I have a nice 24v cab heater unit that I want to use on a machine that is 12v. (just the blower is 24v) I think its draws about 5A. I see on amazon or elsewhere you can get voltage increasers to get 24v. Would that work ok?
It will work at about 50% speed if you hook it straight to the 12v system. It may draw double the amps at 12v. I've done this with several 24v fans over the years without issue other than slightly slower speed. I have no experience with a 12-24v voltage increasing system and I did not even know that they were offered. Most common is a 24-12v voltage reducer.
One of my first cars was a 1966 VW Beetle that had a 6 volt electrical system. I added a 12 volt radio with a cassette player. Had to install a 6v to 12v transformer to run it. Worked great. Wasn’t very expensive. I suspect there are 12-24 ones out there. But the fan will run on 12v.
I think I would rather see about finding a 12 volt motor.
Short answer is yes, it'll more than likely work for simple fan motors. I used the step down versions a lot for prototyping efforts and they work fine. The reason I say more than likely is these systems a bit noisy and have a slight chance of making digital electronics a bit wonky. But if you have a standard dc motor controlled by a rheostat, you'll be fine. Just remember a couple things: If you think you need 60watt capacity, then double it to buying a 120watt capacity converter (or near it) to be on the safe side of heat and longevity. Last long time when you follow this. Also remember that if your cab heater is grounded to the same ground as the 12v input ground, then you will more than likely need to tie all your grounds together so you don't have floating ground. Control systems and other electrical doo-dads do funky things when there is a floating ground.
I'd look at some 12 volt motors first. Might not be too expensive and might find one that turns a little faster too.
Sorry guys but boost converters are common and cheap now days. I think finding a new fan may have been the right way a few years ago but maybe not today. Here's the just the first example I found on Amazon. Can be at OP's front door in a couple days and for under $20. And yes, a 12v fan would work as long it's matched to whatever speed control system it will use. If just on/off, then anything should easily work.