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We recently renovated our condo and replaced the baseboard heaters. An licensed electrician did all of the electrical. Now, during a cold snap, the heaters on one thermostat are turning on and off, but the room is not reaching the temperature set on the thermostat. Currently, the thermostat is set to 30℃ yet the room temperature is below 20℃ (need to wear sweater). The baseboard heaters on that circuit will turn on and then turn off intermittently never getting the room to the temp. I would have expected them to run full on until the room was at the set temp.

Some details: We live in a condo which has 208v 3-phase service to the building, 208v single phase to units. The heaters are supposed to be 208v heaters (ordered special from Stelpro). I've verified that the two legs at the thermostat are 120v each. The thermostat is a very basic Stelpro mechanical thermostat.

There are two heaters (1x1500w, 1x1000w) on the same circuit. Prior to renovations, the circuit hosted 3 x ~1000watt heaters (I didn't check them, but they are smaller than the new 1500w heater so that's a guess).

Why would the heaters not stay on continuously till the room is warm enough?

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  • The thermostat doesn't care what the room temperature is. It only cares about what its sensor is telling it. Is the thermostat on a wall and is the wall is warm? Or the air around the thermostat is warm for some reason. Dec 28, 2021 at 15:39
  • Thanks @SteveWellens. I should have been clearer. The temp at the thermostat is nowhere near what it's set to. No extra heat coming from thermostat and the area near the thermostat is not unusually warm. Turned out to be a defective thermostat. Thanks. Dec 28, 2021 at 20:29

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Here are a few possibilities.

  1. The thermostat is actually switching off early: While the thermostat should be calling for heat, check the output voltage at the thermostat. If you ever find that the thermostat output has switched off then you can focus your efforts on the thermostat. Either it is malfunctioning, or something about the installation causes the thermostat to be warmer than the rest of the room.
  2. Thermostat remains on, but voltage is lost at the heaters: movement caused by heating could cause a defective wiring connection to open, interrupting the current and stopping the heating. While the thermostat is calling for heat, its output is energized, but the heater isn't heating, confirm correct voltage is present at the heater input terminals.
  3. Overheat protection in the heaters may be activating: The heaters may be equipped with a safety switch to prevent overheating. The switch may be faulty or installed wrong, or there may be a problem in the installation of the heaters that actually does cause them to overheat (lack of free air flow for example). Examine the wiring in the heater unit, find the safety switch, and measure voltages to see whether that switch has opened during the intermittent no-heat condition.
  4. It's a long shot, but -- is there any kind of demand management device? These are sometimes installed by electric utilities or by building owners to intentionally interrupt appliances. This can reduce strain on the electric grid during times of high demand, or it can reduce financial strain on the owner's pocket book when time-of-use, demand-based, or other variable pricing is in effect.
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    Thank-you for such detailed trouble-shooting steps. Turns out it was a defective thermostat. It worked, but seemed to be calibrated completely wrong. I swapped in a different thermostat and things are heating properly now! Dec 28, 2021 at 20:27
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Check that the wires are well connected to the thermostat, if the wires not fully tight the current can cause heating which will confuse the thermostat.

Alternatively call the electrician back and explain the problem to them.

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