Why does fan come on when heater is off?

If your blower is constantly turning on/off when the furnace isn’t heating… …you most likely have a bad fan limit switch. The system uses a “temperature probe” to detect temperature changes inside the heat exchanger (see image below) and turns your blower on and off in response.

Why is my AC blower not turning off?

The most common reason that your Air Conditioner fan won’t turn off is because a stuck relay keeping the circuit closed. If it’s not the relay, your thermostat likely has a shorted cable or needs to be replaced.

Why is my AC still running after turned off?

If your air conditioner keeps running when it’s turned off this could be a sign of shorted thermostat wiring. A continuously running air conditioner could also be triggered based on the settings selected for the unit.

What can cause the blower to come back on for a short time after cycling off?

Usually this is a problem of residual heat in the furnace heat exchanger rising into and re-heating the plenum air, causing the fan limit switch to turn the fan back on to try to cool down the system by extending the heat exchanger temperature purge cycle time.

Should I set my fan to circulate?

Mild Temperatures Can Be Optimal For Fan Usage So generally speaking, it makes sense to run the fan constantly, or use the circulate option on a thermostat, if there is minimal heating/cooling taking place and you aren’t opening the windows for ventilation or air movement.

How do I know if my fan limit switch is bad?

The most common sign of a malfunctioning limit switch is the continuous operation of your furnace’s blower. Just as the limit switch won’t allow the fan to turn on until the warm air has reached the right temperature, the limit switch also shuts down when the air cools to a certain temperature.

What is the likely cause if nothing happens when the thermostat is set to heat?

What’s the likely cause if nothing happens when the thermostat is set to heat? If your furnace does not heat at all. Thermostat malfunctions cause most heating system failures. Other causes include a tripped circuit breaker or blown fuse, or—in the case of combustion furnaces—a pilot light that has gone out.

What is fan circulate setting?

Most new thermostats have a “circulate” option that allows you to set the fan to run a certain amount of time per hour. When the heat is on, the fan will only turn on as your home heats up. When cooling, the fan will only run while your air conditioner is running.

What happens when a central air blower does not start?

The blower motor starts, turning fan blades that circulate cold air from the evaporator coils, through the home’s ductwork and into each room. If the blower motor fails to start, nothing happens. The living spaces gets uncomfortable as the cooling cycle stalls. Most thermostats control both central air and central heat.

Why does my furnace blower turn on and off all the time?

But since temperatures inside the heat exchanger should stay relatively stable when the furnace burners aren’t on, your blower fan shouldn’t be needed at all. So if your furnace blower is turning on and off when the furnace isn’t in a heat cycle, it usually means the fan limit switch is defective.

When does the HVAC fan turn on and off?

In normal operation, when your thermostat is set to “auto,” it will tell the blower fan plus the heating or cooling components to turn on and off at the same time when the desired temperature is reached. In many systems, the blower fan runs a minute or so before heating or cooling begins and ends to get air flowing.

What should I do if my blower fan won’t start?

Step 1: I would start by confirming that the blower fan will operate. Your system probably has a manual FAN ON switch on the thermostat, or if not, there may be a FAN ON push pull switch right on the fan limit switch in the air handler.

Why does my a / C blower fan keep running?

There are some reasons (explained at BLOWER FAN CONTINUOUS OPERATION) to leave the blower fan on continuously for long periods, such as to use an air filtering system to improve indoor air quality. And in some buildings we may run the blower continuously for more even or better conditioned (heated or cooled) air distribution.