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BY GEORGE LANTHIER

This was originally done for our textbook Hydronic Systems , An Advanced Primer with some help from my good friend Bruce Marshall .

Primary-Secondary , Figure 1 , systems 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 modulating-condensing boilers . If there is no flow during burner operation in these boilers , heat exchanger failure will occur .

By using 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 .

Figure 1

Primary-Secondary pumping circuits in most hydronic systems rely on the cir- culators to do the work and the reasoning behind 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 pumping should be looked at . In many cases , even laying out a singlefamily residence can lead to a better 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 Primary-Secondary piping and the math involved has also been made easier by a whole new generation 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 fitting . 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 .

Figure 2

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Primary-Secondary piping can also produce longer 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 . As previously noted , for those installing modulating-condensing ( mod-con ) boilers , Primary-Secondary is a must . For those installing conventional boilers , it would put a lot of money in your pocket in fewer repeat calls and less aggravation .

Figure 4

There are a few keys to understanding how the concept works . The first has to do with flow through fittings and those

34 SEPTEMBER 2023 | FUEL OIL NEWS | www . fueloilnews . com