Feeling Separated?

By Bob “Hot Rod” Rohr


Fun fact:

My first “aha” moment was learning about the point of no pressure changes (PONPC) by reading and re-reading Dan Holohan’s Pumping Away, which is a must read for all hydronic pros and wannabes. Just as that knowledge changed the way I understood and piped hydronic systems, so did the introduction to separation. I was learning how to properly pipe systems and those systems were working really well. Of course, my customers also reaped the benefits.

Another “aha” hydronic moment occurred when I was introduced to hydraulic separators. Note that low loss header is another term used to describe a separator device. Knowledge of hydraulic separators majorly changed the trajectory of my approach to hydronics.

Actually, the mechanics of hydraulic separation were wrapped up in the concept of primary secondary (P/S) piping. Credit is given to the Bell and Gossett folks for introducing P/S piping to the industry. It was conceived as a good work around for converted gravity piping systems in larger commercial jobs.

As I recall, P/S piping and closely-spaced tees in residential applications came into play in the 1990s. As low temperature radiant systems became popular, P/S piping was one method to keep a high temperature boiler separated from the low temperature radiant temperatures. When properly designed and piped, the boiler could run without low return temperatures while supplying low temperatures to radiant slabs, for example.

The key to that working properly lies in the closely-spaced tees in the circuit. It is the space between the tees that is critical to the workings of P/S piping. This space provides the hydraulic separation between the various piping circuits. If it is properly sized, installed and controlled, P/S piping remains an excellent piping option. It once again provides value with flow restrictive mod con boilers. That P/S disconnect allowed the boiler to have the correctly sized “boiler” pump. The secondary piping and circulators were hydraulically separated so each side, primary and secondary, was assured proper and adequate flow.

The P/S series circuit piping allowed for multiple distribution piping options. It allowed an indirect tank to be connected and properly pumped. It became the rage to pipe systems with P/S. Boiler installation manuals started showing P/S piping options. All was well in the hydronic world thanks in part to the proper piping P/S could provide.

Somewhere in the early 2000s I was introduced to a hydraulic separator device. A manufacturer arrived at my shop bearing gifts of steel vessels called separators. Basically, all the important features of hydraulic separation were wrapped into a single steel tank or vessel. Hydronic specialty manufacturers did some number crunching to arrive at the correct dimensions for a cylinder or vessel to allow ideal separation to be obtained.

Along with providing this wide spot in the piping, the side benefit of air removal was added. For the designer and installer, a single, fairly simple component became both the hydraulic separator and the central air elimination device.

Why not give a separator a chance to prove its value on your next piping job? While you may not visually see a difference, the fluids and transfers going on inside will benefit.


Scalable with multiple functions

Once you have the calculations sorted out, hydraulic separators are very scalable. Commonly separators are found in sizes from one inch to 24 inches or larger. The hydraulic separator continues to evolve with the industry.

Multiple functions are now built into a single separator that provides:

  • Hydraulic separation.
  • Air removal, down to micro bubble size.
  • Dirt and particle removal down to a five-micron particle size.
  • Magnetic separation is now a feature providing ferrous material removal down to a .05 particle size.
  • Dry wells and wet wells – options for adding sensors or gauges.

These features are available now in five-in-one devices.


Inside a common hydraulic separator

The premise is very simple: provide enough space or room for multiple flows to “get along.”

Three conditions are possible:

  1. Equal A and B side flow. In this condition, flow crosses directly through the separator, and no mixing occurs.
  2. Condition 2 could be more flow in the A side compared to the B side, so some fluid mixing occurs inside.
  3. Lastly the B side could have more flow compared to the A side.

It is important to be aware that under conditions 2 and 3, the temperatures may be mixed at either the input or output side of the separator, depending on flow rates. These temperatures can be calculated. We use a common mixed fluid temperature formula (see Formula 1) to predict temperature at any port at any temperature or flow condition.


Formula 1

Assume 10 gpm from the boiler into the separator at 160ºF.

The distribution side is flowing 25 gpm, returning to the separator at 120ºF.

Notice that the supply to the distribution is blended down from the 160ºF, by the returning 120ºF from the system side.



A benefit of a hydraulic separator is all the secondaries piped off the B side will see the same supply temperature. This is not possible in a series primary loop as each set of tees downstream sees a lower mixed temperature caused by cooler return fluid entering the loop.

Understanding what a multiple purpose separator does gives you a window into the benefits it provides. We know closed loop systems need to be air free, as the smallest micro bubbles will drive down heat exchanger efficiencies, both in the boiler as well as inside the heat emitters. Dirt, rust, sediment, any particle flowing through the system, can cause harm and failures. Pumps, valves, and pipes all prefer a clean working environment.

With ECM wet rotor style circulators, adding the magnetic function helps prevent tiny particles from reducing or preventing adequate rotation of the motor in the fluid.

The connection point for “pumping away” is established at the hydraulic separator. Conveniently, control sensors, pressure and temperature gauges can be centrally located on the separator.

Size the device to the largest flow rate, either A or B side, and do not add the two flow rates to select the size. Mount the unit on the wall and pipe as shown on the manual or the label attached to the separator. Both heated or chilled fluid systems will thank you. Separators add these same benefits to chilled water systems, by the way, as they are not a boiler specific product.