Is your furnace not blowing hot air? Discover the common causes of furnace problems and easy troubleshooting tips to restore warm airflow to your home.
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If your furnace is gas-powered, see this article for a more detailed troubleshooting guide: Why Is My Gas Furnace Blowing Cold Air?
If you have an electric furnace, go here:
Why is My Electric Furnace Blowing Cold Air?
There are several possible reasons why your furnace is not blowing hot air, but the most likely are: a Dirty Air Filter, an issue with the Thermostat, or a Furnace Ignition System (or Heat Strips – on electric) failure.
This article will not only help you troubleshoot common causes of the malfunction, but it will help you recognize when a professional’s help is necessary.
The Furnace Air Filter is Dirty
When your gas furnace’s air filter is covered in gunk, there’s no way that it can blow out warm air, and here’s why.
When a dirty filter restricts airflow within the furnace, the furnace begins to work too hard and can internally overheat.
And as a protective measure, the furnace blows out cold air to cool itself down. This functionality is seen in just about all furnaces.
This is a common reason why furnaces blow out cold air.
The appropriate course of action to take when the furnace filter is dirty is to replace the current furnace filter with a new one.
This can be done at home if you’re up for a bit of labor.
If you have an oil filter, read on- oil furnace issues will be discussed further down in the article.
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Replace Your Furnace Filter
Below are some steps for replacing a furnace filter.
- Access your current filter and take it out. Furnace filters can be easily found inside the furnace or in the air return vent.
- Make a note of the size and type of filter you’ve just removed. Grab a piece of paper and pen and write that down. You’ll need that when you go to purchase a replacement filter.
- Purchase and replace the filter. After purchasing a new filter, make sure that you are putting it in the right way. A quick way to find out whether you’re putting the filter in the right way is to point the edge of the filter in the same direction as the blower motor.
Note: If you put the filter in the wrong way, the furnace won’t be able to blow out air as strongly as it would normally be able to.
The Furnace Ignition System is Faulty
A furnace’s ignition system is extremely important to its functionality. When the ignition system is in flux, the furnace will not be able to blow out cold air.
There are two components of a furnace ignition system that you should know about:
- The pilot light
- The ignition
If your furnace doesn’t blow out hot air, it could be that either the pilot light is out or that some part of the ignition system is not working properly.
Both of these issues require a bit of finesse to troubleshoot and repair. If you have tried everything else, it is wise to reach out to an HVAC technician for assistance.
Thermostat Is Not Signaling the Furnace to Heat
If you’re someone who isn’t experienced in working with thermostats, it’s important to know that your thermostat is in sync with your furnace.
If your thermostat is not signaling your furnace to turn on, the furnace will not blow out hot air.
As time goes on, the average thermostat is getting more intricate and complicated.
On the other hand, if your thermostat is old, you may find that there is a learning curve to learning how to operate it properly.
The Temperature is Wrong
Here are a couple of things that you can do to make sure that your thermostat is set to the right temperature:
- Check the manufacturer’s instructions. If you don’t know exactly how to set your thermostat to the right temperature, you should first look at the manufacturer’s instructions. Alternatively, you can search for the instructions on the web or give the thermostat manufacturer a call.
- Ensure that your thermostat is on “Heat.” Some thermostats won’t prompt the furnace to blow out hot air unless it is set on the “Heat” function, whether you’ve set the desired temperature to 50 degrees or 80 degrees.
After you’ve done the above, if the furnace is still not working properly, something else could be to blame.
The Thermostat is Dead
One common cause of a faulty furnace is a dead thermostat. Thermostats commonly malfunction due to technical problems that only an HVAC technician would understand.
However, there are times when the wires in your thermostat need to be resecured, or it needs a new battery.
These are things that you can check yourself, in most cases.
If you have an electric thermostat, you may be able to easily open up your thermostat, purchase a new battery that matches the one you removed, and replace it with a new battery.
But in some cases, you may need to enlist the help of a technician to replace the battery. Also, removing the face of your thermostat will reveal its wires-make sure that they are secure.
The Thermostat is Dirty
For mechanical thermostats, over time, the inside can become dirty and cause it to malfunction. To get rid of dust and debris, open the front of the thermostat and blow any debris away.
The Thermostat is Not Compatible
If your thermostat is not compatible with your thermostat, your furnace will not function in tandem with it.
The chances of this being the issue are slim unless you’ve recently installed your own thermostat without the help of a professional.
If you aren’t sure whether your thermostat is compatible with your furnace, a professional can confirm this for you.
This is not likely to be the issue if your furnace was working properly before and has only recently started blowing cold air.
Your Oil Furnace Is Not Getting Oil
This section is for households with an oil furnace.
Out Of Oil
For people who have a furnace that operates on oil, it may be worth it to check the oil tank and make sure it’s not empty.
If it is empty, the internal burners will not light. This can cause the furnace to blow out cold air.
Over the winter, an oil furnace can guzzle 3 gallons per day for the average household.
It also varies with the user’s habit and preference. If you have been using the heat more often than usual, you may have run out of oil without noticing.
Clogged Oil Filter
The oil filter can be another reason why your furnace won’t blow out hot air.
If the filter is clogged, this can cause issues with the ignition system within the furnace, and a replacement filter would need to be fitted.
Note: The process of replacing an oil filter is much more involved than replacing an air filter (for those who have a gas furnace).
However, if you have prior knowledge of oil filters, you could replace the filter yourself. But most people call a professional for help with an issue like this.
The Gas Line Shut-off Valve May Be Closed
There are typically two different shut-off valves between the utility gas and your furnace. One is at the meter outside, and one is at your furnace.
Check to make sure that BOTH valves are in the ON (open) position.
First, check the shut-off valve at the gas meter, as it is the easiest to do.
However, if you have other gas appliances in the house (water heater, cooktop, fireplace, etc.), and they are working fine, then you can be sure that the meter valve is on. Then you can cross that off the list and move on to the valve at the furnace.
If someone has been fiddling with your furnace lately, they could have switched off your gas valve and inadvertently left it that way. If this is the case, there is no way for your furnace to generate heat.
This actually happened to me at one of my rental properties last winter. When the furnace wouldn’t ignite, I checked several other (more complicated) components before realizing the service tech, once he had finished working on the furnace, had simply forgotten to turn the valve back on!
To check this issue, you’ll need to verify the valve’s position.
If you don’t know where the gas line shut-off valve is, you can usually follow the gas line from the furnace toward the meter. But it should be very accessible within a few feet or so of the furnace itself.
Whether or not this is easy or difficult will depend on your particular situation and house design. But just keep in mind that the gas line will generally be run in the most efficient and direct way possible.
Once you’ve located the valve at the furnace, observe its position. If it turned perpendicular to the pipe, it is in the “off” position. To turn it back on, turn it so that it is parallel (in-line) with the pipe. This should solve the problem.
Is Your Gas Shut Off at the Source?
Have you paid your gas bill? If your furnace isn’t blowing hot air, it could be that your gas was turned off by the utility, either intentionally or otherwise.
If you find that none of your gas appliances work, and you’ve made sure the meter valve is on, call your utility company. It could be a mistake on their end.
Duct Problems Could Be the Cause
Over time, air duct problems can arise, showing themselves in the form of holes and leaks, or even blockages.
Holes and leaks
These leaks and holes allow external air to directly invade your ducts, internal air to escape, or both. This issue is more common in older furnaces than those that have been recently installed.
Have you had any remodeling or construction work done on your home? Sometimes, that is when ductwork can get disturbed and develop leaks.
Do you have rodent issues? Rats, mice, squirrels, and other pesky varmints can chew right through the soft flexible duct walls in modern home HVAC systems. Be sure to look for signs of unwanted guests.
Blockages
A blocked or restricted air duct will also cause you to lose adequate heat to your space.
Blockages can occur for a number of reasons. Ductwork can easily get squished and pinched – especially when run through an attic space.
It’s inevitable that someone will have to crawl around in the attic for a repair or other project at some point. If they’re not careful, they may inadvertently step on or lean against a duct, causing a bottleneck (or worse, a complete stoppage).
Or, since attics are notorious places to store rarely used items, it’s not uncommon to find boxes or other objects lying on top of ductwork. Over time, the ductwork can sag or kink, limiting airflow.
Whether a leak or blockage, this is not always a problem that can be troubleshot and fixed by a non-professional. It might be best to contact a reputable HVAC company to fix an air duct problem.
To save money, make sure that you check for easily fixed issues before calling in a professional.
The Furnace Needs a Restart
If you have a new, high-tech furnace that has electronic controls, the issue could lie within that system.
This is not a likely issue if you have a standard furnace, though.
Resetting your furnace’s computer system can be the jolt that your furnace needs to blow hot air again.
For many of the newer furnace models, a restart is as simple as pushing the power button on your furnace’s control panel, waiting several minutes, and then powering it on again.
If this does not work, explore other options.
Final Thoughts
So many things can go wrong with a furnace (or the thermostat that controls it) but taking a bit of time and troubleshooting common issues can help you fix the issue yourself.
Yet, many furnace issues will require the expertise of a professional. We hope that this guide will help you successfully pinpoint your furnace issue.