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Discussion Starter #1 (Edited)
I have 3 electric baseboard heaters totaling 4750 watts on a double pole 30 amp 240v circuit. They are controlled by a low voltage Honeywell thermostat via a Honeywell R841C relay. Last night I noticed it getting quite warm, checked the thermostat 25c, set at 20c and reading heat off. Heaters still on strong! Threw the breaker, let the temp cool, turned back on 2 hours later. Temp set to 17c overnight, when warming up to 20c in morning heaters would run for 20-30min then stop even though thermostat says the heat is on. Reset the breaker(not tripped) and heat would come back on. Has been ok this evening when only maintaining temp. I'm thinking relay because of last nights staying on, but this mornings tripping off has me confused.

Thanks for any help,
Jeremy
 

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Discussion Starter #2
So I was in the basement this morning when I heard a pop. Turns out heaters are blowing the breaker. But only one of the two and it moves very little. This system has been in the house for 30+ years, recently I had two of the heaters replaced. System has been on now for over two months why is this happening now? Can a worn relay/contactor cause this or a weak breaker?
 

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If the breaking is throwing the heaters should be off. Either way, if only one of the arms is moving and they're tied to each other something is very wrong. I'd replace that breaker and work from there.
 

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This is just a scientific wild guess, but I'd say it's possible one of your heating elements is shorted to ground, and when it trips the breaker it's only opening one leg. That shouldn't happen. Are you sure it's a double-pole, and not two single-poles with a handle tie? What brand of breaker? If it is a double-pole, it's defective and needs replaced.

If the relay is only controlling one leg of the 240v circuit (single-pole relay), than that would allow a shorted heating element to continue to heat up even when the thermostat is not calling for heat. A shorted heating element would also draw more current than usual, which could cause the breaker to trip.
 
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