Picture of the installed Honeywell Large Dial Thermostat, T87N1026.

Thermostat Clicks But No Heat Or Air

You might notice that you get no heat or air, even though your thermostat clicks as it calls for heat.  This clicking is a relay inside that closes to switch on the furnace. Now this no heat or air situation can happen for many reasons. Incorrect wiring, tripped circuit breakers, blown fuses, loss of gas supply, burned out heating elements, or a broken HVAC system could cause this. Here, we describe some of these causes, and then offer troubleshooting procedures and repair hints to resolve them.

Warning !

Some of the tasks below are quite dangerous if you don’t know what you’re doing.  So if you’re not at ease working with high voltages or natural gas / propane, then please hire the appropriate professionals to solve your heating problem for you.





Why Thermostat Clicks But Then You Get No Heat Or Air: Summary

    1. The thermostat Was Incorrectly Wired.
    2. There is faulty wiring between the thermostat and furnace.
    3. Defective thermostat.
    4. Loss of power / gas to furnace heaters or compressors.
    5. Dirty or defective ignitor or pilot light.
    6. Defective control board in furnace.

Thermostat Clicks But No Heat Or Air: Detailed Explanations

1. Thermostat Badly Wired

Picture of needle nose pliers being used to straighten the ends of thermostat wires before connecting them to the new wall plate terminals. Thermostat Clicks But No Heat Or Air.
Use needle nose pliers to straighten thermostat wires before connecting to new wall plate terminals.

Problem

If the heating problem happened when you installed a new thermostat, you may have erred in how you wired it. Perhaps you’ve reversed the W and Y wires. This would run the cooling stage though the thermostat is actually calling for heat. Other symptoms may arise too. These depend on which wires and how many of them you have connected to the wrong thermostat terminals.

Fixes

Check for correct wiring at both ends of the thermostat cable. At the thermostat, attach all wires to the right terminals in the wall plate. Then, do the same at the furnace end. Get help from a professional heating system repairman if you feel insecure matching these wires up with the right terminals.





2. Broken Thermostat Wiring 

Picture of the Honeywell Smart Thermostat RTH9580WF wall plate, mounted, with wires connected.
Honeywell Smart Thermostat RTH9580WF wall plate, mounted, with wires connected.

Problem

The solid copper wires in the thermostat cable can break if flexed too often. Or, perhaps the installer stapled the cable with too much pressure or stapler misalignment, and nicked the cable.

This issue normally shows up during new thermostat testing. But sometimes, it might not surface for years. Plus, settling of wall studs can flex these wires that installers often fasten to them. So, after decades, this flexing can wear wire coverings, causing faulty heat operation. In this case, the thermostat is still receiving power from the furnace, but the wires that it uses to call for heat are broken.  So while the thermostat itself continues to operate, it is unable to communicate its heat requests to the furnace.

Solutions

Replace the cable between the thermostat and furnace. Avoid tightly stapling the wire, and don’t fasten it more than you must for a secure installation. Lay it loosely.





3. Defective Thermostat

Problem

Though the control relay in the thermostat might click to call for more heat, the relay itself may be worn out, its contacts no longer making a good connection when they close together.

Solutions

    1. Your best bet here, is just to get a new thermostat.
    2. Or, if you want to tinker, and have a known-good t-stat laying around, try swapping it in for your current one and see if things work any better. If they do, then you’ve determined that the original thermostat is indeed faulty.
Picture of a Siemens indoor load center circuit breaker box, front view, with door open.
A Siemens indoor load center circuit breaker box, front view, with door open.

4. Broken or Dirty Ignitor or Pilot Light

Problem

When the thermostat clicks to turn heat on, the ignitor (could either be a spark type or hot metal variety) activates at the same time the gas does (in gas furnaces).  But sometimes these devices burn out.  Or, if your furnace has a pilot light, which is a small flame fed from the gas supply that is situated near the main burner.  And this is what ignites main burner gas when the heat comes on.  But if this flame goes out due to dirt or otherwise clogged piping, then you’ll get no main heating when the t-stat calls for it.

Solutions

    1. Clean pilot lamp jet.  Turn gas off to pilot light and clean the nozzle with a tooth brush.
    2. Check and replace pilot lamp sensor if needed.  This sensor detects the presence of a pilot flame before turning on the gas to the main burner.  So if this sensor does not detect the flame, due to no flame present, or it’s just broken, then you’ll get no heat though the thermostat tries to click it on.
    3. Replace the ignitor. Make sure though that the new ignitor you install is positioned correctly — close enough to the main burner that it will indeed fire up the gas coming from there when the heat comes on.

5. Loss of Electricity or Gas Supply to Parts of Furnace

Problem

Though the furnace turns on, you may notice that the heating elements such as burners, electric coils, or heat pump compressor does not come on as well.  This can happen due to a loss of electric power to said elements due to tripped circuit breakers, failed heating elements, broken relays, or a faulty control board.  Or in the case of gas furnaces, it could be a loss of gas supply or faulty ignitor (the device that lights the gas coming out of the burner when the t-stat calls for heat, and again, faulty wiring or broken control board.

Fixes

    1. Check circuit breakers. Make sure all breakers that feed the furnace and any outdoor components are ON.  For gas heating, assure that your gas supply is okay by checking that your other gas appliances are still working and that no one has mistakenly closed the gas valve that feeds the furnace.  Also, are you up to date on your gas bill? If breakers keep tripping, then seek help from a qualified electrician or HVAC technician.
    2. Check connections in furnace. Most furnaces have at least a couple cables that plug into other cables, to control boards, or relays.  So make sure that these are all tight and properly seated, because one loose connection can prevent heat from activating.
    3. Make sure gas burners are clean.  It’s a useful maintenance practice to clean the burner jets yearly, as some being clogged can cause the protective circuits to prevent the gas valve that feeds the burner from opening.

6. Defective Furnace Control Module 

Problem

We saved this one for last, because it will likely be the most expensive repair. These bords typically run several hundreds of dollars, not to mention the labor costs you may incur to have a professional install them. Still though, they do go bad occasionally.  And since these processors are what orchestrates the entire operation of the furnace, when they fail, the furnace often becomes non responsive to heat calls from the thermostat.

Solutions

    1. Replace the control module.  Now this is likely a job for a professional, since removing the old one and installing the new one without damaging it, is likely a job too advanced for the amateur HVAC tech hobbyist. Further, clearly determining that the module is indeed bad before ordering a costly replacement is a must, because most parts supply centers will not take the module back and give you a refund if you misdiagnose the problem. So if you suspect a bad control board, we strongly recommend calling in the pros to fix it.

Conclusion

As should be clear, the thermostat-clicks-but-no-heat-or-air  problem has many causes. Plus, most of them lie beyond the thermostat. But we hope that now, you know what might cause this. Why?  Because knowing the causes of heat problems often helps in choosing the best fixes. Even though the right fix may cost the most, at least now, you have the data in front of you.

Related Posts to Thermostat Clicks But No Heat Or Air

    1. Honeywell Thermostat Not Reaching Set Temperature Troubleshooting
    2. How to Calibrate Honeywell Thermostat Temperature, RTH9580WF

References for Thermostat Clicks But No Heat Or Air

    1. Four Steps for Troubleshooting Your Thermostat from ComfortTechnologyInc.com

Revision History

    • 2023-02-16: First published.