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Fan ON AUTO switch settings explainedA/C or Furnace Blower Cycling Intermittently

Fan turns on/off when there is no call for cooling nor heating

This article describes reasons that an air handler's blower fan might turn on and off when the room thermostat is not calling for heating nor air conditioning.

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Intermittent A/C or Furnace Blower Fan Cycling On-Off

On 2021-05-26 by Sandee - why is our fan turning on and off sporadically?

Illustration of a house ventilation system showing a motorized damper in an air duct used to admit outdoor air - cited & discussed at InspectApedia.comI didn't see anything about when a fan sporadically turns on and off when the heating and a/c are off.

My thermostat does not have an off selection for the fan, just on/auto.

I have turned the heat and a/c to off but the fan still cuts on several times a day.

I want to have my windows open to enjoy the good weather but I'm nervous to with the fan coming on. I've checked the vents, nothing is coming out of them. Any ideas?

On 2021-05-26 by (mod) - Furnace or Air Conditioner Blower Fan Runs Intermittently for No Reason

Illustration: this Honeywell Fresh Air Ventilation system is easily added to a home HVAC system and is described in the product literature cited below.

[Click to enlarge any image]

@Sandee,

Thank you that's a very helpful question about the causes of the blower fan running intermittently during the day when there is no call and has been no call for heat and no call for air conditioning for a quarter of an hour or longer.

Let's re-state it as: Furnace or Air Conditioner Blower Fan Runs Intermittently for No Reason

First check: for your case you say no air is coming out of the vents but your blower fan is running.

If your blower fan runs normally and air flows out of your supply registers when your thermostat is calling for heat or air conditioning (cooling)

but at other times during the day Your blower fan turns on but no air is delivered,

then

Watch out: that's pretty odd: I suspect something I dislike citing: a combination of problems such as a venting system that's trying to run but an automatic duct damper that is sticking closed. That's unusual. Most-often there is a single, simpler explanation. you will need to call an HVAC service technician to inspect and diagnose your system.

Please confirm that I have understood you correctly.

What to check next: is there a ventilation system?

IF your residential air system for heating and / or cooling includes an outdoor fresh air ventilation system then that system may have its own independent controller.

An example of a fresh air ventilation control that might cause your residential air handler to run intermittently even when your thermostat is not calling for heating or cooling is the Honeywell Fresh Air Ventilation Control system illustrated here.

Soler & Palau ventilatino system controller cited & dicussed at InspectApedia.comSo you can start by a simple visual inspection of controls mounted on or at your air handler (or as your HVAC service technician to check for you) to see if your system has a fresh air ventilation control system like the controls shown on this page - see if one is installed.

You might find this ventilation system installed on a residential building like a one-family home particularly for newer homes that are built very air-tight and that were designed for very energh-efficient operation.

We discuss this situation at INDOOR AIR QUALITY & HOUSE TIGHTNESS

For an indoor residential heating or cooling system that does NOT include a fresh-air ventilation feature, I don't know of any reason that the fan should run when it's not asked to, with two exceptions that we document in two separate articles I'll cite below.

Illustration: Soler & Palau's ventilation control cited below.

So if your blower fan (the indoor air handler) is running intermittently during the day well after any call for heating or cooling (say 15 minutes or longer afterward) and if your HVAC system does NOT include a fresh-air ventilation feature (an extra feature used more-often on commercial buildings) then

I would expect your service technician to look for a loose electrical connection at the thermostat - fan control wiring, and at the fan control board itself;

See also the furnace fan cycling articles listed below.


More about fresh-air ventilation systems:

Some HVAC systems include an additional controller that might cause the air handler blower to run intermittently to bring fresh air into the building. You wont' normally find this on a residential HVAC system.

Recommended below are some of our articles discuss fresh air ventilation for buildings


...

Continue reading at FURNACE FAN CYCLES AFTER HEAT, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

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BLOWER FAN CYCLES ON INTERMITTENTLY at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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INDEX to RELATED ARTICLES: ARTICLE INDEX to BUILDING VENTILATION

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INDEX to RELATED ARTICLES: ARTICLE INDEX to AIR CONDITIONING & HEAT PUMPS

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