How A Furnace Works and Common Problems

Every year, we use heat to keep us warm during the winter.

That may come in many forms, like cozying up under a blanket or sitting in front of the place. But for most people, it means running a furnace.

Perhaps you’re sitting on your couch or in your favorite lazy boy and you begin to wonder how a furnace works.

How a Furnace works

I’m going to assume that if you’ve found your way here that you aren’t an engineer or an HVAC technician.

You’re just curious about how that hunk of metal in your garage or closet manages to heat your house every day.

So with that in mind, we’ll discuss how a furnace works step by step in a simple to understand process that leaves the jargon for the training manuals.

On the off chance you want a hyper technical explanation, Repair Clinic put out a really great video detailing the whole process.

For everyone else, read on.

1. Thermostat reading

The whole process starts with your thermostat. Without it, the furnace has no idea what the temperature in your room is.

When your thermostat registers that the air temperature has dipped below the target temperature, it will trigger the furnace to ignite.

Woman setting furnace thermostat

2. Flame Ignition

At this point the gas valves will open, allowing for the free flow of fuel to the system.

That gas is then ignited by one of a few processes.

Either you have an electric starter that causes a spark or you have a pilot light that remains lit all winter long.

Both are effective sources at then igniting the gas and turning it into flame.

3. Air Heating

Inside your furnace, there’s something called a heat exchanger.

This is a set of pipes that snake bake and forth multiple times before turning into an exhaust that leads outside your home.

Air is sucked in through the same opening the flames shoot into, causing the air to warm and heat up the heat exchanger.

The pipes then become very hot, while the air and carbon monoxide inside the pipes are directed out through the exhaust.

4. Hot Air Delivery

Air from a different location is then blown over the heat exchanger (those hot pipes) heating the clean, breathable air.

Next, that air passes through your ducting and fills your home.

5. Cold Air Recycling

When the air cools, it passes through the return vent and goes back to your furnace.

The same clean air is then heated once more and sent back out through your ducting.

This cycle happens over and over again, keeping your home warm all winter long.

Common Furnace Problems

Now that we’ve detailed how a furnace works, let’s talk about the common problems that might result if one of those processes is interrupted.

1. Thermostat issues

I say thermostat issues because there can be several.

The first is simply that the thermostat has stopped working.

If there’s a short in the electrical line or the batteries are dead, then the thermostat won’t turn on and can’t send a reading to the furnace. Without the thermostat, the furnace won’t ignite.

It could also be that the thermostat needs calibrated.

If you set it to 74, but the unit thinks 74 is 70, then it won’t kick on when the temperature hits 73. This is a mechanical malfunction and just needs to be reset.

But there’s another possible problem as well.

It might be that the thermostat was installed improperly.

Thermostat location is important to effective use. If it was placed in the sun, the thermostat will think the house is warmer than it is, thereby causing the house to be cold.

And if the thermostat is placed near a door and a gust of cold air washed over it every time someone goes in and out, it’ll cause your furnace it kick on prematurely.

The thermostat is a small part of the furnace, but a very crucial one, and can often be the source of furnace problems.

2. Bad Flame Sensor

During the ignition process, the flame sensor ensures that the gas indeed ignited and flame is present.

If no flame is present, the flame sensor closes the gas valves so that raw, natural gas isn’t spilling into your home where it can explode.

But if you have a bad flame sensor, even if flames are present, it’ll read as if there was no ignition and will shut the system down.

Sometimes this can be easily fixed by cleaning the flame sensor.

Other times it needs to be replaced.

3. Bad Blower Motor

The blower motor is basically just a fan that sucks air in and blows it out.

If you have little to no air coming from your vents, but you can hear the furnace running, then likely your blower motor has broken.

4. Dirty Air Filter

Another source of bad air flow could be a dirty air filter.

Your air filter should be changed roughly every 90 days during active use.

If you only use your furnace a month out of the year, like is possible here in San Diego, then you may change it less often.

But people living in the frigid north may find that they’ll change their filter a few times before the season is over.

If the filter gets too dirty, it will impede air flow meaning your system has to work harder to heat your home and it will take longer to get warmth when you want it.

Dirty air filter

5. Strange noises

If you rhythmic squeaking or squealing, or any other strange noises coming from your furnace, you probably have a loose screw on the blower wheel.

Because the blower motor isn’t properly secured, it rocks back and forth as it spins, causing that irritating noise.

Tightening the screw may be all that’s needed.

6. Odd Odors

If you smell gas or any other strange odors coming from your furnace, turn it off immediately.

You likely have a cracked heat exchanger.

This is more common than you might think.

As time goes by, everything wears down. And with your furnace often exposed to high temperature fluctuations, the expansion and contraction of the metal causes it to crack.

When this happens, dangerous fumes are slowly leaking out of your unit.

Not enough to cause your serious harm at first, but if left unchecked, it can become a major hazard.

Don’t take any chances and immediately call your local HVAC contractor to fix it.

7. Bad ignitor

This one is pretty self-explanatory.

The ignitor is like your car’s spark plug.

If the ignitor is bad, the system won’t turn on.

You might also find that every time your try to turn on your furnace, the breaker flips. This could be a voltage issue related to your ignitor.

Need Furnace Help?

Learning how a furnace works is one thing, but fixing one is an entirely different story.

You can troubleshoot a few of the problems above on your own, but for most of them you’ll need a professional, licensed HVAC technician.

If you live in San Diego and need help fixing your furnace or are wondering how much it costs to replace a furnace, call Precision Temperature. Our friendly staff are standing by to take your call 24/7/365.

Call us today at 619-588-5321.

to top