Help diagnosing gas furnace

Hey guys,

My furnace is acting up a bit.  Intermittently when the thermostat clicks on the furnace doesn't start up. If I toggle the thermostat from "Heat" to "Off" then back to "Heat" again it will turn on.

The furnace is a Rheem model #RGDG-07EAUEA.

I've confirmed that the thermostat is sending 24v to the W terminal when it wants heat. So the thermostat is good. What can I check next?

It can act up several times in a row, then be fine for several days. I'm not 100% sure since it's so intermittent and I can't pay attention to it all the time, but I don't think I've heard the inducer motor spin up for a bit then shut off. It either works normally, or nothing happens. I wish I had more information but I don't notice anything it wrong until the house gets cold and from there just resetting the thermostat fixes it.

Since it's so intermittent I'm hesitant to pay for a service tech.
 

Sounds a lot like a bad gas valve to me.

When you are talking about the inducer motor, do you mean the vent motor? Or exhaust fan motor? If that is spinning up and then combustion doesn't start then the gas valve is likely not allowing gas flow, or the ignitor is not working and the gas valve shuts off.

If the exhaust fan doesn't start then the gas valve will not allow flow to prevent CO.

So you need to figure out if the vent motor is starting up normally first and go from there. If it doesn't run the whole process stops.

Thermocouple bad?

NGTD said:

Sounds a lot like a bad gas valve to me.

When you are talking about the inducer motor, do you mean the vent motor? Or exhaust fan motor? If that is spinning up and then combustion doesn't start then the gas valve is likely not allowing gas flow, or the ignitor is not working and the gas valve shuts off.

If the exhaust fan doesn't start then the gas valve will not allow flow to prevent CO.

So you need to figure out if the vent motor is starting up normally first and go from there. If it doesn't run the whole process stops.

^ This

Exhaust fan or damper fail will prohibit gas valve opening.

Ignitors can get crusty from sitting off season. 

Observe step by step

Bad ignitor? Symptoms sound similar to my sister's HVAC issues and replacing the ignitor with the proper one fixed it.

NGTD said:

Sounds a lot like a bad gas valve to me.

When you are talking about the inducer motor, do you mean the vent motor? Or exhaust fan motor? If that is spinning up and then combustion doesn't start then the gas valve is likely not allowing gas flow, or the ignitor is not working and the gas valve shuts off.

If the exhaust fan doesn't start then the gas valve will not allow flow to prevent CO.

So you need to figure out if the vent motor is starting up normally first and go from there. If it doesn't run the whole process stops.

All I know about furnaces I mainly learned on youtube earlier this evening. What I'm calling the inducer motor is the fan that runs for a bit before the burners light on. I don't think it's spinning up.

Do ignitors fail intermittently? Seems to me they would either work or not work at all if cracked. It glows cherry red whenever I've tried to observe operation.

I guess at this point I have to wait and see if I can get it to fail multiple times in a row to watch where it hangs up.

In reply to Run_Away :

I’ve seen an ignitor get crusty & not create a consistently strong enough spark, but as mentioned above if the fan isn’t running start with it. 

Cascade of events goes something like this.  Tstat calls for heat.  Relay engages inside control panel sending 120V to inducer fan to make sure their is no unburnt gas inside combustion chamber.  Vacuum switch senses vacuum in chamber, engages another relay to send 120v power to igniter.  Glow plug warms up for several seconds and then gas valve gets 24V and opens.  Gas hopefully ignites.  Sensor sees flame and sends power to another relay that keeps gas valve open.  If it doesn't see a flame, then valve closes and it tries again.  Tries 3 times and then goes into lockout.  Every high efficiency heating system has some variation on this cascade.  Find out where it stops and you have found your problem.  Sounds like a flame sensor or igniter is getting crusty.  You should be able to see igniter getting red or sparking.  You should be able to hear a click when gas valve opens followed immediately by fire.  Block flue can also cause a shut down.

NGTD said:

Sounds a lot like a bad gas valve to me.

When you are talking about the inducer motor, do you mean the vent motor? Or exhaust fan motor? If that is spinning up and then combustion doesn't start then the gas valve is likely not allowing gas flow, or the ignitor is not working and the gas valve shuts off.

Yup, that is exactly what I just went through with mine.

Take the main cover off the furnace and call for heat.  In my case, the exhaust motor would spin up, and after about 30 seconds, the ignitor would glow for a minute, fade, wait, glow again.  It would cycle like this 3-4 times, and then the exhaust motor would spool down.  This was all easily visible with the main panel off.

If I cycled the main power to the furnace, it would (usually, but not always) light when I demanded heat.  There is also a switch directly on the gas valve which had the same effect. Later, it also lit if I tapped the solenoid body lightly with a wrench.

HVAC guy came out and told me that cycling the power "cleared the codes" so he couldn't read any errors, but that the behaviour sounded like a bad gas solenoid.  He also quoted me $980 to replace it.

I googled the part number and bought a name-brand replacement from an online parts house for $120 shipped.  $3 for a can of pipe dope, 2 pipe wrenches, and 30 minutes later, it's working perfectly and not leaking.  If you can change brake pads you can do this, as long as you are careful, and save a boatload of money.  I got solid advice in my thread linked above.

But as noted above the first thing to check is whether the exhaust fan and ignitor are working.

Okay so I've spent some more time with it since it acted up today. What it's doing is short cycling.

I took a 14 minute video, and took recordings of the time between it cycling. Inducer spins up, glow plug lights, burners turn on, and then the main fan turns on about 15 seconds later. Every time it shuts off the burners, resets, and then turns them back on, the main fan stays running.

First time around the burners are on for 2:46. It takes 1:08 for the system to reset and light off the burners again. Second time around it ran for 1:21. 1:08 for things to reset. 3rd time it ran for 4:21 before kicking off. 1:08 to reset. Then it ran for another minute or so before the thermostat reached it's set temp.

The filter is maybe 3-4 weeks old. I took apart the exhaust vent about an hour ago and looked all the way up my chimney and it's all clear, I can see that screws that hold the cap on. There was a bunch of silt built up at the bottom of chimney and along the horizontal parts so I vacuumed that all up.

So I'm thinking either the flame sensor or maybe a bad pressure switch somewhere. If it was overheating I think the time between cycles would get shorter until it go into "lockout".

I've labeled everything I can see inside. Can anyone help me identify the sensor/switch below? Between the Inducer motor and the pressure switch.

In reply to Run_Away :

most likely limit switch being two wire

best guess for 0.02

In reply to Run_Away :

I’ve had a bad flame sensor cause that exact problem. 

Pete Gossett said:

In reply to Run_Away :

I’ve had a bad flame sensor cause that exact problem. 

Me too. I didn’t even have to replace the sensor - cleaning it took care of it. How I cleaned it I don’t recall. 

In reply to dculberson :

Pretty sure I used sandpaper on mine, but I ended up replacing it anyway a few weeks later. 

Re: Can anyone help me identify the sensor/switch below? Between the Inducer motor and the pressure switch.

That is the high temperature limit. It measures the temp of the heat exchanger. The temp set point is likely printed on it. It can cause the symptoms that you are having. This could be caused by either the limit going bad or a lack of airflow and the heat exchanger is actually reaching the high temp limit.

Here is the startup sequence.

Activate any circuit boards and self-test
Start inducer motor (the little blower for the actual fire and exhaust) which is the centrifugal black cage in your photo
This activates a pressure/flow switch to verify blower operation (red hose to the silver puck top left)
Then ignitor and gas valve.  If it's a spark ignitor, gas and ignition happen about the same time.  If it's a glow ignitor, it will glow first, then gas.
After ignition, a thermocouple verifies that it's burning
Then either the thermocouple or a separate sensor kicks on the blower fan after a certain temp is reached.

So the nice diagnostic part there is that wherever it stops progressing is usually the culprit.

My guess on the ???? switch is a vacuum sensor.  They test the differential between the inlet and exhaust side in case you get an obstruction.

I did a quick google search for that model.  Seems like a relatively common complaint is the circuit board goes bad.  I just had a similar problem last week with mine and it turned out to be the inducer motor.  When I watched it, it was just humming and trying to spin.  When I spun it manually, it was squeaky and dragging.  $50 and 15 minutes fixed mine

Easy tests:  pull the hose off the blower motor and test continuity across the contacts, then blow in the hose and see if it changes.  Is the inducer motor spinning?  If so, verify switch operation with the motor running and not.  Its possible that your mouth generates enough pressure to trip the switch, but if its crusty (the switch, not your mouth) it may not activate with the small pressure from the fan.  If the inducer is not functioning, simple multi-meter tests can tell you if its getting juice.

Is there a flashing LED?  If so, try this:

I’ll echo the “bad flame sensor (aka thermocouple)” diagnosis.   Mine did the same thing, same cycle of three attempts, everything.  Also tried cleaning it which worked until amazon brought me a replacement.  No problems since.

13hr day at work and a large dump of snow to clear so I only cleaned off the flame sensor last night. Watched it after installing and it ran for 3:30 without short cycling before the thermostat shut it down.

Heat came on normally this morning. Another long work day today, hoping to spend time with it Wednesday evening.

No LED to blink codes unfortunately.

Okay so I cranked up the heat on the thermostat, ran downstairs, pulled the cover off, and started a timer on my phone.

25 minutes of non-stop heat!

So I'm going to call this fixed. Thank you so much to everyone who posted with their help!

I'm probably going to buy a new flame sensor and change it out just to be safe. $18 CAD on Amazon it looks like.

An extra thanks to you for starting the thread, and those who contributed. I read this out of sheer boredom and accidentally diagnosed an intermittent heat issue I have been having for years (gas solenoid ftw).

In reply to Nugi :

GRM always delivers.  Seriously, it doesn't matter what topic - I start here first.

In reply to Nugi :

the GRM community has experience with pretty much everything.

In reply to Nugi :

We once had a member looking for cadavers/organs/body parts - something like that, anyway - for some medical/science purpose & another member was able to help out. It was pretty much at that point when I decided that if a question is answerable, or problem is solvable, this is the place to find it.