Thermostat Switch Setting Q&A-3

Why use FAN ON - for continuous operation
Erratic blower fan operation
Which thermostat setting gives most heat?

Reasons for Using FAN-ON or MAN to Keep Fan Running FAQs

Why would we use the FAN ON to keep our blower fan always running?

Need help with an office dispute. Our central system has heat/air and auto/fan/off switches.

Outside daytime temp has been in the upper 90's for about a month, with weeks more to come.

A co-worker insists on moving the "auto" switch to "fan" resulting in warm air filling the suite and the temperature increasing about 10 degrees F.

He says this is to "keep the air circulating".

While this sounds right, I can still feel the uncomfortably warm air every time he moves it. I was taught, firmly, that the setting should remain on auto at all times. Your thoughts? On 2015-08-07 by Sherry

by (mod) - keeping air moving will generally improve dehumidification and will keep indoor temperatures more even

I tend to agree with your co-worker at least in theory; keeping air moving will generally improve dehumidification and will keep indoor temperatures more even. But the specifics of your office HVAC design might make your co-worker shoot herself in the foot with the pistol of my advice.

As my HVAC genius Mark Camer (Tampa Florida where it's plenty hot) says, "... it depends"

If the return air inlet for your system is picking up very hot air from an inadequately cooled space then when the A/C compressor itself is not running it might blow too-warm air especially on occupants under supply registers closest to the hot return innlet.

The thermostat location can also make trouble - if the thermostat is "satisfied" and turns off cooling while some occupied spaces are still too hot that's the problem.

It's especially tricky to balance cooling in office areas if people subvert the system by closing some registers, opening others, or removing suspended ceiling tiles, all of which were not in the plan of the HVAC designer.

Start by lowering the thermostat setting a few degrees while leaving the fan ON.

Our detailed explanation of why and how we sometimes set the air handler blower fan to continuous ON operation is

at BLOWER FAN CONTINUOUS OPERATION

Is it OK to run the fan in FAN ON mode and let the thermostat turn the A/C on and off?

Is it okay to have my FAN ON set on the furnace, and my air conditioner set to come on at a certain temp?

I notice since the air conditioner has been kicking in now that it's hot, the FAN on the furnace blows constantly but makes a tiny "squeak" sound when the air conditioner kicks in and of course the fan goes into high speed or something or more forceful.

I'm wondering if the "squeak sound it makes when switching is normal and safe".? thanks very muchOn 2016-06-06 by ellen

by (mod) - yes but we warn about that squeaky fan

Yes indeed. That tends to give more even temperatures, does more air particle filtration, and ... in this case won't work too long if your HVAC tech doesn't find and fix that squeak: it could be a failing bearing.

A/C or Heat or Blower Fan Erratic On-Off Behavior FAQs

When I set thermostat to AUTO the blower short cycles on/off every 2 seconds

If i set my thermostat to auto it keeps coming on and off every 2 seconds. What may be the reason? On 2016-04-07 by Alpha

by (mod) - Some blower on-off cycling is normal but not every few seconds

Some blower on-off cycling is normal, such as running on for a minute or two at the end of a heating cycle.

But short cycling every few seconds means we have a sensor or wiring error or a control relay that's failed and needs repair. Perhaps a bad thermostat, a bad wire, or a problem at the heater and its controls.

Watch out: that short cycling can burn up your fan motor. It's time to call for help from a trained HVAC Service Technician. I'd leave the system off until then.

See also these two articles

FURNACE BLOWS COLD AIR -

and

BLOWER FAN OPERATION & TESTING at https://inspectapedia.com/aircond/HVAC_Blower_Fan.php

Even with thermostat OFF the blower fan turns on and off on its own

Even though the thermostat is in the off position, the blower fan will turn on and off by itself. What can cause this? - 2016-03-28 by Ned

Reply by mod:

See the diagnostics and repair procedure at BLOWER FAN CYCLES ON INTERMITTENTLY 

The fan cycles on and off even when the furnace burners are not on

thermostat is set to auto ,yet the fan cycles on and off even when the furnace burners are not on is this normal. On 2015-12-03 by Carm

by (mod) -

Carm

I'd say it happens but it's not necessarily "normal" in the sense of "correct" for the furnace fan to keep cycling;

See FURNACE FAN CYCLES DURING HEAT

that explains why a fan cycles on and off when you don't expect it to.

Then check out the fan limit switch for proper operation at

BLOWER FAN OPERATION & TESTING