Primary/Secondary Piping Common Problems
Caution :All drawings are conceptual drawings for illustration purposes only and may be incomplete.More specific drawings may be needed for your application.

Common Mistakes made with Primary/Secondary (P/S) Piping

To see all the right things to do when piping P/S, see . Here we will list some more common mistakes when piping P/S. We need to produce hydraulic separation by keeping the tees very close together and creating a very low pressure drop between the tees. Many times, we fail to remember that all fittings have an equivalent feet of pipe (EFP) rating.
chart for copper fittings and iron fittings

One of the more common problems I experience is one of the tees is rotated 90 degrees. This changes the flow in and out of the secondary zone be it the boiler or a heating zone. As stated above we are looking for a very low pressure drop between the tees. We need the same resistance into the secondary zone as we have out of the secondary zone. If you read the P/S tutorial you know the maximum distance between the tee branches or also called the bull of the tees is not to exceed 4 times the diameter of the primary pipe not to exceed 12” whichever is closer. What is the problem with this first drawing?


Since every fitting has an equivalent feet of pipe (EFP) plus all the piping the water passes through is a resistance to flow, this can create a problem. Since the secondary loop is always piped away and returned back to the branch of the tees, we need to take a look at the EFP of the tee. See the EFP chart link on this page, but if we look at 1-1/4” tees we can see the flow into the tee and out the branch is equal to 5.5 EFP. Remember we need the same resistance into the secondary zone as we have out of the secondary zone to keep the flow correct. The flow into the tee and straight through the tee is 0.6 ft.of pipe. What this means is one tee route is 5.5 EFP and the other tee route is 0.6 EFP. The result is unbalanced flow. In the above example, we can measure the distance and if it measures with a rule to 5” we might think we are OK. But looking at the EFP we are about 6 feet of pipe.

Problem #2

The second most common problem I encounter is some type of fitting between the tees. Since each fitting has an EFP it throws the maximum distance way off. Any fitting will throw this dimension off. I see a valve here many times. It does not matter what fitting we put between the tees we exceed the 4 times rule.

Putting a 1-1/4” elbow between the tees will add about 3 EFP. A valve could add up to 10+ EFP.

Problem #3

I have found some jobsites with pump failures. What I have contributed this too is inverted traps. The boiler secondary piping was piped into the top of the primary pipe like the drawing below.


Bring the secondary boiler piping into the side or the bottom of the primary pipe.

Problem #4

I had a job where the contractor reduced the pipe between the tees. This raises the resistance to flow and created flow problems.

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