The Dunham Differential System is a simple two-pipe power vacuum (air pump) return system working normally at pressures below atmospheric (subatmospheric) in the system and employing orifice supply valves on the radiators. The principal advantage of the Dunham system is that it continuously distributes heat at a variable rate equal to heat losses from a structure. This is accomplished by reducing the capacity of the system by reducing the volume temperature of the steam.

Standard radiators, pipe fittings, and boiler connections are used in the Dunham system. The principal features of the Dunham system that distinguish it from other heating systems are:

  1. The traps and valves,
  2. The condensation pump,
  3. The controller.

Each of these components is differentially controlled; that is, each is actuated by a pressure differential (the difference in pressure in the supply piping and the pressure return piping). The Dunham system distributes the steam proportionately to all radiators, the pressure range being from 2 lbs. gauge to pressures considerably lower than atmospheric. Because of the relatively constant differential in pressure between the steam and return lines, the radiators are filled with steam.

A condensation pump is connected to the differential controller and to the supply and return piping. The controller starts the pump when the pressure difference between the supply and return piping tends to fall or disappear, and stops it when the pressure differential is restored. The condensation pump is a wet air pump, which handles both air and condensation.

The thermostatic radiator traps are actuated by temperature changes within the radiator. Drip traps are installed at drip points to which large volumes of condensation flow. These are combined thermostatic and float traps. A control valve regulates the admission of a continuous flow of steam into the heating main.