dnd 5e – How can I either heat air for a balloon or heat another material to power a machine, without consuming fuel?

Create Bonfire might work

Create Bonfire creates a self-sustaining magic fire that lasts for 1 minute. However, as it is a cantrip, you can repeatedly re-cast it to keep the fire going longer. It can deal fire damage and ignite flammable materials in its area, so it definitely produces heat. However, it continues to burn even if no flammable materials are placed in its area. The spell has no material components. As long as you don’t intentionally place any flammable materials (or flammable creatures) in the bonfire’s space, this spell seems to satisfy all your requirements.

Create Bonfire might not work for a moving vehicle

Since you have added your reason for needing this spell, Create Bonfire may not be what you want. The bonfire created by the spell is stationary, but “stationary” is not clearly defined, leading to a number of questions about how certain spells work on moving vehicles, such as this one and this one. You should ask your DM how they will apply the rules in this case (or if you are the DM, you’ll need to make a ruling).