What's wrong with the combination of components in the title?

The issue I have may not actually be a problem but I'm curious to get another opinion nevertheless.

I had to find a circuit to connect an electrical sauna in the basement. The 1840 Watt sauna required a dedicated 120 Volt, 20 Amp, single phase circuit.

To do this I disconnected three baseboard heaters that were no longer needed in the basement. This gave me a dedicated circuit on a #12 wire. So far so good. However, because the original circuit was dual phase, I had to relocate the connection in the panel to a single phase 15 Amp breaker. Unfortunately I couldn't move the original 20 Amp breaker.

Now here's my question.

Since I have a dedicated circuit on a #12 wire, is it OK to use the 15 Amp breaker, especially if it doesn't trip when the sauna is in use? In other words, am I not "overprotecting" the circuit?

Looking forward to hearing your opinions.

Thanks
Kurt

Short answer, you should be using a 20 A circuit breaker.
Why?
Because the sauna is likely considered a "space heating" or continuous load. Because of that the wiring is sized at 1.25 > the current. 18.85 A = 1820/120*1.2. The next circuit size is 20A and the wire size is 12 AWG.

A "continuous load" is a load that is expected to be on for more than 3 hours at a time. It specifically includes space heating.

1820/120 is a smigen over 15A which further complicates things.

The OCPD or Overcurrent protective Device may mot be rated for 15 A continuous, so it may overheat.

So, are you saying that an undersized OCPD will dangerously overheat before it actually trips open? Obviously this happens with an undersized conductor, but I didn't realize that this can also happen with an undersized breaker.

BTW, I'm not interested in doing an experiment in my home panel. Therefore, based on your opinion I will install a 20A breaker on this circuit before using the sauna.

Thank you for your response.

You did get the short version: Long version here: Sizing a circuit breaker.