Thermostat Switch Setting FAQs-2

Room Thermostat Switches vs
Blower Fan won't start
A/C or Heat Won't Run

A/C or Heat Fan Won't Run When it Should - FAQs

Blower fan won't turn on no matter what

doesn't matter what we do the fan will not turn on, everything seem ok in our thermostat but out home is not cooling down there is a red light on in the furnace. On 2015-12-27 by carla

by (mod) - what to check first when the blower fan won't turn on

Check for an open door to the blower compartment or to a dirty or blocked air filter.

I set my Thermostat to HEAT ON but my furnace won't run

I use Honeywell thermostat, a real simple one.

Thermostat click said "HEAT ON" on the panel.

But the heater is not turning on. Turning on Fan works but the heat does not ever come on. On 2018-03-09 by abuchanan

Reply by (mod) -

AB

Let's start with the diagnostic steps given at NO HEAT - FURNACE https://inspectapedia.com/heat/Furnace-Diagnosis-Repair.php

With thermostat in AUTO the FAN won't turn on but my backup electric heat does turn on

I got a honeywell Focuspro 5000 tstat in the heating mode. When the tstat is in auto the fan wont come on but the heatstrips will On 2017-12-20 by Carl

by (mod) - My fan wont stop when heat is on

Right, Rick; it's normal that in a residential forced warm air heating system the fan will come on shortly after the burner comes on (giving just some time to warm up the heat exchanger so we don't blow cold air on occupants),

and the fan will run all during the call for heat until the thermostat is satisfied

and the fan may run a little after that as well before it shuts down (that's to extract the remaining heat in the heat exchanger and thus to protect it from damage)

See the abnormal conditions for a fan that won't stop at

• FAN WONT STOP - LIMIT SWITCH
• FAN WONT STOP - THERMOSTAT SWITCH

With thermostat in AUTO the FAN won't turn on but my backup electric heat does turn on

I got a honeywell Focuspro 5000 tstat in the heating mode. When the tstat is in auto the fan wont come on but the heatstrips will On 2017-12-20 by Carl

by (mod) - My fan wont stop when heat is on

Right, Rick; it's normal that in a residential forced warm air heating system the fan will come on shortly after the burner comes on (giving just some time to warm up the heat exchanger so we don't blow cold air on occupants),

and the fan will run all during the call for heat until the thermostat is satisfied

and the fan may run a little after that as well before it shuts down (that's to extract the remaining heat in the heat exchanger and thus to protect it from damage)

See the abnormal conditions for a fan that won't stop at

• FAN WONT STOP - LIMIT SWITCH
• FAN WONT STOP - THERMOSTAT SWITCH

With new programmable thermostat & hyrdonic water-to-air heat, in HEAT mode the blower won't turn on in AUTO mode

I just replaced my thermostat with a new programmable thermostat. I have a Hydronic air system, boiler, air handler and air conditioning unit.

In HEAT mode when the thermostat is calling for heat, the blower fan will not turn on in AUTO FAN, but will in FAN ON mode.

In COOL mode the blower fan comes on in both AUTO and FAN ON mode when the thermostat is calling air conditioning. Any idea what would cause this? On 2018-11-16 by John

Reply by (mod) -

Assuming that you remember to put the thermostat in to HEAT mode, if the blower fan never turns on I would start by watching The Fan limit control switch.

You should see the furnace or heat Source come on and the plenum temperature rise and the fan limit switch dial rotate.

If you're certain of that and the fan is not turning on there's probably a control board, relay, or wiring error.

Check our diagnostic details at

FAN LIMIT SWITCH TROUBLESHOOTING

At my Tempstar air handler In AUTO mode, the fan won't turn on in AUTO mode but will run in MAN or FAN-ON mode.

My tempstar nugg075df03 air handler blower won't always turn on in auto mode but will always turn on when fan on switch is actuated.

I have checked all voltages at transformer 110/24vac replaced thermostat suspect fan limiter is the problem. What do you think! On 2016-07-27 by gene woodfin

Reply by (mod) -

Gene

In AUTO mode, the fan won't turn on until other conditions are met, such as the thermostat calling for heating or cooling. The fact that the fan runs in ON or MAN means that the trouble is not in the fan itself.

If your thermostat IS calling for heating or cooling but the fan won't turn on, there may be a problem with the fan relay or control board in the air handler.

If your system is in HEAT mode, then yes the fan limit switch could be in trouble.

by gene woodfin - two different circuits to turn on the Tempstar air handler fan

My Tempstar air handler has two paths to the fan run relay.

In the fan switch on mode your turning the fan on thru a n/c contact on the relay.

In the auto the 24vac takes a different path thru the fan limiter unit n/c contact

Reply by (mod) -

Thanks for that added detail, Gene, it'll help other readers.

I set my A/C fan to AUTO but the thermostat says COOL ON - what's wrong?

I have put the air conditioner fan on AUTO but on thermostat it shows COOL ON,

So I'm trying to figure out what went wrong. - On 2016-07-05 by Elle

by (mod) - Fan set to AUTO means the fan will run automatically when the system is cooling (or heating)

That's right: Fan set to AUTO means the fan will run automatically when the system is cooling.

But the fan setting does not call for cooling;

The thermostat switch needs to be set to COOL and the thermostat temperature control needs to be set below room temp.

See

• AIR CONDITIONER / HEAT PUMP WON'T START - when the A/C won't turn on when you want
• AIR CONDITIONER / HEAT PUMP WON'T STOP - when the A/C won't turn off when you want
• FAN ON AUTO MAN THERMOSTAT SWITCH - instructions on using the thermostat AUTO setting

Furnace won't turn on when the temperature falls low - is it the thermostat?

I have a dual system, oil furnace and heat pump.

Furnace is not switching on when the outside temperature falls under -16 c. On 2018-01-08 by Mil

Reply by (mod) -

That may be a control board or relay or outdoor temperature sensor failure in your heat pump system.

It should be possible to simply turn off the heat pump and switch on your oil furnace. In emergency one may be able to simply move thermostat wires to the furnace control.

Carrier heat pump won't turn on - is it the thermostat?

My heat does not come on at all. I have a carrier heat pump thermostat. It is not working at all.

It will not turn on at all. It was working yesterday morning when I left for work. When I came on yesterday evening it was not working.

My thermostat has two bulbs of Mercury in it. Thanks On 2017-11-29 by Melanie

Reply by (mod) -

Melanie, let's start with some simple diagnostics beginning at NO HEAT - HEAT PUMP

since when a heat pump won't turn on at all it's almost the same as if an air conditioner won't start.

Let me add that if you have a thermostat that uses mercury bulbs, as I'm looking for those old thermostats for technical study, you can contact me by email and I will send you a free Honeywell thermostat that you can try in replacement of the old unit. If interested use the page bottom CONTACT link.

- Update: My study was later completed, so this offer EXPIRED - so other people, please don't also ask me for more free thermostats - Ed.

To get the most heat do we set the thermostat Fan to AUTO or FAN-ON?

To get the most heat, should it be set at Auto or Fan? On 2018-01-30 by Beverly Dunham

by (mod) - changing the fan won't give you more heat

Thank you for an excellent question, Beverly.

Setting the furnace fan from AUTO to MAN or ON, thus causing the fan to run full time will not "get more heat" out of the furnace than leaving the fan on AUTO.

While there are some reasons to run the fan full time in some situations (such as to improve IAQ by filtering dust from the air), generally you should leave your fan control set to AUTO.

During the heating season, running the fan full time means that the fan will be blowing even when there is no call for heat (the thermostat is satisfied). An effect of doing that is that at some points the room occupants might be made uncomfortable when the fan blows cool air out of the supply registers.

Furnace controls (particularly the fan limit switch) are designed (in AUTO mode) to avoid turning on the fan until the furnace supply air plenum has warmed-up, specifically to avoid blowing cold air on occupants.

Take a look at

FURNACES, HEATING - home, to review the basics of warm air heating systems.

Then be sure to read AIR FLOW TOO WEAK since that article WILL help you get the most heat from your existing furnace system.