Heating or cooling room thermostat diagnosis: how to troubleshoot a heating or cooling room thermostat that is not working at all or is not working properly.

Here we suggest step by step procedures in diagnosing HVAC thermostat problems such as a thermostat that is not working at all or one that shows a blank thermostat display or a thermostat that does not turn heat on or off at the right temperatures.

Page top photo: a basic room thermostat showing the red and white wires being switched by the thermostat and a place where it's easy to check for voltage .

Heating & Cooling Control Thermostat Troubleshooting Guide

If your heat won't turn on at all, the problem may not be at the thermostat at all. Start at HEAT WON'T TURN ON. But if you think the problem is at the room thermostat for either cooling or heating, you can also start right here in the steps we give just below.

Question: Wall thermostat is not working - what do I do?

My wall-mounted thermostat is not working. - Ed 8/14/12

Reply: Things to check if a thermostat is just not working

Ed you're not giving us much to go on so my reply is a bit limited. Here are the steps in figuring out what's wrong with a heating or cooling room thermostat:

• Check that the thermostat has Electrical power -
  
  see THERMOSTAT POWER & VOLTAGE. If your thermostat is a programmable unit that uses a digital display, if the display is blank you already know that the thermostat is either disconnected or has lost power.
• If the heating or cooling system has no electrical power check for a blown thermostat or HVAC equipment fuse or tripped circuit breaker; keep in mind that in addition to the fuse or circuit breaker powering the equipment, there may be fuses or switches inside the boiler or furnace itself.
• Next check that power is turned ON to the heating or cooling equipment - power off will often also turn off the thermostat since it's usually powered by a transformer connected to the same circuit as the indoor air handler, blower assembly, or furnace or boiler.
  
  See ELECTRICAL POWER SWITCH FOR HEAT where we explain these electrical switches on heating systems in more detail
  
  If the heating system has electrical power but the thermostat is dead check for voltage at the low voltage transformer that powers the thermostat -
  
  see LOW VOLTAGE TRANSFORMER TEST - separate article.
  
  If your thermostat is a model that relies on its own internal batteries all the time, Check for a dead thermostat battery -
  
  see THERMOSTAT BACKUP BATTERIES - for temporary or full time operation
• Check the continuity of thermostat wiring and that the wiring is correctly installed for your thermostat and the device it controls -
  
  see THERMOSTAT WIRE CONNECTIONS - how-to, separate article. Also check for a short between the thermostat wires
• If the thermostat and the equipment both have Electrical power, identify the two wires that will be connected when the thermostat switch makes or closes and remove them from the thermostat entirely - just jump them together; if the equipment runs then the thermostat was the trouble; if it does not, the problem is elsewhere.
  
  At THERMOSTAT WIRING COLOR CODES we discuss how to identify the different wires in the thermostat and how people jump wires to identify their function - a useful but sometimes risky procedure.
• If the thermostat won't turn ON or won't turn on equipment
  
  see THERMOSTAT WON'T TURN ON
• If the thermostat won't turn OFF or won't turn off equipment
  
  see THERMOSTAT WON'T TURN OFF
• If you are having trouble turning a fan on or off using thermostat switches see
  
  
  • THERMOSTAT SWITCHES, EXTERNAL
  
  
  • THERMOSTAT SWITCHES, INTERNAL
• If your heating or cooling system will run but room temperature is too hot or too cold,
  
  see HEAT ANTICIPATOR Adjustment and
  
  see THERMOSTAT CALIBRATION

Question: what doers "TT" mean in thermostat and HVAC discussions?

I was reading your help forums on thermostat trouble shooting, and needed to know what you are referring to when you say TT? (The Thermostat?) (I have no power at my thermostat)

Can you also tell me the wire colors that I would need to check,or exactly how to bypass the thermostat at the TT?
Is the voltage that I should be seeing 110,or 220 at the thermostat?
Thank You!!! Great Site!!!

Reply:

Scott, thanks for the question - it's an example of how people too familiar with a topic can speak in shorthand that is gobbledygook to everyone else.

TT is simply shorthand for "thermostat" in most discussions; for example in wiring up an aquastat the instructions might at times refer to the TT or "thermostat" terminals.

Most room thermostats operate at 24VDC except line thermostats such as those used to switch some electric heat baseboards.

Useful Heating or Cooling Troubleshooting Articles

Following a list of articles useful in troubleshooting thermostats we include a series of reader questions & answers about troubleshooting room thermostat problems.

Because some controls are used in common on hot water heat, hot air heat, and steam boilers, readers should see these other articles:

Air conditioners & heat pumps: see A/C - HEAT PUMP CONTROLS & SWITCHES

For hot water or steam heat see BOILER CONTROLS & SWITCHES, and also

see BOILER COMPONENTS & PARTS for a detailed list of heating boiler controls, other heating system components, parts such as circulator pumps & draft regulators.

If your building uses warm air heat, 

see FURNACE CONTROLS & SWITCHES

If your building uses steam heat 

see STEAM HEATING SYSTEMS

. Also see HEAT WON'T TURN OFF - Stop Unwanted Heat.

Water heater thermostats are discussed separately at AQUASTAT CONTROL FUNCTIONS and

at ELECTRIC WATER HEATER THERMOSTATS.

Readers needing to find and fix un-wanted air leaks, heat losses, or other energy wasters should

see HEAT LOSS DETECTION TOOLS.

Readers should see ENERGY SAVINGS RETROFIT CASE STUDY and also

see HEAT LOSS DETECTION TOOLS

and INSULATION INSPECTION & IMPROVEMENT for energy saving retrofit detailed guides.

Also see GAS PIPING, VALVES, CONTROLS for more details on how to inspect and test LP and natural gas piping, controls, valves, and tanks.

Room Thermostat Error Code Meanings

Reader Question: How to Translate the error codes on digital thermostats

My Trane digistat has a red button lit and has error code 90? what does that mean? - Tony 7/7/2011

Reply:

Tony error codes are thermostat or heating/cooling device and model dependent. The meaning of HVAC unit error codes are usually printed on a sticker inside the unit cover or control or device cover. If yours has lost that information look for the installation and service manual for the equipment.

If that fails you'll need to call the manufacturer with your model number, serial number, and ask for the manual. That's because there is not a universal standard for digital thermostat error codes - the codes displayed and just what they mean vary among different heating & cooling devices. Some example thermostat and digistat error codes are listed below:

• Trane XL19i A/C Digistat error code 89 or 90 or 91 - check for a loose wire or broken connection in the thermostat wiring;
• Trane 900AC43UA error code 126 - condenser unit lost power in mid-cooling cycle. Turn off all power and wait at least 5 minutes, we recommend 15 minutes, then turn power back on. This may enable the refrigerant pressures in the system to equalize and allow the compressor motor an easier start-up.
• Trane 900AC43UA error code 53 - check for blocked air filter or dirty condensing coil
• Trane Furnace LED Flashing Error Indicators:
  • Slow regular flashing: normal condition, the thermostat is not calling for heat (modern units)
  • Fast regular flashing: normal condition, the thermostat is calling for heat (modern units)
  • Flashing (older units): System has locked out on safety - unable to start and run safely
  • On solid: bad control board
  • Off solid: no power to the unit
  • 2 flashes then pause: System has locked out on safety - unable to start and run safely
  • 3 flashes, then pause: vent pressure switch senses an error
  • 4 flashes: the high temperature limit device is not working or high temperature has been reached - safety off
  • 5 flashes: burner flame sensed when it should be off
  • 6 flashes: reversed hot / neutral electrical power wiring or defective electrical ground connection
  • 7 flashes: gas solenoid valve error or circuit error
  • 8 flashes: burner flame too low
  • 9 flashes: igniter circuit failure & wire marked from terminal "N" to 24VAC voltage is low - check for bad ground
• Digistat 3 water heater Error Code 90 - the unit is not heating the water or the fan is finding airflow resistance that causes it to over-correct. Check for insufficient makeup air, an improperly installed or blocked heater exhaust vent (dangerous), a blocked fan motor or vent, or insufficient BTUs or fuel supply.
• Drayton Digistat+2 Code E1 or E2 - sensor fault, system remains off, call for service [UK Product]

Our image above shows a Trane ComfortLink™ II digital thermostat that uses downloadable software that can be transferred into the device using an SD card or similar memory device.

This is one of at least ten models offered by the company. Contact Trane Corporation at PO Box 9010, Tyler TX 75711, or via their website, www.trane.com or by calling Trane's Thermostat Customer Assistance Helpline at 1-877-880-3381 - don't expect to find detailed thermostat installation guides or error codes for Trane equipment online - the company does not provide that information.

 

Common Heating or Cooling Thermostat Problems, Questions, Answers

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2021-04-07 by (mod) my thermostat will not turn on the heat or the ac

@sunnie,

Your heating or A.C service tech will probably start by removing the thermostat from consideration by disconnecting it at the furnace or AC unit and jumping the thermostat terminals there. If the equipment turns on as it should, then the problem is either the thermostat, or the wires between thermostat and the heater or A/C unit control board.

See detailed diagnostic steps at THERMOSTAT WON'T TURN ON - topic home

On 2021-04-06 by sunnie

my thermostat will not turn on the heat or the ac. when i turn it to either one i can hear the click inside the thermostat but then nothing happens. its batteries are new and it still tells the inside temp on the main screen

On 2020-12-10 - by (mod) -

Gary I don't assume that the problem is the thermostat but it's very easy to remove the thermostat from consideration by disconnecting it at the furnace and jumping the thermostat terminals there

On 2020-12-10 by Gary winstead

I have a 3 year old furnace and ac system. New home. It seems to have a mind of it's own regardijng when it will come on. We have replaced the thermostat, the sensor and the circuit board.
When we replaced each item in turn, the system worked for one or two days. Then upon setting the temp down below 70 at night, it would not come on the next morning for anywhere from a couple of hours until afternoon. so we have just kept it set at 72. It cycles normally.
the thermostat is a Honeywell, one of the smart ones with all the touch controls.

On 2020-11-11 - by (mod) -

Blaine

I'm really sorry but I can't diagnose your system with so little information. I don't know the brand and model of equipment. It's certainly the case that some heat pumps and air conditioning units and Heating units use a circuit board that someone might refer to as a computer. Ask your heating service tech to explain it to you

On 2020-11-10 by Blaine

I have no heat in my home. The thermostat lights up, responds to Fan on but no heat on Auto. The A/C company that came to look at the unit on the roof told me that it was the computer and it would cost $650 to replace it including parts and labor. I don’t know if that’s reasonable or even the actual problem. I never heard that there is a computer component in my 15 year old A/C unit. Please advise if this is true and what would average charges be to replace. Thank you!

On 2020-10-31 - by (mod) -

Annie,

Stand by your furnace while the thermostat is calling for heat and watch what happens. If the blower never comes on but the burner does then the problem is probably the blower and the furnace is shutting off as a safety feature to prevent overheating and damage

On 2020-10-31 by Annie

Why my unit heat come on with air and heat and don't stay on long enough to heat the house go off after 3 minutes

On 2020-10-10 by (mod) - Honeywell CT87 N / CT87K wiring: A/C runs on call for heat - check for shorted thermostat wires

Jeff:

You're right, That TT should work too.

You can check your wiring against those instructions - excerpted below.

Another thing to check is accidental shorting of thermostat wires together anywhere: in the thermostat itself

- a wire end under a screw touching another screw connector, or wires shorted enroute to the control board at the air handler or heat pump.

In our article on HEAT WON'T TURN OFF - topic home we include a step with details on checking for damaged or shorted thermostat wires that in essence has you disconnect the wires from the thermostat

and disconnect them at their other end at the control board

and then use a VOM or DMM in OHMS mode to check for a short between the wires

STEP 3: CHECK DAMAGED THERMOSTAT WIRES

On 2020-10-10 by Jeff M

Wow...thanks for the quick response.

I bought the CT87N. Based on this link on the

owners manual HONEYWELL CT87N / CT87K ROUND THERMOSTAT OWNERS MANUAL [PDF]

CT87N and CT87K are compatible and used for heating & cooling.

The CT87K worked for 15 years.

On 2020-10-10 by (mod) - on call for heat both heat and A/C run on my Honeywell CT87

Jeff:

At THERMOSTAT WIRING HONEYWELL you will find PDF downloads for each HOneywell thermostat including yours.

You'll also find a note from Honeywell in several of their thermostat instructions pointing out that if your home uses both heating and cooling you should have bought a Honeywell CT87B model thermostat.

The wiring table I show below, taken from this Honeywell CT87 thermostat Wiring Guide shows the connections and wires.

Take a look at that and tell me how that matches up with your situation.

On 2020-10-10 by Jeff M

Replaced Honeywell CT87k with The Round CT87 Series Manual Thermostats by Honeywell.

Wiring and baseplate the same. When it calls for heat (hydronic to a common air handler), both the heating coil and air conditioner turn on. Help please.

On 2020-09-08 - by (mod) -

I would like to help but I don't understand the question. Is it possible that there's a Centigrade Fahrenheit setting confusion?

On 2020-09-08 by Tracey

Hello and thank you for reading this! The thermostat downstairs will automatically reset the desired temperature to 85. Yet the air temp side says it's 30 degrees. Any suggestions? Many thanks!

On 2020-08-14 by Anonymous

do you mean the fan for the outside air?

On 2020-08-10 by Debbie Green

Warning sign for air fan?

On 2020-07-21 by Stacy

Temperature on thermostat rises...air working outside...then i flip the breaker twice and then starts to cool to correct temperature...What could be wrong? happened three times in a week...

On 2020-05-30 - by (mod) -

Ralph

Have you had a licensed electrician look at your electrical service? That's where I'd start.

On 2020-05-30 by Ralph Teasley owner

Fuse blown out and run continuously in house hum sound none stoppable to needing repairman come see Trane on wall or other areas of tvs or appliances.

On 2019-11-20 - by (mod) -

Nest stat does not bring on the fossil fuel furnace when the heat pump is set to shut off.

Dennis

Please see your question and my detailed reply suggesting some diagnostic steps now found at

NEST THERMOSTAT SET-UP COOL vs HEAT

Post a followup question or update on what you find and on whatever questions you have next.

Thanks for asking.

On 2019-11-20 by Dennis

The nest stat does not bring on the fossil fuel furnace when the heat pump is set to shut off

On 2019-11-19 - by (mod) -

Lynne

If the thermostat actual setting that's calling for heat at higher temperatures seems to be moving on its own - which would be very unsual - then it's time to replace the thermostat or to try putting a locking cover over it if you think someone is gaslighting you.

If the thermostat setting is not itself going up but the room temperature goes up above the thermostat setting then the wires could be shorted together.

See THERMOSTAT WON'T TURN OFF https://inspectapedia.com/heat/Thermostat_Stuck_On.php

On 2019-11-18 by Lynne

The temperature on my heat setting on my trane thermostat was magically going up and up all by itself. I have always just kept it on auto for heat or cool and in the few years I've had it, I've never just completely turned it off. I tried that and kept it off for several hours. Its been back on for about 3 hours and it has yet to start doing that again. Could it be it just needed to be turned completely off, sort of a reset? Fingers crossed that's all it needed!

On 2019-10-03 by Ralph Teasley

Trane Thermostat and Thermometer has a high rated voltage running on Trane to wall at 905 Hilton Road Anniston, 256-237-9135 call Ralph please.

Question: A/C fan turns on but A/C won't turn off? Is it the thermostat

My fan will turn on and off when i set mode on thermostat, but ac will not turn off. Is it safe to say I need a new stat? - Roll 8/22/11

Reply:

Roll, the thermostat is a simple "on-off" switch as far as your A/C equipment is concerned. I suspect the problem is elsewhere, in a control relay or switch in the equipment itself.

Reader follow-up:

fixed. cleaned contactor, now it does not lock on or off, works as designed. case closed.

Reply:

Glad a simple cleaning step at the contactor fixed the thermostatic control. Just watch out: cleaning of electrical contact points needs to be as gentle and smooth as possible. I used to literally file off burned contactor points, but I learned that the contact point surfaces are plated and that once I too-aggressively filed them down they pitted and burned rather quickly. If the problem recurs, replace the entire thermostat or the contactor relay itself.

Reader follow-up:

Dan: thanks for the advice. I used a small flat head with caution and then used a compressed can of dust off for a final cleaning. This is a 1991 Fedders that purrs like a kitten. I clean every year myself. It feeds to a 1991 Goodman Janitrol that has new fan motor and igniter. Runs great. Key is cleaning annually including electric board. I replace filters regularly.

Reply:

Roll that sounds like a perfect wall thermostat clean-out job, and good advice for others whose thermostats seem to be acting strangely. Indeed on older thermostats I've found amazing clots of dust inside.

Question: My inside air handler blower fan won't stop running no matter what

My inside fan will not shut off even when the thermostat is in the off position and continually blows hot air. The outside unit is not coming on in either on or off positions. I've had to turn the breaker off on both sides. Would this be a problem with the thermostat, the inside fan circuit or the outside unit? - RG 9/6/11

Reply:

RG

- check that no one has pulled the "fan on" switch on the fan limit switch - this only applies if your blower fan is on a combination heating furnace and air conditioning system.

- if that's not your case and the thermostat is not in FAN ON position, and if you are not calling for heating or cooling, I'd try disconnecting the thermostat entirely - it's basically an on-off switch; if it's removed the wires will not be connected; if the fan still runs there's a short in the wiring or a bad control board in the air handler. Most likely; or something else we haven't thought of.

Take a look at HEAT WON'T TURN OFF.

Question: We changed our A/C unit and Thermostat - now it Stopped Working

Hello, Mr. Friedman. I seem to be having a strange problem. We just had our A/C unit and thermostat changed in February, and it was working wonderfully up until today. We just got back from vacation yesterday and had left the thermostat at 80 degrees. We came home, and I switched it from 80 to 75 and then from 75 to 70. And it was working fine up until about three hours ago (it is now 7:15 pm).

The thermostat registers the inside temperature, but there is no side number indicating what temperature I wish to have it at. (Ex.: It's 85 degrees, and I would set it so that it would be 75 degrees, but that number isn't showing up). The fan will run if I switch it to "on", but not on auto

. I have switched the batteries, called Honeywell, flipped all the switches I can find. I'm assuming that this would be the cause of the thermostat, as opposed to the actual unit itself?

We have been having problems for a while now, with strange things. (Last month, the carpet directly across from the unit was sopping wet, and took a week to get up! But there was no leakage directly at the AC unit.) It's been kind of a Murphy's Law joke. I'm not sure what I hope to hear, but any kind of advice would be gladly received.
Thank you.
Jillian 7/9/11

Reply:

I'm not sure about your model but many thermostats show the current room temperature and do not show the set temperature except while you are pressing the set buttons to change the temp up or down. If you have set the temperature well below the current room temp and the A/C won't start, then you will want to take a look at the air conditioning diagnosis suggestions found in the article linked to at the ARTICLE INDEX the bottom of this article and

titled LOST COOLING CAPACITY

About that wet carpeting, if your air conditioning condensate drain system is clogged or leaking that could be the problem; watch out for hidden mold or even insect damage or rot if such a leak is left un repaired.

Question: We changed our thermostat settings and now the A/C unit won't come back on

We had our thermostat set at 79 and it read 79. My wife had some friends over and they were hot so we turned it down to 75. The reading never changed from 79. I decided to turn it off and then on again to see if it would help.

When I turned the switch back on to "cool" it would not turn back on. We usually hear a click and then it starts going. Nothing now. What do you suggest? - Mike 7/11/11

Reply:

Mike on at least some air conditioning systems if you switch the compressor component off and then back on again quickly the compressor can have trouble re-starting against the head pressure in the system.

Usually if you leave the system switched off for 30 minutes or longer you'll get past that problem, but it might be worth looking out for a hard-starting compressor.

Question: My new digital thermostat worked for just 6 hours then stopped

I replaced my thermostat with a digital one and it worked great for like six hours and then before it reached the desired temperature it just shut down. Now the ac nor the fan will come on. The fan will not even come on in manual mode. I flipped the breaker and nothing has worked. What is up with this A/C? Maybe I wired wrong but the unit was working perfectly for a hours. - Mike in Lubbock

Reply:

Check for a wiring error, broken or loose connection, lost power.

Question: A/C system cools for just an hour, suspect thermostat or control board trouble

ok joe this is what i got. Ac system set to 75 get home flipped it on started to cool 1 hour later no more air. Flipped fan switch to on instead of auto on thermo, nothing. pulled the cover off fan unit and saw contactors go on to start fan motor than flipped off.

Decided it must be a bad start /run capacitor, put a new one in and the same thing.

I then pulled out the magnetic coil /contactor /control board and jumped the contactor that was for the fan motor. The motor started right up and ran fine.

Either the thermostat went bad and is not sending the proper signal to the control board or the control board is bad. Got any good tests to figure what is bad? - Ray 7/19/11

Reply:

Hey Ray, try this:

If you suspect a bad thermostat you can just temporarily eliminate it from the circuit: a thermostat is basically an on-off low-voltage switch. If you connect the TT wires to the cooling circuit it's the same as the thermostat calling for cooling.

Or just replace the thermostat - they're not costly.

If the TT is not the problem then from what you described there could be a bad contactor relay or control board itself - that is, if the relay is not damaged then it could be the control board circuit not calling for cooling.

Reader follow-up:

Well I got to your e mail a few hours too late . I replaced the contactor/relay to no avail, after scratching my head for about 20 minutes I decided to find your advice. I found an old thermo and viola!! the system came right on and cooled the house quickly.

Hey i just wanted to say thank you for taking the time to help all of us save hundreds if not thousands of dollars with your expert advice. There are millions of people out there that are willing to tackle problems with a little guidance keep up the good work we all appreciate it even if you don't hear back from us. Ray D

Reply:

Ray D - so glad you got things working, and that our website was helpful. We are dedicated to making our information as accurate, complete, useful, and unbiased as possible: we very much welcome critique, questions, or content suggestions for our web articles. And working together always gives us better information than struggling on alone.

Often a good general approach to trying DIY repairs is to try the easy inexpensive tests and fixes first. Replacing a questionable thermostat is a good example of that approach.

Question: our heat won't stay on

Thermostat has 2 readings, actual temp inside the home and the temp we set that we want inside the home, the heat comes on for 1 to 2 minutes then turns off, and will not warm up the home,we have to turn it completely off then on again for it to start again for another few minutes, what's wrong with the thermostat or is it that? - Sam

Reply:

Sam, take a look at the room temperature and compare it to the "set" temperature of the thermostat.

If the room temp is different from the set temperature then the thermostat is calling for heat (in your case). If the heater wont' keep running and the thermostat keeps calling for heat, the problem is most likely not the thermostat but a start or operating problem at the heater. It may be going off on safety, then automatically resetting - not a cycle that you should continue as it could be unsafe.

Question: My thermostat only works on a call for heat; on call for cooling a fuse blows

when I call for heat the thermostat works, when I call for ac t blows the fuse what is going on - George 4/21/12

Reply:

George,
Calling for cooling turns on the outdoor compressor-condenser unit that is not user during heat calls (presuming it's not a heat pump system). I am guessing your compressor is not starting and is tipping the breaker or blowing the fuse.

Question: How do we balance air temperatures in a triangular shaped room?

hi, i am at work in a triangular room, the shape is /_\, so at the bottom we have the regular wall, and the other 2 sides are formed of long big windows. at the top of the triangle it is always colder than at the bottom... the people in one side are always colder than the others.

We have 5 thermostats; the air vents are on the margins of the 2 glass walls; in the middle of the room we have another air vent; now we were told to keep all of them on Heat or Cold, and at the same temp.

Nevertheless the difference between temp in the room is quite big; for ex we set the temp at 73, and on heat, but the top of the triangle has temp 62 and the bottom 79.... Quite huge difference. could you help in advising what could be done or if it is ok nonetheless to set 2 thermostats on cold in the hotter part of the room and the other ones on heat in the cooler part?
or do we need to do other changes in the room? - Maria

Reply:

Maria balancing temperature in a large but irregularly-shaped open space can be difficult, but the solution usually lies in not only where the conditioned air supply registers are located but also the location of the air returns.

Also whenever anyone changes "just their local area" - like moving a suspended ceiling tile or changing their supply register outlet, they upset the balance of the system. You need an experienced HVAC designer/installer to take a look at the problems you describe.

Question: After installing a new thermostat the A/C worked fine, then started making noise

2 days ago I replaced my old honeywell digital thermostat with a new honeywell digital thermostat because the old one just stopped working - there would be nothing at all on the display and nothing would appear when I pressed any of the buttons.

It was not a battery backed up type. After replacing the thermostat - the AC seemed to be working quite well and I was so pleased with myself. I left to run some errands and when I returned home I could hear the outside unit clicking loudly as if it were trying to come on but then shorting out and then trying again repeatedly. It kept doing this until I got inside and turned the thermostat to "off".

Several hours later I tried again by turning thermostat to "cool" and the ac came on and blew cold air fabulously for about 10 minutes then the outside unit started making that noise again (It's a terrible noise - you know something's wrong). The AC keeps blowing cold, even when the outside unit is making that noise until I turn it to the off position again.

I didn't have the problem with the old thermostat and this only started happening when I replaced the thermostat. Did I buy the wrong Thermostat? What could be causing this?

Reply:

I don't have enough information to make a good guess on this one, but it sounds as if you need a service tech to take a look at the compressor and outdoor fan. Clicking noises could be a bad relay, or even a loose fan blade or a stick that fell into the fan unit.

Question: thermostat won't turn off fan: our attic blower fan won't turn off

Our t-stat will not turn off the blower fan in the attic. The condenser unit ... - Anon 6/8/12

Our condenser unit stopped running so it's not cooling. I tried to turn the blower fan off from our simple non-programmer digital thermostat but it wouldn't turn off. I had to go up to the attic to turn the switch off. - Anon, June 2012

Reply:

Anon,

Check that the "FAN ON" switch is not set on your thermostat. Try setting that switch to AUTO or FAN OFF.

Then if that doesn't correct the problem,

see Fan won't stop: What to do if the Air Conditioning or Heating System Blower Fan Runs Continuously and Won't Shut Off?

or go to our blower fan diagnostic home page

at FAN, AIR HANDLER BLOWER UNIT.

Question: thermostat won't turn off air conditioner

My Honeywell round thermostat is not shutting off my A/C. I have the position set to auto and the A/C works but won't shut the A/C off. Also, it appears not to be calibrated any longer in that I need to adjust the temperature to a much higher temp than actually desired. Is it a "dirty" stat? Help! Roy K, 6/19/12

My outside unit wont shut off unless I turn it off with the breaker. A friend told me it was the thermostat so I replaced it. While in Auto/Cooling the house cools and the fan shuts off, but the outside unit continues to run. Maybe a relay? - Tim 7/16/2012

AC will not turn off when thermostat is at set temperature it run all the time - Scott 8/24/12

Reply:

Roy,

this should be pretty easy to check since thermostat wiring is not complicated. I'm not sure of the age and type of your round wall thermostat - take a look at the photos in the article above - but if it's a bimetallic spring type and dust has clogged the unit it should be possible to simply blow or vacuum debris out of the thermostat. Be careful not to bend anything.

A second diagnostic step: simply disconnect the thermostat wires from their terminals right at the thermostat. That disconnection is equivalent to the thermostat turning "OFF" the call for cooling.

If the cooling system continues to run then the problem is not the thermostat itself.

Other problems that could keep the A/C running include

- a short anywhere in the thermostat wiring that crosses the two wires together (this is equal to a thermostat "ON" call for A/C)
- a problem with the control relay or circuit board in your A/C air handler or compressor/condenser unit

Tim,

If the thermostat is not calling for heat, and PROVIDED that the thermostat wires are not accidentally shorted together somewhere (worth an ohmmeter check), then the AC equipment should not turn on. An indoor air handler blower may keep running if the fan is in ON or Manual rather than AUTO, but the outside unit would shut off. IF the outside unit does not shut off I suspect a relay or control board problem there.

Scott:

- If the thermostat is not properly located it may not properly sense room temperature; for example locating a heating thermostat on a poorly insulated exterior wall or locating an air conditioner thermostat in hot sun can cause problems
- in addition to the advice I give above, check for a dust-clogged thermostat that is not sensing room temperature properly
- a bad control board, relay, or shorted wire could be the culprit

Question: Air Conditioner sometimes won't turn on

BASICS: Apartment A/C unit with heat pump. Inside: Blower, motor, coils, clean drip pan. Outside: One-year-old compressor.

PROBLEM: A/C sometimes doesn't cycle on. Thermostat temperature climbed to 100F during the heat wave. Moving thermostat arm to 60F still doesn't cause A/C to turn on:

The trick is to turn the OFF-COOL-HEAT switch from COOL to OFF to COOL; then, the A/C comes on within a minute and resumes normal cycling (until the next random failure).

NOTE: I can hear fluid moving in the coils (that normal "tingly" sound) when the A/C SHOULD come on.

Any ideas? Apartment maintenance guys' HVAC knowledge seems lacking. Thanks!
- Sleepless in Atlanta, 7/5/12

P.S. I just had them install a brand-new clock thermostat, to replace the old-fashioned manual one. Made no difference in the problem.

Question: Inside Blower Fan Runs in AUTO but not in ON - fan shuts off, coil ices up

why would the inside fan motor quit when in the auto position but yet run fine in the on position? this happens after working fine for 3 cycles in the auto position and then I have to switch to the on position because the fan shuts off and ices up the condenser. I have stat set @ 75 (stat is new) - Woody 7/21/12

Reply:

Woody you've taken a good diagnostic step; we know the problem is in a control, control board, ran relay, or switch, or thermostat wiring, not the fan motor itself.

Let's get clear which fan we're discussing and where icing is occurring. If the indoor air handler fan won't run I'd expect the evaporator coil in the indoor air handler to ice-up. The condenser coil is in the outdoor compressor/condenser unit. You wouldn't expect to see icing there.

Question: Round Honeywell Digital Thermostat - exhaust purge fan keeps cycling

My mother's home has a round Honeywell digital thermostat (just like the one pictured for a previous question). When her a/c is running, the furnace exhaust purge fan comes on and cycles repeatedly (it will run for a short while, then shut off, then start right back up before it has a chance to completely stop). She was told it was a defective thermostat. Does that sound right? - DaveM 7/31/12

Reply:

Dave M

Sounds to me as if the cooling system thermostat wiring is not hooked up correctly - in HEAT mode it would be normal for an exhaust purge fan to start when the TT called for heat. But not when it calls for cooling. It could be mis-wired or it could be that the thermostat model is not one that supports (and has wires run for ) both heating and cooling.

Question: Apartment heat is off for the summer and I don't want the Air Conditioning On but I'm Freezing Cold

The heat is turned OFF for the summer in my apartment building, and I do not want the air conditioning on. The Honeywell on the wall (rectangle), shows 72, but my portable thermostat shows 68 and I'm freezing. What should I do besides using a portable space heater? - Ellen 8/6/12

Reply:

Ellen, in the links listed at the ARTICLE INDEX the bottom of this article under THERMOSTATS see the article titled COOL OFF HEAT, Thermostat Switch

you should be able to turn your thermostat to OFF

Or set the SET temperature on the thermostat to a temperature higher than the room temperature to avoid turning on the A/C

Question: my A/C system won't turn on in response to the thermostat

My a/c contactor is not receiving the "call". If I push in the contactor button, it kicks on and everything seems to work. But, the thermostat won't kick it on. I have a brand new capacitor and contactor. If there was a problem with power getting to the unit, wouldn't it NOT come on if I pushed in the button on the contactor?

Any ideas? I looked for a switch on the blower motor, but none found. Help! - Art 8/30/12

Reply:

Try

- removing the thermostat from the equation by jumping the two on-wires (see our tables of thermostat wire identification) that would be connected if the thermostat calls for heat or cooling - whichever you are testing

- if that solves the problem then the issue is in the thermostat, else I suspect a contactor or control board problem

Question: Heat/AC system cycles on and off too often - short cycling boiler or furnace

New, Heat/AC Honeywell Round Thermostat. (not digital)
Cycles unit off and on too often. Seems to be trying to hold set temperature within 1 degree.
Shouldn't it allow temp 3 degree change change before unit comes on? - Anon - 10/2/12

(Dec 17, 2012) sean said:

The thermostat has been in the same place for 33years, and I replaced the thermostat and it still keeps cycling for heat but they are short cycles less than 3mins. I diconnected the thermostat wires from the board and the furnace still wants to run is it the board?

how does a fan limit switch malfunctioning cause the burner to run I could see the fan running abnormally but not making the burner to run also. again the burner turns on and runs approx 2-3mins then the fan kicks on, directly the burner turns off, then the fan turns off after 1-2 mins then the burner comes on again, and the cycle continues and the temp in my home continues to rise and rise

Reply:

Yep; check for a bad heat anticipator, dust clogged thermostat, improper thermostat location in hot sun or too cold space, or in an area where air from the supply register blows right onto the thermostat

Sean,

Good detective work! Indeed

if you disconnect the thermostat AT THE BOILER OR FURNACE and the system continues to run then there is a control problem at the heater, not a thermostat problem.

But if you disconnected the thermostat AT THE THERMOSTAT then it's possible that the thermostat wires are shorted together somewhere enroute, thus "calling" for heat.

Question: rapid on-off cycling of outdoor compressor/condenser fan

I just bought a new house and I noticed that the fan on the outside unit is cycling on and off in 1-2 minute cycles with the thermostat set to off. This doesn't seem normal to me. I know that there was a freon servicing valve that was stuck and let all the freon escape.

This was fixed prior to us closing and i'm not sure if that has any relation to what is going on now. I'll probably have to call in for warrantee work unless you know why it's doing what it's doing. It seems like the wiring might be incorrectly installed, what do you think? - Joe 10/31/12

Reply:

Joe I agree that rapid on-off equipment cycling is not proper but typically I suspect a bad control or sensor, not the wiring itself.

Question: Our digital thermostat keeps magically switching from heat to cool

Is this a bad thermostat or is it an indication of something else? I have a basic digital thermostat and the electric heat pump system is 5 years old. The outside temperature has ranged anywhere from 14 to 50 degrees outside.

I have the temperature set at 68 on heat and can wake up and the house is in the low 60's. The thermostat will read that it is on cool. I run the settings on the thermostat back to the heating cycle and the house warms back up only to change back to cool maybe 8-24 hours later again. This has been going on for a while. - Janice 12/30/12

Reply:

Janice, be sure that the manual switch on the thermostat is set to HEAT mode;

Question: Gemtech programmable thermostat troubles

I have a Gemtech GTP211D, a programmable thermostat. I set the temperature based on time pf day and day of the week. However, the thermostat program resets itself and loses all the settings. In addition, the heat and air will not be running according to my programmed temperature. Please advise. 12/30/12 - Eunice 12/31/12

Reply:

Eunice,

check for:

• a dead backup battery in the room thermostat
• loose or bad low voltage wires between the thermostat and the heating equipment or low voltage transformer

Question: can I replace my Honeywell round wall analog thermostat with a new programmable one on a steam heating system?

I have a gas-fired steam heat furnace (Burnham - approx 10 years old) with an analogue Honeywell round-dial type thermostat. Is it possible to replace the thermostat with a digital type with programmable heat settings or does the age and type of furnace preclude such luxury? - Anon 2/7/13

Reply:

Replacing an old round analog room thermostat with a programmable unit should be no problem at all - the thermostat is basically just an on-off switch.

The compatibility issues we've come across more often occur if a thermostat had to control a 3-wire Flair-type zone valve - which your steam system won't have.

...

Continue reading at THERMOSTAT BACKUP BATTERIES or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see THERMOSTATS, DIAGNOSTIC FAQs diagnostic questions and answers about wall thermostats for heating & air conditioning

Or see these

Recommended Articles

• MANUALS for HEATING & A/C SYSTEM CONTROLS
• THERMOSTATS, HEATING / COOLING - home
• THERMOSTAT INSTALLATION STEPS - how to replace or install a new thermostat
• THERMOSTAT WIRING COLOR CODES & methods for identifying which thermostat wire is which if yours have lost their labels or have unclear color codes.
• THERMOSTAT WIRE CONNECTIONS detailed room thermostat installation & wiring guide for each heating or cooling system type and each thermostat brand / model
• THERMOSTAT WIRING OPENING SEAL
• THERMOSTAT WIRING in PARALLEL / MULTIPLES
• THERMOSTAT WIRE TERMINAL ID CODES / FUNCTIONS - what are the R, W, and other thermostat wire terminals used for?
• THERMOSTAT SETTING INSTRUCTIONS - Separate article - How to Set the Thermostat
• THERMOSTAT TROUBLESHOOTING - Separate article: What to check if a thermostat is just not working
  • HEAT ANTICIPATOR ADJUSTMENT
  • HEAT ANTICIPATOR ADJUSTMENT - T87
  • THERMOSTAT ERROR CODES - How to Translate the error codes on digital thermostats
• THERMOSTAT WIRE CONNECTIONS - how to wire up all types and brands of heating & cooling thermostats
• THERMOSTAT WON'T TURN OFF - too much heat
• THERMOSTAT WON'T TURN ON - no heat

Suggested citation for this web page

THERMOSTAT DIAGNOSTIC GUIDE 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 HVAC THERMOSTATS

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