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Thread: Pilot light wont stay on

  1. #1
    Join Date
    Aug 2004
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    Kansas
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    Default Pilot light wont stay on

    Hey guys, neighbor just came over a few hours ago and couldnt get his pilot light to stay on. The furnace is a Ruud/Rheem Model # UGDA 100C ER. Went over to see what I could do. This is typical upflow natural gas furnace with gas valve and thermocouple. The pilot will light but is very weak flame. No matter how long I hold in the on/off knob on the pilot position when I release, the pilot immediately goes out. The knob is very stiff and wont "pop" back out you have to pull it back out. I've held it in for up to five minutes to heat the thermocouple but shouldnt have to do that. They have a maintenace policy so just told them that would be the thing to do as should get the benefits of their maintenance plan. I was just curious if you would run what some of the things that might be causing this by me for my education needs. Seems most likely to be a thermocouple. How do you go about knowing what thermocouple to chose if you are to replace one? Not real sure what happens in the valve itself once the thermocouple gets warm enough to produce the needed Mv's. All the gas supply valves are open.
    Last edited by Roger; October 16th, 2004 at 10:23 PM.

  2. #2

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    These pilot burners can get crudded up with white crusty crap. They make a pilot burner cleaning wire type kit to clean the little beggers. Looks like the wires that you use to clean a cutting torch nozzle. Perhaps the pilot burner could use a good cleaning? Also, since the gas valve button won't pop back out, there could be additional issues with the gas valve. The gas valve does have pressure adjustment set screws on it for both the pilot burner and the main burner. If the furnace has worked in past years, these shouldn't need fussed with. They're only for initial setup. If, when the pilot burner is cleaned, you still have a weak pilot, the gas valve is the only suspect. Since you say that you have a weak pilot flame, the thermocouple is doing it's job if it won't let the furnace run. You can check the thermocouple output with your meter if you want. Just unscrew it from the gas valve and have a helper hold the pilot button in to put a flame on the thermocouple. Should have around 13mv min.
    "Nothing is foolproof to a sufficiently talented fool"

  3. #3
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    Thanks Marc maybe I'll go over tomorrow and clean the pilot tube out and then check the thermocouple. When you buy a thermocouple I'm sure they are not created equal so what do I need to know? Just asking since doesnt sound like this furnace needs one. So if I clean the pilot and the thermocouple checks out then we start thinking gas valve. So if it comes to that will let their maintenance plan foot that bill. Thanks....

  4. #4

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    When buying a thermocouple, there aren't many differences. The length of the thermocouple tube has to be considered. They come in five standard lengths (only 3 of which are normal stock). When I did mostly HVAC work, I only carried the 36" length. Thermocouples in lengths greater than 36" are generally for commercial equipment, such as process baking lines and such. The entra can just be left coiled up at the gas valve. You have to be careful not to kink the thermocouple lead, since you can short it out. I like the Honeywell brand, but for no particular reason. I just ascribe a certain value to that brand. The generic ones from Sid Harvey's and the like are fine. There's no difference whatsoever between a thermocouple used on a furnace and one used on a gas water heater. With the exception of the lead length (which doesn't really matter, unless the replacement is too short) they are exactly the same. It would pay a person to have a spare thermocouple laying around the house for the 2 a.m. on Saturday night no heat times. They're only a few bucks, and are sometimes replaced as a PM measure on important equipment.

    Don't confuse the relatively cheap thermocouple with the much more expensive thermopile. A thermopile (as the name might imply) is constructed like a "pile" of thermocouples. Thermopiles have a 750mv max output (as compared with the thermocouple's 30mv max output), and run the thermopile output directly through the thermostat on the wall in the home. These are popular on gas, wood, and coal fired steam boilers because in the event of a power outage, you'll still have heat. You don't have to rely on a 24v transformer for the control voltage. The thermopile makes it's own control voltage. The thermopile lead is just two thin wires protected with what looks like tiny MC cable armour. They generally connect at the gas valve with spade terminals instead of the flare button arrangement that the thermocouple has.
    "Nothing is foolproof to a sufficiently talented fool"

  5. #5
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    Thanks for your time and clear explanation

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