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I have two electric baseboard heaters in my house that are both controlled by a single digital programmable thermostat (King ESP-230). I believe each heater is 1500w so 3000w total. Twice now in the last year or so, it got really hot in my house when I realized the heaters weren't turning off at the specified temperature. The thermostat would be set around 65 degrees, and the house temperature would get up to around 75 degrees as shown on the thermostat itself.

What's weird is the red light that indicates the heaters are on, will be off. Here's a list of what I tried, the last item being the one that eventually fixed the problem:

  • Pushed the power switch on the thermostat itself to Off
  • Turned the temperature down to minimum
  • Shut off the heaters at the breaker, and turned back on
  • Pushed the Reset button on the thermostat, and set the day and time.

Only after setting the date and time after a reset did the heaters shut off. Does this just sound like I need to replace a thermostat that occasionally goes haywire, or could there be something wrong with the heater wiring that's causing the thermostat to mess up? Both times this happened, it was around 9pm on a Sunday evening, but I would imagine that's just a weird coincidence.

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  • Is that the thermostat you want, or would you rather have a 24V smart 'stat like any of the Honeywells clear on up to the Nest? There's a way to make 240V heaters work on 24V stats. – Harper Feb 5 '18 at 23:26
  • @Harper -- The one I have does everything I need it to do, plus I don't believe I have the correct wiring setup for a 24V thermostat. – connorblikre Feb 5 '18 at 23:42
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It sounds like the relay contacts are fused, this is not uncommon with loads that are constantly switched, the fact that the controlls are saying the relay is not being commanded on but it is heating points to the relay. (It could also be a solid state relay these usually short when they fail, then if there is enough current blow the junction open). The fix is a DIY get a replacement thermostat, turn the circuit breaker off, install the new stat and turn the power back on. I try to get thermostats that have a much larger rating than I need, make sure to add the ampacity of both heaters and get a thermostat that has at minimum this value (I will pay more for a 2x thermostat so it will last longer).

  • Thank you! Does that mean there is a relay built into the thermostat? – connorblikre Mar 14 '18 at 23:07
  • Yes there is normally a relay most of the time they are mechanical but some models use solid state switches. – Ed Beal Mar 15 '18 at 12:59
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I have a similar set-up (two baseboard heaters controlled by one digital 24-v thermostat). If it's like mine, chances are there's a Relay switch next to the main electric panel (mine's in my garage) that actually turns the heaters on and off when the thermostat "tells" it to. I've had that relay go south at least once and had to have it replaced. NOT a do-it-yourself project as you'll be dealing with 240 Volts.

  • Why is it not DIY? Can't you isolate the circuit with its breaker? – isherwood Mar 12 '18 at 13:48
  • It certainly CAN be a DIY, as long as the homeowner is skilled enough to identify and turn off the associated breaker(s). I was just defaulting to not DIY due to the potential safety issue. – user82453 Mar 12 '18 at 14:18

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