Honeywell room thermostatHeat Won't Turn OFF: troubleshoot the thermostat
4 Steps to take if the heating system won't turn off

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Diagnose & fix heat that won't turn off when it should: here we discuss various causes of too much heat or of a heating system that does not shut off whenit should.

We explain how to diagnose and fix thermostat switch settings, switch settings right at the air conditioner or furnace air handler, and we review thermostat wiring problems that cause a heating system or cooling system to stay ON when it should not.

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 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 OFF

Honeywell digital room thermostat (C) Daniel FriedmanHere we discuss how to diagnose & fix a thermostat that keeps calling for heating or cooling when it should not.

A separate article, HEAT WON'T TURN OFF, gives diagnostic procedures for other reasons why building heat wont' turn off besides problems at the thermostat itself.

Article Contents

First Step: Change the thermostat setting up or down

If the heating system won't turn OFF, that is if heat keeps being delivered even when you don't want it, set the thermostat to its lowest temperature setting and wait three to five minutes.

Or if your thermostat has a HEAT OFF COOL or FAN AUTO OFF control, set the control to OFF or COOL.

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.

If the air handler, blower unit, or fan won't stop, see FAN WONT STOP - THERMOSTAT SWITCH

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.

If the cooling system wont' turn OFF, that is if cool air keeps being delivered even when you don't want it, we do the opposite: set the thermostat to its highest temperature, or if your thermostat has a HEAT OFF COOL or FAN AUTO OFF control, set the control to OFF or HEAT.

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.

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

Forced Warm Air Heat Won't Stop

Programmable thermostat furnace fan limit switch

If your heating system is by forced warm air (a furnace system) then check two switches that can cause the blower fan to run continuously even if the furnace is not heating the air:

While you may find good reason to keep the blower fan running continuously (discussed at BLOWER FAN CONTINUOUS OPERATION) for now, to diagnose the problem make sure that both of these switches are set to FAN OFF.

Watch out: not all forced warm air furnace heating systems have a Manual-On switch on the limit switch, and not all thermostats include a FAN-ON switch on the thermostat.

All Heating Systems: forced air, warm air, hot water, steam, heat pump, electric: stuck "on" step 2 - continued

Aquastat showing wiring connections (C) Daniel FriedmanDisconnect the thermostat from the heating or cooling system - at the heating or cooling equipment.

It's easy to isolate the room thermostat and its wiring from the heating or cooling system: we simply remove the two thermostat wires right at the primary control for the heating or cooling system involved. Typically the two thermostat wires are red and white and are connected in the controller to terminals T1 and T2.

Our photo (left) shows the red and white thermostat wires connected to a heating boiler aquastat control - look at the top left corner of the photo. [Click to enlarge this or any photo or chart seen at]

Zone valve wire connections (C) Daniel FriedmanOK so it's not always as trivial as I just said. Multi-zoned heating systems may mean you have to follow the individual thermostat's wires to find just which control they operate.

If you are diagnosing a heating system that is controlled by a single room thermostat, then there will be one wall thermostat, one pair of wires that run between the thermostat and a primary heater or air conditioner controller, and one primary controller at the heater or air conditioner.

Typically these are an AQUASTAT CONTROLS for hot water heat (heating boilers),

or a FAN LIMIT SWITCH for warm air heat (furnaces).

An air conditioner or heat pump will be controlled by thermostat wires connected to a control board or relay in the air handler and/or compressor/condenser unit.

But in multi-zoned heating systems there may be multiple hot water zone valves or multiple air duct zone dampers. In those systems you will need to follow the individual problem thermostat wires to the specific controller such as an individual hot water heating zone valve to make the disconnection we discuss here.

Keep in mind that a zone valve can become stuck in the OPEN position or it may even be manually latched "open" to continue to allow heat to flow.


Our photo above shows a rats nest of wires at an older hot water heating system zone valve. Two of these, red and white, will be coming from the thermostat and will tell the zone valve to open on a call for heat. Additional wires from the zone valve tell the circulator pump to run.

Many room thermostats for heating and cooling have more than two wires connected to the thermostat. But in just about every case, just two wires are actually being used to turn the heater, air conditioner, or heat pump ON or OFF.

Help in sorting out thermostat wires is provided in our articles starting at THERMOSTAT WIRE CONNECTIONS


and at SHORTING THERMOSTAT WIRES to ID R W B.In those articles you'll also find thermostat wiring diagrams and details for just about every type of room thermostat.

Step 3: check for damaged thermostat wiring

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

Disconnect the thermostat wires at the thermostat: when the heat is on and running and the thermostat is set to say 68F, when the room reaches 68F (measured at the thermostat on its scale), just disconnect the thermostat wires right there at the thermostat. (See our warning at Step 2.)

Our photo (left) 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.

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.

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 Off Electrical Power to the Heating or Cooling Equipment. Call your heating or air conditioning service company for further diagnosis and repair help.

The equipment should STOP immediately.

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 troubleshooting, wait at least five minutes between on-off cycles;

For cooling system troubleshooting wait at least fifteen minutes to minimize the risk of equipment damage.

On rare occasions malfunctioning controls on heating equipment may continue to feed fuel to an oil or gas burner, requiring that fuel also be shut off. Turn off the fuel supply following instructions at OIL or GAS SHUTOFFS.

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.

Examples of Diagnosing Too Much Heat or Heat Won't Turn Off - starting at the room thermostat

Reader Question: how do I fix a thermostat that I have to set lower than the level of heat I want?

My traditional honeywell simple dial-type thermostat on the wall has two pointers and two temp. scales. However I have to set it on 60 degrees to get heat temp to 70 degrees. What can I do? It was working fine until my condo changed the air/heating system this summer. - Jeannette 1/23/2013

Reply: check these reasons for a room thermostat that keeps calling for heat


On most two-scale room thermostats, one scale reads the current or actual room temperature while the other scale is the set temperature - and is adjustable. When you move or adjust the thermostat to call for a different room temperature you will see the scale pointer move to your new setting on the "set" scale.

You are describing having to set the thermostat lower (60F) than the desired temperature (70F) to get the proper level of heat in your home. In other words, you say that the room gets too hot - hotter than you asked-for. Here are some things that would cause that problem:

Reader Question: possible shorted thermostat wires

Our furnace thermostat failed after an adjacent water heater installation. I noticed that wires were pinched between the water heater and the gas pipe.

After I loosened the safety straps and freed the wire, the thermostat works. (I reset the furnace).

Were squished wires the cause of a temporary short and will the wires keep working if they are undisturbed? - Larry 8/2/11

Reply: check for shorted wires and for thermostat transformer damage as well as for blown fuse

Yes it's surely possible that your thermostat wires were shorted. Now a short in those wires sometimes lets you off the hook with no trouble, since the wires and thermostat are basically an "on-off" switch that calls for heat (or no heat).

But a thermostat wire short at the transformer could burn out the low voltage transformer that powers the thermostat and heating controls or maybe even damage the thermostat itself.

Find and check the low voltage transformer (often it's mounted near your furnace, on an electrical junction box, or it may be integral in your heater control unit).



Continue reading at HEAT WON'T TURN OFF or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


FAN ON AUTO MAN Thermostat Switch




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