Haven't searched for a while but 12vdc to 24vdc converters were not easy to find.
Here are a couple of links.
First one was my choice, appearance of rugged industrial construction, lots of detailed spec's but since I last looked they've added the notation. "Motors: Running motors from these units will void the warranty.http://www.powerstream.com/dc24.htm
This one looks a little simplistic and not much info' on specs but there are "Pump controller versions" so obviously manufacturer approved for motor loads.http://www.solarconverters.com/product_frame.html
Pump controller versions by this manufacturer and sold by a number of the Solar outfits are controlled by a float switch across 2 flying leads. Your heating thermostat would do the same thing.
Haven't seen or handled either of these units. I use a 110vac to 24vdc power supply bought at auction.