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

Honeywell Smart Thermostat Not Turning ON Heat

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

Honeywell Smart Thermostat Not Turning ON Heat: Possible Causes

Summary of Causes

    1. Honeywell Smart Thermostat Temperature Offset Set Too Large
    2. Power Failure Keeping Heat from Coming On
    3. Honeywell Smart Thermostat Temperature Offset Set Too Large
    4. T-stat Not Level on Wall
    5. Temperature Set Too Low
    6. Bad Electrical Connections
    7. Mechanical Thermostats can Bind Up
    8. Not Set to Desired Operational Mode (Heating or Cooling Mode)

Thermostat Temperature Offset Set Too High


The thermostat temperature display seems to show that the current room temperature never reaches the set heating temperature. Yet the heat never turns on. So in this case, the display offsets may be too high.  That is, the 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 heat ON.

Consider this.

    • Let’s say that you set the thermostat to heat to 74 degrees.
    • Further, you have set your display temperature offset to +3 degrees. This makes your 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 thermostat would show 77 degrees.

Now the thermostat calls for heat 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 heat. 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 heat working if you set the thermostat to 74.


Decrease the display offset temperature. This helps assure that the temp reading on the thermostat shows as closer to actual room temp.  Plus, an accurate display reading helps prevent the situation where you heat 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 heat to 74 degrees. But with the +3 degree offset, you’ll need to turn the 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 Heat from Coming On

Picture of a Honeywell smart thermostat, mounted but powered down.
A Honeywell smart thermostat, mounted but powered down.


The screen on Honeywell smart thermostats looks blank or dark. No readings or status messages appear. Further, the furnace heater 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, no heat turning on. That is, that the furnace never runs, no matter the 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 thermostat could glean power from either one, although usually not both. A power outage in either system could cause Honeywell Smart Thermostat Not Turning ON Heat. By checking that the heating control circuits have power, you’ve ruled out this cause of thermostat power loss. In this way, a faulty cooling system can make the heating system fail too.

Thermostat Poorly Wired or Installed

Picture of a Honeywell smart thermostat wall plate mounted, but with wires not yet connected.
A Honeywell smart thermostat wall plate mounted, but with wires not yet connected.


If the heating problem happened when you installed a new 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 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 thermostat terminals.


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

Thermostat Not Level May Cause Honeywell Smart Thermostat Not Turning ON Heat


Poor leveling on older 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 heating 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 thermostat. If so, then you should replace it with a current all-electronic model that has no moving parts. Today’s 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 heat 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 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 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 thermostat toward level. Then, mark the wall with an erasable pencil.  Next, while holding the thermostat in that marked position, tighten the screws again.  Finally, use an eraser to clean off any visible wall markings.

Thermostat Temperature Set Too Low

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


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


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

Bad Electrical Connections Might Cause Honeywell Smart Thermostat Not Turning ON Heat

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


The solid copper wires in the 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 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 heating.


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.

Mechanical Thermostats can Bind Up


Now and then with smart thermostats, a snap of static electricity near the thermostat, 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 heating system may not come on. No matter how far from set temperature the room temperature is, the heating system may not run at all.


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


When this no heat problem happens, try rebooting the thermostat. Turn off its power for ten to twenty seconds. Then switch it on again. This power cycling is a literal snap with many 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 thermostat.

Not Set to Desired Operational Mode (Heating or Cooling Mode)

Picture of the Honeywell Smart Thermostat, showing its -System Heat / Cool Mode Setting -screen. The system mode is OFF.
The Honeywell Smart Thermostat, showing its -System Heat / Cool Mode Setting -screen. The system mode is OFF.


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


Set the t-stat’s operational mode to Heat.  On our t-stat, we do this in the System tab.  We show that screen pictured above.  Just tap the Heat option and press Done to make the new setting effective.  You should soon thereafter feel some heat.

Other Honeywell Smart Thermostat Posts

    1. Honeywell Smart Thermostat Wiring Instructions


    1. Official Honeywell Smart Thermostat Support Page
    2. Where to Buy the Honeywell Smart Thermostat