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I have a Marco gas fireplace model# 794019C. It is the millivolt pilot generator type. I first changed the thermopile and cleaned the spark ignitor to get it to light. I have checked continuity and everything checks out. Damper switch opens and closes, burner wall switch is good. When I jump the the redundant gas valve with jumper wire from one of the three terminals it does turn the burner on. Some of the time it is hard to keep pilot light lit, it will trip gas valve when you move from pilot to on position. The only thing that I think it could be is the gas valve! Please give me some advice. Thank you. Mike

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  • Anonymous Apr 08, 2011

    Advice was given to me in regard to a possible bad thermocouple! I do not have a thermo couple I just have a thermopile as I explained. I replaced the thermopile before. The advice that I recieved did not make sense. Please try to help me understand. Thank you so much.

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Mike, I suspect that the problem you're having is not related to the new Thermoplie (Pilot Generator) you installed. Its with the Thermocouple. The clue was "Some of the time its hard to keep the pilot light lit". The Thermopile controls the opening and closing of the gas valve, when the wall switch is turned ON/OFF. The Thermocouple, is what keeps the gas valve open and pilot light lit all the time. If there's no flame heat on the Thermocouple, it closes the gas valve. Rather than replace it, you can try polishing it with some very fine sand paper, then wipe the tip with a cotton ball soaked in alcohol. If the Thermocouple is really pitted, don't attempt this. Just replace it. If you do replace it, when you make the connection to the gas control valve, finger tighten the nut, then tighten only 1/4 turn more with the appropriately sized wrench.

By the way, MARCO is no longer in business as MARCO Fireplace. It was purchased by Lennox Hearth, a division of Lennox Heating. And your model firsplace is no longer manufactured. If you continue to have problems with it contact Angie Parrish at Lennox via email angie.parish@lennoxhp.com.

Hope this helped you solve the problem. Please be kind enough to rate my response to you. Thanks and have a good day!

Posted on Mar 25, 2011

  • Rich Landrum Apr 08, 2011

    If you only have a Thermopile are you certain that you connected the 2 wires to the TH and TPH Terminals on the control valve milivolt block. If not , it won't work. Now, lets get down to the nitty gritty. Where did you purchase the Thermopile and how did you determine that you purchased the right one? Not all Thermopiies generate the same amount of voltage. If the one you purchased was below the minimum required, that would be the problem.

    If, as you think it's the gas control valve, I can tell you this, they are not inexpensive to replace. With a fireplace as old as yours and is no longer being manufactured, you may want to consider replacing the entire firebox with one that's new and more efficient.

    Also, I would be curious as to what Angie Parrish advised you to do? Please advise. Thanks

  • Rich Landrum Apr 08, 2011

    If you only have a Thermopile are you certain that you connected the 2 wires to the TH and TPH Terminals on the control valve milivolt block. If not , it won't work. Now, lets get down to the nitty gritty. Where did you purchase the Thermopile and how did you determine that you purchased the right one? Not all Thermopiies generate the same amount of voltage. If the one you purchased was below the minimum required, that would be the problem.

    If, as you think it's the gas control valve, I can tell you this, they are not inexpensive to replace. With a fireplace as old as yours and is no longer being manufactured, you may want to consider replacing the entire firebox with one that's new and more efficient.

    Also, I would be curious as to what Angie Parrish advised you to do? Please advise. Thanks

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Did you get the right gas valve for a thermapile and not a 24 volt gas valve? Also I would recheck my wiring.... Here is a article that may be of HELP....... Hope this Helps.....
Thermocouple: A thermocouple is a device made of two different metals which creates a small electrical charge when heated at one end.
Thermopile: A thermopile is a probe that contains multiple thermocouples, therefore it can produce a larger electrical current. Millivolts: 1/1000 of a volt - thermocouples and thermopiles typically produce from 25 to 600 millivolts of power.
Piezio - a spark producing device often used to ignite gas pilots and burners.
Gas Valves Types:
A. Single Thermocouple Only - Used on some gas logs
B. Valves with Thermocouples and Thermopiles - Used on most hearth appliances and gas logs with switches or remote controls or thermostats.
C. ODS Systems - Used on Mostly Vent-Free. Available in manual control or thermostat/remote/switch (combination) valves.
valvepict.jpg
Typical Gas Valve A. Thermocouple-Only
Found in: Most gas log sets with standard safety pilot knob control. Also found in certain gas space heaters and construction-site portable heaters.
Explanation: This type of gas valve used a single thermocouple. A thermocouple is a device made of two different metals which creates a small electrical charge when heated at one end by the gas pilot. This small charge causes an electromagnet inside the gas valve to open and allow gas to flow to the main burners. Since the thermocouple must be heated before the burner will start, gas appliances often have a startup mode, during which a knob must be depressed and held for 30 seconds or so after lighting the pilot. At the end of the 30 seconds, the pilot should be generating enough electricity for the valve to operate correctly. At this time, the startup knob can be released and the valve turned to an "on" position for appliance operation.
pilotclose.gif Troubleshooting:
Most problems with this type of valve are due to thermocouple problems. Check the following:
1. Connection from the thermocouple to the valve. Clean the threads of the connecting nut with a pencil eraser and re-tighten.
2. Pilot hood and flame direction. The pilot should engulf the top 5/8" of the thermocouple with a decent flame. If the flame hits the thermocouple too low, this can cause the appliance to go out or not generate enough millivolts for valve operation. The pilot hood and orifice should also be clean from soot which could slow or block the pilot flame.
3. Pilot pressure. Many of these valves have an adjustment screw to adjust the pilot flame. A pilot that is too short may allow the pilot to stay lit after ignition, but may not create enough charge to allow the burners to ignite.
4. Overheating: If the unit works for a few hours and then shuts down, it's possible the thermocouple has become overheated. Repositioning of the gas valve and/or pilot may be needed to avoid this problem.
Problems with LP units can also be due to a tank that is nearly empty or a bad regulator at the tank.
B. Thermocouple & Thermopile valve
gas1.gif Found in: Most modern VENTED gas stoves, fireplaces and fireplace inserts as well as vented gas log sets with thermostat or remote control.
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Startup is similar to the thermocouple-only valve. A piezo spark ignition is used to ignite the pilot after the gas knob is turned to the "pilot" position and depressed. Once the pilot is lit, the knob is held in for 30 seconds to "prove" the heat and then released and turned from the "pilot" to the "on" position. The main burner will then respond to the switch, thermostat or remote control.
Troubleshooting
Since there is both a thermocouple and thermopile in this valve type, it is important to isolate where the potential problem may be. If the pilot can be lit and stays on after the knob is released, then the problem is probably with the thermopile side of the valve. Here are the most common problems and solutions.
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Check carefully that gas to appliance is on and that the valve is in he correct (pilot) position and fully depressed when lighting.
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ods.gif ODS stands for "Oxygen Depletion Sensor" , a term which accurately describes this valve type. The valve itself is similar in many ways to the two valve types above...with one exception. The pilot tube is a precision mechanism that creates a very stable flame as long as the room air contains the proper amount of oxygen. If the oxygen level in the room air drops even slightly, the pilot becomes unstable and lifts off of the thermocouple (see diagram) causing the gas valve and appliance to cease operation. This type of valve is very reliable, and there have been very few failures of this system - even with tens of millions in use worldwide.

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1 Answer

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