Primary/Secondary Basic Tutorial

Caution: All drawings are conceptual drawings for illustration purposes only and may be incomplete. More specific drawings may be required for your application.

Primary/Secondary Piping

The idea of primary/secondary (p/s) piping is not a new concept. It has been around for decades. P/S piping gives us the option of using smaller circulators and controlling the flow in our different zones even with different flow rates. This piping can be done with a cast iron boiler, steel boiler or the new modulating/condensing (mod/con) boilers. I believe it is a good way to pipe any heating system as it always has proper flow through the boiler at all times. Since the whole concept of p/s works with the use of hydraulic separation we should first explain what that is.

Hydraulic separation allows several circulators of different sizes or flow rates to "co-exist" within the same system without interfering with each other.  The closely spaced tees allow each circuit to be thought of as a separate or stand alone circuit, not connected to the other circuits. A circulator in a given circuit of a P/S system "doesn't know the other circulators even exist."

The primary pipe is the pipe where the run of the closely spaced tees are installed. The run of the tee is straight through the tee. The secondary piping is the circuit piped off the branches, sometimes called the bull or side of the closely spaced tees. There are rules we must follow when piping p/s systems if we want them to work well. First let's understand which is primary and which is secondary. Remember the primary has the closely spaced tees installed on it and the secondary comes off the branches of the closely spaced tees. Some people call the boiler piping the primary pipe as that is where the boiler is connected. That may or may not be true. Let's take a look at some examples of different p/s piping ideas.


Drawing 1

 In this example the pipe coming from the boiler is the primary pipe as it has the closely spaced tees on it. It will leave the boiler and go back to the boiler. As the secondary circuits (piped off the closely spaced tees) are running the water temperature in the primary pipe will drop. This is best when applied to a system with multiple water temperatures. The secondary piping would be arranged so the hottest shortest loop is first and the longest coolest loop is last from right to left in this drawing. This may be an example of a baseboard loop first, a duct mounted hydro-coil next, a cast iron radiator zone third and last a radiant floor loop last on the left.




Drawing 2

This is the same as above drawing where the boiler loop is the primary pipe. The only difference here is I am showing how to pipe two zones of the same water temperature. The first two zones on the right may both be 180f baseboard zones and the lower temp zones going left. I also show a three way valve for cast iron boiler protection. For more information on cast iron boiler protection click here. If this installation was a modulating condensing (mod/con) boiler, it would not use the three way valve because we want as cool  of water as possible to get back to them. The cooler the water the more efficient the mod/con boilers operate. The cast iron boilers we must be concerned with flue gas condensation.


Drawing 3




In this drawing the primary pipe, again being the one with the closely spaced tees would be the loop. There are four secondary heating zones on the top and of course the boiler is a secondary zone on the bottom.  Why is the boiler piping here called a secondary loop instead of the primary loop? Remember the run, straight through the tees is always primary and the side of the tees is always secondary. I have also heard this called boiler injection.


Drawing 4

This is also primary/secondary (p/s) as it is connected with closely spaced tees for the boiler connection. The boiler would be considered secondary piping as it is piped out of the branch of the closely spaced tees. This piping is a good application if all the zones are looking for the same temperature water. The piping around the close spaced tees should be of ample size to allow proper flow in either direction. See below.

Drawing 5


This drawing shows a device called a hydraulic separator. It will take the place of closely spaced tees. The advantage of the hydraulic separator is it will offer an air vent on top and possible dirt drain on the bottom (not shown).


Drawing 6

Flow Left to Right

If using close spaced tees on a primary loop it is important to follow these measurements. The boiler is piped as secondary piping would also follow the distance between the closely spaced tees as per drawing #3 or #4. The most confusing is the distance between the closely spaced tees. You need to follow 4 x the diameter of the primary pipe not to exceed 12" which ever is less. It does not mean you have a choice between 4 x dia. and 12". It means you follow the 4 times rule, if the result exceeds 12" you apply rule # 2. For example, 4 x 2" pipe is 8", this is the maximum distance between the tees. Four times 3" pipe is 12" maximum. Four times 4" pipe is 16" you cannot exceed 12". Never put anything between the tees. This will create a larger pressure drop between the tees and could create ghost flows in the secondary piping. The best idea on the tees is as close as you can get them all the time. The minimum distance between multiple sets of closely spaced tees is a minimum of 6 pipe diameters.

Another important point is equal resistant into the secondary and exiting the secondary. In other words the pressure drop or Equivalent Feet or Pipe (EFP) into and exiting the secondary piping must be equal. Every fitting has an EFP and has a chart different for copper fitting and iron pipe fittings. If the resistance entering and exiting the tees is equal flow will not be affected.

Let's review what is happening here. If we look at the EFP chart for copper pipe and assume these are 1-1/4" copper tees. Looking at the chart we will see that the flow straight through the tee is equal to 0.6 ft straight pipe. To make the turn to the right is equal to 5.5 ft .of pipe. The resistance to flow must be equal on the exiting side of the secondary loop. This creates a balance into and out of the secondary piping.


Flow in Relationship to Tees

Now with all the above explained let's look at what is happening in the system. We will look first at the primary loop drawings 1 - 3. There are two rules we must remember when piping p/s.  The first is "What enters a tee exits a tee". It has nowhere else to go and cannot stay in the tee. If 10 gpm enters a tee it must exit at 10 gpm. It does not have to exit in equal flow rates out of the tee as you will see later. But, for ease of explanation let's assume as the 10 gpm enters the tee, 5 gpm goes out the branch and the other 5 gpm moves between the tees. The second rule is all the water in the piping starts to move at the same time. Look at the drawings below and look at the difference in flow rates between the tees. pay special attention to the last one.


Flow Between The Tee's

Flow From Left To Right


 In this first example the primary loop is moving 10 GPM. When the secondary loop circulator starts all the water starts to move at the same time in the secondary loop. It steals 5 gpm from the primary loop and the other 5 gallons continues between the tees. In the second tee the two 5 GPM flows join together and the flow continues in the primary loop at 10 gpm. The primary circulator had no clue this even happened as it did not affect the flow in the primary loop. It does not know the secondary circulator is even there much less any flow happening in the secondary loop. The only thing going on with the primary circulator is it is getting 10 GPM and is discharging 10 GPM.

No Flow Between The Tee's


 In this example the secondary circulator is pumping the secondary loop at 10 GPM. The flow in the secondary and primary are equal. As the 10 gpm enters the tee all 10 gallons passes out the branch and there is technically no flow between the tees. When the flow enters the return tee all the water goes to the right and there is 10 gpm flow in the primary loop to the primary circulator.



Reverse Flow Between The Tee's

In this exampleit is important to remember the two rules. What enters a tee exits a tee and all the water in the piping moves at the same time. The primary loop is moving 10 GPM. The secondary will move more than the primary loop. It wants to move 12 GPM. As 10 GPM enters the tee from the left and the circulator starts it takes all 10 GPM the primary loop circulator will supply. As the secondary circulator starts all the water in that piping moves at the same time so it dumps 12 GPM into the tee on the primary loop. The primary loop circulator takes away 10 GPM of the 12 GPM leaving 2 GPM to reverse flow between the tees and mix with the 10 GPM entering the tee from the primary circulator thus giving us the 12 GPM flow required on the secondary loop. If this is designed this way it is a good thing. Unfortunately this happens by accident more than design. If not part of the design the temperature in the secondary loop has a hard time getting hot enough to satisfy the load. In most cases the temperature of the water entering the pipe will be the same temperature of the water between the tees. If this is not so you have reverse flow. The question is "Was it designed that way or not"?

Here is something different

You may at times see something like this. This will allow enough flow between the tees to blend the secondary water temperature down to where it needs to be. The flow between the tees is greater than the flow into and away from the tees. This will normally be used in what we call crossover piping which is another form of P/S piping.


The reverse flow would take place on the radiant low temp zone. Pay attention to the arrows.


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