I designed and made a control panel for a furnace a year ago and it has worked well except for one relay in the 3phase control box. This relay powers up a contactor to keep the fan running for the air supply. The furnace is on 24/7 and that relay has burnt out with a coil overheat twice. This happened for the second time today, while I was visiting the workshop, so fortunately a spare relay could be plugged in without the furnace cooling down too much.
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The only thing I can think of that caused the problem was that the relays are 24vAC and the power supply is a solid 24vDC. The next relay will be DC rated, and I can only think that the AC relay coil limits current to a 'safe' level due to its' inductance.

I would welcome any input on this problem - in fact I will relay it on to the furnace owner (GROAN !!!).

Cheers,

Rob

BTW I made the front panel on my CNC router !