How to Diagnose a Bad Furnace Gas Valve (10 Things To Check in 2021)

Hey guys, today we’re going to talk about
Troubleshooting a Furnace Gas Valve. I wanted to expand on our recent gas furnace
troubleshooting series by going into each part of the sequence of operation of a furnace. In this video I’ll fill you in on what the
gas valve does and why it’s important. And towards the end of the video, I’m going
to give you 10 things to check when you’re troubleshooting a furnace gas valve. That's coming up here on Fox Family Heating
& Air.

If this is your first time watching our channel,
please click subscribe down here on the bottom right. And if you click that little bell next to
it, you'll be notified of all our videos as they come out. The furnace sequence of events
First, as a technician, you have to know the sequence of events that occurs for a gas furnace
to start up properly. It's really straightforward, and you should
have this memorized before you can even consider being qualified to troubleshoot. 1. Power to the furnace control board
2. Thermostat signals the call for heat
3. Inducer motor kicks on
4. Pressure switch proves the inducer operates
correctly 5. Ignitor activates
6. Gas valve energizes
7. Flame pours across burners
8. Flame sensor proves all burners are lit
9. The blower forces air through the ducts
First the inducer motor starts 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. Next, a safety device called a pressure switch
activates when the diaphragm inside of it recognizes the suction or purging action of
the inducer motor.

When the “all clear” signal is given to
the control board, high voltage is sent to the ignitor – be it a hot surface ignitor
or a spark ignitor. The hot or sparking ignitor stands in the
way of the gas that is getting ready to pour over it. This is where the gas valve comes in to play. Modern gas valves typically have a printed
circuit board in them that receive a 24-volt signal to activate the valve inside of it. Remember the video I did on printed circuit
boards? If not, I’ll attach it to this video, so
you can brush up on what they are and the things that can go wrong with them.

Now this sequence right here is going to happen
in three stages – and even if one step of this doesn’t perform, each part is still
going to do their thing sequentially once the signal is given by the board. So, 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. (And we have a video about that, which will
explain much more about ignitors) Next gas valve opens. The gas that’s coming from the utility company
or the propane tank in the back yard is free to flow on to the ignitor.

That gas valve is what’s regulating the
flow of the gas. The flame sensor senses whether the flame
is correctly burning. 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 on to the control
board. You see, only a certain amount of gas can
be allowed to go on to through the manifold and on to the burners.

The manufacture of the furnace determines
what that will be. It is pretty standard though. About 3.5” water columns (wc). The natural gas pressure coming from the street
is somewhere around 7”-10” wc., but the gas valve itself specifically allows that
3.5” wc onto the burners. There are some situations and equipment where
I’ve been told to bring the outlet pressure down to 3.25” wc. But I only did it on the advice from the technical
support rep from that equipment. Specifically, it was Ruud equipment. The rollouts were getting too hot, because
the hood that covers the flame would trap the heat and make the safety open. Modifying the hood and adjusting the gas pressures
what was recommended to us, and it seemed to fix the problem. 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
– Emerson, White-Rodgers, Honeywell, and other makers of valves will usually have it
pre-set to that 3.5” wc, some installers forget to do this.

We can’t depend 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. Got 24 volts? Then something downstream of that 24 volts
is not working. What’s the next thing that’s supposed
to be working? The printed circuit board or electric solenoid
attached to the gas valve isn’t telling the valve to open OR that 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 these make repairs. Some people will literally 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. If we try to fix these ourselves and something
goes wrong with gas valve, and it somehow caught the house on fire, 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 customers claim. I know that sounds a little drastic, but it
could happen, and why put yourself in that situation anyways. I see people try to fix control boards, and
ignitors and such, but with such a sensitive instrument, we shouldn’t try to fix gas
valves ourselves. 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 the cracked or frayed? That could mean a couple things.

pexels photo 3964341

You have a REALY old furnace, or something
could have scorched the wires. Things like that. Replace the wire and continue your diagnostic. 2. 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, you have a bad coil. There are more involved things here but let’s
keep this straightforward. 3. The gas coming into the valve should be at
utility line standards. In my neck of the world, it’s around 7”-10”
wc for natural gas. There’s a port on the inlet side to check
it. 4. The burner orifices could be plugged. A furnace that has sat for the summer without
being run can be the victim of a spider spinning a web inside the burner orifices. Now, that’s a tiny spider, I know, but I
promise, 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.

5. 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. That’s something to check for sure. And you can do that 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. 6. 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. 7. 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 that drifts off to the left or right. This will set the switch off. That doesn’t mean remove the switch, that
means fix 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 that
solves it. Then place the burner correctly into the channel. 8. 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 I would check is to see is
if the evaporator coil is dirty. I have a great video that shows what a dirty
evap coil looks like and what it takes to clean it. 9. 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.

10. Check the ductwork too. These last three have all dealt with airflow. If the return duct is crushed, then we’ll
have low airflow again. 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. What else should folks check? Leave me a comment down below to share your
expertise. When you’re installing the new gas valve,
there are few things to keep in mind. It’s a like for like changeout, but 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. I’d really recommend not over-tightening
the gas valve to the manifold. You could bend the manifold, but also remember,
someone might have to get that thing off someday and you’d be creating a tough situation
for that tech if they have to come out and service it in a few months. Some guys get a little over the top and really
crank down on it. 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 flow through it. If it’s a natural gas set-up, the spring
that comes 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. I can’t say it enough. 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.

So, just to 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 that the flame exists.

If this all goes well, you have heat blowing
into the house about a minute later when the blower kicks on. If this is your first time watching our channel,
please click subscribe down here on the bottom right. And if you click that little bell next to
it, you'll be notified of all our videos as they come out. Thanks so much for watching, and we'll see
you on the next video..

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