Picture of the Honeywell RTH9580WF Internet Thermostat, being snapped onto wall plate.

Smart Thermostat Not Turning ON

The smart thermostat not turning ON problem can happen for many reasons. A faulty Smart Thermostat, a broken HVAC system, power losses, damaged wiring, et al. Here, we list some of these causes, and offer repair tips to solve these no-run issues. First, we look at what might have happened with the Smart Thermostat itself. Then we explore the furnace issues that can also cause no HVAC running at all.  Here, we discuss these problems, not partial-but-insufficient-heat issues.  These address the case when you get no heating or cooling. But they do not cover the cases where you get some yet not enough temperature control.

Smart Thermostat Not Turning ON: Possible Causes

Summary of Causes

    1. Smart Thermostat temperature offset too large.
    2. Power failure keeping HVAC unit from coming on.
    3. Smart Thermostat not level on wall.
    4. Temperature set too low or high.
    5. Bad electrical connections.
    6. Mechanical Smart Thermostats can bind up.
    7. Not set to correct operational mode (heating or cooling mode).

Smart Thermostat Temperature Offset Set Too High


The Smart Thermostat temperature display seems to show that the current room temperature never reaches the set temperature. Yet the HVAC never turns on. So in this case, the display offsets may be too high.  That is, the Smart Thermostat  acts as if the current indoor temperature is higher than it really is. And when the t-stat thinks that the room is warmer, it will not turn the HVAC ON.

Consider this.

    • Let’s say that you set the Smart Thermostat to 74 degrees.
    • Further, you have set your display temperature offset to +3 degrees. This makes your Smart Thermostat display a current temperature reading that is three degrees warmer than the environment actually is. In this case, when the current house temperature is 74 degrees, the Smart Thermostat would show 77 degrees.

Now the Smart Thermostat calls for HVAC when the temperature it reads drops below 74 degrees.  But a +3 degree offset means that the actual temp must fall to 3 degrees below 74 to get warmer. So though you set the temp to 74, you won’t get heat until the actual room temp falls below 71.  Now in apartment buildings, a room on the upper floors might easily hold at 71 degrees. So you may rarely or never feel the unit working if you set the Smart Thermostat to 74.


Decrease the display offset temperature. This helps assure that the temp reading on the Smart Thermostat shows as closer to actual room temp.  Plus, an accurate display reading helps prevent the situation where your furnace does not come on enough. We suggest that you avoid setting the temperature offset to more than one degree either way (+ or – offset).

Or, just turn up the set temperature.  In this scenario, let’s say you want your place to be 74 degrees. But with the +3 degree offset, you’ll need to turn the Smart Thermostat higher than the temperature you desire.  So you’ll have to set it to 77 degrees to get that 74 degrees actual temperature.

Power Failure Keeps Smart Thermostat from Turning On the HVAC

Picture of a typical Smart Thermostat, mounted but powered down, snapped onto wall plate. Smart Thermostat Not Turning ON.
A typical Smart Thermostat, mounted but powered down, snapped onto wall plate.


The screen on Honeywell smart Smart Thermostats looks blank or dark. No readings or status messages appear. Further, the furnace does not run. Plus, you hear no humming at the furnace. But for non intelligent t-tats you may no power loss indication, except that their backlights don’t glow. Yet the effect is still the same. The HVAC Not Turning ON. That is, that the furnace never runs, no matter the Smart Thermostat setting.


Honeywell thermostats often get power right from the furnace system. So, check that all furnace systems are getting electricity. Often there’s a single circuit breaker that feeds the furnace. Then there’s another that feeds the outside compressor unit in HVAC arrangements. So verify that all these breakers are ON.

Furthermore, the heating and cooling subsystems often get electricity from separate circuit breakers. So, the Smart Thermostat could glean power from either one, although usually not both. A power outage in either system could cause Smart Thermostat Not Turning ON. By checking that the HVAC control circuits have power, you’ve ruled out this cause of Smart Thermostat power loss. In this way, a faulty cooling system can make the HVAC system fail too.

Smart Thermostat Poorly Wired or Installed

Picture of a Smart Thermostat wall plate mounted, but with wires not yet connected.
A Smart Thermostat wall plate mounted, but with wires not yet connected.


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


Check for correct wiring at both ends of the Smart Thermostat cable. At the Smart Thermostat, affix all wires to the right screw terminals in the mounting plate. Then, do likewise at the furnace. Seek help from a professional HVAC technician if you feel insecure matching these wires up with the right terminals.

Crooked Mounting May Cause Smart Thermostat Not Turning ON


Poor leveling on older Smart Thermostats can affect how well they respond to room temperature changes. Those early models used a bi-metal spring to sense temperature. Manufacturers would affix a mercury switch to the outside end of that spring. Then, as room temperature rose and fell, the spring changed size. This caused its outer end to move back and forth in accordance with temperature changes. This in turn tipped the mercury switch ON or OFF, to bring room temperature nearer to the set temperature.

You needed proper leveling for this tipping to occur as designed. Indeed, if tilted too far off of plum or the horizontal, the HVAC system might never turn on. In this case, the mercury inside does not make the contacts as it should when the room gets too cold.


Now you may still have a mercury and bi-metal Smart Thermostat. If so, then you should replace it with a current all-electronic model that has no moving parts. Today’s Smart Thermostats have more accurate sensors and many more features. Plus, modern t-stats work well, no matter how far off of level you install them. They’ll turn on the furnace correctly in the house, no matter their position.  Mount them on their sides or upside down. Or, let them dangle by the wires (although we do not recommend this).  But the point is that they will regulate your heat well, in any position.

If you cannot upgrade from an older Smart Thermostat, then try drilling new mounting holes. To do that, remove the screws and wires if they get in the way. Then re position the Smart Thermostat over the new holes. Finally, drive the screws into those new holes. Use wall anchors in these holes for a stronger hold.

If your t-stat is not too far out of level, try loosening the screws that hold it to the wall. Then, slide the Smart Thermostat toward level. Then, mark the wall with an erasable pencil.  Next, while holding the Smart Thermostat in that marked position, tighten the screws again.  Finally, use an eraser to clean off any visible wall markings.

Smart Thermostat Temperature Set Too Low

Picture of the temperature at 69 degrees.
The temperature at 69 degrees.


You can set Smart Thermostats way lower than the ambient temperature in your home.  During moderate days outside for instance, you might set the room temperature to 72 degrees.  But if you live in a tall building, your apartment may never fall below 72 degrees.  Thus, your furnace may never come on.


    • If you need more warmth, try setting the Smart Thermostat higher.
    • Or just be happy your furnace is not running any more than it is, especially if paying your own bills.

Bad Electrical Connections Might Cause Smart Thermostat Not Turning ON

Picture of the Honeywell T8775C1005(2) Smart Thermostat wall plate, front view, showing screw terminal connection points.
Honeywell T8775C1005(2) Smart Thermostat wall plate, front view, showing screw terminal connection points.


The solid copper wires in the Smart Thermostat cable can break if flexed or bent too much. Or, maybe the installer stapled the cable with too much pressure, and shorted out the individual conductors in that cable.

This issue normally appears during new Smart Thermostat testing only. But sometimes, this problem may not show up for decades. Plus, settling of wall beams can flex these stapled wires. So, after many years, this flexing can wear wire coverings, causing faulty or no warming.


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

Mechanical Smart Thermostats can Bind Up


Now and then with Smart Thermostats, static electricity near them can freeze the touch screen. Also, a power surge in the home’s electrical system can do this as well. Then too, the data shown onscreen no longer updates. Further, the HVAC system may not come on. No matter how far from set temperature the room temperature is, the furnace may not run at all.


Picture of snapping a Smart Thermostat onto the wall plate.
Snapping a Smart Thermostat onto the wall plate.


When this no run problem happens, try rebooting the Smart Thermostat. Turn off its power for ten to twenty seconds. Then switch it on again. This power cycling is a literal snap with many Smart Thermostat models. Why? Because they can detach from furnace power without having to unscrew them. Just unsnap them from wall plates. Then, wait a few seconds. And finally snap them back into place to reboot. Or, if your t-stat does not come off of the wall easily, power cycle it at the breaker box. Doing this reboots the Smart Thermostat.

Not Set to the Correct Operational Mode (Heating or Cooling Mode)

Picture of the -System Heat / Cool Mode Setting -screen, with the mode set to OFF.
The -System Heat / Cool Mode Setting -screen, with the mode set to OFF.


Certainly you might see Smart Thermostat Not Turning ON, when it’s not currently n heating mode!  On our smart Smart Thermostat,  we can set it to either heating mode, cooling mode, or OFF.  If OFF, you won’t get warmth.  Nor will any cool air flow from your registers in Cool mode.


Set the Smart Thermostat’s operational mode to Heat.  On our Smart Thermostat, we do this in the System tab. Just tap the Heat option and press Done to make the new setting effective.  You should soon thereafter feel some heating.

Other Smart Thermostat Posts

    1. Honeywell Smart Thermostat Wiring Instructions


    1. Where to Buy a Smart Thermostat