Gas Furnace

Flame Sensor and Thermocouple

Questions and Answers

Image of Mercury Flamesensor gas Valve System used on a Lennox Furnace
 Below is the result of our feedback form. It was submitted by  on Sunday, December 31, 2000 at 23:16:14
Type: hot air furnace
fuel: natural gas
location: shed
problem: ignition delay gas, pilot out
 Thank You for your time,  I have a lennox conservator III G16 that has a burner that will not ignite after the electric ignition lights the pilot. I took the thermocoupler and had it checked (parts stores said it was ok -did  not  have replacement) and had free lennox service technician check unit out. He stated that the gas valve unit should be replaced with some type  of kit for about 250-350 dollars. He then stated that since the unit was 15+ years old, the whole unit should be replaced. I do not mind fixing the problem, however I do not want to randomly replace valves, modules, relays etc.. much less purchase a whole new system. Should I replace the thermocoupler regardless of the parts store testing. Can it realistically be tested? > > > > Thank You. > > > > > wrote:To the best of my knowlege only standing pilot gas equipment have thermocouples.
 (Like the one shown below)
Just like a car with an automatic transmision doesn't have a clutch pedal.
This may be some special gas valve system or you were told you have a
Thermocouple and you don't.
 Picture of standard standing pilot thermocouple.
 feel free to call me at 240-460-3820
    My close friend and colleague Scott Cc'd me the response he sent to you. I have one other suggestion and will be Cc'ing my response to you and sending it to Scott. I am curious that you may have one of the lesser used White Rodgers brand "3000" or "900" series "Mercury Flame Sensors." They are used in conjunction with a number of their gas valves such as the "36C series and other valves. I have never seen them used with any other brand gas valve than their own, White Rodgers, so if you have another brand gas valve what is written below probably does not apply. They come in a direct plug-in type which plugs directly onto the gas valve and a spade connector type which has the push on flat blade "spade" connectors. The spade connector type are really scary because you can inadvertently plug the wires onto the wrong terminals (bad idea= potential BOOM!)

    Often folk confuse these with "Thermocouples" because the pilot flame end looks exactly like a thermocouple, and there is the apparent "capillary tube" similar to a thermocouple which comes from the sensor end which senses the pilot flame. Also, because these are not as common as thermocouples, less informed supply house men who work the Parts Counters sometimes even think these are "Thermocouples" and call them "Thermocouples." I know Scott has worked on many of these and is intimately familiar with them. He just didn't pick-up on the possibility that you may have one of the lesser used Mercury Flame Sensor setups for whatever reason. Also, with the advent of Hot Surface Ignition systems with their accompanying solid state control modules, I am not aware of any manufacturer still using MFS systems in current production furnaces and rooftop equipment. They are, however readily available as are the compatible gas valves which they mate to and which I will describe below.

    Here's the BIG difference between a "Thermocouple" and a "Mercury Flame Sensor:"  The part of the "Thermocouple" which actually senses the flame is really technically called a "thermocouple junction". It is two dissimilar metal wires which are fused (or welded) together at the end where they are (enclosed and protected by an outer metal case) immersed in the hottest part of the pilot flame. When heated, the thermocouple junction actually produces D.C. voltage from the energy of the flame similar to the way a solar panel cell produces electricity from the energy of light. The output of a thermocouple is a very minute voltage (30 millivolts) but it is sufficient to operate a very small electro-magnet which is the "holding" part of the "Push to Light" mechanism of most standing pilot gas valves found on older furnaces and water heaters. What appears to be a capillary tube which goes from the thermocouple end (which is immersed in the pilot flame) to the gas valve is actually a coaxial wire with the copper exterior being one conductor and the inner insulated wire being the other. Most Thermocouples connect to the gas valve with a hex nut and if you look closely you will see that the "button" type end has a piece of insulator disk to separate it from the flared copper exterior. When the nut is tightened the electrical connection is made firm to the internal circuitry of the gas valve.

    The "Mercury Flame Sensor" on the other hand is an entirely different animal all together. The only thing that is the same is the appearance of the sensor end which senses the flame and what appears to be a capillary tube which goes from there to the gas valve. I emphasize "appearance." Unlike the tube of the thermocouple, what appears to be a capillary tube on the MFS is actually just that, a capillary tube which carries the pressure from the sensor end to a set of "Bellows" at the gas valve. The bellows or diaphragm actuates a single pole double throw switch mechanism which first energizes the ignitor, be it spark, hot surface, or glow coil, via the normally closed contact, and then, upon proving the pilot to have successfully lit, energizes the gas valve to send gas to the main burners via the normally open contact (and other related circuitry). There are some MFS's which have spade connectors with field or factory connected wiring, but with most, the bellows/switch assembly simply plugs onto the gas valve. If you took what you thought was a "thermocouple" off of the gas valve by using a 3/8" wrench to loosen a hex nut you do in fact probably have a thermocouple. If, however on the other hand you "unplugged" it from the valve and if it has three either round or flat prongs on it and a 3XXX- series or 9XX- series number on it, you have a MFS.

    Here's how a MFS works. The sensor end, capillary tube and spdt switch assembly at the gas valve end are filled with mercury. When the flame sufficiently heats the sensor end, the mercury in the sensor end tube vaporizes and forces the remaining liquid mercury through the capillary tube to the bellows/switch assembly at the gas valve end of the capillary tube. The bellows move against a non-adjustable calculated spring tension and with sufficient pressure exerted "snap" a single pole double throw set of switch contacts from one position to the other thereby opening the normally closed contact and closing the normally open contact. This then de-activates the ignitor, be it spark, hot surface ignitor or glow coil, and energizes circuitry which allows the gas valve to open for the main burners. There are other critically important things which happen to allow the main burner gas valve to open, but for general explanation purposes, it's a simple switch-over from pilot lighting and proving to be on, to the main burner gas valve then opening for the main burners.

    If this is what you have, the MFS's themselves are very reliable, and I would dare say that most of those which are replaced are probably not defective. They are even more reliable than the operating thermostat in your kitchen refrigerator. Have you had to ever replace one? Probably not. I've seen guys replace both the gas valve and the MFS simply because they did not know how to diagnose which was bad. If you have the MFS unplugged from the gas valve and have a propane torch or even a heavy duty cigarrett lighter handy and a continuity light or ohm meter and are familiar with how a spdt switch works, just find out which two of the three plug pins are "made" with the sensor "cold" and then put the end of the sensor in the flame as if it was actually in the pilot flame itself. CAUTION!! be sure to keep it vertical as it would be in the pilot flame- what you want to do is have the vaporized mercury vapor push down on the liquid, not have the vapor go up the capillary tube!!! When the end is heated up to the temperature the pilot flame would heat it you will lose continuity on the two pins you had it on when the end was cold, but should then "have" continuity between the third pin and one of the original two pins. Whichever of the two original pins gives continuity to the third one will be what is known as the "common" pin because it is "common" to both the normally closed and the normally open pin.
    Having said all of that about the MFS, on this type of system the gas valves are usually the culprit. Many have an integral gas pressure switch which you cannot get to and which fail frequently. I've seen them where the epoxy or plastic which hold the tiny gas pressure switch in place gets loose and slightly moving the wires will allow it to temporarily "make" contact and function. They also have a "pic" coil which energizes for the pilot gas and electrically latches in and must stay latched in when the MFS unit switches over from "no pilot sensed" to the "pilot proven on position." The reason for the integral gas pressure switch is obvious, but it together with the pic coil protect the system from unexpected momentary power drops or gas pressure losses which could have the gas valve be "open" upon a momentary loss and resumption of either power or fuel. Were it not for the pilot gas pressure feature and the Pic coil feature raw un-ignited gas could roll into the combustion chamber and wherever else building up for the eventual "retry of the pilot when the MFS cooled down and switched back to the "no pilot sensed position." The result could be disastrous! When working on this kind of stuff it is really scary to think what could happen, so I must caution you to be sure you know fully what you are doing and if in doubt, just compare the cost of a service call to the cost of a house or worse lost lives. In our immediate area at around 3:00 AM a house blew up and killed two people apparently while they slept. When I say "blew up" I am not exaggerating. The house was literally flattened and the surrounding trees were littered with clothes, pieces of furniture and building materials hanging where they snagged in the tree limbs and branches. Scott and I rode over that way this evening and although we could not get really close because the area was ribboned off, it was a very ugly and sobering scene with even the two houses on either side substantially damaged from the force. The cause has not been determined, but it will most likely either be a natural gas build-up or something like a propane gas grill stored inside with the tank valve accidentally left on together with a ruptured or leaking rubber hose. Because we work in the industry and are acutely aware of the need for extreme care and caution when working on gas units, we will be following the incident to find out what was the cause of this tragic incident. Be careful, and do not construe anything written herein as advice to do any of this yourself. I am writing this only as courtesy advice for your qualified technician to only remotely consider in his diagnosis of your problem, and because I have not seen or even heard first hand of your problem, can in no way have any part in what or how it gets fixed.

I hope the above descriptions and especially the cautions prove helpful- be sure you have a good technician on the job.
Scott's friend Forest Grauel
Happy New year

Related Pages:
    Identify which gas valve system you have.
    Learn how furnace fan and blower controls work.
    See pictures and description of a system like the one described above.
    Read about gas systems that use a combustion blower.
    Learn about thermostats.   
    Control wiring colors.

Written By: Forest Grauel for:
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                 This text edited by: Scott Meenen * G & S Mechanical

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