Electric baseboard wiring
Sorry, the ONLY way to do this properly is to get the heater on a properly sized 240V circuit. This can be a 15A or 20A depending on what is shared on the circuit and what the distance is (less than a 150' or so circuit run this is not an issue). 

If you don't want to do it right and find how the t-stat is wired you may be able to put an integral thermostat right on the unit and un-wire the wall t-stat. This depends on the age and brand of the heater if a t-stat is available for it. It's worth investigating thought. You may find the t-stat is wired right to the unit and has nothing to do with the receptacles.