Make sure the wiring to them can handle the amp load. Quick Example: If heater use 10a each then circuit needs 25a or more w/o melting contacts and/or insulation.
You may need to add another circuit and fuse, maybe a relay, etc.
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
The Ogre's Fiero Cave