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ARTICLES - Gas - How Gas Appliances Work

NOTE: This article is meant to provide an overview on operation of gas appliances and valves. It is NOT meant to be used as a field service guide. Only qualified service persons should attempt work on gas appliances. Vent-Free appliances are factory adjusted and should NEVER be tampered with in the field.

Definitions: (Note, more definitions at our glossary)

B-Vent Appliance - An appliance that usually uses natural draft to vent byproducts of combustion up a metal chimney. These chimneys terminate above the roof line of the home. Most B-Vent appliances use room air for the gas combustion.
Direct-Vent Appliance - A gas appliance using a two pipe system which takes combustion air from the outside(one pipe) and then exhausts out the other pipe. These system do not use room air for the combustion process.
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.

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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.

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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

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Found in: Most modern VENTED gas stoves, fireplaces and fireplace inserts as well as vented gas log sets with thermostat or remote control.

This valve is similar to the thermocouple only valve, however has a pilot which heats up BOTH a thermocouple and a thermopile. The thermocouple still acts to prove that the pilot flame is on and allows this flame to continue after startup. The thermopile is used to power a second circuit which is used to open the main valve. This second circuit is powerful enough (300-600 millivolts) to allow the use of a thermostat, wall switch or control switch to operate the main valve. Control of the valve is obtained simply by hooking a pair of wires to two terminals located on the valve.

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.

1. Pilot does not light or stay lit after knob is released - Make certain that the Piezo igniter works by checking for a spark between the igniter tip and the pilot hood. If there is no spark, the piezo may be bad or the piezo wire might be shorting to the appliance chassis. It is also possible that the igniter tip needs to be bent slightly toward the pilot hood so the spark jumps to it.

Check carefully that gas to appliance is on and that the valve is in he correct (pilot) position and fully depressed when lighting.

If pilot ignites but does not stay lit after knob is released, then the problem is with the thermocouple not generating enough voltage to the valve. It may be that the pilot adjustment needs to be turned up, or the thermocouple replaced. Another possibility is that the thermocouple may not be being “bathed” fully by the pilot flame. Check your owners manual for a diagram and description of the proper pilot flame and hood adjustment. It is also possible that there is soot or other blockage in the pilot tube, orifice or hood which is reducing the size of the pilot (and also the voltage of the thermocouple).

2. Pilot stays lit, but appliance will not turn on - There are two common causes for this. It is possible that the thermopile is not producing enough millivolts to power the control circuit. The millivolts can be checked with a simple voltmeter (consult the owners manual for proper setting) and adjusted with the pilot adjustment screw. Improper millivolts will also cause the appliance to shut down in the middle of operation.

Another common problem is loose or poor connections or circuits to your appliance switch, thermostat or remote transceiver. This can be isolated by simply using a small piece of wire to jump the “TP” and “TH” terminals located on these valves. If the appliance turns on when these terminals are jumped, then you can be sure that your problem is not in the appliance itself, but further down the switch circuit. Make certain you have used the suggested gauge of wire and that the length for your control runs does not exceed the specs given in your manual.

3. Other possible problems - include wind or back drafts affecting the pilot flame and checking of “spillage” circuits which may be wired into the valve in most B-Vent units.

If all the above checks out, and your valve is still acting weird (i.e., works some of the time), then you may have a defective gas valve in the appliance. Problems with LP units can also be due to a tank that is nearly empty or a bad regulator at the tank.

C. ODS System

Found in: Unvented (Ventless, Vent free) gas logs, fireplaces and stoves. These systems are available in manual or remote control.

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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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