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Troubleshooting the Furnace Gas Valve | Red Deer Heating & AC

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Troubleshooting the Furnace Gas Valve

How to Diagnose a Bad Furnace Gas Valve

GasValve

In this article, you will know what the Gas valve does and why it’s important. There are 10 things you can check when you’re troubleshooting a furnace gas valve.

The Furnace Sequence of Events

First, you have to know the sequence of events happening for a gas furnace to start up properly. It’s straightforward and you should have this memorized before you can even consider being qualified to troubleshoot.

  • Power to the furnace control board.
  • The thermostat signals the call for heat.
  • The inducer motor kicks on.
  • The pressure switch proves the inducer operates correctly.
  • Ignitor activates.
  • The gas valve energizes.
  • Flame pours across burners.
  • The flame sensor proves all burners are lit.
  • The blower forces air through the ducts.

When a furnace begins a new cycle, the inducer motor is the first thing you should see kick on. 120 volts is applied through the wires coming from the control board. This starts the inducer motor for up to 60 Seconds before anything else even happens.

Then a safety device called a pressure switch activates when the diaphragm inside of it recognizes the suction or purging action of the inducer motor.

 

Now, this sequence is going to happen in three stages and even if one step of this doesn’t perform, each part is still going to do its thing sequentially once the signal is given by the board.

After the board senses the pressure switch and inducer motor are working;

  1. 120 volts is given to the ignitor (on some Package units it’s 240 volts)
  2. 24 volts is given to the gas valve.
  3. The flame sensor starts detecting if there is a flame or not.

The ignitor is supposed to come on for a set amount of time 30 to 60 seconds

Then the gas valve opens. The gas coming from the utility company or the propane tank in the backyard is free to flow onto the ignitor.

 

At the opposite end of the burner assembly, the flame sensor also stands in the way of the flame. The rod which should be cleaned annually, will heat up and send a millivolt signal down to the ceramic base of it and onto the control board.

 

Only a certain amount of gas can be allowed to go on through the manifold and onto the burners. The manufacture of the furnace determines what it will be. It is pretty standard, though, with about 3.5” water columns (wc).

 

The natural gas pressure coming is somewhere around 7”-10” wc, but the gas valve itself specifically allows the 3.5” wc onto the burners. Some furnaces are different from others so please check your furnace installation and service guide for the specifics of your system.

 

This is something you don’t want to be wrong on. The gas valve can be adjusted and usually, the installer of the equipment will dial in the outlet pressures on start-up because the manufacturer of the gas valve will usually have it pre-set to 3.5” wc. Some installers forget to do this.

 

We can’t depend and assume the valve is set up perfectly every time. That’s why you can have issues with your furnace-related to your gas valve because it was never set up right by the installer the first time it was used.

 

Troubleshooting a Furnace Gas Valve

If 24 volts is coming from the board to the gas valve terminals, and you don’t hear that little clicking noise the internal valve makes, you could have a bad Gas Valve. To double-check, take the leads off to the Gas valve and check there.

GasValve2

If you got 24 volts, then something downstream of the 24 volts is not working. The printed circuit board or electric solenoid attached to the gas valve isn’t telling the valve to open or the Gas valve board is telling it to open, but the valve is stuck somehow.

 

If something is wrong with the internal components of the gas valve, it should be replaced. The gas valve cannot be repaired in the field. Only the gas valve manufacturer or someone certified by the manufacturer of the gas valve can make repairs.

 

Some people will take a wrench and bang on the Gas valve to get it to open up. This is extremely dangerous. Gas is nothing to play around with, but if you do decide to try this and it does kick on – please replace the gas valve now, rather than later.

GasValve3

If we try to fix these ourselves and something goes wrong with the gas valve, it might somehow put the property on fire. Also, the investigation could come back to the furnace if they wanted to know who worked on it last and what was done to it.

The manufacturer of the gas valve could claim innocence and the homeowner’s insurance could deny the customer’s claim.

 

It sounds a little drastic, but it could happen and why put yourself in that situation anyway. We see people trying to fix control boards and ignitors and such, but with such a sensitive instrument, we shouldn’t try to fix gas valves ourselves if we’re not yet experienced.

 

10 Things to Check

Here are 10 things we can check when we think we have a bad gas valve before condemning it.

  1. Check the wires to the gas valve.

Are they cracked or frayed?  It could mean a couple of things. You might have a really old furnace, or something could have scorched the wires. Replace the wire and continue your diagnostic.

 

  1. Check the coil at the gas valve.

If you check the resistance of the coil by putting your two-meter leads on each terminal and it reads OL, then you have a bad coil. There are more involved things here but let’s keep this straightforward.

 

  1. The gas coming into the valve should be at utility line standards.

It’s around 7”-10” wc for natural gas.  There’s a port on the inlet side to check it.

 

  1. The burner orifices could be plugged.

A furnace which sat for the summer without being run can be the victim of a spider spinning a web inside the burner orifices. It is just a tiny spider but it happens. Take a small piece of thermostat wire and gently poke inside the holes of the orifices attached to the manifold and try to fire up the system again.

Orifices

  1. The flame might be coming on for a few seconds but then shuts off.

Is there a dropout of voltage or gas pressure to the gas valve?  It is something to check for sure.  and you can do it by putting a “T” fitting in line with the hose to hook your manometer up to.  Check the inlet and the outlet side to see if the pressure is dropping on either side of the valve.

 

  1. Check the flame sensor.

Another reason the flame could drop out after only a few seconds of burning is the flame sensor.  If the sensor doesn’t detect the flame, the gas valve will be told to shut down by the control board.

flamesensor

  1. If the flame does anything but shoot directly into the hollow metal heat exchanger, a safety can trip.

One safety is the rollout switch.  Sometimes you’ll get a little part of the flame which drifts off to the left or right.  This will set the switch off.  It doesn’t mean removing the switch, it means fixing the problem.

 

Clean the end of the burner assembly nearest the heat exchanger. Rust will sometimes build up on the crossover channels.  Use a wire brush to clean and see if it solves the problem. Then place the burner correctly into the channel.

burner1

  1. The high limit switch.

The other safety that can cause the system to cut the gas off to the valve is the high limit switch.  If the furnace runs for a few minutes then shuts off, something could be causing the inside of the furnace to get too hot.

The first thing to check is to see is if the evaporator coil is dirty.

 

  1. The other reason the high limit could open is the blower motor speed could be set too low.

Check your installation guide as a reference for where the settings should be.

 

  1. Check the ductwork.

These last three have all dealt with airflow.  If the return duct is crushed, then we’ll have low airflow.  Visually check the return duct and feel around it if it looks questionable.  If the duct is not perfectly round, then this could be the problem. The furnace is suffocating.

 

Installing the Gas Valve

When you’re installing the new gas valve there are a few things to keep in mind: Gas leaks are a serious issue so make sure to use some pipe dope or pipe tape to seal the fitting. Also, don’t bend the manifold when you’re trying to remove the gas valve or put the new one back on.

 

Use two wrenches to get a proper hold on the manifold and the gas valve. We recommend not over-tightening the gas valve to the manifold. You could bend the manifold and also remember someone might have to get it off someday.

You’d be creating a tough situation for the tech if they have to come out and service it in a few months.

 

Some people get a little over the top and crank down on it but it is not necessary. Check for gas leaks with an electronic gas sniffer or soap bubbles. This will assure you the fittings are snug and leak-free.

 

And don’t forget to check the outlet side when the gas valve is on. It doesn’t help when the valve is off because no gas is flowing through. If it’s a natural gas setup, the spring coming inside the valve will already be the right one.

 

If you’re using LP gas, you’ll need to make sure you put the right spring in it. It’ll come in the box, Check the manifold orifices to ensure they are the right ones for LP too. And put the sticker on the gas valve that says LP:

This will help HVAC technicians in the future when they have to service the furnace.

 

And lastly, check the gas pressure on the new valve after you’ve replaced it. It’s simple to do with the right tools. Don’t just change the valve and not check the pressures. On two-stage units, there’s also a setting for low fire and that also needs to be checked.

 

If the gas pressure is too low, your furnace’s efficiency will go down. More condensation than normal will build up because the air in the air-fuel mixture will be too high. This will cause corrosion, creating a possible heat exchanger replacement in the future.

 

High gas pressure can be just as bad for your furnace because it greatly increases the risk of the furnace overheating. When this happens, high limit switches will start opening causing intermittent operation.

 

It can also crack your heat exchanger since it’s only rated to handle a certain amount of heat. And cracked heat exchangers can introduce the spent gasses inside the heat exchanger to be carried along with the heat blowing into the house.

 

Recap

When a furnace begins a new cycle, the inducer motor is the first thing you should see kick on. A safety device called a pressure switch activates when the diaphragm inside of it recognizes the suction or purging action of the inducer motor.

 

Next, the three parts of the ignition sequence begin. The ignitor kicks on, the gas valve opens and the flame sensor senses the flame exists. If this all goes well, you have heat blowing into the house about a minute later.

 

If this is your first time doing things on your furnace and you’re not sure, please call for the help of experts. Call us at Red Deer Heating and AC and we’ll fix any of your HVAC problems.