furnace pilot light keeps going out

Page 1 of 3  
1969 Lennox gas furnace
The pilot light won't stay lit.
I light it easily enough, and it makes a nice two-inch blue flame.
Ten minutes later, with the thermostat still turned off, I hear a clunk, and the pilot light goes out.
It has happened about like that three times.
Please advise.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Possible defective "keep alive" sensor. Gas company should be able to replace it for you.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Fri, 08 Feb 2013 09:44:04 -0600, themattfella

The thermocouple has probably failed and needs to be replaced.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Gordon Shumway wrote:

Hi, Or connection is loose at the gas valve body.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Does the flame contact the thermocouple, and keep the sensor bulb hot?
Doesn't do much, unless the flame is heating the sensor bulb.
Christopher A. Young Learn more about Jesus www.lds.org .

Hi, Or connection is loose at the gas valve body.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 2/8/2013 10:19 AM, Gordon Shumway wrote:

Especially if it's the 1969 original. O_o
TDD
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 2/8/13 10:44 AM, themattfella wrote:

Most common, easy fix, is the thermocouple
See
http://www.youtube.com/watch?v=5j29RILOGOU

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

+1
Thermocouple is the most likely cause. It can be tested for putting out the correct voltage when the pilot light is lit.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 02/08/2013 10:36 AM, Retired wrote:

Thanks for that video.
The guy in the video calls it a "wire". But is it electrical? I thought it was probably a tube filled with gas and that the pilot heats the gas, which operates a pressure switch in the valve body. Is that right?
The pilot gas line is aluminum tubing about 1/4" o.d., which was replaced a couple years ago without good reason.
There are two 1/8" copper lines (which I presume are tubes, not wires) going from the valve body They are connected with compression fittings having 5/16" and 3/8" nuts.
Those two tubes together with the pilot gas line go to a little metal box (say 1.5 in. x 1.5 in. x 2 in.).
There is a bulb from one of the copper lines situated in the flame of the pilot when it is burning.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
themattfella;3010727 Wrote: >

> thought it was probably a tube filled with gas and that the pilot heats > the gas, which operates a pressure switch in the valve body. Is that > right?

No, your understanding is not correct.
Welcome to Gas Valves 101:
That copper "wire" that goes from the thermocouple to the gas valve is a coaxial cable. It has a central insulated copper conductor and and external copper jacket. When heated, the thermocouple generates a tiny voltage between those two copper "wires" which carry that voltage back to the gas valve.
A gas valve may look complicated, but it's really just two electromagnetic valves in series built into the same block of aluminum. The upstream electromagnetic valve is called the "safety" valve or "safety magnet" and the downstream valve is called the "main" valve or "main magnet". The small diameter pipe that allows gas to flow to the pilot light is connected to the gas valve BETWEEN the safety valve and the main valve so that gas will always flow to the pilot light as long as the safety valve remains open. It's the power generated by the thermocouple that keeps the safety valve open. Power is applied to the main valve to get it to open only when there's a demand for hot water or a demand for space heating from a boiler or furnace.
The pilot light/thermocouple combination is a safety system. If the pilot light goes out (for whatever reason, including an old thermocouple) that safety valve loses power and closes. That prevents gas from flowing through the main valve to the burners even if there's a demand for hot water or space heating. That's why with so many furnaces and hot water heaters being fired by natural gas in North America, there are relatively few house explosions.
If the thermocouple is in good condition, then it generates enough voltage to keep the safety valve open, and whenever the thermostat on your water heater calls for heat, power is then applied to the main electromagnetic valve which opens to allow gas to flow to the burner trays. The pilot light ignites that gas to heat the water or provide space heating to the house.
Quite often, water heaters which rely on the electrical power generated in the pilot light to operate both the safety valve and the main valve will use a "thermopile" instead of a thermocouple. A thermopile is an electronic device that is really just a hundred or so miniature thermocouples built into the same metal housing. The result is that a thermopile generates about 1.2 volts instead of the few millivolts producted by a thermocouple. A thermopile sits right inside the pilot light flame just like a thermocouple.
Unlike water heaters which normally don't have 120 volt power available to them, the gas valves on furnaces and boilers will typically use a thermocouple to power the safety valve and 24 volts AC to power the main electromagnetic valve. It's important to have at least the safety valve powered by the heat of a pilot light to ensure there's a flame present to ignite any gas that's released into your house.
The voltage produced by thermocouples gradually diminish with use, and when the power they produce is no longer sufficient to keep the safety valve open, the pilot light will repeatedly go out until that thermocouple is replaced. The pilot light will always be easy to relight because the button your push down or dial you turn on the gas valve to re-light the pilot simply over rides the electromagnet and opens the safety valve manually so that gas can flow through the safety valve to the pilot light.
A pilot light burning continuously uses a fair bit of energy. High efficiency water heaters, boilers and furnaces do away with standing pilot lights and use a spark or hot surface ignitor to ignite the fuel when there's a call for heat. Consequently, high efficiency appliances won't have a pilot light or thermocouple.
There, now you know more about how the gas valve on your water heater and/or boiler or furnace works than most homeowners.
--
nestork


Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 2/8/2013 2:41 PM, nestork wrote:

Kudos for a good explanation but you left out the fact that a thermocouple operates when a junction of two dissimilar metals is heated. ^_^
https://en.wikipedia.org/wiki/Thermocouple
TDD
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Friday, February 8, 2013 2:41:56 PM UTC-6, nestork wrote:

<snip>

-

You did mention or explain "thermo-piles" (not necessarily a burning hemorrhoid)!
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 02/08/2013 02:41 PM, nestork wrote:

Coax cable, valves in series, port for pilot is between the two valves, thermocouples wear out.
Thank you for a great explanation, nestork
I think I just have one more question. There are two 1/8" copper lines coming out of the valve body and going to the little pilot box along with the 1/4" aluminum pilot-supply gas line. Both copper lines are connected to the valve body by what look like (but apparently are not) compression fittings. Does that mean that there are two thermocouples? One fitting has a 5/16" nut, and the other nut is 3/8".
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

s

t

a

ny

e

he

r

s

a

ces

e

n

a

t

n

lve

es

text -

It takes two connections to make a circuit. Each of those two is one of them.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Most thermocouple I've seen, have one copper tube, and the electrical is pin and shield. Might this themocouple have two wires? I've never see none like that. But, most 1969 equipment near me has long since been replaced.
Christopher A. Young Learn more about Jesus www.lds.org .
wrote:

It takes two connections to make a circuit. Each of those two is one of them.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Feb 9, 8:44 am, "Stormin Mormon"

is

e

I just took a look at my water heater and you're right. The thermocouple is just one nut connection. It's been a long time since I replaced one and I guess the two connections thing I had in my mind was from the two connections I needed to take off apart to do the repair. One was the TC, the other the pilot light gas line.
So, what he has is a mystery. When he removes it and looks at it should be easy to determine if it's one TC or two. Depending on the climate where that 1969 furnace is located, it might be time for a new one. Can't imagine the efficiency of that beat. I replaced by 25 year old gas furnace two years ago. It made a huge difference in gas bills.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Saturday, February 9, 2013 8:28:35 AM UTC-6, snipped-for-privacy@optonline.net wrote:

NEVER tell a top-posting Mormon he's right...that's NUTS!
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
I'll be picking flowers off the wall paper for weeks.
Christopher A. Young Learn more about Jesus www.lds.org .
wrote:

NEVER tell a top-posting Mormon he's right...that's NUTS!
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Thanks for going, and taking the time to look. With a unit that old, I was totally willing to belive a two wire thermocouple.
Christopher A. Young Learn more about Jesus www.lds.org .
wrote:

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Sat, 9 Feb 2013 08:44:08 -0500, "Stormin Mormon"

Nope. The thermocouples are CoAxial connectors.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Site Timeline

Related Threads

    HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.