Baseboard Heater Won’t Turn Off? Thermostat, Size or Wiring

Turn it all the way down, and wait

The test. Cadet’s instructions for a mechanical thermostat are to turn the knob all the way to the right, then, once the room is comfortable, turn it to the left just until it clicks and the heater turns off. King and Qmark give a check for a newly installed system: with every thermostat at LOW or NO HEAT, wait 3 to 5 minutes, and none of the heaters should be running.

No off position. Dimplex says a single pole thermostat breaks only one side of the power line, so power is continuous to the heater, and its knob has a low setting but no off. Marley’s Qmark thermostat notes say single line break thermostats have no OFF position and will operate at a temperature below their minimum set point.

So to stop it. On a single pole thermostat, a room colder than the lowest setting can still turn the heater on. To cut the power fully, turn off its breaker. Dimplex says a double pole thermostat breaks both sides of the line, so its off position cuts all power to the heater.

Heating a warm room at the lowest setting. That is not the single pole design at work. Cadet lists a defective thermostat for a heater that does not turn off, and King and Qmark say that if a heater runs with its thermostat at LOW or NO HEAT after installation, disconnect the power and check the wiring. Turn its breaker off and call an electrician.

Check where the thermostat is

The same room. Cadet’s instructions for wiring several heaters to one thermostat say the thermostat and the heaters must be in the same room. Qmark recommends no more than one thermostat in each room, and says not to keep different rooms at different temperatures unless the doors are kept closed; with the rooms open, all the thermostats should have the same setting.

Drafts and heat. Honeywell says its RLV4300 digital thermostat shows the wrong temperature if it is in a draft, if the sticker on its screen has not been peeled off, or if it is near or above a heat source such as a light dimmer. Its installation instructions put it about 5 feet high on an inside wall facing the heater, away from drafts such as the top of a staircase or an air outlet, dead air spots such as behind a door, and direct sunlight.

King’s chart. King says sunlight or other heat sources can throw a thermostat off, and to move the thermostat or remove the heat source. It also lists improper calibration, fixed with the calibration screw under the thermostat’s cover.

Moving or adjusting one. A line voltage thermostat is wired to the heater circuit, so moving it or opening its cover is a job for an electrician.

A digital thermostat: check the schedule

The installation check. Honeywell’s check after installing its RLV4300 is that the heat turns on when you raise the setpoint with the Up button, and off when you lower it with the Down button.

A change that does not stick. In Automatic mode, Honeywell says a setpoint changed with the Up or Down button is used only until the end of the current period, and Manual mode is the permanent override of the schedule. Its default schedule asks for 70°F at 6:00 every day and again at 18:00 on weekdays.

Heating before you expect it. Honeywell says that with Early Start on, in Automatic mode, the thermostat starts heating early so the room is warm at the start of periods 1 and 3.

The wrong times. If the temperature does not follow the schedule, Honeywell says to make sure the thermostat is in Automatic mode, and to check the clock and the schedule: on the 12-hour format, PM must show on the screen for afternoon times. It says the time shows wrong if it was never set or the thermostat was without power for more than 5 hours.

Is the heater big enough for the room?

Too small. Cadet lists the wrong heater wattage for the room size as a cause of a heater that does not turn off, and its fix is a higher wattage model or more heaters, if the circuit allows. Stelpro’s ABA guide says a heater too small for the room runs continuously, and may fail sooner and turn yellow.

A rough size. Dimplex’s general rule is 10 watts of heat per square foot: a 1,000 watt heater heats 100 square feet in a standard insulated room. Stelpro gives the same 10 watts per square foot, based on a standard 8 foot ceiling, and says insulation and windows change what a room needs.

Getting it right. Stelpro says a specialist can work out a room’s heat losses, and to heat a large room it recommends several heaters instead of one, for example two 1,000 watt heaters instead of one 2,000 watt heater.

Adding a heater. Cadet says the load on one breaker is limited to 80 percent of the breaker’s capacity or the thermostat’s maximum rating, whichever is lower. Adding a heater is a job for an electrician.

A faulty thermostat, or none at all

A thermostat that has failed. Cadet’s fix for a defective thermostat is to replace it, and Stelpro’s ABA chart lists a defective thermostat or a wrong setting for a heater that runs continuously. King says a room that swings from too hot to too cold points to a defective or low quality thermostat, and that an electronic thermostat is the best.

No thermostat. Cadet says a thermostat is required for every heater, built into the heater or on the wall. King says its K Series heater does not come with a thermostat, and that baseboard heaters must be thermostatically controlled for efficient, safe operation.

A wiring fault. King and Qmark say that if any heater runs with its thermostat at LOW or NO HEAT after installation, disconnect the power and check the wiring. Turn its breaker off and call an electrician.