I have a 240v 1500w 6.5a electric baseboard heater with a wall mounted thermostat. What I found out is that the the thermostat and all the outlets in the room are on the same 20a breaker with this heater, it never gets really hot, only warm, I'm thinking someone wired it to 110 because of the outlets. How can I remedy this to get more heat? If I just hook the wires to a double pole 20a breaker for 240 it would make the proper heat but the outlets?? Humm, if I run a separate line for the heater for 220, I'll lose the thermostat, how would I control it? I'm not busting the wall apart to find where the thermostat wiring is. I did have the cover off the thermostat and its 12/2 wired. I can provide pictures of the info plates on both the heater and thermostat.
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.
Low heat output could very well indicate a improper voltage connection. Especially, as you noted, that the 110V receptacle's and the 220V heater are on the same breaker. The repair would consist of separating the outlets and the heater. Depending on the distance to the heater, this is done by running a 10/3 cable and controlling the circuit with a dedicated 20 amp breaker. It would be wise to seek the help of a licensed electrician if you have any doubts of your abilities for electrical work.