Electric baseboard - 1 of three units won't turn off

Hi, I have a set of three 1500w electric baseboards that are tied to a single thermostat. I noticed this weekend that one of the three baseboards is running constantly and won't turn off when I turn the thermostat way down. The other two units properly shut off when I turn down the t-stat. I'm assuming the problem is not the thermostat because 2/3 of the units respond properly, but don't know enough to be sure. I shut off the breaker for the time being (these are in an outbuilding, so we can shut them off without too much disruption).

Googling hasn't turned up much in the way of things to look for, or maybe I'm a terrible googler. Any thoughts as to what could be causing this?

These units are King Electric, model 8K2420 Series 4.

Thanks!

 

Baseboard heaters are either 120 or 240 vac? Elements should be wired in parallel The thermostat has two wires? If there 120vac thermostat breaking the white wire?

 

Your unit is 240 volt at 2,000 watts each. That is 8.3 amps each. So your at about 25 amps on a 240 volt circuit, probably a 30 amp breaker.

What is the model number of your thermostat? One thermostat usually cannot handle this type of load. By knowing the thermostat it will tell if it it operating three 24v relays. It could be three 120v or 240v relays. Look around the breaker panel for a box that might feed these heaters. We had one post a few years ago the relay(s) were inside the breaker panel. By operating the thermostat you might hear and feel a click noise if there are relays. For the little information, I'm suspecting a set of burned contacts of a relay have welding together.

On King Electric website they list a "transformer relay"
https://king-electric.com/products/thermostats/transformers-relays/

https://king-electric.com/product/model-k-240-volt/
8K2420A ALMOND 240/208 2000/1500 8.3./7.2 8 ft.

King Electric does have some thermostats rated at 22 amps.
https://king-electric.com/products/thermostats/line-voltage-thermostats/

KING ELECTRICAL MFG. CO.
9131 10th Avenue South, Seattle, WA 98108
Phone: 206.762.0400
Fax: 206.763.7738

 

Thanks - unfortunately I can't get the model of the thermostat as I can't get back to the house right now, but I'll take a look when I'm back up there. There is definitely a metal box right near the panel that clicks when the heat goes on and off. It looks like one of those relays - it's wall mounted just above the panel. I'll check the relay for any burned contacts. Thank you very much for the guidance!