3 Things to Check When Your Furnace Stops Working

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Modern high-efficiency furnaces are complex pieces of machinery, and most major repairs are best left to a professional furnace technician. But before calling the furnace company and incurring a substantial service call charge, it makes sense to check out three very common problems that are often easy to remedy yourself: power supply interruptions, thermostat malfunctions, and problems with the condensate drain system.

Is the Power Interrupted?

Just like any other appliance in your home, a furnace that suddenly stops running may be experiencing an electrical power interruption. Sometimes this is as simple as the ON-OFF toggle switch on the furnace being accidentally shut off—this can happen, for example, if you have kids who play around in the utility room. The furnace's switch looks just like an ordinary light fixture switch, sometimes located on the furnace cabinet itself, sometimes on the wall near the furnace. Make sure the switch is in the ON position before calling the furnace repair service.

Another common power interruption occurs if the circuit breaker controlling the furnace has tripped and shut off. Locate your main electric service panel and open the door. Find the circuit breaker that controls the furnace, and check to see if it has tripped to the OFF position.

To reset this circuit breaker, first turn the thermostat to the lowest setting to prevent the furnace from starting up at the instant you reset the breaker. Then, flip the furnace's circuit breaker back to the ON position. Set the thermostat to a high temperature setting, and observe the furnaces's behavior. In many cases, simply resetting the breaker will fix the problem.

However, you should consider the reasons why the furnace circuit breaker tripped. Sometimes this can indicate a circuit breaker that is beginning to fail, and means it will need replacing sometimes soon (a job for an elecrician). Or, it may indicate that the furnace motor is laboring for some reason. For example, a badly clogged furnace filter can cause the motor to strain, which can potentially overload the ciruit breaker. Or, a bad capacitator or other component, or an electrical short inside the furnace's wiring, can cause the circuit breaker to trip. Replacing the filter is an easy solution you can try, but if the circuit breaker trips repeatedly, it is time to call a professional to examine the furnace or circuit breaker for more serious problems.

Do Gas Furnaces Work Without Electricity?

Both gas and electric furnaces require electricity in order to work. For gas furnaces, even though gas is the fuel that provides the heat, electricity is needed to operate the blower motor, relays, circuit boards, and more. Gas furnaces cannot operate without electricity.

Is the Thermostat Functioning?

When a furnace stops working, the problem very often lies with the thermostat. Checking the thermostat is fairly easy to do, and many of the fixes are well within the reach of a DIYer.

  • Look at the clock time: Start by simply checking the automatic clock if you have a programmable thermostat. If the clock is off or set incorrectly, then the schedule will not operate correctly. Resetting the clock may return the furnace to normal operation.
  • Check the thermostat's batteries: Many older thermostats operate off of the low-power current from the wire that attaches to the thermostat, so they will have no batteries. But newer programmable thermostats usually do have on-board batteries designed to keep the inner clock and program intact in the event of a power outage. If the batteries go dead, the thermostat may stop working. In many cases, simply replacing the battery will return the thermostat—and your furnace—to perfect operating condition.
  • Check the wire connections at the thermostat: Your thermostat may also malfunction if any of the low-voltage wire connections have come loose. Carefully detach the thermostat body from the wall and check each connection. Depending on the complexity of your system, there can be as many as six low-voltage wire connections linking your thermostat to the furnace, or as few as two wires. Make sure each low-voltage wire coming out of the wall is securely attached to its corresponding terminal on the thermostat.
  • Check the thermostat wire connections at the furnace: It's also possible that the thermostat wire connections might have loosened at the other end—at the furnace itself. Locate the control panel on the furnace and check the low voltage thermostat wire connections to make sure they are all secure.
  • Consider the transformer: Finally, it is possible that the thermostat is malfunctioning because of a problem with the transformer that steps down 120-volt power to the 24-volt current required for the thermostat. Usually built into the furnace, the transformer is found inside the access panel and will have both 120-volt line voltage wires connected to it, as well as low-voltage thermostat wires. Replacing the transformer is usually a matter for a furnace technician, as it involves handling 120-volt line voltage wires, testing the transformer with a multimeter, and disconnecting electrical connections. Don't attempt this unless you are very experienced with electrical wiring.

Is the Condensate Pump Working?

Modern high-efficiency furnaces have a two-stage heat exchange design that creates condensed water that is removed from the furnace by a drain line. This drain line may include a small pump if the furnace installer ran a tube for the condensate water to a utility sink or other plumbing fitting. (If there is a handy floor drain near the furnace, the installer may have installed a simple gravity drain tube from the furnace to the floor drain.)

If your furnace is equipped with an electric condensate pump, you may have heard its faint buzzing—a sound similar to that produced by the pump for an aquarium or decorative water fountain. This sound is usually heard after a long heating cycle—the sound runs for a few minutes, then stops.

But if the condensate pump becomes plugged or malfunctions, the condensate water may simply spill onto the floor around the base of the furnace. This can be an alarming symptom, but it's actually not all that serious. It's usually easy enough to get the pump working again—or even replace it entirely.

The condensate pump and pan is about the size of a shoebox, usually located on the side of the furnace. Sometimes, it will be powered by an auxiliary outlet next to the furnace. If this is a GFCI outlet (it should be), check to make sure the GFCI hasn't tripped. If it has, merely resetting the outlet may return the pump to operation.

Other times, the condensate pump is hard-wired into the furnace and is powered directly by the furnace. If so, turn off the circuit breaker (or breakers) controlling the furnace before examining the pump. Start by checking the tube that runs from the condensate pump to the outside and make sure that it is not pinched or blocked.

Open the lid to the condensate pan (usually removing the condensate pump in the process). Check to see if the draw tube is still at the bottom.The condensate pump pan might be overly full and leaking on the floor. If so, it means the automatic switch on the pump isn't working correctly—manually flip the switch back and forth a few times to loosen it. If the condensate pump still does not work, it will need to be replaced entirely. Condensate pumps are relatively inexpensive and easy to replace. If the pump is hard-wired into the furnace, though, you may need to hire an electrician.