Master Plumber . Net
The Boiler or furnace Thermocouple, what it does
why it fails and how to replace it.
thermocouple is a probe that
sits directly in the flame of
the pilot light of your gas
fired boiler or furnace or water
heater or unit heater. That has
a copper wire that goes from the
probe to your gas valve. It is
important to note: not all gas
boilers, furnaces and so on,
have a thermocouple or a light
able pilot. Some have electronic
ignitions and some have spark
ignitions. trying to light this
type of system can de dangerous
and a total waste of time.
Lighting a pilot light and or
replacing a thermocouple is
potentially dangerous. Fire,
Explosion, death and destruction
of property can result. If you
have any doubt in your ability
to do this safely stop and call
a professional to replace
thermocouple. Master Plumber and
I are not responsible for any
problems you may have as a
result of reading this
information and using it.Working on
any gas appliance can be
The few things that I am warning
you about may not cover all the
possible things that can go
wrong. again if you are not 100%
sure of your ability CALL A
PROFESSIONAL!!! To replace
On Any Image To See A Larger
A warning about
Propane gas: Propane is heavier then air
and can sit on your floor all around
your boiler and or furnace or water
heater, and can explode when you go to
light the pilot light, and if that is
not bad enough it can, and does, loose
it's odor, making it impossible to
detect and an extremely dangerous
Natural gas can
build up in a room and can explode. it
has lifted houses off of there
foundations. I'm not trying to scare
you, Gas appliances are perfectly safe
under normal operating procedures. The
danger comes in when someone who does
not know what they are doing messes
around with them, changes settings or
removes safety items that are required.
What does it do?
send a low voltage signal to the gas
valve, letting the gas valve know there
is a flame and it can stay on.
Why do they go
out? Nothing lasts forever. after a
while a thermocouple is no longer able
to send enough voltage to the gas valve
to keep the pilot button set. and it
needs to be replaced. Some times you can
just relight the pilot and it will hold
for a few days then go out again. Most
of the time this is a week thermocouple.
How to replace
thermocouple scroll down for
First thing that I need to make clear
is the difference between a "Gas Valve"
and a "Gas Cock" Many people call the
gas cock a gas valve, but for the
purposes of this page I will refer to it
as the gas cock to avoid confusion. all
images on this page are thumb nails
click on the image to see a larger
version of it.
The Gas valve is the main
control of the gas going to
your appliance. There are
many different kinds of gas
valves. The one in this
picture is from a Weil
McLain boiler, it is a 24
volt standing pilot gas
valve. Not all gas valves
are 24 volt standing pilot.
and not all Weil McLain
boilers have this type of
gas valve. Some are mili
volt and use a thermopile
instead of a thermocouple
and are not interchangeable.
Others are 120 volts. and
are also not
The gas cock turns the gas to the
appliance off. This gas cock should be
located out side the jacket of the
appliance it is serving.
There are many different types of gas
cocks, but most of them turn off with a
1/4 turn the gas cock in the image above
is in the off position. parallel with
the pipe is open across the pipe is off.
Ok lets get started If you are unsure
if you have a thermocouple or other type
of gas control you can send me a picture
of the gas valve and lower front of your
appliance and I can tell you what type
of system it is. Send pictures to
For most thermocouple
replacements You will need a 3/8" open
end wrench and a 7/16" open end wrench.
And for water heaters you will need a
3/4" open end wrench as well. Turn the
power to the appliance off if
applicable. from this point we will have
to go to 1 specific boiler. The boiler I
have chosen to use as the example for
this is a Weil McLain cg series boiler.
once again I need to mention that not
all Weil McLain cg boilers have the same
gas valves they are not all standing
pilot and they are not all 24 volts If
you are not sure send me the pictures.
and I will help you. The following image
is from the Weil McLain boiler.
here to continue.
Website by: Thomas Petito Reardon