Furnace turning off randomly

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Old 04-05-18, 10:15 PM
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Furnace turning off randomly

Hi everyone,

I am having an issue where I will notice my house feels colder than normal. When I look at the thermostat it shows I have it set at 70, but the temp is only 68, and the furnace is not running. The thermostat shows it should be running the heat.

Diagnostics:
  • Volt-metered the furnace circuit board where the thermostat wires in, it read 24v between "W" and "C" terminals.
  • Watched the furnace turn on, flames light and about a minute later they turn off. Then it tries to light again in another minute or so.
  • I cleaned the flame detector rod thing.
  • When I power down the furnace and power it back up, it seems to work for a day or so.

One interesting thing I have recently changed is wiring my humidifier into the thermostat wiring. Before, the humidifier was tied directly into the main power, thus running 24/7. My humidifier is now tied into the "C" and "W" wires, so it only runs when heat is called for. Is that pulling too much power away from the furnace and causing the furnace not to realize it should be running? Do I need a relay on that wiring?

I'm not completely sure where to go from here. Any help would be greatly appreciated!
Jake
 
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Old 04-06-18, 06:06 AM
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It could be the fact you have paralleled the humidifier contact with the t’stat contact which changed the circuit resistance seen by the furnace control board. As a start, disconnect the humidifier contact, If the problem goes away, you need to find another way to turn on the humidifier. If the problem still exists, details about the t’stat and furnace will be needed to troubleshoot further.
 
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Old 04-06-18, 07:00 AM
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I disconnected my humidifier yesterday and checked my furnace today. It is still constantly turning on and off.

Furnace model number: frigidaire FG7TC080DVC
Thermostat model: honeywell rth2510b1000

A few things I noticed this morning:
  • lights on furnace circuit board are both lit which means normal operation
  • Voltage differential is 28vac between C and W terminals. I thought 24vac was expected.
  • Voltage differential between G and C terminals is 0vac. This seemed wrong, because the fan should be running while the heat is on, right?
We've been having warm enough weather (40's) that I haven't noticed the house getting too cold. I'm assuming the furnace is turning on and off all the time to try to keep up.
 
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Old 04-06-18, 11:48 AM
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Welcome to the forums.

24vac-30vac is normal and ok.
G is only energized for A/C or in some cases.... FAN ON mode.
The control board uses a timer to control the blower in heating mode.

It sounds like your burner is shutting down on overheat and that should set a fault code. You need to check for a fault code before blower door is opened or stat is turned off or the fault will be cleared.

As a start... have you checked/changed the air filter ?
 
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Old 04-06-18, 05:46 PM
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I'm not sure if I am going crazy or if something has changed, but now the green light is flashing and the red light is solid which means the pressure switch is open. I volt metered both pressure switches and the one that comes from the collector box is not closing when the furnace is starting.

Update: I cleaned out the hoses and put them back and the pressure sensor isn't an issue anymore. Hopefully this was my issue all along, but I'm not confident. I'm thinking this because with my original issue, the flame would light and then shut-off quickly. With the pressure sensor never closing the flame wouldn't light at all. I suppose it could've been working temporarily, lighting the flame, then pressure sensor faults, and it all turns off.
 

Last edited by Vandykej; 04-06-18 at 06:19 PM. Reason: pressure sensor started working
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Old 04-07-18, 07:09 AM
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Looks like the pressure sensor fault was a red herring. Furnace still not working.

Originally Posted by PJmax View Post
G is only energized for A/C or in some cases.... FAN ON mode.
The control board uses a timer to control the blower in heating mode.
Good to know, thanks!

Originally Posted by PJmax View Post
As a start... have you checked/changed the air filter ?
I haven't changed it since January. I normally change them every 4 months since they are 20x25x5 filters. I have googled on how often to change those large filters, and I'm still not sure. I will pick one up today at Menards and see if that helps.
 
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Old 04-07-18, 11:36 AM
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After changing the filter today, the problem persists. Any other ideas before I call a furnace repair company?

I should also note that after I turn the power off to the furnace (I have a light switch next to the furnace) and turn it back on, it will run normally for a while. Then after about a day or so it goes back to the cyclical issues.

When the furnace unexpectedly turns off, I can hear something click off and then a second later the flames turn off, and finally the whole system shuts down. Does that sound like a solenoid issue where it shuts off the gas flow incorrectly?
 

Last edited by Vandykej; 04-07-18 at 12:02 PM.
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Old 04-07-18, 09:51 PM
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So now when the furnace shuts off.... there is no fault code flashing ?

If it were a gas valve problem.... usually the blower would keep on running.
 
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Old 04-08-18, 05:10 AM
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The two lights are constant, which according to the thing on the back of the door panel means it is working correctly.

I'll go listen to it shut off and see if the fan continues running afterwards.
 
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Old 04-08-18, 05:29 PM
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Here's where I'm at now. I have checked on the furnace multiple times now and sometimes it has the original cyclical issue, sometimes it has worked normally, and sometimes I find it blinking its green light saying the pressure switch is incorrectly open.

This leads me to believe that the pressure switch has to be the ultimate culprit. When the furnace is blinking the green light, I can hear the internal connections of the pressure switches continuously clicking. When I hook up a continuity checker to it, it shows the same thing: they are switching from open to closed very fast.

I wonder if sometimes it is "tricking" the control board into thinking they are closed, when really they are switching from open to closed quickly. Then the cycle begins and during the cycle it notices they are open and everything shuts down.
Other times I am guessing the control board thinks they are open from the get-go and it does not ever allow the cycle to begin.
And finally other times the pressure switches somehow work the entire cycle and nothing is wrong.

Do you think the the diaphragms are working as designed, but the suction from the tube is not consistent enough? Or do you think the pressure switches themselves are bad?

These pressure switches are connected to my collector box, which to my knowledge is where condensation drips to and leaves through a tube. If I blow into the tube, into the collector box, the air seems very restricted. And every time I do that, the furnace seems to finish a heating cycle correctly.

Any suggestions on fixing this issue with the collector box not creating enough low-pressure to keep the pressure switches closed?
 
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Old 04-08-18, 05:56 PM
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The pressure switches rarely go bad. A plugged fitting or water (even one drop) in a line can cause a problem. We use a manometer which monitors the actual vacuum to see what it is and what affects it. It can be hard randomly guessing at the problem.

I need to find the service manual.
 
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Old 04-12-18, 05:41 AM
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I decided i should finally call a professional. So I put everything back together, but now it has been working for 3 days straight without any issues. Still not sure what the issue was/is but I guess I can't call them about an issue if it's not happening...


Thanks for everyone's help so much!
Jake
 
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Old 04-12-18, 06:23 AM
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I really hate it when things fix themselves; and refuse explain what happened !
 

Last edited by Vermont; 04-12-18 at 07:58 AM.
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