Troubleshooting A Furnace Gas Valve That Is Not Opening

Faulty gas valves are a common issue when using gas-fired furnaces and heaters. Whether it’s a modern heater that uses a state-of-the-art electronic ignition valve or printed circuit board, or a traditional model that uses a standing pilot, the valve can malfunction, compromising the entire heating process. 

One of the most common malfunctions is a gas valve that refuses to open. When the valve refuses to open, it cannot release gas to the burner, meaning you can forget about heating on that fateful day. 

So, what can cause the gas valve to refuse to open, and what can you do to prevent or reverse the issues? Let’s find out. 

What is a Furnace Gas Valve?

The furnace gas valve is a component of the furnace’s fuel system. Its primary purpose is to open and close the gas line to allow or cut the gas flow to the pilot light and burner(s). 

The component itself is straightforward. However, it forms a critical part of the gas furnace, without which the furnace may not function properly. Indeed, if the gas valve malfunctions, the furnace cannot run until the issue is diagnosed and fixed. 

Furnace gas valves are only present on gas-fueled forced air furnaces and gas-powered boiler systems. However, some gas-powered hot water systems and fireplaces also have gas valves. 

How Does a Gas Valve Work?

The gas furnace valve operates by electromagnetics. It comprises two valves positioned in series, one after the other. The primary valve, known as the safety valve, supplies gas to the pilot light, while the second one, known as the main valve, permits gas to flow to the burner trays. 

A thermocouple (or thermopile) within the heater generates power that keeps the safety valve open. The thermocouple remains immersed in the pilot light flame and is designed only to open if it receives a pre-defined heat level.

If the thermocouple can’t detect any heat or is underheated, it remains closed. When the thermocouple is closed, it cuts off the gas supply to the pilot light, thus preventing gas poisoning in the home. 

Thus, the safety valve is primarily a safety feature. Its primary purpose is to prevent the buildup of gas within the home. Some people also call it the pilot light valve because it primarily controls the gas supply to the pilot light. 

Meanwhile, a 24-volt VAC transformer (or thermopile) powers the main valve, keeping it open or closed depending on the request from the circuit board. The valve allows gas to flow to the burner trays via a larger tube than the pilot light valve. 

The main valve is installed in series with all the other furnace safety controls such that if there’s a problem with the system, it interrupts the valve’s circuitry. The system will cut off power to the main valve when this happens – while keeping the pilot valve open. 

However, the valve opens and closes programmatically under regular operation in line with thermostat directions. For example, if the thermostat calls for heat, it opens to release gas to the burner trays. When the thermostat says “stop,” it closes to prevent overheating. This process helps to maintain the desired temperature in your home. 

Don’t forget that the main valve also regulates the pressure of gas flowing into the furnace. This, too, happens in line with thermostat requests. It will allow gas to pass through at a higher pressure if more heating is needed (perhaps when temperatures drop significantly) and reduce the pressure on days when it’s not too cold indoors. 

Types of Gas Valves

There are a few types of gas furnace valves. However, the most common ones are the gas chain valves and combination gas valves. 

Gas chain valves consist of a manual valve, solenoid valve, and pilot safety valve. They require the homeowner to manually turn on the handle to open or close gas flow to the furnace. 

The solenoid valve opens only when the furnace calls for heat, meaning that gas only flows through the system if all other valves within the system are open. Also of note is that the pilot safety valve only stays open if the thermopile or thermocouple located in the pilot flame heats to a predetermined temperature. 

Although it’s electromagnetically operated, you can manually override the valve to relight the pilot light. 

Meanwhile, combination gas valves, which gained popularity in the 1960s, perform all the functions of the gas chain heater in one package. It contains a valve knob/handle, a regulator, solenoid valve, thermocouple, and electric terminals. 

Unfortunately, the popularity of combination valves has been recently decimated by technology as their role is taken up by electronic ignition controls and integrated furnace controls (IFCs).

Problems and Solutions: Troubleshooting a Gas Furnace Valve that Won’t Open

One of the most common issues with gas furnace valves is the refusal to open. As we’ve seen, the furnace valve needs to open to allow gas to flow to the pilot light and burner trays. If the valve stays close, heating cannot proceed no matter how hard you try because gas flow is blocked.

The first thing to check when you first find out that your gas furnace valve won’t open is the 24-volt wiring connecting the circuit board to the gas valve terminals. If you notice that the 24-volt is present, but you can’t hear that little clicking noise the internal valve makes, you can be sure that you have a bad gas valve. 

You can double-check, though. Take the leads off the gas valve and check there. If the 24 volts is present, then it likely has a problem. Perhaps the printed circuit board or electric solenoid attached to the gas valve isn’t telling the valve to open. If not, then maybe the gas valve board is signaling it to open, but the valve is somehow stuck. 

The good news is that you can replace most internal components of the gas valve. The only problem is that you cannot do it alone. Instead, you must return the heater to the manufacturer for further troubleshooting or repair. Alternatively, find someone certified by the gas valve manufacturer to make the repairs. 

So, don’t make the mistake of attempting to repair the gas valve yourself. We’ve seen a few people take a wrench and bang it on the gas valve to get it to open up. Don’t make the same mistake. Doing so is like playing with fire as gas leaks pose a massive fire risk. 

If you attempt to fix the issue yourself and the house catches fire, even your insurance provider will not come to your rescue. The investigations would also conclude that you were the last to work on the furnace, guaranteeing the gas valve manufacturer’s innocence.  

What Can You Do?

However, since you also can’t wait and watch, you need to know a few steps you can take to keep things under control. Here’s what to consider;

Check wires to the gas valve

You want to determine whether there’s a connection between the gas valve and the control panel (circuit board). So, check if wires are connecting the two pieces. If the wires are present, check their condition. Are they in good condition? Or are they cracked and frayed?

If the wires are in good working condition, then the issue likely lies elsewhere. However, if the wires are badly damaged, it can impact the functioning of the furnace valve. Damaged wires could point to a few issues. Perhaps you have a really old furnace that needs replacement. Or, maybe something scorched the wires. 

If the heater is still in good working condition, replace the wires and continue your diagnostic. Make sure to use quality wires. 

Check the coil at the gas valve 

As we’ve seen, on the gas valve are one or two coils (solenoids) that open and close the valve to control gas flow to the pilot light and burner. If the coil fails, gas will not flow, and heating cannot happen. Fortunately, you can check whether the coils are faulty in a reasonably straightforward manner. 

Begin by checking the coil’s resistance by putting your multimeter leads on each terminal and checking your meter reading. If the reading is OL, you have a bad coil. 

You can also watch the burner assembly immediately after starting the heater. The ignitor should begin to glow, followed by a click of the gas valve coil, and then the flame ignites. If the igniter glows for several seconds and then goes out, the coils (solenoids are likely damaged). 

You may have plugged burner orifices 

Gas burner orifices are brass fittings responsible for the amount of gas that flows into the burners. They screw into (spud) or onto (hood) valves and have varying hole sizes depending on the heater and fuel type. 

Orifices are designed to remain open to facilitate gas supply. However, a gas heater that has been off all summer can have cobwebs spinning inside the tiny holes. This may seem like a minor issue. However, considering the size of the orifice hole, even a cony cobweb can impact gas flow into the burners. Tiny dirt particles can also fill the holes. 

Take a small piece of thermostat wire and gently poke inside the holes on the orifices attached to the manifold to remove the dirt. Then try to fire up the system again. 

Is there a voltage or pressure drop?

Sometimes it happens, and sometimes it doesn’t. However, it would be best to rule out the possibility of either as you narrow down on the potential issue causing your gas valve to remain closed and thus preventing heating. 

One of the main signs that you may have pressure or voltage drop across the system is a flame that comes on for few seconds then goes out. You can measure the pressure level by placing a “T” fitting in line with the hose to connect to your manometer. Pay special attention to the inlet and outlet side to see if the pressure drops on the sides of the valve. If so, you need to call an HVAC professional. 

Is it a damaged flame sensor?

The flame sensor is a tiny feature within the gas heating system that generates a small current to determine the presence of a flame during heating. It’s a short length of thin metallic rod that generates a small current, typically around 6.0 microwatts, to confirm that there’s a fire burning in the furnace.

If there’s a fire, it signals the gas valve to release gas. However, if no flame is detected, it signals the valve to stay shut to prevent possible gas poisoning. 

However, imagine a situation where the flame sensor itself is damaged! It wouldn’t detect a flame even if there’s one. Thus, the gas valve may remain closed throughout. It’s best to have a professional look at and repair the flame sensor. 

Is it a tripped rollout switch?

The furnace is a highly delicate appliance with multiple safety features to prevent gas leaks and potential fires. One of the most important safety mechanisms is the rollout switch, also known as the rollout limit switch.

The rollout switch’s primary function is to cut the gas supply if the flame goes out of control. For instance, the switch will disconnect and thus cut off the gas supply if it detects overheating inside the gas chamber. This way, it averts potential damage to the chamber and prevents the risk of fire. 

So, if the gas valve won’t open no matter how many times you try, the problem could be in the rollout switch. Maybe it blew off to save you from a fire. Or perhaps it’s rusted or too worn to function correctly. For instance, a rusted rollout switch loses its capacity to detect a flame.

Other Potential Issues

Besides the six above, the gas furnace valve can also refuse to open if the high limit switch is triggered or the blower motor speed is set too low. A blower motor speed that’s too low can cause heat accumulation within the gas chamber, creating unnecessary risk, thus causing the valve to close.

Finally, the gas valve may also refuse to open if the furnace is suffocating. This usually happens when the ductwork is blocked or compromised in another way. Arming yourself with basic troubleshooting skills can save your life in these situations.  

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