OK, You have two elements inside the heater. You have one switch and one dial. The dial is the thermostat.

The switch will send all power to only one element when it is in the low position. Each element will draw 750 watts when on. In the high position, the switch sends power to both elements for a total of 1500 watts.

There are two types of dials. It could be a thermostat or a rheostat. Most likely it is a thermostat. In almost all heaters that I have seen, turning the dial all the way counterclockwise (left) turns off the heater. That is your off switch.

The low high switch and the thermostat dial are in series, that is the power passes through one, then the other. If the switch is the first one in the series, then you will always find voltage on it, even when the heater is off.

The dial could be a rheostat, which is a variable resistor, but that is very unlikely. If it was, you wouldn't need a high/low switch because the rheostat would be like an infinitely variable low to high selector. Also it has to dissipate a lot of heat itself because of its resistance.

More likely the dial is a thermostat which is a switch with a variable spring on it. The spring expands and contracts with temperature turning the switch on and off. Because it is in the heater itself, it is controlling the temperature inside the heater and not in the room. Because of this, you have to turn it up on colder days so the heater stays on longer. That is also why there isn't a temperature scale on it, just a cool to hot on the dial, like the heater in your car.

If the thermostat doesn't have an off position, then you should put a switch in to turn it off during the summer. But most of these, if you turn them all the way to the left (CCW) you should feel it go over a detent right at the end, that is the off switch. You will feel the voltage on the low/high switch, but with the dial in the off position, the circuit is not complete so no current flows, therefor no power is consumed.