A method for compressing a gas by using energy produced from a heat source. A boiler is provided. The boiler is segregated into an upper chamber and a lower chamber by a barrier such as a piston, a bellows, or a diaphragm. The lower chamber is filled with a liquid having a suitable boiling point and other properties. The upper chamber is filled with a gas to be compressed. Heat from any suitable source is applied to the liquid in the lower chamber in order to bring the liquid to a boil, and thereby produce pressurized vapor in the lower chamber. The rising pressure in the lower chamber moves the barrier in the direction of the upper chamber, thereby compressing the gas in the upper chamber.