• 1. Hydraulic Separation
• 2. The flow of one circulator does not affect the flow of another circulator
• 3. Guarantees the proper flow through the boiler, not dependent on system flow
• 4. Allows multiple Delta-Ts within the heating system. For instance, the system could be one delta-T and the boiler could be at a different delta-T if required

In this example, the pipe coming from the boiler is the primary pipe, as it includes the common pipe. The water will leave the boiler and return to the boiler through the connected loop. As the secondary circuits (piped off the closely spaced tees) are running, the water temperature in the primary pipe will decrease after any running secondary zones. This diagram is most effective when applied to a system with multiple water temperatures.
The secondary piping would be arranged so that 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 large cast-iron loop second, and a radiant floor loop last, on the left.

This is the same as the previous drawing, where the boiler loop is the primary pipe. The only difference here is that I am showing how to pipe two secondary zones of the same water temperature. The zones to the left will receive a lower supply temperature. See more information if this is a cast iron boiler installation, as we will need Protection against flue gas condensation. If this installation is a modulating condensing (mod/con) boiler, we would not be concerned about condensing because we want the cooler water returning to the boiler for higher efficiency. The cooler the water, the more efficient the mod/cons run. The cast iron boilers would be destroyed by condensate from return temperatures less than 130f.

In this drawing, the primary pipe, again being the one with the closely spaced tees, would be the primary loop. There are four secondary heating zones on top, and, of course, the boiler is a secondary zone at the bottom. 

This is also 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 require the same temperature water. The piping around the closely spaced tees should be of sufficient size to allow proper flow in either direction.

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This drawing shows a device called a hydraulic separator. It will replace closely spaced tees and common pipe. The advantage of the hydraulic separator is that it provides air elimination and, potentially, bottom-dirt elimination (not shown), in addition to hydraulic separation.

The first rule when installing close-spaced tees is that they must be mounted end to end. If piping the boiler with close-spaced tees, the system flow must travel through the run of the tees (see drawing 4), not the boiler water. Boiler water should flow in and out of the bull or branch of the tees. If using close-spaced tees on a primary loop, it is essential to follow these measurements. The boiler or heating zones can be piped as secondary piping and would follow the distance between the closely spaced tees. The most confusing is the distance between the closely spaced tees. You need to follow the 4 x the diameter of the primary pipe rule. It does not mean you have a choice somewhere between 4 x dia. and 12". It means you follow rule one, if the result exceeds 12" you apply rule # 2, Do not exceed 12". 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, leading to ghost flows. The best idea for the tees is to keep them as close as possible. The minimum distance between multiple sets of closely spaced tees is 8 - 12 pipe diameters.

Flow in the Tees

Now that all the above has been explained, let's look at what is happening in the system. We will look first at the primary loop drawings 1 - 3 above. 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 gallons per minute (gpm) enters a tee, it must exit at 10 gpm. It does not have to exist in equal flow rates out of the tee, as you will see later.

The second rule is that all the water in the piping starts to move at the same time. Examine the drawings below and note the difference in flow rates between the tees. Pay special attention to the last drawing #9.

Drawing 7

In this first example, the primary loop is moving 10 GPM. When the secondary loop circulator starts, all the water in the secondary loop begins to move simultaneously. It steals five gallons per minute from the primary loop, and the other five gallons continue between the tees. On the second tee, the two 5 GPM flows join, and the flow continues in the primary loop at 10 GPM. The primary circulator was unaware of this event because it did not affect flow in the primary loop. It is unaware that the secondary circulator is even present, let alone that any flow is occurring in the secondary loop. The primary circulator is operating at 10 GPM, discharging the same volume into the loop.

Drawing 8

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

Drawing 9

In this example, it is important to remember the two rules. What enters a tee exits a tee, and all the water in the piping moves simultaneously. The primary loop is operating at a flow rate of 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 that the primary loop circulator will supply. As the secondary circulator starts, all the water in the piping moves simultaneously, dumping 12 GPM into the tee on the primary loop. The primary/loop circulator removes 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 providing the 12 GPM flow needed on the secondary loop. If this is designed this way, it is a good thing. Unfortunately, this happens by accident more than by design. If not part of the design, the secondary loop struggles to reach a sufficient temperature to meet the load. In most cases, the water entering the pipe will have the same temperature as the water between the tees. If this is not the case, you have a reverse flow. The reverse flow would take place in the radiant low-temperature zone. Pay attention to the arrows.

The question is, "Was it designed that way or not?

Drawing 10

You may, at times, see something like this. All supply temperatures in the crossover pipes will match those in the manifold piping shown in Drawing #4. This will allow sufficient backward flow between the tees to blend the secondary water temperature down to the desired level. This will not affect the flow on the crossover pipe. The flow between the tees may be greater than the flow into and away from the tees. This will normally be used in what we call crossover piping or Primary/Secondary Parallel piping. This is normally used when one circulator is larger, moving more water, and is sized for a higher head than the other circulators.

When the need for a higher head pump is needed, and this was piped on a manifold as shown in drawing #4, the larger could pull water through the smaller zones or reduce the flow through the zones with smaller circulators.

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