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It's not uncommon for a fan controlled by your Honeywell thermostat to run before heating kicks in or after cooling stops. But, a HVAC fan that won't turn off and runs on its own for hours can be indicative of a problem. 

In addition to the annoying sound, if a fan is running continuously, it will add to your energy bill and also reduce the lifetime of your equipment. So it’s better to check why your fan is continuously running and then solve the issue.

How your HVAC fan normally works

Although a HVAC fan is designed to work in sync with your heating and cooling system – it's a distinct device that is controlled separately. 

Basically, your thermostat is programmed to detect the temperature in your home and see if cooling/heating is needed. The thermostat then sends signals to the appropriate HVAC device to turn on/off. The fan is used to transfer hot or cold air from your HVAC devices to your home. If the thermostat detects that the desired temperature has been reached, it’ll turn the fan off to save energy. If your fan is continuously on, this might be due to a fault in this mechanism.

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At SmartHomePerfected we always advise readers not to undertake any work they are not competent in performing and if in any doubt, it is best to seek the assistance of a professional.

Reasons your HVAC fan controlled by a Honeywell thermostat won’t turn off

1. Thermostat Settings

Typically, the fan selection key on Honeywell thermostats will have two (or three) settings: On, Auto, and on specific models, Circ (for circulate). 

The On setting will help switch the fan on to run continuously. When you choose this function, the fan will run, notwithstanding whether the cooling or heating units are on or off. 

Auto Setting

The Auto setting allows the thermostat to run the fan along with the heating or cooling unit whenever they're switched on. This function won't allow the fan to run without one of the temperature units working. This is the preferred setting since it saves energy.

Finally, the Circ. setting will allow the fan to run around 20 minutes per hour. It is often reserved for seasons where the indoor temperature doesn't need any heating or cooling to stay comfortable, such as the spring or fall. 

That's why, if you've selected the On setting instead of Auto on your thermostat, your fan will run continuously and won't turn off.

You can find the manual for your specific Honeywell Thermostat here.

2. Extreme Temperatures

Another common cause why your Honeywell thermostat fan won't turn off is extreme weather events. 

For instance, if the temperature takes a drastic drop or rises uncontrollably – it can force your HVAC system to keep the fan working overtime to maintain your preferred indoor temperature. 

Size

This problem generally occurs when the HVAC system is too small to handle climate control for the given space or hasn't been serviced or maintained for some time. 

In both these cases, the HVAC system will continue to rely on the fan to sustain the temperature. That is until the weather takes a turn for the better.

3. Stuck/Damaged Relay

The relay switch helps turn the HVAC equipment on or off. That's pretty much what a thermostat does too, but the difference is that a relay switch helps the thermostat and HVAC equipment function together – despite their voltage differences. 

HVAC equipment is generally line voltage, whereas thermostats are low voltage. That means that they can't be placed in the same circuit together – unless there's a relay switch involved.

How it works

At the risk of being too technical, we’ll briefly explain what a relay does. A relay’s functioning consists of an electric current going from your thermostat to the relay coil, where it's changed into a magnetic field. This magnetic field then switches the metallic switches on and off. Thus, depending on the signal from the thermostat, the relay switches the HVAC component.  

Relay switches are generally found on the sides or the back of your air conditioning unit. You can also refer to your equipment's user guide to discover a relay switch's location. 

Broken switch

A broken relay switch can be the reason why the settings you're choosing on the thermostat aren't being communicated to the fan. There can be several reasons why your relay switch is malfunctioning. But, more often than not, natural or electrical wear and tear are to be blamed. 

Replacing a broken relay switch might require seeking support from the manufacturer or referring to a qualified technician.

4. Bad Sensors

Bad sensors can also be why your HVAC / Honeywell thermostat fan won't turn off. Think about it, if your temperature sensor is faulty, then your thermostat is running blind. 

If a sensor cannot gauge the temperature of a room correctly, it's sending inaccurate results to the thermostat. Which, in turn, can prompt the thermostat to keep the fan operational despite there not being a need for it. 

The easiest way to check for a faulty sensor is to use a digital thermometer for the most accurate reading. If room temperature in the thermostat display and the thermometer don't match – you've found the root of the problem. 

5. Thermostat Wiring

Most modern thermostats have a separate wire for controlling the fan. It's green in color and is attached to the G terminal at the back of your thermostat. However, most new thermostats also require an additional C-wire, which many household systems do not have. 

So technicians often refurbish the fan wire as the C-wire, and do not connect any wire to the G terminal. In this case, the fan remains on indefinitely, since it is not being controlled exclusively by the thermostat.

You can easily ascertain if this is the case by first checking to see if your thermostat needs a C-wire to run. If yes, then remove the thermostat's cover to reveal the backplate. If there's no wire attached to the G terminal, you may need to change the wiring to settle the fan issue.

Impact of a Honeywell thermostat fan that won’t turn off

One of the biggest drawbacks of your thermostat fan running continuously is energy wastage and higher costs. Another disadvantage is that you'll need to spend more to keep replacing your filters because air is constantly passing through them. 

There are also weather-specific problems that you might face. For instance, a fan that won't turn off during the winter may lead to your indoor temperature being cooler than you'd like. On the other hand, in summers, your home temperature can get uncomfortably warm – which will lead to you relying more on your cooling unit than is necessary. 

Also, let's not forget that a fan that won't turn off can be a sign of more significant concerns, which will require immediate attention to keep from snowballing into something costly to fix. 

The Verdict

A fan that runs constantly may not seem like a big concern, but it's not the kind of problem you should ignore. If your fan won't turn off no matter which thermostat setting you try – it's best to bring in skilled technicians to handle the situation for you. 

Need Help with a Honeywell Thermostat?
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Sources

Honeywell

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