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No power to the thermostat - Heating & Cooling

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  • dgallant96 Jul 25, 2008

    it is an air flow air confitioner.

  • B Carson May 11, 2010

    What kind of air conditioner is it?


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1) I meant, is it a window air conditioner or a central air conditioner.

2) I haven't tried yet. Thanks for the low rating.

3) With the information you have provided, I can safely say, "connect the power to the thermostat".

Posted on Jul 25, 2008


Posted on Jul 25, 2008

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1 Answer

My white rogers f189 11 thermostat is off- nothing is lit up?

Remove the cover from the thermostat and check for voltage (usually 24VAC) on the red "R" wire. It comes from the transformer at or in the furnace. Place one probe on the screw marked "R" and the other probe to any of the other wires coming into the thermostat. Depending on how old and the type of system you have, you may have a black wire (Common or neutral), green (fan), yellow (cooling call) and white (heating call). Heat pumps and staged system will have more wires and other colors. If there is power at the thermostat, check that all connections are tight and you have a fresh battery installed if it has one. If it still does not work, replace the thermostst. If there is no power at the thermostat, check for power at the furnace. There should be a disconnect switch near the furnace, it may or may not be fused. Check with a voltmeter to see that you have power at the disconnect first and that any fuses are good. If it is ok, move on to the transformer at the furnace. Check the "Line" side of the transformer for 120VAC power and the "Load" side of the transformer for 24VAC power. If it is ok, and you have no power at the thermostat, the wires to the thermostat are bad or you have a loose connect at one end or the other.

Jan 10, 2015 | Heating & Cooling

1 Answer

My air conditioner thermostat keeps losing power

The power for thermostat come to an from the thermostat with the red and common wire. Check for proper connection to thermostat and check thermostat wire from the thermostat to the air handler. Technicians sometimes do not install splices well and sometimes the splices comes loose. I prefer using rtv loaded wire nuts to make splices when I can find them. Another cause for this problem is bad insulation on thermostat wire allowing wire to short out. If this is your problem, you need to replace the thermostat wire itself. If these do not fix your problem, you may need to replace the thermostat itself.
Hope this helps

Oct 20, 2014 | Heating & Cooling

1 Answer

The display went blank on my honeywell t8600c thermostat. when the batteries were changed, the display shows the time and -AC. Nothing else functions

I believe this thermostat will display -ac if it loses power from the furnace or air handler. Your thermostat will operate on low voltage(same as your door bell) and constantly powers your thermostat. I would check your furnace breaker, and fuses to see if there is power to the equipment

Nov 03, 2013 | Heating & Cooling

1 Answer


This model of Honeywell thermostat uses the 24v from the unit to power it. One of three things must have happened.

1. You have lost power to the furnace, therefore no power to thermostat.
2. You only have 4 wires hooked up on the thermostat and not 5. A lot of older homes had just 4 wire running to the thermostat, which will not work for this thermostat. You should have a wire connected to the (C) or (B) terminal on the thermostat, which ever one they have, I can't remember off hand. These are the Common terminals. They have to have the common from the furnace 24v power to operate.
3. Faulty thermostat, which will obviously have to be replaced.

Hope this helps and gets you to cooler days!

Jun 28, 2012 | Honeywell Heating & Cooling

1 Answer

Rudd thermostat says battery power

Most new digital thermostats are design to run with battery or A/C power, unless the A/C power is connected to the thermostat via a connection to the common "C" connection to the furnace common the thermostat defaults to battery operation and that is normal. Batterys last from 3 to 5 years in the average thermostat.

Oct 01, 2011 | Ruud UBHC Air Conditioner

1 Answer

My air handler isn't working, red light is flashing so it appears to have power, but the thermostat LED display is blank????

Does the thermostat run off battery power or does the transformer supply power to the thermostat. You can check this by removing the thermostat face and checking for a wire connected to the C terminal. If there is no wire there then it is battery only. Replace the batteries. If you have the brand and model # of the thermostat, you can do a Google search for the owner's manual. Of course there is always the possibility that the thermostat has gone bad.

Sep 06, 2010 | Amana Heating & Cooling

1 Answer

Thermostat no power

Hello Friend,

No power to air conditioner ? Reset circuit breaker or change the fuse.

Thermostat dirty ? Clean the thermostat.

Thermostat faulty ? Replace the thermostat.

Thanks & Regards,

Apr 03, 2010 | Carrier Heating & Cooling

1 Answer

Heater and AC run even though thermostat is off

It sounds like you have a short in the control unit at the ceiling pack. Remove the inner shroud from the AC and pull down the control unit. Remove the cover from the control unit to access the relays. With the thermostat off, check for power at the white (or orange depending on the unit) wire coming from the thermostat to the relay board. Also check power from the yellow and gray wires from the thermostat to the control board. None of these should be hot with the thermostat off. If there is power then you have a short in the wiring between the tstat and the upper unit. If there is no power, then you have a bad control board.

Hope this helps,


Rate please!

Jun 24, 2009 | Dometic Rooftop RV Air Conditioner

1 Answer

My ac unit does not have power to the thermostat. There is 200 volts going into the transformer and 24 volts going out. There is 24 volts on the back side of a terminal with thermostat wire running to the...

Please exercise due caution in checking voltages in live circuits!

24 volts sourced by the transformer, through the thermostat, then through the contactor (turns on power to compressor/fan) and returns to the transformer, If the thermostat is closed (calling for cool) you will see no voltage across it, but instead the contactor should be energized. Please note, 200 volts seems odd, you should have between 215 and 235. It sounds more like the contactor relay coil may be open. Is the contactor/transformer one piece?

May 15, 2009 | Heating & Cooling

1 Answer


Check your furnace for power. will be 4 wires. power ground and 2 thermostat wires. Check for power at the time delay relay, if you have power there check for power going in the fan motor, if power in motor wire, motor is bad. If no power in motor wire and power going to circuit board take the 2 thermostat wire apart and wrap the 2 together comimg from the thermostat. Fan will stat in about 30 seconds. If it does you need a control kit in the ac. But sounds to me like you have a furnace problem.

Jan 14, 2009 | Dometic Heating & Cooling

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