Multi-phase modular comfort controlled heating system

A circulating fluid that is heated by a number of gas-fired burner assemblies is used to heat a space either through a hot water system or a forced air heating system. Each burner assembly includes a housing with a heat exchanger mounted therein and a gas burner for heating the fluid which flows through the heat exchanger. The heat exchangers are connected in parallel in a closed loop forced circulation system. A microprocessor based thermostat control compares the temperature in the space heated by the air with a reference temperature. Dependent upon the magnitude of the difference between the temperatures, one or more of the burner assemblies are energized. The burner assemblies are sized such that during most of the heating season only a fraction of the burner assemblies are used. Burner assemblies are cycled on and off as needed to return the temperature to the steady state during the warmer and colder parts of the heating season at least one burner assembly is usually in operation during a majority of the heating season.