Diagnose & fix a room thermostat that that won't turn the heating or cooling system ON when it should - room thermostat troubleshooting:

here we explain how to diagnose and fix hermostat wiring problems that cause a heating system or cooling system to stay OFF when it should turn on.

We provide a sequence of diagnostic steps that determine if the problem is at the thermostat, the thermostat wiring, or at other heating or cooling system controls. We include some simple VOM tests or DMM tests to check for a broken or "open" thermostat wire and tests to check for thermostat wires that are shorted together.

Thermostat Troubleshooting: heating or cooling system won't turn ON - Step 1

Here we discuss how to diagnose & fix a thermostat that seems to refuse to call for heating or cooling when it should. We explain how to rule out the thermostat in a "NO HEAT" diagnosis procedure.

If you're confident that the thermostat is not the problem, the see this separate article, HEAT WON'T TURN ON where we give diagnostic procedures for other reasons why building heating or cooling system will not turn on when it should.

Change the thermostat setting up or down

For heating: if the heating system won't turn ON, that is NO heat (or cooling) is being delivered when you want it, set the thermostat to its highest temperature setting and wait three to five minutes. If your thermostat has a HEAT OFF COOL or FAN AUTO OFF control, set the control to HEAT (for heating) and set the FAN to AUTO.

For help locating these thermostat switches

see THERMOSTAT SWITCHES, EXTERNAL.

• If the heating system starts  tthen the problem is most likely the thermostat setting,
  
  Optional - see THERMOSTAT CALIBRATION
  
  or THERMOSTAT WIRE CONNECTIONS, or external influences like position of the thermostat on the wall, exposure to drafts, etc.
  
  You might also want to
  
  see TEMPERATURE RESPONSE of ROOM THERMOSTATS.
• If the heating system does not start continue
  
  to STEP 2 below.

Watch out: for warm air heat blower units behave differently than hot water or steam heat systems: to avoid damage to the heat exchanger the air handler or blower unit will normally keep running for a minute or even a few minutes after the thermostat has stopped calling for heat.

So for warm air heating systems, give the system a few minutes to shut down.

For hot water and steam heating systems, because the heating baseboards, convectors, or radiators will be hot and because they have some thermal mass, heat continues to be delivered for a longer interval even after the heating system itself has stopped.

Therefore to check whether or not setting the thermostat temperature down to its lowest setting (or to OFF) has turned OFF the heating system you need to listen to the heating unit itself to observe that it has stopped.

For air conditioning: if the cooling system wont' turn ON, that is if no cool air is being delivered even when you want it, we do the opposite: set the thermostat to its lowest temperature, or if your thermostat has a HEAT OFF COOL or FAN AUTO OFF control, set the control to COOL and FAN to AUTO.

• If the cooling system starts then the problem is most likely the thermostat setting,
  
  THERMOSTAT CALIBRATION,
  
  THERMOSTAT WIRE CONNECTIONS, or external influences.
• If the cooling system does not start continue
  
  to STEP 2 below.

Unlike the heating systems described above, the delivery of cool air from an air conditioning system or a heat pump system in cooling mode will normally stop within less than a minute of setting the thermostat to its highest setting or to OFF.

Thermostat Troubleshooting Step 2: confirm that the problem is in the thermostat or its wiring

Remove the thermostat face or body to expose the thermostat wires themselves. If you simply connect the two thermostat wires, usually the red and white (for heat), the heating system should start or the zone valve open or boiler or furnace start operating.

When you connect the thermostat wires you're doing the same thing that the thermostat does itself when it "calls for heat".

Our photo shows the simplest case: a red and white wire from the wall thermostat turn a heating unit on (if they are connected by the thermostat or manually by touching them together) or off if they are disconnected.

Pulling the thermostat off of its mounting plate in this instance accomplishes "disconnect the thermostat wires at the thermostat" instruction - you can see that the red and white wires simply end at individual screw terminals.

(Those terminals are contacted and connected or not by the thermostat when it is restored in its normal position).

Since a room thermostat is basically an ON-OFF switch that calls for heating or cooling in response to room temperature, when we disconnect the thermostat itself, that's the same as NOT calling for heating or cooling.

If when you connect the red and white wires the heating system operates then the problem was the thermostat itself.

The problem may be

THERMOSTAT CALIBRATION, or the

THERMOSTAT WIRE CONNECTIONS - at the thermostat, or external influences like position of the thermostat on the wall, exposure to drafts, etc., OR the thermostat itself is defective.

For heating problems check that you were setting the thermostat all the way UP and that actual room temperature is BELOW the thermostat SET temperature.

• For warm air heat check that you have not set the fan control to OFF at the thermostat
  
  See FAN ON AUTO MAN Thermostat Switch
  
  If the air handler, blower unit, or fan won't start,
  
  see FAN LIMIT SWITCH TROUBLESHOOTING
• Check the thermostat settings & batteries: some room thermostats include a "safe" mode that will call for heating at a pre-set temperature should the system lose power.
  
  If the thermostat requires batteries to operate, be sure that it has new fresh batteries installed.
• For cooling problems check that you were setting the thermostat all the way UP and that room temperature is BELOW the SET temperature.
• And if the cooling system air handler, blower unit, or fan won't stop, also
  
  see FAN WONT STOP - THERMOSTAT SWITCH

If when you connect the red and white wires the heating system does not operate then we suspect a problem with the thermostat wires (a wire is cut or open), or with the wire connections at their other end - at the heating system.

Go to STEP 3

Thermostat Troubleshooting Step 3: check for damaged thermostat wiring: shorts or open wires

Reader question: the thermostat won't trigger the boiler nor the zone valve

I have a hot water heating system with oil furnace for heating water. I replaced the old low voltage thermostat (circular mercury one) with low-voltage Honeywell RTH 5+2 day programmable thermostat. There are only two wire white and red which I correctly connected to the new thermostat.

Now I do not have heat upstairs as thermostat does not trigger either boiler or the zone valve. I called Honeywell tech support and they asked me to check the voltage across the wire and I did. The voltage is between 8-10 VAC.

The rep at Honeywell told me that for RTH to work the voltage should be in the range of 20-30VAC. Later, I checked the transformer and it showed me a voltage of 24VAC.

I tried manually opening the zone valve to see if it will trigger the system but no avail. I have old thermostat in the basement when I turn that to higher temperature the system seems to run fine for the basement (different zone valve for the basement).

I am wondering if old thermostats ran on very low voltage or there is substantial loss in the voltage for the upstairs or the zone valve is gone bad. Also, I am wondering for upstairs if I remove the thermostat and connect two wires together then the circuit must be complete and the system must trun on continuously?- Akram Khan, Fredericton, New Brunswick, Canada. 12/4/2013

Reply: thermostat wire integrity tests: check for open thermostat wires & for shorted thermostat wires

I suspect you may have mis-wired the thermostat connections at the thermostat, the zone valve, or the boiler, but assuming you've first double-checked those connections, we'll check out the thermostat and its wires for a problem.

Since you found 24V at the transformer but not at the wires at the thermostat one suspects that there is a bad connection or perhaps even a partial short circuit between the wires somewhere in their passage.

See details at LOW VOLTAGE TRANSFORMER TEST

Here are a few thermostat wire integrity tests you might try:

• Disconnect the thermostat wires at the heater end and disconnect them from the thermostat upstairs at their other end. Tie the thermostat wires together at one of those two locations.
• Measure resistance (OHMS) using your VOM or DMM at the other end - between the two wires. It should be close to zero - showing continuity. Now disconnect the thermostat wires where they were tied together - so both ends of the pair of wires are unconnected to anything.
• Measure resistance again between the two wires - you should get infinite resistance - no current flows. If you find anything less there is probably a short between the wires.

Finally, yes, if the thermostat wires are intact and un-damaged throughout their length, then presuming the wires are connected properly at the heating system controller, if at the thermostat end you simply connect the two wires together, that should turn on the heat - you are doing exactly what the thermostat itself would do when room temperature falls below the set point and the thermostat calls for heat.

More details and some warnings about connecting thermostat wires for diagnostic purposes are

at SHORTING THERMOSTAT WIRES to ID R W B

Thermostat Troubleshooting Step 4: what to do when the problem is not in the thermostat or thermostat wiring

Turn On Electrical Power to the Heating or Cooling Equipment. Check that the heating or cooling equipment actually has electrical power.

Details about this switch are

at ELECTRICAL POWER SWITCH FOR HEAT

Watch out: Don't cycle heating or cooling equipment ON and OFF rapidly using the electrical power switch or any other control. For heat, wait at least five minutes between on-off cycles; for cooling wait at least fifteen minutes to minimize the risk of equipment damage.

• The primary controller on the heating system may not be working properly; on occasion we find that the primary control on a heating system, for example an internal relay, is refusing to pull "in" to turn heat on even though the thermostat has said "gimme heat!".
  
  Depending on the type of heat your building uses your service person will check one or more primary or additional heater controllers for a stuck relay or other defects.
  
  Details about the heating or cooling device controller operated by the thermostat depend on which type of equipment the thermostat is controlling, and are found at
  • BOILER CONTROLS & SWITCHES
  • FURNACE CONTROLS & SWITCHES
  • STEAM HEATING SYSTEMS & CONTROLS
  • A/C - HEAT PUMP CONTROLS & SWITCHES
• If the warm air heating or air conditioning or heat pump air handler, blower unit, or fan won't start, see these two diagnostic procedures:
  
  
  • FAN WONT STOP - LIMIT SWITCH - a manual push-pull switch right at the limit control in the air handler can set the blower fan to run continuously to at least confirm that the blower motor and fan can operate
  
  
  • FAN WONT STOP - THERMOSTAT SWITCH - separate switches right on the room thermostat can turn the blower fan on continuously

Watch out: as we warned earlier, warm air heat blower units behave differently than hot water or steam heat systems: to avoid damage to the heat exchanger the air handler or blower unit will normally keep running for a minute or even a few minutes after the thermostat has stopped calling for heat.

For warm air heating systems, give the system a few minutes to shut down.

• Other causes of continued delivery of un-wanted heat are detailed
  
  at HEAT WON'T TURN OFF.

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2020-11-12 by Jack Holmes

how to hook up two wire thermostat for the wood burner redd and white

On 2020-09-19 - by (mod) -

Dan

Odd but plausible; many forced air heating system controls include a fan limit switch that will not permit the fan or blower to turn on until the plenum has heated-up because the designers didn't want to blow cool air on chilly building occupants.

Take a look at your forced air furnace; see if it has a fan limit control that includes a FAN ON button; that one will turn the fan on full time (It's not present on all fan limit controls)

See details

at FAN LIMIT CONTROL SETTINGS https://inspectapedia.com/heat/Fan_Limit_Switch_L4064B_Settings.php

On 2020-09-19 by DanK

I have a new Emerson/White-Rogers P210C thermostat. Installed in July, 2020.

We always have the fan setting to always ON. However as soon as we set the switch from cool to heat, the fan will not stay on?

when the temp is lower the furnace will start and fan will run fine, but I want the fan on for circulation.

Any Ideas what to do, or do I buy another brand thermostat.

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

Sounds as if you have a heat pump and the outdoor compressor unit needs repair

On 2020-09-09 by Cynethia Thomas

So my thermostat is showing that my a/c isn’t work. I’ve moved the switch to heat and it turns in just fine.

The emergency heat and the a/c won’t come on though. It happened a few days ago and eventually it cut on after a few hrs but today it hasn’t worked at all.

On 2020-09-01 by Steve T

I have a strange problem. I have 3 zone air conditioning. The same thermostats control the heat. One thermostat is the master meaning that it must be set cool to push the system to ac. Suddenly the ac condensor outside will not turn on unless that one thermostat is calling for cool air.

The other two thermostats will kick on the air handlers but not the compressor causing warmer air into those zones. If I lower the temperature on the master thermostat it will send cool air to the other 2 zones. I. Thinking its a relay issue but I'm not an hvac tech..any ideas?

On 2020-07-20 - by (mod) - thermostat manuals are free to download at InspectApedia.com

Karen

There's no cost to you at all to Simply download a printable PDF of the right carrier manual for your system. Just use the on Page search box to search for air conditioner manuals and you will find in that article a link to your friend and from that link a list of all of the manuals we've been able to find for Carrier

On 2020-07-20 by Karen Vogeli

Is there a cost to send me a Carrier manual? I moved into a new apt complex. They threw all the manuals away. Until now I haven't needed the air. But we are in a heatwave.
2611 Freedom Dr. #103 Charlotte NC 28208

On 2020-06-20 - by (mod) -

Cheryl

I don't know what's the problem with the thermostat, but my very best, organized diagnostic steps are on the page above. Let's try looking through those

Often, if nothing else has changed, we find that a themostat wire has become disconnected, loose, or shorted, or that the control board in the air handler is damaged. I would start with the wiring tests I describe.

On 2020-06-20 by Cheryl MacDonald

I have a Honeywell programmable thermostat it is on Cool and Auto. I have all 4 temperature settings set it says Cool On below temp and nothing happens. Chk batteries, turn air cond circuit off n on still nothing. My air conditioning worked last year. Heat works. Whats the problem?

On 2020-05-13 - by (mod) -

Gloria

Tell us the brand and model of your thermostat or post a photo of it so that I can take a look.

On 2020-05-13 by Gloria McAllister

I have a programmble thermostat that I purchased last year. It had heat, cool, and off. This year, it is not showing the cool option. Only heat and off.
What is wrong?

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Continue reading at AIR CONDITIONER WON'T START if you're having a cooling problem, or see HEAT WON'T TURN ON if you're having a heating problem, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see THERMOSTAT WON'T TURN ON FAQs - questions and answers posted originally on this page

Or see these

Recommended Articles

• AIR CONDITIONER BLOWS WARM AIR
• AIR CONDITIONER WON'T START
• HEAT WON'T TURN ON
• THERMOSTAT SWITCHES, EXTERNAL
• THERMOSTAT WON'T TURN OFF

Suggested citation for this web page

THERMOSTAT WON'T TURN ON at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

Or see this

INDEX to RELATED ARTICLES: ARTICLE INDEX to HVAC THERMOSTATS

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