Common Furnace Problems & Troubleshooting

In an ideal world, we’d never have to troubleshoot a problem with our furnaces. Furnaces can be found in most homes around the country, being the most common type of heater. And most people don’t give much thought to their furnaces for the vast majority of the time as long as it provides them with the heating in the fall and winter that they need. But that’s why once this stops happening it can be all the more disconcerting because suddenly something that worked effortlessly without a second thought, something that most people take for granted, is acting up and you may have no idea what your next step should be. Luckily there are patterns of behavior when it comes to furnace problems

Short Cycling

Depending on who you talk to you might hear that the absolute most common issue that people encounter with their furnaces is something called short cycling.

What Is a Normal Furnace Cycle?
  • Normal furnace operation means your furnace will switch on once the thermometer in the thermostat reads that the temperature in the house is below that which you set it at (assuming that the thermostat is set to “auto” and “heat”)
  • The furnace will then run until the thermostat reads that the temperature has risen above the temperature you set it to
  • This will trigger the furnace to turn off
What Is Short Cycling?

Short cycling is the commonly agreed-upon name for the phenomena of a furnace turning off and starting back up again more frequently than it should. It happens when the furnace turns off prematurely before the set temperature has been reached, then quickly turns back on again because the thermostat is reading a lower temperature than what it has been set to. This causes the furnace to turn on and off again in short cycles, hence the name. 

Is Short Cycling Bad for a Furnace?

Well, short cycling, if left alone, can slowly erode the integrity of your heating system and cost you more money in the long run. If your system is constantly turning on and off, it is putting unnecessary strain on the components of your furnace. This extra strain can lead to system failure and expensive repairs. Additionally, the inefficiency caused by a system that constantly turns on and off can increase your energy bills, costing you money.

What Causes a Furnace to Short Cycle?
  • It has a dirty air filter inhibiting airflow to your blower motor
  • Your thermostat is malfunctioning
  • You have poor insulation
  • Your furnace is oversized.

Luckily, the most common cause of short cycling is also the easiest and cheapest to fix and that’s a dirty air filter. The furnace’s air filter is what filters out dust and other particles in the air which keeps the air flowing through the system clean. If the filter gets too dirty then airflow will be restricted and air won’t be able to flow over the heat exchanger. This will cause the furnace to overheat and turn off prematurely. This is an easy fix, however.

How to Fix a Furnace That Is Short Cycling
  • The first thing you should do is check the air filter. If it’s dirty, replace it and see if that fixes the problem.
  • If that didn’t do it, check to see if your thermostat has batteries. To do this, simply replace your thermostat’s batteries and turn it off and on again. Let your system run for an hour and see if that fixed the problem. If your thermostat is malfunctioning or low on battery, it could be telling your system to shut off prematurely.
  • You can also check your thermostat’s location. Your unit should not be located near any heat sources like a space heater, heat register, or direct sunlight. These may be causing your thermostat to inaccurately read the temperature of your home and shut down your system.

If none of these components are the culprit then you’ll need to call in a heating professional. Either the furnace is sized too big for the house, there’s a problem with the insulation, or something else more major is going on and only a professional is going to be able to finish the diagnosis.

No Heat

If your furnace is blowing air but not hot air that can be extremely frustrating but it is, unfortunately, a common problem. It means most of the furnace is working. The blower is moving air through the vents and the thermostat is telling it to do so, but because there’s no heat it isn’t doing you any good and may even be making you cooler. This is a waste of energy and a major inconvenience. 

Why Is My Furnace Not Blowing Heat?
  • The first thing you’ll want to rule out is an issue with the thermostat settings. This is the easiest fix and a common cause, so before you call a professional make sure this isn't the problem. Specifically, you’ll want to make sure your thermostat is set to the “auto” setting, not the “on” setting. The “on” setting is going to keep the blower fan blowing air even when the rest of the furnace is dormant and not heating the air. If your furnace only sometimes blows cool and not warm air then this is most likely the cause.
  • Another possible cause of your furnace blowing cool is that the pilot light is out. The pilot light is what ignites the flame that burns the fuel, thereby causing the air in the heat exchanger to warm up before it is blown by the blower fan. The pilot light is the start of this chain reaction and if it is malfunctioning then your furnace will produce no heat. The first possibility you’ll want to rule out is whether or not this was a temporary incident or if there is something more seriously wrong For example, the pilot light on a gas furnace can go out because of drafts. In this case, all you’ll need to do is manually restart the pilot light. To relight the pilot, follow the manufacturer's instructions exactly; they are usually fastened to the furnace. If this fixes the problem then you’re all done, if not then move to the next troubleshooting step.
  • If the pilot refuses to light, ensure that gas is flowing into the furnace. Make sure the furnace’s gas valve switch is in the “on” position (parallel/in line with the gas supply pipe.) if the switch is on, then check to see if the pilot light is dirty. If it is, clean it to allow gas to come through. If the pilot lights, but won’t stay lit, then there’s probably a problem with the thermocouple (the copper rod you see the pilot flame hitting). It needs to be adjusted or replaced. This is most likely when you’ll want to contact a heater repair professional.

No Air

They may seem like they’re related but there are often very different causes to the two common furnace problems of no heat and no air. A furnace with no heat is still blowing air but something is off with the part of the furnace that heats the air that it blows. A furnace that doesn’t blow air at all might be producing heat but something is preventing this heat from being distributed through the ducts.

Why Is My Furnace Running but Not Blowing Air?
  • The first thing you’ll want to check is the thermostat. You’d be surprised how often the fix is as simple as righting a wrong setting. If the thermostat is off or set to “cool” instead of “heat” or the temperature is set too low, then the furnace isn’t going to kick on at all.
  • If that isn’t it, then you might have an internal furnace issue, which in this case, is going to be with the belt that connects the blower to the heat exchanger. If your furnace’s motor runs but the blower doesn’t move air, the belt that connects the two might be broken. Replacing it is an easy fix, which we detail below.
  • If the blower is working however but you’re still not getting any air then the problem is likely with the airflow being restricted, which means the air filter is the most likely culprit. Either the air filter is so clogged that there is little air making its way into the furnace or the restricted airflow is causing the furnace to overheat and switch off. Both of these can easily be solved by replacing your dirty air filter.
How to Replace Furnace Blower Motor
  • Turn off all power to the unit
  • Turn off the gas at the gas valve that serves the furnace
  • Remove the door on the front of the furnace cabinet to give you access to the blower
  • Check the number stamped on the belt and get an exact replacement from a home center or heating supply outlet
  • Slip the belt on the motor’s (smaller) pulley first and then start it on the blower pulley.
  • Rotate the blower pulley by hand, holding the belt in place but keeping your fingers from getting caught between the belt and the pulley.
  • The belt should slip right into place.
  • If it seems to be too tight or difficult to set in place, it may be necessary to adjust the motor mount to provide more slack. Then you can re-tighten the tension once the belt is in place. Check the manufacturer’s specifications for proper tension.

Professionally Resolving All Your Heating Problems

Fixing a furnace isn’t always as difficult a task as it may seem on the surface. It can be frustrating when the weather takes a turn for the worst and out of nowhere your heater stops working but that frustration can be significantly reduced if you are armed with the knowledge of how to troubleshoot your furnace and get to the bottom of the problem. In some cases, you can solve an issue with your furnace on your own. In many cases however you’ll need the help of a heating service professional. If so, reach out to the expert team from AC Express at (512) 866-5520.