Furnace Blowing Cold Air? Common Causes and How to Fix Them

Furnace Blowing Cold Air

Is your furnace blowing cold air sometimes or all the time? We can help.

You likely searched something like a furnace not blowing hot air to find this article. You chose ours because it ranked well or seemed to have useful information. We hope both – because of the many, many articles available on the topic of the furnace blowing cold air – or not blowing hot air – ours strives to be three things:

1. Comprehensive – We walk through the potential reasons your furnace is not blowing hot air from the most common and easy to solve to more tricky issues.

2. Easy to understand, so homeowners like you have the opportunity to DIY and not call for an expensive repair.

3. Complete with repair cost information.

Thank you for choosing PickHVAC to help you discover and solve the “furnace not blowing hot air” problem. It’s one of the 12 most common furnace problems.

Consider bookmarking our site for later when you want to learn more about gas furnaces, air conditioners, air purifiers and much more. We’ve got it all!

Reasons for the Furnace Not Blowing Hot Air

Reasons, solutions and costs. Here they are. Once you read it you will be able to do a few simple troubleshooting steps that if they work, they will get that nice warm air you love flowing again. Hopefully you can also save yourself the money you would have spent on an HVAC service call.

First Things First – An HVAC technician will first check your thermostat and its settings. If the thermostat is compatible, functioning properly and the settings are correct, the next step is to check your furnace’s error codes for clues to what is wrong.

So that’s where we begin. Let’s do some gas furnace troubleshooting when the furnace not blowing hot air is the issue.

Thermostat Fan Set to “On”

Your thermostat has a couple buttons/settings. One controls the fan.

Most homeowners set the fan to Auto. This means that the fan will turn on only when the furnace or air conditioner is running. However, it also has an On setting, which means the fan will run all the time, and if the furnace isn’t making heat, it will blow cold air.

Thermostat Fan Set

See the Fan setting in on this thermostat is set to Auto. The button below the setting can be used to switch between Auto mode and On mode where it will run continuously.

Solution: Change the setting from On to Auto.

Cost: Free

Thermostat Set to Cool

Another switch on the thermostat determines what mode the HVAC system is in – Heat or Cool. If it is early in the heating season, perhaps the mode wasn’t changed. Or maybe it got switched accidently.

Solution: Change the setting from Cool to Heat.

Cost: Free

You Have an Incompatible Thermostat

If the thermostat was just changed and the furnace blows cold air when it should be heating, the thermostat was either installed improperly or, more likely, it isn’t the right one for the HVAC system you have.

Solution: If it was professionally installed, call back the installer. It’s their responsibility to give you the right thermostat and install it properly.

If you installed it, see our Thermostat Buying Guide to learn about HVAC system and thermostat compatibility.

Cost: If it is an installer’s fault, there should be no additional cost to make it right. If you are buying and installing the thermostat, cost for a new thermostat ranges from about $30 for a cheap non-programmable thermostat to about $200-$300 for the most popular smart thermostats like nest, Honeywell lyric, ecobee and Lux Geo. Top of the line, touchscreen WiFi thermostats cost more than $500.

You Have a Bad Thermostat

Your gas furnace will blow air through the system periodically, even if you do not have the heat on. It will circulate air constantly on certain air settings too. If you turn the heat on and this circulated air is not warm after a few minutes you may have a thermostat problem.

To check this, turn your thermostat all the way up and also turn off the constant air circulation setting. After that go stand near your furnace. You should hear your heater’s airflow system come on and you may hear clicking noises that indicate the heater control system is telling the burner it’s ok to come on and produce heat. If there’s only silence then there is a good chance you have a defective thermostat.

Solution: To replace a defective thermostat you will have to replace, at minimum, the entire control head found on your wall. Pull it off, and you’ll see it detaches from the wiring connections. If you buy the same brand and a similar model, the new “front” might work with the “back.”

If not, you’ll have to remove the wiring from the back, connect the wiring to the back of the new thermostat, and attach the new thermostat front to it.

But don’t worry. Installation and more is covered in the PickHVAC Thermostat Buying Guide and Reviews. It is loaded with useful information.

Cost: As noted above, thermostats start at about $30 and range to $500+. Most good-quality programmable and smart thermostats cost between $75 and $325.

Carrier Infinity Thermostat

Your Furnace is Showing an Error Code

Error codes are clues to the problem, not the problem themselves.

Hang in there. This is a little bit of a long answer, though not complicated, and it really will help you get to the problem and solution for many common furnace issues.

Most furnaces manufactured in the last few decades feature self-diagnostic technology. They’ll tell you what is wrong with them, though they don’t cover every possible problem.

Each brand is a little different, but the typical way it works is this. Inside the cabinet of the furnace, there are one or two lights. The furnace cabinet door might have a small window through which you can see the lights – or you might have to remove the cabinet door.

When the furnace is working properly, the light or lights will be steadily on or might blink in an on/off/on/off way.

When there is an error or fault, the light(s) will emit a series of blinks. For example, the single light might blink three times quickly and then pause, then repeat. That’s error code 3 for that furnace. If there are two lights, a green one might blink three times and a yellow one blink twice before repeating. That’s error code 32.

So far, that’s entirely unhelpful. However, inside your furnace, on the inside of the cabinet door or on the control panel (the place all the wires are attached to), there will be a code key like this one telling you what the codes indicate.

most common error codes

Here are some of the most common error codes taken from the image above. Other furnace brands will have different codes to indicate the same problems. Here is what each problem means plus their solutions and costs.

  • Rapid Flash: A false flame code indicates the furnace “thinks” the burner is on when it isn’t. This usually indicates a bad flame sensor.
  • Solution: Replace flame sensor yourself for the cost of the part, $10 - $30, or call an HVAC technician to do it for the cost of a service call plus the part, so about $150 - $200. Believe it or not, some homeowners have these parts replaced preemptively when having the furnace cleaned or another repair made. We think it makes sense, since the parts are cheap. If the technician is already at your home to clean or repair the furnace, the cost will be about $50 to $75.
  • One Flash: Limit Switch is open. This means that the switch is bad or that the switch is good, but the furnace has been overheating.
  • Solution 1 and Cost: Replace the limit switch for the cost of the part, about $30-$50. Here’s a tutorial. https://www.youtube.com/watch?v=pJzP1jzYPqg Or have a professional make the repair for around $200 to $250.
  • Solution 2 and Cost: If the limit switch is doing its job because your furnace is overheating, then you have to determine why it is overheating. The most common reason is that airflow is restricted – the heat isn’t getting out of the furnace. And restricted airflow points directly to a very dirty air filter. Replace the filter at a cost of $3 to $15 depending on the filter you choose. Our Air Filter Rating Guide will help you choose the right air filter for your furnace and lifestyle needs.
  • Two Flashes & Three Flashes: The pressure switch is bad, either open when it shouldn’t be or stuck closed.
  • Solution and Cost: Replace the pressure switch for the cost of the part, which ranges from $20 to $50. Or call an HVAC technician to confirm the diagnosis and replace the pressure switch for $115 to $200.
  • Four Flashes: Failed ignition is usually the fault of a bad igniter. Most furnaces have a hot surface igniter. When the thermostat calls for heat, the igniter heats up, the gas valve opens, and the gas fires up.
  • Solution and Cost: Igniters cost $10 to $30 and are easy to replace. We’ve completed a gas furnace troubleshooting guide on How to Tell if Your Furnace Igniter is Bad and Where to Buy One. Cost to have an HVAC technician make the repair ranges from $125 to $225.

The Air Filter is Dirty Enough to Restrict Airflow

Furnace Dirty Air Filter

Your furnace needs two elements to fire.

The first of those, of course, is the natural or liquid propane gas that comes into your furnace. Air is the other key ingredient that is used to regulate how hot the flame is that your furnace produces.

The air that feeds your burner system is pulled into your furnace through the same intake vents (sometimes referred to as registers) that bring in the air

This air first passes through a filter. If a register is blocked or the air filter is very dirty it will shut down the airflow to your system. When this happens the system will detect it and not let the burners’ fire up.

Solution: There is a pretty simple resolution to this problem. Just take out the dirty air filters and either clean or replace them. You have to also make sure there are no boxes, furniture or other clutter blocking the air from getting into your air intake vents.

Cost: $3 to $15 for most air filter replacements.

The Furnace Isn’t Getting Gas, part A

Your furnace not blowing hot air could be caused by the fact that it has no gas to burn.

Was the furnace recently worked on – or shut down for the summer? The gas valve on the gas line (or oil valve/oil line) might be turned to the Closed position – which is when the valve is perpendicular to the line.

Solution: Turn the valve to the Open position.

Cost: Free

The Furnace Isn’t Getting Gas, part B

You might be out of fuel – or the gas to your home has been shut off.

A gas furnace will either be supplied by connecting to a city’s natural gas line or by being supplied by your own personal LP storage tank and gas line. An oil furnace has an oil tank.

If you have your own LP gas tank or oil tank, you need to check to make sure it’s not empty. You just locate the tank in your yard and open up the domed cover on top of it. Under the domed cover, there will be a gas or oil fill connection and a pressure gauge. Check the pressure gauge; if it reads less than 15%, it is probably a good idea to call your LP gas or home heating oil supplier for a fill-up. Still, the tank should feed fuel to the furnace until it is just about empty. If there is more than 5% left in the tank, this probably isn’t your problem.

Solutions: Check the tank outside, if you have one. If your gas is supplied by an underground gas line, call your gas provider to see if the gas has been turned off.

Cost: Call your gas or oil supplier to get a fill. They can tell you about current pricing. If it is natural gas, call your provider. Perhaps there is a “past due” on your gas bill that needs to be paid.

Check for Major Holes in Ductwork – Or a Lot of Little Ones

Just like you will have trouble heating your home if the doors are open, so too will have problems generating the warm air flow from your furnace if you have a major hole or a seam has come undone in your ductwork. That cozy, warm air might be leaking into the attic, basement where nobody is or outside.

This is a fairly rare occurrence on ductwork that’s not exposed to outside elements but you cannot rule it out if you have a flame at your furnace and it’s still blowing cold air.

Most ductwork in your house will either run through your attic or in a crawl space under your home. You will have to grab a strong flashlight and go into these spaces to actually examine all the exposed lengths of ductwork in these areas.

Solution: How do you fix it if you do find a sizable hole or seam opening in the ductwork coming from your gas furnace?

You could replace those sections of ductwork that are causing the problem but that may prove to be difficult and expensive. A simple but effective fix is to just seal any holes or separated ductwork seams up by covering those exposed areas with professional grade duct tape.

Cost: Your ductwork should be in good condition to ensure your HVAC system – furnace and air conditioner – are working as efficiently as possible and your heated and air conditioned air isn’t being wasted. We recommend having ductwork professionally repaired and insulated. It’s an investment that will pay you back in just a few years through lower energy costs. The cost to replace ductwork ranges from about $10-$15 per linear foot. An average 2,500 square foot home will have about 100 feet of exposed/replaceable duct, so that’s $1,000 to $1,500 to replace it.

Make Sure The Condensation Drain Line Is Not Plugged

This one only applies if you have a modern high-efficiency gas furnace. These systems heat so efficiently in their burner area that it results in much condensation being formed. So much condensation is produced that they actually have a catch pan and dripline to help remove it.

There is a sensor built into this line that will not allow your gas burner to fire if it senses that drip line setup has a blockage.

Solution: This catch pan and drip line setup are located directly under your gas furnace’s burner tray. If your furnace is blowing cold air you can try gently cleaning out any blockages in this dripline. Once the blockages are removed the sensor will allow your gas furnace to fire up again if that’s what was causing the problem.

There’s more detail on this topic in this AC Drain Line Clogged? The air conditioner uses the furnace drain line, so we’re talking about the same problem. DIY is discussed in the guide.

Cost: There’s little cost if you DIY. A pro technician will charge $115 to $200 for the work.

What About the Pilot Light?

Furnace Pilot Light

We didn’t cover pilot light issues in this guide. If you read a gas “furnace not blowing hot air” article that mentions the pilot light, it’s an indication the information might be out of date.

Furnaces with pilot lights are mostly in landfills now.

If your furnace happens to have a pilot light, we recommend that you talk with local furnace installers about replacing your furnace, not repairing it.

Why? First, your furnace is old and will likely continue to break down until replacing it is inevitable. We recommend skipping the 3-5 years of high repair bills before replacing a furnace and go straight to furnace replacement.

Secondly, if the furnace is old, it is likely inefficient. Today’s gas and propane furnaces offer tremendous efficiency for whatever climate you’re in.

  • In warmer climates, an 80% furnace is fine.
  • In moderate climates, we recommend a furnace in the low-90s.
  • In cold climates, mid-90s efficiency makes more sense.

See our Gas Furnace Reviews and Prices post. It is a complete Gas Furnace Buying Guide with the most comprehensive information you’ll find in one place.

Nothing Worked? Contact Local HVAC Service Companies for Diagnosis and Estimate

If you have tried all of the above steps without success you probably want to stop trying to fix the problem yourself. That’s unless you are very familiar with how a gas furnace works.

The reason for this is that further troubleshooting to figure out why your gas furnace is blowing cold air takes advanced furnace knowledge and the right troubleshooting equipment.

Not to mention, that if you do not know what you are doing and you continue trying to make repairs, you could actually make things worse. So at this point call a trusted HVAC service company and have them send one of their skilled service techs to your home to help. If you don’t have a local company you trust, use our Free Local Quotes form or phone number, and prescreened, licensed and insured furnace companies will be happy to provide consultation, diagnose the problem and give you an estimate on repairing or replacing your furnace.

While that service person is at your house working on your gas furnace, it’s also a great time to inquire about getting routine maintenance done once a year on your gas furnace heating system. Many times proper routine maintenance can help you avoid ever having problems like your gas furnace blowing cold air in the first place.

Related Article: Why is My Electric Furnace Blowing Cold Air?

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