The combination gas valve and pilot assembly
In order to understand the combination gas valve, we must know where it came from.
When gas appliances were first designed, there was a need for 4 things.
One, we needed a manual valve to be able to shut down all gas when necessary.
Two, we needed to control the gas pressure from the line pressure to the manifold pressure.
Three, we needed an electric solenoid that could be controlled from outside the appliance such as a thermostat.
Four, we needed a flame safety control to be sure gas would not flow if there was nothing available to ignite the gas.
The orignial design used to control all these functions was the gas chain.
This consisted of all the parts described above mounted as separate parts in series with each other.
When looking at the gas chain below, the first part on the left is the pilot safety control. The thermocouple attaches to the bottom of the part and the button to be pressed to light the pilot is on top. The next part is the manual valve. It is a quarter turn valve that can shut off the gas flow to the pilot and burners. The next part is the regulator. In natural gas, this part reduces the line gas pressure (usually 7 in wc) to manifold pressure.
the video below shows how this part works.
When gas appliances were first designed, there was a need for 4 things.
One, we needed a manual valve to be able to shut down all gas when necessary.
Two, we needed to control the gas pressure from the line pressure to the manifold pressure.
Three, we needed an electric solenoid that could be controlled from outside the appliance such as a thermostat.
Four, we needed a flame safety control to be sure gas would not flow if there was nothing available to ignite the gas.
The orignial design used to control all these functions was the gas chain.
This consisted of all the parts described above mounted as separate parts in series with each other.
When looking at the gas chain below, the first part on the left is the pilot safety control. The thermocouple attaches to the bottom of the part and the button to be pressed to light the pilot is on top. The next part is the manual valve. It is a quarter turn valve that can shut off the gas flow to the pilot and burners. The next part is the regulator. In natural gas, this part reduces the line gas pressure (usually 7 in wc) to manifold pressure.
the video below shows how this part works.
Manifold pressure is the pressure that the burner orifices are designed to flow the proper amount of gas to the burner. The pic below shows a gas valve and manifold with 5 orifices that feed gas to the burners.
The next part is the gas solenoid valve. This part is electrically operated and opens upon a call for heat from the thermostat. Gas will only pass thru this valve to the manifold and burners if the manual valve and the thermocouple pilot safety valve are open.
After a number of years of using this type of control, the combination gas valve was designed. This control incorporated all the functions of the gas chain into one compact part. The pic below shows one of the common designs for these valves.
The pic below is an overview of this type of valve. On the lower left, is the manual valve. In this case, the valve can be placed in 3 positions. In the farthest clockwise position, lol gas to the burners is turned off. In the pilot position, main gas flow is shut off and if the knob is pressed down, the pilot gas will flow as long as the knob is pressed down or if the pilot is then lit and the thermocouple heated up. If the pilot is lit and the valve is held down by thermocouple, the knob can be turned to the on position and the main gas will flow.
Above the knob is the fitting for the thermocouple. The pilot safety is located just below this part. On the upper right is the regulator. The slot head cap is removed to adjust the manifold pressure.
Below right the regulator is the electric solenoid. If you look close, you can see the terminals for the the electric coil for the solenoid. They are marked "th" and "tr".
Below right the regulator is the electric solenoid. If you look close, you can see the terminals for the the electric coil for the solenoid. They are marked "th" and "tr".
for an explanation of how the combination gas valve is made and a comparison to the gas chain check the video below.
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The pilot assembly
The pilot assembly performs 2 functions. It warms the thermocouple and lights the main burner. Above left , is a pilot assembly. The burner itself is on the left with the pilot tube from the gas valve below. The thermocouple is to the right. The gas flows into the burner and mixes with air. As seen on the left, the pilot light covers the thermocouple well. There is a small fitting with an orifice at the end of the pilot tube that limits the gas flow into the pilot. The pilot assembly is placed close to the main burner(s) so the flame can light the burners.
The pilot safety dropout
In order to provide burner shutdown if the pilot goes out, the gas valve must shut of all gas to the pilot and main burner in a maximum of 3 minutes after the pilot burner shuts off. This is so if there is a call for heat, the gas valve will not allow gas to flow if the pilot is not lit. The video below shows how this works.
lighting the pilot
Above I noted the positions of the knob of the combination gas valve on a specific valve. Each manufacturer makes valves somewhat differently. Lighting the pilot of the gas appliance is considered to be an owner capable operation. Instructions are written on the appliance. That does not mean all owners can effectively do it. However, if the instructions are followed it can be safely done.
Below is a video explaining how to light a pilot on a gas furnace.
Below is a video explaining how to light a pilot on a gas furnace.
tRoubleshoot of the combination gas valve, thermocouple and pilot assembly
Most of the problems relating to this valve concern the pilot safety. Problems such as pilot light going out and not staying lit after being relit or nuisance pilot outages are the most common.
First we will look at pilot outage and upon relighting, it will not stay on.
First we will look at pilot outage and upon relighting, it will not stay on.
look at the pilot burner flame while you are holding the gas knob down and have lit the pilot. Is the flame blue and maybe a little ruffled? Or is it yellow and lazy like a candle flame? The lazy flame indicates a dirty pilot assembly. The assembly could have lint or possibly small bugs in the assembly. The flame should cover the thermocouple at least 3/8 of an inch.
If the pilot will not light when the knob is depressed, you have a supply problem. Is the gas cock on? Do other gas appliances operate normally? You may need to remove the pilot assembly to see if the pilot orifice is plugged. The video below shows the pilot orifice, where it is and how to troubleshoot it.
if the burner lights and the flame is blue and covering the thermocouple but the flame will not stay on when the knob is released, the thermocouple may have failed or the thermocouple may not be tight on the fitting at the valve. The fitting should be tight but not excessively so. The button on the end of the thermocouple should be clean but should not be cleaned by any abrasive. A piece of cloth will clean it well.
The thermocouple may be tested with a millivolt meter. Most modern multimeters are capable of testing these parts. The meter must be set to DC volts. As can be seen below, the button on the end of the thermocouple that fits into the gas valve is held by the black test probe. The other probe is clamped to the copper tube. The pilot is then lit and the knob held down. If you let up on the knob, the pilot will go out. Note the voltage as the thermocouple warms. 10 millivolts is generally considered to be the minimum. However, a good thermocouple should read 18 to 22 millivolts. The thermocouple should be covered by the pilot flame at least 3/8 in (2 cm). The video below shows how to do this.
At this point, if the pilot is burning normally and the thermocouple shows enough power but the pilot will not stay lit, replace the thermocouple. The reason for this is it is the cheapest part. If that fails to stop the problem, the pilot safety in the gas valve has probably failed. Replace the gas valve.
interrmittent pilot outages
When the pilot lights and stays on normally, but goes out after a period of time, hours or days, and the thermocouple has been replaced, you could have an interrmittent pilot safety failure. However, sometimes pilot outages may be unrelated to the appliance itself. Negative pressure in the structure can cause a reverse air flow in the appliance venting. This will cause vent gas dumping out of the draft hood. This will usually cause a pilot outage due to the dumping of burned gas into the space around the appliance.
The cause of this problem could be a tight structure with exhaust fans creating the negative pressure inside. Think clothes drier, stove exhaust and bathroom fan. It could also be wind conditions. When intermittent pilot failures occurs, look to both the controls or outside influences.
The cause of this problem could be a tight structure with exhaust fans creating the negative pressure inside. Think clothes drier, stove exhaust and bathroom fan. It could also be wind conditions. When intermittent pilot failures occurs, look to both the controls or outside influences.
Gas valve problems
Problems with the gas valve are fairly simple.
Electrical problems are limited to the electric solenoid. If there is power to the valve, and no gas flows, is gas available(is pilot light on?). If so, gas is available. On many valves, the solenoid is operated by DC power to avoid vibration or buzzing. The voltage supplied to the gas valve is AC. There is a rectifier in the valve. For this reason, continuity of the solenoid cannot be checked by an ohmmeter. Below is a video demonstrating how this works.
Electrical problems are limited to the electric solenoid. If there is power to the valve, and no gas flows, is gas available(is pilot light on?). If so, gas is available. On many valves, the solenoid is operated by DC power to avoid vibration or buzzing. The voltage supplied to the gas valve is AC. There is a rectifier in the valve. For this reason, continuity of the solenoid cannot be checked by an ohmmeter. Below is a video demonstrating how this works.
If the pilot won't stay lit, and thermocouple problems are eliminated, the gas valve pilot safety has failed. 24 volts to the valve, no flow, valve has failed.
Replace the combination gas valve
If you have gotten to the point that you need to replace the valve, be sure the furnace is worth the repair. Check the heat exchanger carefully. Be sure the fan motor works normally.
Replacing the combination gas valve is a rather complicated operation.
First, the gas cock must be shut off. It will be located outside the appliance and is a quarter turn valve. If the handle is at 90 degrees to the gas line, the valve is off. Gas cocks are shown below.
Replacing the combination gas valve is a rather complicated operation.
First, the gas cock must be shut off. It will be located outside the appliance and is a quarter turn valve. If the handle is at 90 degrees to the gas line, the valve is off. Gas cocks are shown below.
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Turn the thermostat down as low as it will go. Shut off all power to the appliance. There should be a disconnect near the furnace. Remove all wires, the thermocouple, the pilot tube, break the union and unscrew the valve. The replacement valve can now be installed. All threads must be thoroughly cleaned. New pipe dope should be placed on the male threads only. When installing the valve to the threads of the piping, the connection should be quite tight as pipe threads seal best when tight. Most valves can be installed with the knob upright or up to 90 degrees. All valves have an install arrow that must point towards the gas manifold. There should be a new thermocouple with the new valve. Remove the old thermocouple from the pilot assembly and replace. The new thermocouple should fit into the same position as the old one. Screw the other end of the thermocouple into the fitting on the gas valve. Tighten the fitting snug but not tight. Next, the pilot tube should be connected to the fitting marked "pilot" on the gas valve. You may have to re route the tube as different valves have the pilot tube fitting in different places. Careful here. This connection is easy to cross thread.
Sometimes the pilot tube may have to be replaced. The tubing must be 1/4 in aluminum and uses a special ferrel fitting. The video below shows how to install the tubing.
Sometimes the pilot tube may have to be replaced. The tubing must be 1/4 in aluminum and uses a special ferrel fitting. The video below shows how to install the tubing.
The wiring that was removed should now be reconnected. Once these operations are finished, you are ready to fire off and test.
If all the piping is installed, you can now turn on the gas cock. After turning on the cock, smell around the fittings you have loosened to be sure there are no large leaks.
The first test is the pilot shutdown test. Turn the gas valve knob to the pilot position. Press down on the knob or button whichever valve you have. Attempt to light the pilot. The pilot will not light right away as there is air in the gas lines. You will have to hold the knob down until the pilot lights. Once it lights, continue holding the knob down for 30 seconds to 1 minute. Release. The pilot should stay lit. Let the pilot burn for another minute. Do not turn the gas valve to on yet. Shut off the gas cock. The pilot will go out. Wait until you hear a click. That should be the pilot safety valve closing. Turn on the gas cock. Without touching the knob at all, try to light the pilot. What you are doing here is testing to see if there is any leakage of the pilot safety. If the pilot lights, The gas valve is defective and should not be used. If it won't relight, go thru the pilot lighting again. Once the pilot is lit, turn the gas valve to the on position. Do not turn on the power to the furnace. Turn up the thermostat above the room temperature. This sequence is important. If you turn on the thermostat with the power on, the unit will start. You do not want the furnace to light off unless you are in front of it. If you have made a mistake, you need to be there to shut it down. After the thermostat is up, turn on the power. The burners should light. They should light quickly and smoothly.
This is the time to leak check your work. Leak check with soap bubbles or an electronic detector. Check every fitting you have moved. There should be no leaks anywhere. Especially be sure there are no leaks in the pilot tube. Be sure the burners light normally. You will also need to check the gas input. This can be done 2 ways. You can check manifold pressure and compare to the model plate or you can clock the meter. The meter should correspond to the BTU input on the model plate. You should also check all limits and safeties. Disconnect the fan and wait for the limit to shut off the burners. On this type of furnace, usually the only load that is in the thermostat circuit is the gas valve. This means that the amperage draw of the circuit may have changed with the new gas valve. If you have a mechanical thermostat, the anticipator may have to be adjusted. To finish up, check amp draw of all motors, be sure the burners are clean and, if it has a draft hood check the draft. There will probably be a pilot lighting sequence sticker that came with the valve that should be installed over the original instructions. Clean up the area. The video below shows how this is done.
If all the piping is installed, you can now turn on the gas cock. After turning on the cock, smell around the fittings you have loosened to be sure there are no large leaks.
The first test is the pilot shutdown test. Turn the gas valve knob to the pilot position. Press down on the knob or button whichever valve you have. Attempt to light the pilot. The pilot will not light right away as there is air in the gas lines. You will have to hold the knob down until the pilot lights. Once it lights, continue holding the knob down for 30 seconds to 1 minute. Release. The pilot should stay lit. Let the pilot burn for another minute. Do not turn the gas valve to on yet. Shut off the gas cock. The pilot will go out. Wait until you hear a click. That should be the pilot safety valve closing. Turn on the gas cock. Without touching the knob at all, try to light the pilot. What you are doing here is testing to see if there is any leakage of the pilot safety. If the pilot lights, The gas valve is defective and should not be used. If it won't relight, go thru the pilot lighting again. Once the pilot is lit, turn the gas valve to the on position. Do not turn on the power to the furnace. Turn up the thermostat above the room temperature. This sequence is important. If you turn on the thermostat with the power on, the unit will start. You do not want the furnace to light off unless you are in front of it. If you have made a mistake, you need to be there to shut it down. After the thermostat is up, turn on the power. The burners should light. They should light quickly and smoothly.
This is the time to leak check your work. Leak check with soap bubbles or an electronic detector. Check every fitting you have moved. There should be no leaks anywhere. Especially be sure there are no leaks in the pilot tube. Be sure the burners light normally. You will also need to check the gas input. This can be done 2 ways. You can check manifold pressure and compare to the model plate or you can clock the meter. The meter should correspond to the BTU input on the model plate. You should also check all limits and safeties. Disconnect the fan and wait for the limit to shut off the burners. On this type of furnace, usually the only load that is in the thermostat circuit is the gas valve. This means that the amperage draw of the circuit may have changed with the new gas valve. If you have a mechanical thermostat, the anticipator may have to be adjusted. To finish up, check amp draw of all motors, be sure the burners are clean and, if it has a draft hood check the draft. There will probably be a pilot lighting sequence sticker that came with the valve that should be installed over the original instructions. Clean up the area. The video below shows how this is done.