Honeywell room thermostatNo Heat Problem Diagnosis & Repair
- start here to diagnose & fix a no-heat problem in a building

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Heat won't turn on - what to do: how to diagnose & fix "no heat" problems in buildings. This article describes what to do if you can't get your building heat to turn on.

We give a simple sequence of steps in the order that is most likely to succeed, starting right at the wall or room thermostat and continuing through making sure there is electrical power and fuel.

For more complex no-heat problem diagnosis we link to in-depth articles on heating boiler, warm air furnace, heat pump and electric heat repair procedures.

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No-Heat Diagnosis & Quick Repair:

Thermostat COOL OFF HEAT settings explainedIf your heating system won't turn on at all, first check the following things in this order:

  • Is the thermostat working at all - is it on?

    Check that the thermostat has power: if your thermostat is a programmable model or has an LCD or electronic display and the display is blank (red arrow in our photos) then the thermostat has no power - its main power source is off and its backup batteries are dead..

On an older analog dial type thermostat you can't determine if the thermostat is working just by looking.

But turning the thermostat UP well above room temperature should turn the heating system on. Thermostats are usually powered by a low-voltage system wired from a transformer near or right on the heater.

If the thermostat has no power, check that its wires are connected, intact and that power is turned on at the heating system since power off there will often also turn off the transformer that powers the thermostat. Then

  • Check that the Heating Thermostat is set to "HEAT (on)". If a room thermostat is in the OFF position or is set to a temperature that does not call for heating (or cooling) then the heating or cooling system is just not going to run.

    For the room thermostat shown at above left, push the clear plastic switch to the right towards HEAT as far as it will go.

Thermostat COOL OFF HEAT settings explained

  • Check that the Heating Thermostat FAN switch is set to AUTO if the switch is present on your thermostat.
    If your thermostat has a FAN control switch but your building only has hot water or steam heat, most likely this switch does absolutely nothing.
  • For forced warm air heating systems, AUTO will let the fan turn on and off when the heating furnace runs while ON forces the fan to stay on constantly.

See FAN AUTO ON THERMOSTAT SWITCH for details about what turns the blower fan on and off on forced warm air heating systems or on air conditioning systems.

Separately, a FAN LIMIT SWITCH, we describe how this additional control, located on the heating furnace itself, should turn the forced warm air heating blower on or off. Don't bother with that switch yet.

Thermostat COOL OFF HEAT settings explained

  • Check that the thermostat is not in "HOLD" or "Standby" mode.

    If the set temperature is down lower than room temperature and the thermostat is in HOLD or STAND-BY mode (a feature present on most electronic thermostats) the device will not turn on heat until room temperature is below that HOLD number
    until you return the thermo sat to normal or RUN mode.

    In our photo my pencil is pointing to the "Hold" button and the red arrow shows that the display is telling me the thermostat is in HOLD mode.

    Press the "Run" button (below my pencil point) to go back to normal operating mode for this thermostat - that is to get out of "HOLD" mode.

Thermostat COOL OFF HEAT settings explained

  • Check that that the set temperature is at least 5 degrees above the room temperature. Later you can set the temperature lower if needed. of how to set all types of room thermostats are

Watch out: on some heating or cooling thermostats like the one shown at left, the device is so simple that the meaning of the temperature shown in the display may confuse you.

Left untouched and in HEAT mode, and controlling just a hot water heating boiler in this installation, the 67 degrees shown on the thermostat is the present room temperature.

Thermostat COOL OFF HEAT settings explainedSo we cannot tell the SET temperature of this thermostat just by walking up and taking a look. So we don't know if the thermostat should be calling for heat or not, nor if it has been SET above the present room temperature.

Luckily and without even reading the instructions (a last resort for many of us) it's easy enough to figure out what's going on.,

The heating boiler was not running - I'd already checked that, and I'd also noticed that the radiators in the building were tepid or cool.

Just pressing either the "down" button (blue arrow pointing down in the photo above) or the "up" button (red arrow pointing up in both photos) will change the display to show me the present SET temperature.

Notice that the SET temperature is all the way down to 56 degrees? This thermostat has been told "don't turn on the heat until temperature in this room drops below 54F! No wonder the heat was not running. The room was already much warmer than that.

What if the thermostat doesn't turn on the heating (or cooling) system?

Now if I press the "up" button repeatedly until the SET temperature is well above 67F then the heat should turn on. It did.

If the thermostat does not cause the heating (or cooling) system to run when it should, there could be any of a number of problems (as we outline here) but among them you may need to check the thermostat itself or its wiring - starting

Watch out: to avoid embarrassment, don't forget to check electrical power switches including service switches that someone may have set OFF. For example

  • Check that the heating system is now running. If it is not, Check that electrical power to the heating system is turned on and for oil or LP gas check that there is fuel in the tank.

    Just take a walk over to your boiler or furnace room and listen - you should hear the equipment running, and for oil or gas fired equipment the metal "stack pipe" (properly, the flue vent connector between the heater and the chimney) will be hot.
  • If none of these steps fixes the no-heat problem, depending on what kind of heat you have, see the explanation of no-heat troubleshooting in the articles listed below.

If you are confident that the no-heat problem is not due to electrical power having been turned off and not due to a thermostat not properly set, then you'll want to choose a no-heat diagnostic procedure depending on the type of heating system your building uses - listed just below.


Continue reading at one of the articles listed just below depending on the type of heating systerm your buildnig uses, or select a topic from the More Reading links or topic ARTICLE INDEX shown below.

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HEAT WON'T TURN ON at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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