How does the thermocouple/ OH stat work?

[Nick the teacher]

OK, so my pilot light wouldn't light. Bought new thermocouple and had it fitted yesterday. Had central heating on for maybe 2 hours and then turned off. I later found the pilot light had gone out again so now I'm thinking about replacing the OH stat.

With the number of plumbers I've had looking at my boiler over last 2 week or so I'm becoming pretty familiar with the inside of my boiler (and no I'm not going hunting with a spanner and screw driver!!). I just wondered how the OH stat works. I'm guessing that the thermocouple expands due to heat and makes a contact at the far end? Is this right? What happens next?

Thanks for your help


Boiler type: Myson Apollo 50B

 

[Baxi-Remeha]

thermocouple works by 2 disimilar meatalt, which when heated create about 30mv. this then charges an electro magnet inside the gas valve. when you push and hold the button, your holding the valve open against this eletro magnet, hence why it takes a small amount of time to hold it open.

over heat stats normaly work using electrical resistance........

your pilot going out should have nothing to do with the o/h stat, its more than likely going to be a faulty mag unit in the gas valve, dirty pilot of dodgy thermocouple, ALL of which require a CORGI to sort out.

 

[Nick the teacher]

Thanks for the reply Jason

The plumbers I've had out have all been CORGI. Unfortunately its an intemittent problem and on two occasions now i've had the plumber get the pilot light lit and within an hour or so its gone out. I don't know whether its relevant but the previous owner of this house built an 8ft wooden gate about 2 feet from the air (inlet?outlet?) that goes to the boiler. One of the plumbers pointed out that this wasn't a very good bit of planning. Is it possible the air hitting the gate could be blowing the pilot light out?

This boiler has been nothing but trouble. I originally bought this thermocouple

//www.diynot.com/forums/viewtopic.php?t=23663&highlight=apollo+50*

but the first plumber pointed out that there was nowhere on my boiler for the interruptor to plug into so it must be the wrong part. Two or three people, including the staff at partscenter told me it is the right part so I'm none the wiser now. Anyone know why my apollo 50B might have a different setup to what its meant to?? One of the plumbers said it could be a very early or a very late model.

So all in all I'm sick of it. Where's a cheap place to get a replacement, modern gas boiler from? If I decide to get it all moved into the loft (ie combi) out of the way how much can i expect to pay? (I currently have gas boiler in kitchen, tank in cupboard upstairs and water butt in loft)

Thanks

 

[doitall]

Apollo 50B dosen't have an interupt.

Thermocouple part no is 309 S 189

GC No 386 515

 

[Nick the teacher]

Selected Product: 455372


Description THERMOCOUPLE THORN APOLLO TYPE
MYSON APOLLO FANFARE 15/30/30/50,15/30B,15/30C,30,50B,30/50C
50/65B,50/65C,50/80C,65/80B,GEMINI,381884
Retail Price
(Excluding VAT and quantity discounts) £19.16



This info is from the Partscenter website. This is the thermocouple with the interrupt in.

Doitall you're the first person to tell me I don't need an interrupt. Thank you!! I felt like I was going mad. [/img]

 

[doitall]

Don't get too excited, it just doesn't show one in the manual, and also has an seperate overheat device which connects to the gas valve.

 

[ChrisR]

Overheat stats are usually temp operated switches.
There were loads of different Apollos.
Your boiler probably needs a service if the pilot is going out - when was one last done?
You won't get a cheap boiler unless you're corgi registered.
New combi in loft perhaps £3000, depending a lot.

 

[ollski]

Selected Product: 455372


Description THERMOCOUPLE THORN APOLLO TYPE
MYSON APOLLO FANFARE 15/30/30/50,15/30B,15/30C,30,50B,30/50C
50/65B,50/65C,50/80C,65/80B,GEMINI,381884
Retail Price
(Excluding VAT and quantity discounts) £19.16



This info is from the Partscenter website. This is the thermocouple with the interrupt in.

Doitall you're the first person to tell me I don't need an interrupt. Thank you!! I felt like I was going mad. [/img]

well the second anyway ho hum/;
//www.diynot.com/forums/viewtopic.php?t=24711

 

[DP]

What is needed here is someone who will look at the overall system. To change system components without solid reason, often leads to dis-satisfaction. This is because a component might be changed as a matter of course, but the defect might be elsewhere.

In the first instance, employ a heating engineer (plumber will often fit a boiler but will balk at carrying out repairs). He will need to investigate possible causes. A fault that is not there at all times is often difficult to locate

 

[Nick the teacher]

Apologies Ollski!! Its been a long week!

ChrisR - as I said I just moved in recently. Turns out next door but one is a gas fitter who fitted the boiler 8 years ago. It was 2nd hand then and he hasn't been out to service it since. Hence the question about getting a new boiler!!!

DP- Didn't even know there was a distinction betwen a heating engineer and a plumber (I can almost hear the tuts of disbelief!). Thats probably what I've been doing wrong. I've not been overly impressed with the people I've been getting round - hence the number of posts I've made on here regarding the problems. I'm going to see if heating engineers, or central heating bods are in the yellow pages right now!

 

[grainish]

I was getting the models mixed up my self any one who read this post before edit please ignore it.

I agree the Myson 50B does not have an interrupt on it.


David

 

[Slugbabydotcom]

Cutting out after 2 hours suggests to me that your boiler stat may be dodgy.
Is your central heating stonking hot when it cuts out?
Try running it on low for a few days if you normally run it on high or vice versa

Is the flame reaching the thermocouple properly.
I change quite a lot of pilot burners on apollos as a dirty flame often causes it to rust also pilot injector could be partially blocked.

 

[COG]

The 50b does have an interrupter (limit stat) it may not be a part of the thermocouple itself but still sits in series with the thermocouple before the final gas valve connection, often found the interrupter thermocouple replaced with the std type thermocouple and elmwood limit stat as its cheaper if the thermocouple is replaced regular especialy on LPG