I have a heater that is a 220vac can I use it on a 120vac? I think it will but the heater elements will be at half heat.

Not quite that easy. When voltage is cut in half, amperage doubles assuming wattage stays the same. For example, if something draws 10 amps at 240 VAC, it will draw 20 amps at 120. Amps = watts /volts. If there is a fan in the heater, it will not function. Better to get a 120 heater or pull a 240 line.

Additional information:

The reason the amperage is important is because if the amperage is to high for the circuit, it could cause the conductors to get to hot. Or cause the breaker to trip as in a 15 amp circuit carrying a 20 amp load.

Okay, the math behind the first response is incorrect.

In a purely resistive circuit (such as a heater), reducing the voltage by half will reduce the amperage by half, not double it.

To rephrase the first response; if a 240 volt 2400 Watt heater (2400W/240 Vac = 10 Amp) heater is reduced to 120 Volts, it would require 20 Amps at 120 volts to generate 2400 watts.

To solve this properly, you must know the resistance of the heater coil. To get this, divide the Voltage, squared by the Wattage. (240*240/2400 = 24 Ohms)

The device's resistance is not going to change because the voltage changes, so according to Ohm's law 120V/24 Ohms will generate a 5 ampere current.

120V*5A = 600 Watts. So the max amount of Watts (heat) would be 600 Watts. The Wattage is reduced by a factor of 4.

Note I stated "assuming wattage stays the same".

Not a problem. Because the OP was using a specific heater, I just thought I could give a clearer answer by showing that the heat given off would drop considerably because the resistance of the heater stayed the same and the voltage dropped.

No intent to offend.