The Furnace Takes a Long Time to Turn On (Do This First!)

I don’t know about you, but when I realize I’m cold, it is all of a sudden. Never mind that I have been gradually getting colder all along, I only notice when I’m already freezing. So when I go over to my thermostat to turn it on, I want the heat to come on immediately.  

Except that when I click it on, I hear nothing. Is this normal? How long am I supposed to wait?  

A small delay between the furnace turning on and its activation is normal. A furnace should turn on within five minutes of activation. If the furnace takes longer than that, there may be something wrong with it. Older models may take slightly longer, but if the heat does not come on within five minutes, you should call a professional.  

How Long Should It Take for a Furnace to Turn On?

When activating your furnace, it takes a minute to heat up and start. During this time, electrical signals are running between the thermostat and the furnace as the furnace begins its ignition sequence.  

Air gets drawn into the burner assembly by a fan. Once this fan reaches the maximum speed, the vacuum switch (the part that prevents fumes from leaving the assembly) is closed. The ignitor is then started, and a gas valve is opened, which produces the flame.  

Assuming everything has worked well to this point, the circulation fan will begin to spin, sending heated air through your house. The entire process takes roughly five minutes from start to finish.  

Sensors are set up at every point to ensure that all parts are functioning correctly. A failure along the process means that the process starts over or is delayed at that step. 

The thermostat will keep a check on the set temperature and start the ignition sequence if it detects that the temperature has dropped below this point. Once again, the process takes about five minutes. While older models may take a minute or so longer, failure to ignite in the five-minute window means that there are potential issues.

What are Some Warning Signs That Indicate a Problem?

Failure to come on within the five-minute window is the biggest indication that there is a problem, but there are a few others that can alert you as well.

Unusual Noises

If you are hearing something outside the norm when you turn on your furnace, it could mean that something is dirty or stuck, or just generally in need of repair.

Uneven Heating

All rooms within your house should be getting the same heat. If the heat in your rooms is uneven, this can mean there is a problem with the sensors or the way that the air is circulating.

Unusual Odors

When you first turn on the furnace at the start of the winter season, you may smell something akin to burning dust. This normally goes away after a few days of consistent use. However, if you are smelling something moldy or something other than dust burning, then you should consult a professional.

Cold Air / Cooler than Normal Air

Your furnace should be kicking out heat. If the air from your vents is cooler than you expect, there may be something wrong with the furnace.

The Furnace Runs too Long

If the furnace is running longer than expected, there may be something wrong with the sensor. A furnace runs in 10-15 minute cycles. If it is running longer than this or constantly running, there is a problem with your furnace.

Furnace Diagram

Reasons Why Furnace Takes a Long Time to Turn On

There are a number of moving parts that are involved with your heating system. An issue at any one of these points can cause your furnace to take longer to turn on than normal. In addition, there are other problems that can affect how long it takes your heater to turn on.

Moisture Buildup

When your furnace isn’t used for some time, it can allow for moisture to build up. Moisture can also accumulate due to the difference in temperature between the air in your ducts and the outside air. The moisture in the cool air outside your furnace condenses on the surfaces of your heater.  

This can corrode the heat exchanger or firebox, preventing gas from entering the burning area. Without gas, the burners will not ignite right away. If you see water condensation dripping from your heater or puddling on the floor, this is a sign of a major issue with your system.

Sulfur Buildup

Natural gas usually has traces of sulfur for safety reasons. As the natural gas normally used to heat your home is odorless, sulfur is added to it so that you can detect a gas leak.

If you have a gas furnace, over time, the burning of natural gas will cause this sulfur to build up on your burners. The tale-tell sign of this is a white layer near your burners or pilot light.

Too much sulfur buildup will cause blockages, which will prevent the burners from firing off. This in turn will cause a delay in your heater from coming on.

Electronic Ignition Problems

There are multiple reasons why you may experience problems with the electric ignition. Simple wear and tear is the primary cause of issues with your electronic ignition.  

Another type of problem is delayed ignitions. In this type of fault, there is a buildup of gas in the combustion chamber when the igniter fails to come on immediately.  

There are usually safety mechanisms in place to ensure this does not happen. However, if they malfunction, you may have to replace your ignition system.

Delayed ignitions are recognizable by the large bang that they produce when the ignitor finally comes on. This is actually a serious safety issue and should be directed to a professional immediately. 

Dust and Lint in the Furnace

When your furnace isn’t used for some time, it is normal for dust and lint to accumulate in the system. This dust can play a big part in why your furnace does not turn on automatically.

Your furnace has an air filter to help keep the dust and other particulates that are taken in by the system circulating in your house. After a while, this air filter gets clogged up, reducing airflow. When it gets too clogged, your furnace will fail to come on altogether.

As cold weather approaches, you should have your furnace cleaned out and check your air filter. This will prevent dust and lint from causing issues with your furnace.

Is a Furnace Delay Dangerous?

When your furnace does not come on when it should, it is not only a sign of a malfunctioning ignition. It also can damage your furnace and pose a safety hazard.

The delayed ignition of gas sounds like a bang for a reason – it is an explosion. Left unchecked, the issues causing the delay get worse, allowing for more gas buildup, and subsequent larger explosions. This will cause vibrations in your furnace, loosening the internal components and eventually causing something to break.

The heat exchanger is also susceptible to breaking from these explosions. They are the most costly part of the system to replace and can cause carbon monoxide fumes to escape during installation. A break or crack in the heat exchanger will mean that you will likely have to replace your furnace. 

Newer systems are equipped with safety mechanisms to prevent significant issues from occurring when your system is failing. However, if you own an aging system, these gas build ups also pose a fire hazard.

If you hear your heater banging when it comes on, make sure that you address the issue immediately. Regular care and maintenance of your furnace will also prevent problems from occurring in the first place.

What are Some Things to Do to Prevent this Issue?

While you can hire a professional to maintain your system, there is a lot that you can do yourself to make sure that your furnace stays in good shape. Maintaining it will not only help you avoid this issue but also extend the life of your system. Here are some tips on taking care of your furnace.

Clean the Vents

Cleaning your vents is fairly simple. Make sure that you have the right equipment before starting.

First, turn off your furnace. Depending on the type of furnace you have, there may be a switch to toggle off – you want to ensure that it cannot come on while you clean. 


Take off your air vent covers and vacuum them. Get some cleaning solution, spray it on, then wipe it off. 

Once this is done, you can reinstall the covers. However, if you are cleaning your vents, you should also consider changing your furnace’s air filter so as to save you some time.

Change the Filter

Remember that a clogged filter will prevent your heater from coming on. It is important to know how often to change your filter – follow the manufacturer’s guidance. It will most likely be about once a month.

When going to change the filter, make sure that your heater is off. The filter is located just inside the air vent.  

Airfilter Of AC.

Before you remove it, you may want to make sure that you know the direction of airflow. The filter usually has an arrow pointing in the right direction. Mark that direction on the furnace so that you do not forget it.  

Grab the filter and slide it out of place, then replace it with the new filter. Finally, close your heater back up and turn it on.

Check Your Thermostat

There are a few simple tests that you can perform to make sure that your thermostat is functioning normally. These checks can be done on a yearly basis; set a time in either spring or fall to check the functionality of both your AC and heat.

First, set your thermostat higher than normal to check if the heat comes on. If you want to check your AC at the same time, do the same but at a lower temperature.

Double-check that the thermostat is on and on the desired settings. While this may sound unnecessary, it will save you from having to call a professional to fix what turns out to be a silly mistake.

Man Checking the thermoatat

Finally, turn the breaker off and open the cover to check the wiring. When you put it back together and turn the breaker on, the thermostat should come back on again. 

Check the Blower

Checking the blower may require some more specialized tools, but is still doable. First, make sure that you have a multimeter before you begin.

Once again, ensure that the furnace is off. If you want to play it extra safe, turn off the breaker that your furnace is on.  

Open the furnace, then open up the furnace blower motor. If you aren’t sure what your furnace blower motor looks like, it’s located next to the air filter at the base of your furnace.  

AC Blower

Make sure that there is no visible damage inside, then use the multimeter to test the motor. Once you’ve tested it, seal it back up.

Check the Surfaces

Look for dust or moisture buildup inside your furnace. Take precautions when touching the surfaces to wipe them – use a damp cloth, and make sure that the surfaces are cool before touching them so as not to burn yourself. 

Use a vacuum with wand attachments to cover the base of the furnace and the burners. You can also vacuum the blower to make sure that there is no dust accumulation there.  

As you are vacuuming and wiping, look for soot. If you see any, you may need to call a professional to check out your heater – this is a sign of poor combustion.

Clean the Pilot or Igniter

While you are vacuuming and wiping down surfaces, avoid the pilot or ignitor. It can be easily broken, so when you are cleaning it, be gentle. As with all the maintenance procedures mentioned here, ensure that the furnace is off.  

AC Igniter

The best way to clean this is to use a wire brush on the mouth of the pilot light, and then use compressed air to blow it out. This is preferred so that you can get any dirt or dust that is deep inside. If you do not have compressed air, you can also use a drinking straw to blow out dust.  

Clean the Flame Sensor

The flame sensor is  located in the rear of your unit, near your burner, and is rod-shaped. Disconnect the flame sensor and carefully pull it out of the furnace. 

AC Flame Sensor

Gently scrub the flame sensor using some light-grit sandpaper, taking care not to scratch the sensor. This will get all the residue off. Use a lint-free cloth afterward to wipe it down. 

Reconnect the flame sensor and ensure that it is secured. Because of the delicate nature of this piece, you may want to test the furnace. As long as it turns on and off properly, you have installed it correctly.


When your furnace is taking more than five minutes to turn on, there may be an issue with it. While there are a number of reasons why the furnace may be taking too long to start, a delayed start can create serious problems for your furnace and for yourself.  

There are some easy ways that you can maintain your furnace. This will not only extend the life of your furnace but will also help you avoid problems with furnace delay. Finally, if you are encountering serious delays or hear a loud bang when your furnace turns on, call a professional to investigate the issue.

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