig out a diagram

Customer:

Ok. I thought you bad given up hope!

XXXXX XXXXX :

Where it connects to the gas valve are there any wires - it is hard to see from the picture and some of these early models have overheat thermostats

Customer:

Are no wires.

Customer:

If the therocouple is glowing, which it isn't currently, would that be in effect creating a circuit for the system to fire up? I'm teying to get a better jnderstanding of how the system works.

XXXXX XXXXX :

The thermocouple sits in the flame

XXXXX XXXXX :

This creates a small electrical voltage of about 30mV

XXXXX XXXXX :

There is an electromagnet in the back of the gas valve

XXXXX XXXXX :

This electromagnet is powered by the thermocouple

XXXXX XXXXX :

When you push in the button to let the gas through and light it, the electromagnet holds the inside of that button

XXXXX XXXXX :

so when you let go and the pilot is lit it is held internally and you are only releasing the button and not the inside of the gas valve also

XXXXX XXXXX :

Once the inside of the gas valve are held open and you let go, it lets you turn the gas knob from pilot to main flame

XXXXX XXXXX :

With it being held internally you cannot turn the button

XXXXX XXXXX :

*without it being held internally you cannot turn the button

Customer:

I'm not sure what do now in prder to source thd fault.

XXXXX XXXXX :

You may need someone to look at it.

XXXXX XXXXX :

You need to prove the thermocouple is producing the right voltage

XXXXX XXXXX :

If it is and screws directly into the gas valve then it will be a problem with your gas valve.

XXXXX XXXXX :

Can you not get the original guy back and tell him there is something wrong with the thermocouple he has fitted?

XXXXX XXXXX :

He would then need to look at it and diagnose the fault correctly

XXXXX XXXXX :

He should have checked the voltage from the old one and not just assumed that was the problem

Customer:

Ideally I need a multi metre 1st then take it from there?

XXXXX XXXXX :

One that measures mV if you are going to tackle this yourself

Customer:

I can't get an engineer until next week, I need to tey and get hot water for my children to bath lol.

XXXXX XXXXX :

But if you spend £30 on a meter just to find out if it is the gas valve at fault then you are no better off?

XXXXX XXXXX :

Do you not have an immersion heater?

Customer:

We have a tank that the boiler heats.

XXXXX XXXXX :

Is there an immersion heater in the top?

Customer:

I don't think so.

XXXXX XXXXX :

A round cover about 3" across and stands up about 2"

XXXXX XXXXX :

With a wire coming out of it?

XXXXX XXXXX :

Do you want to try another photo?

Customer:

On top of the water tank?

XXXXX XXXXX :

Yes on top of the cylinder to one side of the pipe cominbg out of the top

Customer:

No, no wires, only pipes. I need to go out for a couple of hours, I will contact you when I get back.

XXXXX XXXXX :

No problem, just come back on when you are back

Customer:

Hi, sorry it took so long, I'm home for the evening now.

XXXXX XXXXX :

so you have no immersion heater?

Customer:

No, the water is heated off the boiler.

XXXXX XXXXX :

if you have seen the thermocouple in the flame and the thermocouple is connected to the gas valve then it must be a problem with the new thermocouple or the gas gas valve

Customer:

We have replaced thw thermocouple with a new one, irs still not glowing red or anything else... I dont know where the gas valve is?

XXXXX XXXXX :

The gas valve is the part wit the button that you push to light the gas and the thermocouple screws into

Customer:

I see, its a whole unit, looks like we're going to have to get an engineer asap.

XXXXX XXXXX :

I think you have tried hard, but you are at the end of what you can do by yourself

Customer:

I think you're right, thanks for you're help.

XXXXX XXXXX :

no problem

Andrew Smith, HVAC Technician
Category: HVAC
Satisfied Customers: 3612
Experience: Heating and Controls engineer
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