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AIR HANDLER / BLOWER UNITS
AIRBOUND HEAT SYSTEM REPAIR by WATER FEED VALVE
BACKDRAFTING HEATING EQUIPMENT
BACKUP HEAT for HEAT PUMPS
BLOWER FAN OPERATION & TESTING
BOOKSTORE - InspectAPedia
CIRCULATOR PUMPS & RELAYS
CONVECTOR HEATERS - HYDRONIC COILS
COOL OFF HEAT, Thermostat Switch
DEFINITION of HEATING & COOLING TERMS
DIAGNOSTIC GUIDES A/C / HEAT PUMP
DIAGNOSE & FIX HEATING PROBLEMS-BOILER
DIAGNOSE & FIX HEATING PROBLEMS-FURNACE
DUCT SYSTEM & DUCT DEFECTS
ELECTRIC MOTOR OVERLOAD RESET SWITCH
ELECTRICAL POWER SWITCH FOR HEAT
FAN, AIR HANDLER BLOWER UNIT
FAN AUTO ON Thermostat Switch
FAN, COMPRESSOR/CONDENSER UNIT
FAN CONVECTOR HEATERS - HYDRONIC COILS
FAN LIMIT SWITCH
FLOODED HEATING EQUIPMENT REPAIR
HEATING LOSS DIAGNOSIS-BOILERS
HEATING LOSS DIAGNOSIS-FURNACES
HEATING SYSTEM INSPECT DIAGNOSE REPAIR
LOW VOLTAGE BUILDING WIRING
LOW VOLTAGE TRANSFORMER TEST
MANUALS & PARTS GUIDES - HVAC
NO HEAT - BOILER
NO HEAT - FURNACE
ODORS FROM HEATING SYSTEMS
OIL FILTERS on HEATING EQUIPMENT
PULSE COMBUSTION HEATERS
Reset Switch - Heater Primary Control
Reset Switch Broken - Quick Repair
RESET SWITCH - ELECTRIC MOTOR
Reset Switch - Stack Relays
STEAM HEATING SYSTEMS
THERMOSTATS, HEATING / COOLING
THERMOSTATS, WATER HEATER
THERMOSTATIC EXPANSION VALVES
ZONE VALVES, HEATING
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.
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Here 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.
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.
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.
Forced Warm Air Heat Won't Stop
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
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 InspectApedia.com]
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 CONTROL for hot water heat, or a FAN LIMIT SWITCH for warm air heat. 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 (ZONE VALVES, HEATING) to make the disconnection we discuss here.
Our photo (above left) 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 THERMOSTAT WIRE CONNECTIONS and at THERMOSTAT WIRING TIPS & COLOR CODES 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.
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.
Thermostat Troubleshooting Step 4: what to do when the problem is not in the thermostat or thermostat wiring
The equipment should STOP immediately.
Details about this switch are at ELECTRICAL POWER SWITCH FOR HEAT
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).
Green link shows where you are in this article series.
Frequently Asked Questions (FAQs)
Question: we replaced the thermostat and now our living area is too hot - and the heater won't turn off
Our living area has been really hot and the old thermostat did not have an ON/OFF switch. We changed the thermostat today and switched it off but the heater is still on. We cannot find a reason why and we have verified the wiring and all is correct. Is it possible that something else may not be working? Maybe the transformer or some kind of regulator?
First disconnect the thermostat wires right at the heater itself, at it's control. That's the same as turning down the thermostat. If the heater stops then you know the trouble is in the thermostat wiring thermostat settings, is the thermostat itself.
As your problem showed up after installing a new thermostat, I would be sure to check that you made the right connections, that the thermostat switches are properly set, AND that the thermostat wires were not accidentally shorted together.
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