Flow Centers: Pressurized vs Non-Pressurized (Part 2)

Monday, June 19, 2017

As described in the first part of this article, flow centers require a certain minimum level of inlet pressure in order for the pumps to function properly. The method of maintaining this pressure is what distinguishes a pressurized flow center from non-pressurized flow center.

In some cases, the choice of which type of flow center to use is based on application constraints. But most of the time, the decision is based on preference. When faced with the decision, consider the pro’s and con’s of each type of system.

Pressurized Flow Centers


  • No monitoring and/or maintenance required by the homeowner
  • Smaller size reduces space needed for installation
  • Completely closed and sealed design prevents air and debris from entering the system after installation
  • Typically packaged to include three-way isolation/flush valves
  • Flexible location and orientation installation options
  • Can be installed vertically or horizontally (12-, 3-, 6-, and 9-o’clock positions)
  • Can be plumbed in series or parallel
  • Provides single point isolation location between the ground loop and heat pump
  • Very familiar to plumbers and hydronic technicians


  • No built-in provision for air elimination
  • Thermal expansion and contraction of loop piping along with insufficient startup pressure may lead to a flat loop1
  • Poor installation practices that result in leaks can cause pumps to air lock, flat loop service calls, etc.
  • Requires a qualified technician for maintenance (fluid levels, antifreeze concentration, etc.)
  • Requires flushing, purging and re-pressurization after servicing

Non-Pressurized Flow Centers


  • Water column provides inlet pressure
  • Allows direct measurement of fluid levels, flow rate, and antifreeze concentration, even during operation
  • Fluid reservoir facilitates natural air removal and accommodates loop expansion and contraction
  • Homeowner can perform basic maintenance (such as adding make-up fluid)
  • More forgiving of poor installation practices (incomplete flushing, small leaks in piping, etc.)
  • Pump service does not require the system to be re-flushed
  • With the right manifold, may be able to flush and purge the loopfield without an external flush cart2


  • May promote poor installation practices due to forgiving nature
  • Direct access to loop fluid opens possibility for contamination
  • Requires more space for installation
  • Limited installation locations and orientations3
  • May require additional field-installed isolation valves for flushing/purging, servicing, etc.
  • Individual flow centers cannot be installed in series or parallel

So, Which Flow Center Is Better?

All differences aside, both types of flow centers will perform well when installed properly. Each has its own advantages and disadvantages to consider. In the end, it is up to the installation contractor, system designer or building owner to decide which system best meets their needs.


  1. The installation of an expansion tank is recommended to alleviate the concern of a flat loop.
  2. Requires an inside building header with isolation valves placed on each individual loop.
  3. Piping must not be located more than 30 feet (approx.) above the reservoir, which can only be installed vertically.

About the Author

Jeff Hammond
Geo-Flo Products Corp.

Mr. Hammond is currently Director of Business Development and Marketing at Geo-Flo Products Corporation, a manufacturer of flow centers and accessories for the geothermal heat pump and hydronics industries. He started with the company in 2012, and has been in the geothermal heat pump industry for over 30 years.

Previous to Geo-Flo, he was at Enertech Global for five years, ClimateMaster for nine years and WaterFurnace International for twelve years. Mr. Hammond’s experience in the industry consists of positions in R & D, engineering, product management, training, sales, and marketing. His education includes a bachelor of business administration from the University of St. Francis and an associate of applied science in electrical engineering technology from Purdue University.

Mr. Hammond has been a member of ASHRAE since 1990 and has served on CSA, AHRI , and IGSHPA marketing, technical and advisory committees.