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My gas furnace won't turn on when it's set to heat. My girlfriend and I both noticed that it was a little cold today. I went to flip the heat up a notch after dinner, and that's when I noticed that it's 63 in the house when the thermostat is set to 68. I've been doing some troubleshooting/watching YouTube videos and googling. I stumbled upon this board, so I thought I'd ask to see if some kind soul can help us out. Here's what I did so far:

  1. I flipped the breaker switches in the electrical panel for both the A/C and the furnace
  2. I switched the batteries to the thermostat (it powers without them, but i figured worth a shot).
  3. I flipped the fan from "Auto" to "On" on the thermostat. Interestingly, the fans in the house will kick on if I have the theromstat set to "Cool", but not if it's set to "Heat".
  4. I took the cover off the furnace (where I change the furnace filter). I am noticing the panel has a light that is blinking green 4 times when I hold down the sensor that tells the furnace if the cover is on or not. I'm not sure if the 4 times means something or not, but worth a shot.

That's as far as I've gotten. Any ideas?

We're trying to avoid a cold night...

Photo of the label inside:

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  • What make and model is your furnace?
    – ThreePhaseEel
    Commented Dec 12, 2020 at 5:23
  • Not sure how I tell that. It says "Ruud" on the cover.
    – noel
    Commented Dec 12, 2020 at 5:53
  • If you take the access cover off, there should be a label somwhere inside the furnace blower compartment with the model number and other pertinent information on it -- can you get us a photo of that label?
    – ThreePhaseEel
    Commented Dec 12, 2020 at 5:55
  • Thank you for the help. I added a picture. Not sure if it's clear enough. I can try to get a better one.
    – noel
    Commented Dec 12, 2020 at 6:02
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    Probably going to have to snuggle to keep warm for the night. In one photo there's an "Integrated Furnace Control Troubleshooting Guide" with a flow chart of steps you can take to determine what's wrong. Try following those and report what you find. (Also, the photo is blurry and I can't read what it says.) If you need to have the door off the blower compartment for any tests note that there's a safety switch. Furnace power is disconnected while the blower door is off. You may have to manually hold the switch to test, taking care to keep clothing, long hair, etc from being sucked in.
    – Greg Hill
    Commented Dec 12, 2020 at 6:46

2 Answers 2

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Sounds like the furnace may have failed the first diagnostic test for the draft inducer. If the system doesn't detect a positive pressure to the pressure switch after the inducer motor starts, it won't let the burner igniter operate and throws a 4 flash code. Sometimes the tech just needs to remove the vacuum hose from it and clean out any debris that may be plugging up the hole or the hose and put the hose back on and it will run.

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  • This was exactly what the repair guy did. He pulled the hose off, he blew into it to remove anything that may have been clogging it, and then when he put it back together, it magically started working again.
    – noel
    Commented Dec 17, 2020 at 23:53
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Rudd search online 4 blinks , pressure switch stuck closed will not open. You might try tapping on the switch and cycling the power (the switch that the cover holds on, push in or pull out will usually do it for a power cycle). Pull out is a normal maintenance override.

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    This more or less was the answer. I ended up calling a repair guy. He pulled hoses off, cleaned the crap out of the inside of the furnace, and whatever was stuck got reset. Not sure that it was anything that me as the homeowner could have fixed on my own. But it was a pretty easy fix for the professional.
    – noel
    Commented Dec 12, 2020 at 21:10

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