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Started By
Message
Anyone troubleshoot HVAC furnace?
Posted on 11/11/25 at 6:45 pm
Posted on 11/11/25 at 6:45 pm
I’m doing a little charity handyman project for someone down on their luck.
They have a natural gas furnace that will not light and run.
In the past, it is apparent the evaporator drain stopped up and water ran down the furnace. When I opened it, it is obvious the fan motor had been replaced recently.
The board was obviously bad. I replaced it with one of those White-Rodgers universal boards and programmed it to the furnace. Also replaced the igniter with the one that came with it. Plugged everything up and flipped the furnace back on.
Inducer fan comes on, runs for a bit, cuts off. Nothing happens.
So I looked at the board. Gave a rollout switch error. Fair enough, it was in the path of the water from the plugged drain.
So I replaced both rollout switches and went ahead and changed the inducer fan vacuum switch. Switched it back on and called for heat.
Inducer fan does the same thing. Runs a bit and shuts off. However, I’m not getting an error now. Ran the troubleshooting and it found nothing.
I shut off the safeties in the board and it fires up and runs just like it is supposed to. Flame looks good, blue, and smooth.
I turned the safeties back on and pulled the vent pipe off the inducer fan vent and ran again to see if there was an obstruction. Same thing. Replaced with old vacuum switch. Same thing.
I’m now friken lost. Board says nothing is wrong and it runs great with safeties off. So it thinks something is not right and it doesn’t cut the gas on and light the igniter. I chased the wires between the switch and the board and there is connection.
WTF now?
They have a natural gas furnace that will not light and run.
In the past, it is apparent the evaporator drain stopped up and water ran down the furnace. When I opened it, it is obvious the fan motor had been replaced recently.
The board was obviously bad. I replaced it with one of those White-Rodgers universal boards and programmed it to the furnace. Also replaced the igniter with the one that came with it. Plugged everything up and flipped the furnace back on.
Inducer fan comes on, runs for a bit, cuts off. Nothing happens.
So I looked at the board. Gave a rollout switch error. Fair enough, it was in the path of the water from the plugged drain.
So I replaced both rollout switches and went ahead and changed the inducer fan vacuum switch. Switched it back on and called for heat.
Inducer fan does the same thing. Runs a bit and shuts off. However, I’m not getting an error now. Ran the troubleshooting and it found nothing.
I shut off the safeties in the board and it fires up and runs just like it is supposed to. Flame looks good, blue, and smooth.
I turned the safeties back on and pulled the vent pipe off the inducer fan vent and ran again to see if there was an obstruction. Same thing. Replaced with old vacuum switch. Same thing.
I’m now friken lost. Board says nothing is wrong and it runs great with safeties off. So it thinks something is not right and it doesn’t cut the gas on and light the igniter. I chased the wires between the switch and the board and there is connection.
WTF now?
7
Posted on 11/11/25 at 7:32 pm to CleverUserName
Bad board? It happens.
Posted on 11/11/25 at 7:35 pm to CleverUserName
Can you tell if the fan is moving enough air to trip the vacuum switch? I know you said you checked for obstruction, but could the fan be blowing poorly? Beyond that, I don't know what other safeties you have. Maybe it's sensing that the inducer fan is pulling different amps than it expects, I don't know.
Posted on 11/11/25 at 8:04 pm to calcotron
Yea. I checked the power coming across the switch when the inducer comes on. It’s sending power back to the board when the fan comes on.
I didn’t know it would check for this. That could be something. What is the amperage that would be considered too much?
The inducer fan does run smoothly. No slow starting, squeaking, or other noises. But I know that’s not a good check for its particular condition.
quote:
Maybe it's sensing that the inducer fan is pulling different amps than it expects, I don't know.
I didn’t know it would check for this. That could be something. What is the amperage that would be considered too much?
The inducer fan does run smoothly. No slow starting, squeaking, or other noises. But I know that’s not a good check for its particular condition.
This post was edited on 11/11/25 at 8:11 pm
Posted on 11/11/25 at 8:07 pm to VABCHTIGER
quote:
Bad board? It happens.
I hope not. I’d hate to drop the cash for another one after I’m footing the bill for all of this anyway.
Posted on 11/11/25 at 8:53 pm to CleverUserName
quote:
Inducer fan comes on, runs for a bit, cuts off. Nothing happens.
There is a small hole that gets clogged. It can be cleaned out with a paperclip. I can take a pic of it tomorrow if you need me to.
This post was edited on 11/11/25 at 8:54 pm
Posted on 11/11/25 at 8:55 pm to BigBinBR
If you would, I’d be grateful.
Thanks!
Thanks!
Posted on 11/11/25 at 11:02 pm to CleverUserName
Make sure all the burners are clean and not rusted. From when the first burner lights the last burner has about two and a half seconds to ignite or a sensor sends a shutdown signal. Sometimes if the jets are corroded with rust not enough gas makes it through for the flame to jump to the 2nd, 3rd, or 4th burner.
Posted on 11/12/25 at 6:18 am to CleverUserName
Check the hose connected to the pressure switch. I had a similar problem with my furnace and it was caused by a hole in the hose. The hose was touching the outside of the fan motor so when it ran the spinning part was rubbing on the hose and created a small hole.
Posted on 11/12/25 at 9:11 am to CleverUserName
Mine would do that. The flame sensor was dirty and not picking up that it was getting hot. The machine does not want to just spew gas so it tells itself to shut down. I cleaned it and it works.
Posted on 11/12/25 at 10:03 am to TigerFanatic99
quote:
Make sure all the burners are clean and not rusted. From when the first burner lights the last burner has about two and a half seconds to ignite or a sensor sends a shutdown signal. Sometimes if the jets are corroded with rust not enough gas makes it through for the flame to jump to the 2nd, 3rd, or 4th burner.
When I disabled the safeties and ran it. The burners lit pretty quickly and the flames looked good. I’ve looked over the burners and they look good. I let it run for 10 minutes or so and nothing looked out of the ordinary.
I don’t think it’s related to the burners or the valve. The board will not get as far as to operate the igniter. The igniter only works when I disable the safeties or run it through the troubleshooting process where it runs the tests.
However, here is something I thought of. They said that someone came and tried to replace the board with a used one and it didn’t work. The board had the same plugs as the furnace and looked right. Everything hooked up as it looked like it should.
What I did do was program the board to match that board. Is it possible it could be programmed wrong? It doesn’t give any errors, works everything on the troubleshooting function and lights and works minus the safeties.
This post was edited on 11/12/25 at 10:14 am
Posted on 11/12/25 at 11:30 am to CleverUserName
Check the polarity on your 120volt feed. Do that at the plug or where you wired the 120 to the circuit board.
Simply reverse the black and white.
Simply reverse the black and white.
Posted on 11/12/25 at 12:57 pm to CleverUserName
Check that the exhaust pipe that goes up to the roof isn’t clogged. I had to pay for a service visit this week because my pipe is a popular hangout for birds who drop their seeds down the pipe. System won’t run bc it detects the exhaust can’t vent.
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