A method and system for operating a modulating furnace from a single-stage thermostat. The furnace control system may include a processor, a gas valve, a variable-speed motor, and a thermostat. The furnace control may execute a thermostat algorithm comprising the steps of: determining if a heating load needs to be satisfied; running burners at an intermediate input rate, a modulated input rate, and a maximum input rate for each respective first, second, and third time intervals until the heating load is satisfied; calculating a heating load requirement as function of the time intervals; and determining an initial input rate for a next cycle based on the calculated heating load requirement.