Component Location

Location of Circulator, Expansion Tank and Air Purger

Definition: A well designed hydronic heating system operates without any air related problems. An air-free system provides an efficient transfer of heat from the boiler to the heat distribution units. An air-free system will enable the designer to accurately predict and ensure desired hydraulic performance for the life of the system.

Relevance: The actual location of the basic components, particularly the expansion tank, air purger, boiler, circulator and make-up water line have important relationship to one another in a well designed system. The effective use of the basic principles will dictate the best location for these basic system components.


  1. The junction between the expansion tank and the system piping is "the point of no pressure change". Its pressure remains constant regardless of whether or not the circulator operates.
  2. When the "point of no pressure change" is located close to the circulator suction, then the circulator suction pressure is fixed by the expansion tank pressure. The circulator's differential pressure will appear as an increase above system static pressure.
  3. When the circulator's differential pressure appears as an increase in system static pressure it provides the best possible condition to effectively eliminate system air. It avoids reduced pressure (even in some situations a negative pressure) at the circulator suction and makes heat distribution units practically self-purging.
  4. The circulator on the boiler supply provides the point of low solubility between the boiler and the circulator. By locating the air purger there, all air will be eliminated and the system will be air free.
  5. The diaphragm-type expansion tank with a permanently sealed-in air cushion is required in order to operate an air-free system.
  6. The 'point of no pressure change' provides the best location to connect the make-up water line to the piping system. It's the only place in the system where the circulator does not affect the pressure.


  1. Lower installation costs. The heating system control module can be prefabricated either in the shop or on the job.
  2. Effective use of the circulator differential pressure to keep the system free of air-related problems.
  3. Easy to purge. Pressure purge in one direction through the system piping and up from the bottom of the boiler. Then purge any additional zones one at a time.
  4. Easy access for service. All the mechanical components that may require servicing such as circulators, flow control valves (or zone valves), expansion tank, air vent and fill valves are all accessible.
  5. Quieter. An air-free system will not have noise related sounds of water and air flowing throughout the system piping.
  6. Efficient. An air-free system will ensure the desired flow of heat is carried to the heating distribution units. Air bound piping will cause the boiler to cycle excessively without effectively transferring its heat energy.