Help with thermostat + baseboard heater install

Question

Hoping to get some help with my recent thermostat installation. I live in a house built in the 70's and the wiring is not straightforward for a layman. Upon installing two new baseboard heaters and thermostats, I was greeted by a burning plastic smell a few minutes after turning the breaker on and what I think was a flash (I was concentrated on something so not sure if the flash was my lights or just being tired). Naturally I turned off the breaker immediately and went to investigate. I didn't find any burnt wire nuts or components in either the heaters or the thermostats. The smell is also not a strong one either, and dissipated within 30 minutes or so.

Can someone look at my wiring diagram and tell me if I did something wrong? I emulated the wiring configuration from the previous knob thermostats, but maybe I'm missing something?

EDIT 1: Adding details that are important given comments received, namely that there is a third thermostat and heater coming from the bathroom on this circuit (this one has not been changed and is the original heater + tstat) and that the wire gauge running from the heater to the tstats are all 14-2 with all other connections being 12/2.

Answer 1

Based on your diagram and comments, I don't see anything that would cause a burning plastic odor. It could have been a harmless "new heater" odor from residue on the heating elements from the factory.

All the AWG 14 wire is problematic and contrary to code. Under the NEC, no wire smaller than AWG 12 is allowed anywhere on a branch circuit that is supplied by a 20 amp breaker, except connecting wire internal to a UL (or other lab) approved device. For 14 AWG wire, the breaker must be no greater than 15 amps, but the way it's currently wired, a 15 amp breaker would not be allowed because your total current when everything is running is 90% of 15 amps. The limit is 80% continuous load.

When everything is running according to plan, none of the AWG 14 is currently overloaded, but the NEC plans for the unforeseen.

There are contributors here who contend that insurance companies can choose not to cover a loss caused by wiring that is clearly not up to code, as yours is now. I've never seen independent proof of that, but be forewarned.

If it's too difficult or expensive to replace the AWG 14 with 12, you might consider powering the bathroom heater from another source conveniently located in the bathroom. That may involve replacing the 500W heater with a 120V unit. Then you could replace the 20 amp 2-pole breaker for the two new heaters with a 15 amp 2-pole. Your AWG 14 wire comes into compliance with a maximum continuous load of 76% of 15 amps.

As a final check on the new install, you might power it up again, let it run for a few minutes and then carefully feel for any warmth at the wire nuts. They can overheat if not properly made up.