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Primary - Secondary

Primary - Secondary

Systems - Part 1

Systems - Part 1 by

George Lanthier

This is from our Firedragon Academy textbook “Hydronic Systems’. It’s from one of the many great Chapters in the book was done with the help of my good friend Bruce Marshall of Emerson-Swan who checked everything over, thanks Brucie.

Multiple circulators for zone control has been a popular way to go for a long time, and the pros and cons of us- ing both a single pump and multiple pumps to overcome system restrictions (feet of head), and to move the Btus (gpm). But, there is another way that a circulator can be used and that is in a Primary- Secondary pumping arrange- ment, Figure 1.

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away also allows you to get rid of any system air that works its way into the piping.

Primary-Secondary sys- tems were first used for the zoning of large buildings initially in the early 1950s. In the recent past, the systems were being used as a way to ensure flow through small diameter, high pressure drop heat exchangers used in modu- lating-condensing boilers. If there is no flow during burner operation in these boilers, heat exchanger failure will occur. By us- ing Primary-Secondary pumping systems, it is also possible to supply various water temperatures from a hotter Primary loop to cooler Secondary loops. We’ll look at this concept later.

Primary-Secondary pumping circuits in most hydronic systems rely on the circulators to do the work and the reasoning be- hind these systems is how and why the circulators are laid out the way they are. Primary-Secondary circuits can be quite complex and may require a minimum of math to size them so that they work correctly. Whenever you have a large system or you would like to control the costs of your installation, the use of Primary-Secondary pump- ing should be looked at. In

many cases, even laying out a single-family resi- dence can lead to a bet- ter job by using multiple circulators or zone valve combinations in a Primary- Secondary configuration. In most cases the size of the circulators can be smaller than they would be with conventional systems. The installation of Pri- mary-Secondary piping and the math involved has also been made easier by a whole new genera- tion of fittings, Figure 2. These fittings for many residential applications eliminate having to do any math at all. In addition, the fitting ensures that the most important spacing in the Primary-Secondary system is met by creating ‘closely spaced tees’ and I’ll refer to them as ‘double tee fittings’ throughout this Chapter. Figure 3 shows what happens inside the fit- ting. The great thing about the fitting shown in Figure 2 is that it also includes a full-port ball-valve that will help you when it comes time to fill and bleed these systems.

Primary-Secondary pip- ing can also produce lon- ger life for the boiler by acting as a pumped by- pass and when the boiler is used for the production of domestic hot water, a pumped by-pass is really hard to beat, as shown in

Figure 4. In addition, I truly believe that if all of us were installing all of our boilers with a Primary-Secondary system we would eliminate or at least reduce most of our combustion related problems regardless of fuel. For those installing modulating-condensing (mod-con) boilers, Prima- ry-Secondary is a must. For those installing con- ventional boilers, it would put a lot of money in your pocket in fewer repeat calls and less aggravation. There are a few keys to understanding how the concept works. The first has to do with flow through fittings and those closely spaced tees. The operat- ing trick to P-S (Primary- Secondary) systems is the use of closely spaced tees. There’s nothing compli- cated about Primary-Sec- ondary pumping. It is really not that difficult to under- stand. It is all about the ‘path of least resistance’ and is also nothing but a modified diverter system as I described earlier. The trick is to place the tees no more than 12 inches apart and with a fitting like that shown in Figure 2, you just can’t go wrong.

One thing that should be noted is that the expansion tank, air separator and the fill valve all belong in the Primary loop. Make sure you install your Primary cir- culator so that it is pumping away from the expansion tank like in Figure 5. That way, you can take advan- tage of the Primary loop circulator’s full differential pressure. If you put a fitting in like Figure 2, you’ll gain a full-port ball valve in the common piping between the primary and secondary circuits, and that will allow you to be able to get the air out of the system a lot quicker as well. Pumping

Copper Pipe Gpm

1.5 15,000 3.5 35,000

8 80,000 8 13 130,000 20 200,000 45 450,000 80 800,000

Another way to think of these sys- tems is that Pri- mary and Secondary

loops end up becoming separate, with neither hav- ing an effect on the pumping head of the other. The Pri- mary loop actually serves the heat source, (boiler or heat exchanger), while the Secondary loop serves the load(s). Primary-Second- ary pumping arrangements allow for increased system temperature design drops, decreased pumping sizes and increased system con- trol. The Primary loop and its pump is dedicated and sized to serve the flow and temperature differential requirements of the heat- ing equipment. This allows the Secondary pump(s) and loop(s) to be sized and controlled to provide the design flow rate and temperature differential required to satisfy the heat- ing load(s). Let’s look at a diverter system and see what hap- pens. The most important thing to remember as we go from using a diverter tee system to a P-S system is that in the diverter system the diverter tees direct the flow of water and act as the “flow boss” where in the P-S system, the circulators are in charge and act as the flow bosses. Looking at Figure 6 and Figure 7 you can see that there is only one circula- tor for the entire system or a zone since the flow is directed through the radiation by the on-pipe fit- tings, Figure 8 and Figure 9. With a single circulator it would have to be large enough to pump all of the gpm through the entire foot of head of the system. By zoning, the pumps get smaller because now they only have to pump the gpm of the zone and the length of the longest zone. With P-S systems the pump only has to pump the Primary loop or the Secondary loop,

Steel Pipe

Btus Gpm Btus 2 4

16 24 47 75

Figure 10

20,000 40,000 80,000

160,000 240,000 470,000 750,000

Pipe Size ½" ¾" 1"

1-¼" 1-½" 2"


June 2015

but we need to separate them since water always seeks the path of least resistance. By spacing the tees no more than 12 inches apart, we decouple the Secondary loop(s) from the Primary loop(s). Let’s take another look at the diverter system as an introduction to build- ing a Primary-Secondary system. The most impor- tant part of any system is proper pipe sizing that will carry the Btus required to heat the loads. In Figure 10 we can see that by properly sizing our piping, we can in fact control the amount of heat that we want to carry.

“Primary-Secondary Systems - Part 1” will be continued in the July Issue of the Empire/Metro New York HVAC Insider. See a list of available classes in our calendar. George Lanthier can be reached at 508-421- 3490 and his website can be found at FiredragonEnt. com.

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