Flow Centers: Pressurized vs Non-Pressurized (Part 1)

Tuesday, May 30, 2017

In terms of residential flow centers, the geothermal heat pump industry is divided into two camps: pressurized and non-pressurized.

A flow center is a device that produces system flow and facilitates the removal of air and debris (through built in flush/purge ports)1. The terms “pressurized” and “non-pressurized” indicate whether static pressure is held in the piping system, which is measured when the circulating pumps are off.

Pumping Basics

Most of the pumps used in residential applications are wet rotor circulators. These pumps require that the inlet (suction side) pressure exceeds a certain minimum value, which is specified by the manufacturer (NPSHr). The amount of pressure available at the inlet (NPSHa) must be greater than the minimum for pumps to function properly. If the inlet pressure falls below the minimum, cavitation can become an issue.

There are two ways to maintain pressure at the pump inlet, which is where our conversation begins:

Pressurized Flow Centers (Static Pressure > 0 psi):
Static pressure is induced in the piping during start-up with an external source (i.e. a flush cart).

Non-pressurized Flow Centers (Static Pressure = 0 psi):
Suction-side pump pressure comes from the weight of a standing column of water (in a reservoir).

Both types of flow centers have been used with great success in the residential GSHP market. A brief history lesson may explain why both types exist in the first place.

The Early Days2

In the late 70s and early 80s, pressurized flow centers were the only option available. Most contractors installed standard hydronic components as part of the system (image courtesy of Waterfurnace International).

While these systems worked well initially, the condensation that formed during heating mode operation caused steel expansion tanks to rust and prematurely fail. Additionally, barbed and/or threaded connections were common, which tended to leak over time with the wide swing in ground loop temperatures (and pressures) from winter to summer.

The Shift to HDPE

In the late 1980s, the industry moved from polybutylene pipe to HDPE. Due to its thermal expansion capabilities, many industry practitioners concluded that an expansion tank was no longer needed, especially for residential and light commercial applications.

During this time, the “hydronic specialties” (expansion tank, air separator, etc.) were removed from most flow centers (image courtesy of Waterfurnace International).

Since HPDE pipe expands and contracts, it behaves like an expansion tank. However, it expands more quickly than the fluid, causing the static pressure in a system to drop in the summer. HDPE is also viscoelastic, meaning that it will stretch but not return to its original shape. Unfortunately, these characteristics led to loop pressures that fell below the minimum (NPSHr), causing what many referred to as a “flat loop”. Flat loops caused pump cavitation, GSHP lockout (due to low flow) and premature pump failure.

Making things worse, air bubbles still in the system after startup or maintenance tend to expand with decreasing pressure. The larger bubbles created noise, air-locked pumps and in extreme cases, they even blocked ground loop circuits.

Introducing Non-Pressurized Flow Centers

Being wet rotor circulators, most residential systems require very little suction pressure for proper operation (typically around 1 psi or less). The weight from a small column of water is all that is needed to maintain inlet pressure for this style of pump3.

Armed with this knowledge, frustrated contractors began adding reservoirs to alleviate issues caused by static pressure loss and HDPE thermal expansion/contraction. Following suit, flow center manufacturers began production of non-pressurized options in the 90s. The use of non-pressurized flow centers has grown steadily since then (image courtesy of Geo-Flo).

Today’s Choices

Since the introduction of non-pressurized flow centers, industry veterans have begun to revisit the use of hydronic specialties (expansion tanks, air separators, etc.) to minimize flat loop service calls with pressurized systems. This shift has led to a very high rate of success with both types of flow centers.

Since both work well, the contractor is left to choose the system that provides the best fit for the application. When choosing between the two, careful consideration of the pros and cons of each may help with the decision4.


  1. ANSI/CSA C448.0-16, Design and installation of ground source heat pump systems for commercial and residential buildings
  2. Based upon the author’s experience in the industry since 1986.
  3. A 2.3 ft. column of water above the pump inlet provides the required NPSHr for proper operation (1 psi = 2.31 foot of head).
  4. This is the first in a two-part series. Part two of this article will cover the advantages and disadvantages of pressurized and non-pressurized flow centers.

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.