A gas valve controls the flow of gas to the furnace's burners. A single-stage gas valve has only one setting, which is either fully open or fully closed. In contrast, a two-stage gas valve has two settings, a low and a high, allowing for a varying flow of gas to the burners.
The use of a two-stage gas valve in a furnace allows for more precise control over the heating output, providing more consistent and efficient heating. In most cases, a two-stage furnace will run on the lower setting, using less gas, and switch to the higher setting when the demand for heat is greater.
In summary, the main difference between a single-stage and a two-stage gas valve is the level of control over the flow of gas to the burners, with a two-stage valve providing better temperature control and energy efficiency.