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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 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.
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Here we discuss how to diagnose & fix a thermostat that seems to refuse to call for heating or cooling when it should. A separate article, HEAT WON'T TURN ON, gives 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
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 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.
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.
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
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 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
Details about this switch are at ELECTRICAL POWER SWITCH FOR HEAT
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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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