Recently had a problem with the boiler....

Pilot light would not stay lit when finger removed from the button.

Output from the thermocouple was the correct no. of millivolts
when hot, more or less, and the new one I got was pretty much the same.

I then followed the electrical path (measuring volts output WRT ground) from
the
thermocouple around the boiler until it went into the gas valve. By the time it
had
reached the gas valve, it was down significantly, below the threshold of
working,
(so there had been a voltage drop along its path). I bridged the overheat
cut-off
switch (NC) with a small cable with croc clips and measured again. It didn't
hold
the pilot light valve open either, so I measured again, and was astonished to
see that
there was still 50% of the original volts being dropped across my croc clip
lead,
inferring quite a high current passing (and being produced by the
thermocouple).

I replaced that with a very short thicker cable and even then still some
voltage drop
but enough to get it working again. I "heat cycled" the cut-off switch and now
it
doesn't drop any significant millivolts, and all is currently working OK...

Anyway the question is ...... can anyone tell me the typical output in
millivolts
and current from the average thermocouple, or what is required to keep
a gas valve's pilot light solenoid open ? It must be tiny !

I forgot to measure it and my meter / leads would have probably been

too high a resistance to measure it anyway.

Thanks,

Nick

"nick smith" wrote in message
...
Recently had a problem with the boiler....

Pilot light would not stay lit when finger removed from the button.

Output from the thermocouple was the correct no. of millivolts
when hot, more or less, and the new one I got was pretty much the same.

I then followed the electrical path (measuring volts output WRT ground) from
the
thermocouple around the boiler until it went into the gas valve. By the time
it
had
reached the gas valve, it was down significantly, below the threshold of
working,
(so there had been a voltage drop along its path). I bridged the overheat
cut-off
switch (NC) with a small cable with croc clips and measured again. It didn't
hold
the pilot light valve open either, so I measured again, and was astonished to
see that
there was still 50% of the original volts being dropped across my croc clip
lead,
inferring quite a high current passing (and being produced by the
thermocouple).

I replaced that with a very short thicker cable and even then still some
voltage drop
but enough to get it working again. I "heat cycled" the cut-off switch and now
it
doesn't drop any significant millivolts, and all is currently working OK...

Anyway the question is ...... can anyone tell me the typical output in
millivolts
and current from the average thermocouple, or what is required to keep
a gas valve's pilot light solenoid open ? It must be tiny !

I forgot to measure it and my meter / leads would have probably been

too high a resistance to measure it anyway.

Thanks,

Nick



Clean the soot and other debris off the thermocouple probe. This should also
help.

I did the cleaning bit as a first thing - the old thermocouple probe insulator
was bright white and clean in the main and was producing the same output as the
new one....
Thanks anyway,

Nick


"BigWallop" wrote in message
k...

"nick smith" wrote in message
...
Recently had a problem with the boiler....

Pilot light would not stay lit when finger removed from the button.

Output from the thermocouple was the correct no. of millivolts
when hot, more or less, and the new one I got was pretty much the same.

I then followed the electrical path (measuring volts output WRT ground)
from
the
thermocouple around the boiler until it went into the gas valve. By the
time
it
had
reached the gas valve, it was down significantly, below the threshold of
working,
(so there had been a voltage drop along its path). I bridged the overheat
cut-off
switch (NC) with a small cable with croc clips and measured again. It
didn't
hold
the pilot light valve open either, so I measured again, and was astonished
to
see that
there was still 50% of the original volts being dropped across my croc clip
lead,
inferring quite a high current passing (and being produced by the
thermocouple).

I replaced that with a very short thicker cable and even then still some
voltage drop
but enough to get it working again. I "heat cycled" the cut-off switch and
now
it
doesn't drop any significant millivolts, and all is currently working OK...

Anyway the question is ...... can anyone tell me the typical output in
millivolts
and current from the average thermocouple, or what is required to keep
a gas valve's pilot light solenoid open ? It must be tiny !

I forgot to measure it and my meter / leads would have probably been

too high a resistance to measure it anyway.

Thanks,

Nick



Clean the soot and other debris off the thermocouple probe. This should also
help.

"nick smith" wrote in message
...
Recently had a problem with the boiler....

Pilot light would not stay lit when finger removed from the button.

Output from the thermocouple was the correct no. of millivolts
when hot, more or less, and the new one I got was pretty much the same.

I don't know how much current the thermocouple will deliver, but my
Potterton manual suggests that when the pilot jet is correctly adjusted
(i.e. when "the pilot flame completely envelops the top 15mm of the
thermocouple"), it should generate 18-32 mV open circuit and 10-15 mV closed
circuit. I presume "closed circuit" in this context means when connected to
the gas valve. I have got another year's life from what I thought was a
burned out thermocouple by simply re-making the connection to the gas
control valve, but I've never actually measured the voltage to confirm the
numbers.

" Anyway the question is ...... can anyone tell me the typical output in
millivolts
and current from the average thermocouple, or what is required to keep
a gas valve's pilot light solenoid open ? It must be tiny !"

I find that my Fluke multimeter is temperamnental at this also. I boght a thermocouple tester from Acorn Gas Spares in Leeds. It's a Regin product, and basically it is a solenoid same as the power unit in the multifunction control valve (yes I know, we all call it a gas valve). A friend just uses a power unit from an old gas valve. It's a very simple and effective method of proving a thermocouple.

Sometimes we can be fooled by the overheat stat, I find it's best to remove the leeds of the o.h. stat from the interuptor before shorting it, because if there is insulation break down it tracks the current to ground and a perfectly good thermocouple is discarded when a new leed for the oh stat was what was needed. I've done it myself. I've even blamed the power unit when it was completely innocent!

I was once told how many milliamps proves a thermocouple, but the thermocouple tester is a more reliable indicator, it proves it can do the job.

nick smith wrote:
I did the cleaning bit as a first thing - the old thermocouple probe
insulator was bright white and clean in the main and was producing the
same output as the
new one....

If you can measure continuity between the copper outer sheath of the
cable and the button connected to the inner conductor, it is very likely
that the thermocouple is OK. However, it operates in a very
low-resistance circuit (high current but very low EMF) so attempts to
make measurements with a normal meter and test prods are likely to stop
the solenoid valve from working.

The most important thing is to get two solid electrical contacts to the
TC. For the end contact button, it can sometimes help to build up the
face of the button with a small blob of solder, so you can screw it down
more firmly into the valve. The current return path is through the
general metalwork and/or the outer cable sheath, and this can sometimes
be the less reliable of the two contacts - there's touching contact all
over the place, but often not much deliberate contact pressure anywhere.

The final question is whether the tip of the TC is in the right place
relative to the pilot flame. It must obviously be hot enough to hold in
reliably, but not too hot or else it will burn through prematurely. You
may also find that the TC is quite a sloppy mechanical fit, so you have
to muck about with bending the cable until it's willing to stay in the
right place.


--
Ian White
Abingdon, England

nick smith presented the following explanation :
Recently had a problem with the boiler....

Pilot light would not stay lit when finger removed from the button.

Output from the thermocouple was the correct no. of millivolts
when hot, more or less, and the new one I got was pretty much the same.


That all put together sounds very much like the solenoid has become
shorted in the gas valve. Rather than voltage being dropped, the
solenoid (due to shorted turns) is overloading the thermocouple and
pulling the voltage down.

The same symptoms occured on my boiler a few months ago, I replaced the
gas valve and all was well. I pulled the old one apart and their was a
brown mark on the solenoid, where it had developed a short.

--


--

Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.org

Harry Bloomfield wrote:

The same symptoms occured on my boiler a few months ago, I replaced the
gas valve and all was well. I pulled the old one apart and their was a
brown mark on the solenoid, where it had developed a short.

That possibility had never occurred to me, considering how few
millivolts are available.

I've never had one of these valves apart... it must require very special
engineering to make a solenoid that's even *capable* of being shorted by
such low voltage.


--
Ian White
Abingdon, England

Ian White explained :
That possibility had never occurred to me, considering how few millivolts are
available.

I've never had one of these valves apart... it must require very special
engineering to make a solenoid that's even *capable* of being shorted by such
low voltage.

The thermocouples might only produce a tiny voltage, but the produce a
surprising quantity of current. The solenoid coil winding inside mine,
was about 1" diameter, wound with very fine wire, with a metal slug
inside it. You provide the energy to open the gas valve, then the metal
slug just needs to trap it to hold the gas valve open. If the current
decreases, the metal slug is no longer able to hold the gas valve
against its spring so the valve closes.

If anyone is curious and happens to have an old valve... The solenoid
on the most common type of gas valve can be accessed by removing the
cover plate on the bottom, which is held on by about 6 screws, with a
rubber seal beneath it. Once the seal is broken, the valve should of
course be scrapped.

--


--

Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.org

Paul Barker used his keyboard to write :
" Anyway the question is ...... can anyone tell me the typical output
in
millivolts
and current from the average thermocouple, or what is required to
keep
a gas valve's pilot light solenoid open ? It must be tiny !"

I find that my Fluke multimeter is temperamnental at this also. I boght
a thermocouple tester from Acorn Gas Spares in Leeds. It's a Regin
product, and basically it is a solenoid same as the power unit in the
multifunction control valve (yes I know, we all call it a gas valve). A
friend just uses a power unit from an old gas valve. It's a very simple
and effective method of proving a thermocouple.

The current is rather difficult to measure due to the voltage being so
low and the current so small....

Any attempt to measure the current will introduce resistance into the
circuit from the leads, clips and instrument. The measured values will
be meaningless. As Paul suggested above, the best test is one of
substitution with known to be working items.

--


--

Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.org

On Sat, 08 Jan 2005 14:34:03 +0000, nick smith wrote:

Recently had a problem with the boiler....

Pilot light would not stay lit when finger removed from the button.

Output from the thermocouple was the correct no. of millivolts
when hot, more or less, and the new one I got was pretty much the same.

I then followed the electrical path (measuring volts output WRT ground) from
the
thermocouple around the boiler until it went into the gas valve. By the time it
had
reached the gas valve, it was down significantly, below the threshold of
working,
(so there had been a voltage drop along its path). I bridged the overheat
cut-off
switch (NC) with a small cable with croc clips and measured again. It didn't
hold
the pilot light valve open either, so I measured again, and was astonished to
see that
there was still 50% of the original volts being dropped across my croc clip
lead,
inferring quite a high current passing (and being produced by the
thermocouple).

I replaced that with a very short thicker cable and even then still some
voltage drop
but enough to get it working again. I "heat cycled" the cut-off switch and now
it
doesn't drop any significant millivolts, and all is currently working OK...

Anyway the question is ...... can anyone tell me the typical output in
millivolts
and current from the average thermocouple, or what is required to keep
a gas valve's pilot light solenoid open ? It must be tiny !

I forgot to measure it and my meter / leads would have probably been

too high a resistance to measure it anyway.

25mV would be typical. After that check all the connections including the
one on the gas valve which can come loose quite easily.

You might even be able to prove the gas valve is OK using a universal
t/couple even if the part is a bespoke unit with o/h contacts.



--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html