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Learn About Primary/Secondary Pump Systems

      By the 1930’s, most hot water heating systems used circulating pumps instead of relying on gravity circulation.  In the early 1950’s, small circulating pumps were installed on radiator branch runs, along with the primary or main system pump.  The primary pump ran all the time, and the secondary pumps could be cycled on and off to create independent zones.  The introduction of zone valves reduced the number of primary/secondary systems in residential and light commercial hot water heating systems, but there are still many in use and being installed every day.

There is really nothing very complicated about primary/secondary pumping systems.  Basic principles are involved and an understanding of these principles is all that is necessary to understand primary/secondary-pumping systems.

Think of the main as an extension of the boiler from which you can pull hot water whenever and wherever you need it.

Figure 1.

In Figure 1, we see a simple loop system.  When the circulator runs, all water flows around the loop.  The water just goes around and around.  Let’s add a second loop to the main loop by using regular tees and a valve between the tees as in Figure 2.  Depending on whether the valve is open, closed, or somewhere in between, water may or may not flow through the secondary loop because the DP is greater in the loop than the DP between the tees.  Throttling the valve will increase the DP between the tees and will determine how much water will flow in the secondary loop.  You could also get the same effect by using a smaller sized pipe between the tees, because a smaller pipe will produce a greater DP at the same flow rate than a larger pipe.  None of those methods gives any “control” of the loop.  You can’t conveniently start, stop, or change the flow through the loop.

Figure 2.

Let’s add a pump to the loop as in Figure 3.  “P” is the primary pump, which will run continuously.  “S” is the secondary pump.  When the secondary pump is off, no water will flow through its loop because the DP of the secondary loop is greater than the DP between the tees.  When the secondary pump is on, water will flow in the loop because the pump changes the DP relationship.

Figure 3.

What’s very important is what the DP is between the tees.  That piece of pipe is common to both loops, so its DP must be very low!  The piece of pipe should be the size of the main and 6 to 12 inches long.  We want the pumps to work together.  When we have a system with two pumps, one more powerful than the other, if we are not careful, we can create problems.

Figure 4.

As an example, in Figure 4 the primary pump is a high head pump and the secondary pump is a low head pump.  When the high head pump, P, is running and the low head pump, S, is off, P produces high-pressure at “A” but low pressure at “B.”  The frictional resistance of the piping causes the DP from “A” to “B.”  Water wants to flow through the low head pump’s piping because the high-pressure leg and the low-pressure leg are in parallel, therefore the pressures will equalize out.  This equalization will occur when the water travels through the low-pressure leg.  To prevent this, a check valve is in the low head pump’s piping.  Now, the secondary pump comes on.  It produces pressure, but it is not enough to open the check valve and overcome the DP created by the primary pump between “A” and “B.”

Figure 5.

Now look at Figure 5.  The pressure produced by the high head primary pump at “A” and “B” is virtually the same.  (Remember, “A” to “B” is now 6 to 12 inches long.)  The high head pump P won’t circulate water through the secondary loop, because the common piping, “A” to “B,” is the path of least resistance.  When the secondary pump comes on, it can pump away from the common piping, around its loop that includes “A” to “B,” and back to its own suction.  The high head pump can’t shut down the secondary pump.  They operate as if they are two independent systems.  Depending on the flow rates of the primary and secondary pumps, water can move forward, backwards, or not at all!  If both pumps are sized for a flow rate of 10 GPM, when the primary pump is on and the secondary pump is off, 10 GPM will flow across “A” to “B.”  See Figure 6.  When the secondary pump is on, there will be no flow across “A” to “B,” the common piping.  All the water will flow through the secondary loop.  See Figure 7.  Let’s change the primary pump to one that can pump 20 GPM.  Now, when both pumps are on, the flow through the common piping, “A” to “B,” will be 10 GPM.  Remember a simple rule.  “What enters a tee must leave a tee,” or “what leaves a tee must enter a tee.”

Figure 6.

Figure 7.

Let’s switch the pumps.  The primary pump is now the 10 GPM pump.  See Figure 8.  When the primary pump is on and the secondary is off, 10 GPM will flow across “A” to “B,” the common piping.  But now the secondary pump, the 20 GPM pump, comes on.  20 GPM flows in the secondary loop!  How can this be?  10 GPM reverse flow will occur in the common piping, “B” to “A.”  Remember, what leaves a tee must enter a tee.  If we draw 20 GPM out of the branch of the tee at “A,” we must have 20 GPM enter the tee from one or both sides.  10 GPM is being supplied in one side of the tee from the primary pump, so 10 GPM must come from the other side of the tee.  The secondary pump has to draw that 10 GPM from its own flow, creating backwards flow across the common piping when both pumps are on.  This could be useful where a two-temperature system is needed.  Return and supply water can be blended to have a two-temperature system without using a three-way valve.

Figure 8.

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