We occasionally see a failed gas valve as the cause of a furnace or roof top unit (for commercial applications) not operating.
The photo above is of a two stage gas valve in a residential furnace. The natural gas line runs from the natural gas meter outside of the home to the furnace. The gas valve controls the flow of natural gas to the furnace burners and also lowers the natural gas pressure to the pressure that is prescribed by the furnace manufacturer. This prescribed pressure is identified on the furnace nameplate sticker as the manifold pressure. The gas valve also functions to shut off the supply of natural gas to the furnace in the event of some safety condition where the furnace control board would send a signal to the gas valve to close.
WARNING: These repair activities involve working on high voltage equipment that poses serious electrical shock risk. Follow all safe electrical work practices and ensure that only qualified, trained service technicians perform these repair activities. Hazardous Voltage! Failure to disconnect power before servicing could result in death or serious injury. Disconnect all electric power, including remote disconnects, before servicing. Follow proper lockout/tagout procedures to ensure the power cannot be inadvertently energized.
The furnace control board sequences the startup, operation, and shutdown of the furnace in a specific order. If no natural gas is admitted to the furnace, there could be any number of reasons why this is the case. There could be no call for heating from the thermostat. There could be a high temperature condition in the furnace that locks out the operation of the furnace. A pressure switch inside the furnace could be locking out the furnace. A failing or failed inducer motor could be preventing the furnace from operating. The natural gas could be shut off to the furnace or to the home. There could be a problem with the circuit board where no low voltage signal is being sent to the gas valve. There could also be a problem with the low voltage wires connecting the gas valve with the circuit board. The gas valve itself could also have failed.
Gas valves can exhibit mechanical modes of failure or electrical modes of failure. There are single stage gas valves, two stage gas valves, and modulating gas valves that are commonly used in residential furnaces. Both the single stage gas valve and the modulating gas valve will usually have two low voltage wires connecting the gas valve with the furnace control board. The two stage gas valve will normally have three low voltage wires connecting the gas valve with the furnace control board.
Let’s talk about troubleshooting of a gas valve in a residential furnace. The first indication that there may be an issue with the gas valve is when no flame is established in the burners of the furnace. A typical furnace startup sequence on a furnace that was built within the last 15 years or so involves many steps. However, the most obvious steps are that a call for hear is established from the thermostat, and then possibly after a time delay the inducer fan starts. Then after a short pause, the hot surface igniter begins to glow bright orange and stays orange for several seconds. While the hot surface igniter is energized, the gas valve will receive a signal from the furnace control board to open which admits natural gas to the burners. The glowing hot surface ignitor will ignite the flame going to the burner where the hot surface ignitor is located and then the flame travels to the other burners igniting them.
If no natural gas flame is established then there may be a problem with the gas valve. By listening carefully, the gas valve can be heard to open as the solenoid valve inside the gas valve repositions. Then the sound of natural gas flowing can often be heard. If the flame sensor does not detect a flame within a few seconds then the furnace control board sends a signal to close the gas valve to prevent dumping natural gas into the furnace if the burners do not light.
WARNING: it is important to keep in mind that there are many safety controls that are included in the furnace startup sequence (as well as the normal operation sequence and the shutdown sequence). So be very cautious and deliberate about taking a troubleshooting action that would bypass a safety function. Again, ensure that only qualified, trained service technicians perform these repair activities.
Place the two leads from your DMM (digital multi meter) which is set to AC voltage on the two low voltage leads that connect to the gas valve. One of the leads will be the low voltage power wire and the other electrical lead will be the neutral wire. If you are checking voltage as the inducer fan starts, you should find 0V across these two terminals.
The photo above is of a two stage gas valve showing voltage between the high fire terminal on the gas valve and a chassis ground of the furnace. It may be difficult to connect the DMM leads to both the high fire terminal and the common terminal of the gas valve as these two terminals are so close together. Clamp on probes (alligator clip style) are used in these photos to allow my hands to be free to take the photo. Normally while in the field the standard DMM probes are used which provide easier access to both these two terminals. The photo was taken before the gas valve received the low voltage signal to open. The voltage reading of 0.2V is considered a ghost voltage and are feedback signals in the electronics and can be ignored.
The photo above shows the standard DMM probes used to take measurements on the gas valve connection terminals.
The photo above is a close up of the clamp on probe on the high fire terminal of the gas valve. The center blue wire is the common wire and it has a plastic housing over the connector that helps to prevent inadvertent shorting from the common connection to either the low fire connector or the high fire connector. There are slender style DMM probes available that can be inserted into the plastic connector cover for voltage measurements.
The photo above shows one alligator clip style probe on the high fire connector and a micro style probe inserted into the terminal connector of the blue common wire on the gas valve.
The photo above shows a better view of the tip of the micro style DMM probe. There are other designs for the micro probes that are longer and more slender to allow better access to hard to reach connectors.
If micro style DMM probes are not handy then there are other steps one can take to obtain the needed measurements. The connector above can be connected to the terminal in question and then the connector on the end of the wire can be plugged onto one of the end terminals of the above connector. This will provide some bare metal surfaces to contact the standard DMM probes with. Alternatively, a short jumper can be fabricated to go between the spade terminal on the gas valve and the existing wire connector where bare metal is available.
Back to the gas valve troubleshooting, as the hot surface igniter energizes and a low voltage signal is sent from the furnace control board to the gas valve, the low voltage signal can be measured at the gas valve.
The photo above shows a 25.6V AC signal across the high fire terminal and the chassis ground. Natural gas will then pass through the gas valve and be admitted through the manifold to each of the burners.
There is a troubleshooting distinction that can be made between the voltage across the high fire terminal and common (or ground in this case) when both low voltage leads are removed from the gas valve and when both low voltage leads are connected to the gas valve. It is possible to measure full voltage (25.6V AC in this case) across these two leads when the leads are removed from the gas valve. This is telling us that the furnace control board is sending full voltage to the gas value when the gas valve is not connected as a load in the circuit. But also be sure to measure voltage between the two low voltage leads when the leads are actually connected to the gas valve terminals. In this case, with the gas valve coil in the electrical circuit, confirm full voltage is present in this case. We have seen cases where full voltage from the furnace control board is present when the two low voltage leads are not connected to the gas valve, but when the gas valve coil is connected as a load in the circuit (when the leads are connected to the gas valve) then voltage is something less than full voltage. We found voltage to be 0.7V AC when the low voltage lines were connected to the gas valve. In this case, full voltage was not being sent to the gas valve from the furnace control board. The furnace control board had failed and was sending a weak signal to the gas valve. This was an intermittent problem where the furnace worked fine sometimes but then would not work on an intermittent basis. Intermittent problems are among the more difficult to troubleshoot. In this case, replacing the failing furnace control board resolved the issue.
The above photo shows a current draw of 0.19A through the coil inside the gas valve indicating that the coil is energized. The coil is part of a solenoid that repositions a valve open admitting natural gas. The amp draw shows that in this case, the gas valve is opening. The amp draw reading is another way to establish if the gas valve is operating normally.
For a case where full voltage is being delivered to the gas valve but the gas valve is still not opening, we can measure the resistance through the gas valve coil.
The above photo shows infinite resistance (OL on the meter stands for Open Loop which means infinite resistance) across the gas valve coil. The DMM probes are connected to the high fire terminal and the common terminal. This may appear to be telling us the coil open and thus bad. This measurement would seem to be telling us that the gas valve failed. However, the case with gas valve coils is that they offer very high resistance that is above the indicated range for this particular DMM. This particular DMM measures resistance values between 0 ohms and about 4,000 ohms. The range of this instrument is not sufficient to determine the resistance of the coil in this gas valve.
The photo above shows the resistance reading of the gas valve coil as 6.97 Mega ohms, which is a very high resistance reading. The point of this is to be cautious about which instrument is used to measure resistance values. It would be bad news to condemn a gas valve as having a bad coil only to replace the gas valve and find this does not resolve the issue. An OL meter response from the DMM shown above would be an accurate indication of an open within the coil of this gas valve and would indicate a failed gas valve that would need to be replaced.
Note that a simple coil of wire forming a solenoid coil would have much lower resistance than the 6.97 Mohm shown here. It is due to the presence of other electronic devices within the circuit that contains the gas valve coil that results in such a high resistance reading.
If the equipment in question is a roof top unit instead of a residential furnace, it is good practice to ensure that any low voltage transformer high side is wired for the correct line voltage. It is common for the low voltage transformer that is shipped with the roof top unit to have options for either 230v or 208v primary voltage. Based on results of a voltage test using your voltage meter, ensure that the correct primary side terminals are connected.
For information on troubleshooting a failed low voltage transformer, refer to a blog on Troubleshooting a Failed Low Voltage Transformer.
Before hot surface ignitors were common on newer furnaces, the direct spark ignitor was used. The direct spark igniter allowed the natural gas furnace to move away from a standing pilot system. In a standing pilot furnace, the pilot flame was always lit which meant that a small amount of natural gas was used to keep this pilot flame burning and this small amount of natural gas that was always burning lowered the efficiency of the furnace. The direct spark ignition system involved a high voltage ignition system and the high voltage wire could interfere with nearby electronic equipment in newer furnaces with circuit boards that were used to control the operation of the furnace. Modern natural gas furnaces are all equipped with a hit surface ignitor that glows bright orange hot to ignite the burner flame. For more information on the older direct spark ignition system (link under development) follow this link.
An astute reader might notice that the furnace shown in the photos above has a direct spark ignition system as indicated by the photo showing 0.19A AC that shows the large orange electrical lead that connects to the SPARK terminal on the furnace Ignition Control Module.
Are you wanting to decode the Trane furnace model numbers on your residential furnace but cannot locate the furnace installation manual? The Trane Furnace Model Number Description blog may help you to decode these complicated designations.