Duo Stats for the Home

Johnson Service Company introduces the “three-point controller”

Duo-Stat 1948

Johnson Service Company’s Duo-Stat brochure from 1948

August 6, 1948 (PD: 201208)

Although previous operations and products were directed at the commercial, public and industrial markets, the Johnson Service Company (today’s Johnson Controls) announced on August 6, 1948 that they were creating a new electronic automatic temperature control for homes and small buildings.

An adaptation of the company’s “Duo-Stat” control, which the company had sold since 1934, the new “three-point controller” continuously measured outdoor, indoor, and heating system temperatures (the “three points”) with temperature measuring elements connected to a simple electronic amplifier.  Whenever a temperature change at any of the three points took place, the electronic amplifier “telegraphed” to the heat source the proper relationship between the temperature points, so that heat input was constantly adjusted to compensate for heat loss from the house or small building.

Thus, the three-point controller “changed the size” of the heating system in accordance with fluctuations in outdoor temperature, while maintaining comfortable room temperatures using as little fuel as possible.  The three-point controller could be used with all hot water and warm air heating systems.