Honeywell room thermostatThermostat Won't Turn Heat (or A/C) ON
What to check if the room thermostat does not turn on the heat or A/C

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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.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

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.

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.

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

Room thermostat with unit removed to show the two thermostat wires (C) DanieL FriedmanRemove 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.

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 (LOW VOLTAGE TRANSFORMER TEST) 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. Here are a few thermostat wire integrity tests you might try:

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

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

Emergency electrical power OFF switch for heating equipment (C) Daniel FriedmanTurn 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

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.

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.


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 closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


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