Why does the blower fan keep running even when the AC isn’t? Why won’t the air handler fan shut off? This issue is addressed, and we hope solved, in this Pick HVAC FAQ post.
Life for many of us is hectic much of the time. Because of this, you might not notice at first that your AC is off but the fan keeps blowing.
But eventually, someone in the family is going to notice that the fan that circulates air through the house never seems to shut off. That may sound like a simple issue, but there are quite a few reasons why this may be happening.
Reasons your AC Fan Won’t Shut Off (Indoor Fan)
Here’s a list of reasons “why AC fan won’t turn off,” starting at the simplest and easiest to fix.
Thermostat Settings Wrong
On the thermostat located on an inside wall of your house there is one setting to control the fan and another to control the temperature. If you have a digital thermostat – especially if it’s a smart thermostat – these may be digital settings that you find by toggling through the settings. On others, these may be manual settings that you select by means of a small switch. In either case, there are thermostat settings that can cause your fan to run all the time.
1. Fan Setting
Here, you have the choice between “ON” and “AUTO”.
In the “AUTO” position, the fan will normally only run when your AC outside unit is running and the thermostat is not satisfied that your room temperature has reached what it is set to. As soon as that temperature is reached – as soon as your indoor air is as cool as you want it to be – the outside unit will shut off and the indoor fan will stop running 20 to 60 seconds later. It runs for those seconds to push the remaining cooled air out of the ducts and into your living space.
But if the fan setting is in the “ON” position, the fan will run all the time. There might be a situation when you want this to happen for a while, but normally, the fan switch should be in the “AUTO” position, or “AUTO” should be selected on a smart thermostat. Otherwise, you will discover that your fan is not turning off, regardless of the room temperature.
Pro Tip: We recommend leaving your thermostat in the AUTO mode most of the time. There’s an ongoing debate about this among HVAC pros and homeowners too. In ON mode, the air in your home will constantly cycle through the filter, so it will be more thoroughly “cleaned.” Secondly, the humid air in your home will constantly cycle over the cold evaporator coil, which will condense more moisture out of the air making it drier and more comfortable. Those are the “pros” of leaving the thermostat in FAN ON mode.
However, this mode will use more electricity, creating higher utility bills, especially if you have an older blower motor. They’re usually PSC motors that use much more power than ECM motors. Learn more about the difference here.
Secondly, eventually the air will be well-filtered and the coil will warm up, at least until the AC kicks on again. Your “gains” from the ON mode will be lost.
So, if for some specific reason you want short-term increased air filtration or the air is extra humid on a particular day – maybe you’ve been away and had the AC off, and you come home after a few days to a warm, dank house – then it makes sense to run the system with the fan in the ON mode for a few hours until indoor conditions improve.
2. Thermostat Temperature Setting is Too Low (Too Cold)
In a properly insulated and ventilated house, an efficiently functioning AC unit can be expected to lower the indoor temperature about 20 degrees lower than the outside air. So if it is 80 outside and your thermostat is set on 70, your AC should be able to cool your house to 70 easily. And if the day heats up to 90 degrees outside, your air conditioner should still be able to cool your house to 70. But if it gets up to 100 degrees outside, you will probably experience a constantly running AC system with both outside and inside units not turning off. An exception to this is if you live in a very hot climate. In this case, the system that was installed may be large enough to accommodate higher average outside temperatures.
The solution to this is to be realistic when you set your thermostat. Normally, a setting between 70 to 75 will result in a comfortable atmosphere indoors. But if outside temps are expected to get above 90, it’s best to raise the setting on your thermostat to prevent overworking your AC system and having the fan run all the time.
It’s also possible, however, that the setting on your thermostat was accidentally changed to a very low temperature which may be literally impossible for the system to reach. In this case, simply reset it to a more normal temperature, and you won’t have the situation where the system runs all the time.
Thermostat is Bad
Since the thermostat contains components that tell the indoor fan when to start and stop, a defective thermostat may be the cause for the fan to not shut off. If you have first made sure that the settings are correct, you can then check to see if the thermostat is the problem quite easily.
Take the cover of the thermostat off by grasping it on its sides and pulling straight away from the wall – towards you. If the fan immediately stops running, you have just proved that the problem is in the thermostat itself. Putting the cover back on will cause the fan to start running again, though it might be after a delay of up to a minute or so. If the fan starts running again – the ac is off but fan keeps blowing – it’s time to replace your thermostat.
Shorted Wires to the Thermostat
On the backplate of your thermostat, there is a terminal where several small wires come from the air handler, which could be the furnace, in your house. This is usually part of your heating system, since the same fan is used to circulate both warm air (Heat Mode) and cool air (Cool Mode). When the thermostat wants the fan to run, components inside connect two of these small wires, usually R, red (24 v) and G, green (fan).
These wires are inside a common insulated sheath, but are very close together. If they become “shorted” – touching each other outside their insulation – this would result in the situation where the AC is off but fan keeps blowing. This can happen if the wires have become pinched anywhere in the wall, if a rodent has chewed the insulation or if a nail or screw has been put in the wall and through the insulation. It might also happen if the thermostat is loose on the wall and wiggles every time someone adjusts its settings.
Pro Tip: If you have a multimeter and want to see if this is the problem, you can do it by removing the cover of the thermostat. Then check for a reading of 24v between the G (fan) and C (common) terminals on the backplate. With the cover off, there should not be voltage there; the multimeter should read zero volts. A 24v reading between these two terminals is probably caused by shorted wires.
Before you call an HVAC technician, make sure all the wires are firmly secured to their posts and that no bare sections of different wires are touching one another. If a wire has been stripped too far down its length, causing it to short with another wire, wrap the over-stripped area in electrical tape to insulate it. That might be all it takes to solve the “AC fan won’t shut off” issue. If you believe there’s a short after testing it, but you can find it, then you’ll likely have to call a pro.
Fan Control Relay Defective or Stuck
Inside your furnace/air handler are several components that tell the fan to turn on or off under various conditions. The fan control relay is one of them. It is located on a special printed circuit board along with many small components. If the fan is constantly running because the fan control relay is defective, it would mean that the relay contacts are closed all the time and needs to be replaced by a competent technician.
It is also possible that the contacts are stuck in the closed position. This can happen if the contacts have become pitted from years of use. The relay is usually a square plastic box on that circuit board. If you can locate it, a tap (not a smash) with the handle of a screwdriver might possibly cause the stuck contacts to separate and work normally again. That’s a short-term fix, of course. If the relay can be replaced and you’re fairly handy, it can be a cheap DIY job. The part costs $10 to $25 depending on the model, and is pretty easily switched out.
Here’s a short video on how to test a fan relay switch.
3 Amp Fuse Blown
This small fuse – similar to one used in most vehicles today – is considered a “low voltage” fuse that is designed to protect the small, sensitive components in your control board. It might be on the board itself, or it might be connected to the board, just sort of “hanging” by two wires. In some systems, it could also be a 3 amp circuit breaker mounted on the side of a metal panel inside your air handler. Here’s the fuse installed on a circuit board.
If you can find this small fuse, you can tell if it is blown by looking at it carefully. Inside the plastic body you will see an “S”-shaped line that connects the two terminals of the fuse together. If it appears blackened, or if you can see a break in the “S”, that means it is blown. You can replace it with another fuse of the same amperage, but many times it will blow again as soon as the system is turned on. This is an indication that there is a more serious problem that should be addressed by a reputable service technician.
Extreme Weather Conditions and Poor AC Sizing
As mentioned before, but worth repeating for emphasis, AC systems are only designed to deliver cool air to the inside of your house that is 20 degrees lower than the outside air.
When your AC system was installed, it should have been sized to be appropriate for the square feet of living space in your house, your home’s layout, building materials, number and type of windows and other factors. This was to cool your house efficiently using the least amount of energy required, and keeping your energy bill as low as possible. What is considered “appropriate” may differ somewhat in different climates. But unless you have an oversized AC system, extreme outside temperatures that exceed the ability of your system to maintain the thermostat setting can result in your fan – and even your entire system – to not shut off.
Pro Tip: If your AC runs “all the time” on days that aren’t ridiculously hot, then it might be undersized. This page about the AC Size You Need is a good place to start learning more about this common air conditioning problem.
There is also more information on AC sizing, efficiency, performance and quality in our Air Conditioner Reviews and Buying Guide, one of the most visited pages on Pick HVAC.
What Is Normal?
Under normal circumstances, you should hear your home AC system cycle on and off throughout the day. Naturally, it will cycle on and off more times during the hot days than during the cooler nights. Also, when the outside temperatures are higher, it will normally run for longer periods than when they are somewhat lower.
But if you discover that the AC is off but the fan keeps blowing, that is an indication of a problem somewhere in the system. The above items may help you find the cause and you may be able to fix it. If not, be sure to contact a local HVAC repair company that will be able to resolve the issue.