Why Won’t the Furnace Run?

It’s a bad sign if a furnace is starting and stopping too often (short cycling). It’s bad if it’s just running all the time. What could be a worse sign? Well, what’s it mean if the furnace won’t start in the first place? At least if your furnace short-cycles, you’ve got heat and you know it’s not a catastrophic failure. Things get scary when it won’t cycle at all.


The Easy Suspects

Your furnace isn’t running. Don’t panic yet. If we were to round up every contractor in our books and ask about simple break downs, the stories would never end. Simple, cheap, and even silly things can bring on the chill. We think the most common ones are:

  1. Ran Out of Fuel
    1. Empty tank
    2. Clogged fuel line
    3. Gas main shut off
  2. Electrical Shut Off
    1. Someone bumped a shut off switch
    2. Breaker tripped during storm
    3. Failed switch or contactor stuck in the off position
  3. Thermostat Wire Cut
    1. Wires chewed by a mouse
    2. Wires damaged with nails or screws in the wall
  4. Chimney blocked
    1. With debris
    2. With a birds’ nest
  5. Minor Hardware Failure
    1. Bad Pilot light
    2. Bad ignitor
    3. Bad flame sensor


When we say these are common events, we really mean it. I’ve experienced at least three of these. My home furnace is a nice little oil burner, and we’ve run it dry during the winter. We’ve discovered our fuel pipe doesn’t dip to the bottom of the tank. We’ve bumped the emergency shut off switch (located right by our front door, if a fire breaks out we can kill the furnace without approaching the flames). We’ve even had a flame sensor fail.

In all of these cases, it took at most four days to get everything up and running again. We were able to buy a few gallons of heating oil locally when we ran low. We noticed the switch and flipped it back to the on position. We contacted our furnace manufacturer and got a free replacement sensor under warranty.

In large part, these failures can be done by a home or business owner, provided they’re well versed and understand the work at hand. These repairs are like changing the oil in your car. If you know what you’re doing, it’s easy.


The Contractor Required Suspects

Don’t panic yet, but call your contractor or maintenance guy. If none of these things have stopped your furnace, we’re not entirely comfortable telling you to break out the tools. The next major suspects are going to require careful testing, voltmeters, and could start a fire if you’re not careful. So, what are these deeper issues?

  1. Failed Safety Switch
    1. High Limit
    2. Roll Out
    3. Low Water Cut Off
  2. Failed Controller Board
  3. Failed Burner
  4. Failed Blower
  5. Failed Boiler (the furnace casing can fail)

These things can be difficult to test and confirm as the issue. Removing a burner can involve dismantling large portions of the furnace. If the boiler/tank itself has failed, there’s no easy fix for that.

In these situations, you’re looking at probably a few hours of the contractor’s time, maybe a cheap $200 part, or in the worst case a pricey new system altogether. It’s hard to say. You’ll really have to ring up the contractor and see what they tell you.

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