Heat is caused by the I2R, and I=E/R. If R is high enough the I canbe low enough for the I squared R value to be low. Therefore little heat.
Power, Watts: W=I*E=I exp 2 * R 
Higher voltage, less current, smaller wire, same heat.
Voltage same 120 VAC. with a poor connection the R value will increase. That does not cause the supply voltage to go up it still is 120 vac. Same voltage low current low heat not same heat.
I beg your pardon, I thought the question was how the same heat could be obtained with less current and more voltage.
Sorry if I wasn’t clear. What I was perplexed by was that I have a low voltage situation, and my rudimentary understanding is that to get this, I need to have a short or loose connection (or similar) which would cause increased resistance (thus the voltage drop). In a situation like that, I would expect a very warm situation, leading to possible fire. Like when a stranded connection loosens, so that all the electricity flows through a single strand, causing it to get hot. The surprise (to me) is that I have this situation and it is not burning down the house.
Anyway, I pulled the breaker today and it was nasty. I don’t know how, but the connection on one led had no continuity from where the breaker connects to the panel and the wire connection. Also the wire looked a little cruddy, but I am not sure if I was imagining that. I put in the new 20A breaker and checked the continuity between the panel and the t-stat. Everything is fine, I have 240v between the hots, and the heater is working great.
Thanks again for all the help, everyone.
Awesome. I was about to ask you to take a photograph of the 30amp breaker in the panel. With the panel cover off. We would have seen your problem.