John Siegenthaler, P.E. 2018-01-05 14:19:48
Some hydronic system designers still use problematic system piping configurations. Einstein’s definition of insanity was doing the same thing over and over and expecting different results. If that’s true, there are some “insane” hydronic system designers in North America. They cling to certain system piping configurations, even though existing projects using those configurations have produced problems. One incorrect piping layout I’ve seen many times could be described as a “morphing” of primary/secondary piping and a classic header-type multi-zone distribution system. I’ve seen it as installed hardware and in neatly prepared CAD drawings created by professional engineers. The latest rendition of this problematic piping layout popped up in an email sent to me for review. The piping error I’m referring to is represented by Figure 1 (on Page 39). This piping layout is neither primary/secondary nor a “header-type” multi-zone system. It’s undefined among proven hydronic piping designs. My theory as to how this rogue layout manifests itself is the designer begins thinking about primary/secondary piping and therefore thinks he/she needs a primary loop. The heat source(s) will inject heat into this loop and the load circuits will extract heat from it. So the designer proceeds to sketch out the loop and puts in a primary loop circulator. Next it’s time to add some load circuits. This is where the designer’s memory flashes back to zone circulators all neatly lined up along a wall. With that in mind, the designer connects the supply side of each zone circuit to the upper part of the loop (thinking it’s a header) and the return side of each zone circuit to the lower portion of the loop (again thinking of it as a header). The fact the “headers” are connected at their ends doesn’t seem to matter. Strange behavior: One problem with this design can be envisioned if you consider the pressures within the primary loop when only the primary loop circulator is operating. There’s a pressure drop between the upper portion of the loop, where the supply side of the load circuits connect, and the lower portion of the loop, where the return side of the load circuits connect. This is illustrated in Figure 2 (on page 39). If only the primary circulator was running, the pressure differential would be highest between points A and B due to the head loss along the longest loop path. It would decrease to some minimum value between points C and D. However, the pressure differential across any given load circuit at any given time also will be influenced by the on/off status of the load circulators and thus highly variable. Still, it’s entirely possible and even likely the pressure differential between points where a load circuit begins and ends could be several (psi). If the pressure at point A is higher than the pressure at point B, the water “wants” to move from A to B. And, if nothing blocks its path, the water will flow from A to B. The result is heat delivery into a circuit in which the zone circulator is off and there is no need of heat. Call it heat migration, ghost flow or whatever you want. It’s not supposed to occur and customers have every right to complain when it does. It’s even possible all the zone circuits could have some flow through them when only one zone is actually calling for heat. Flow would occur in any load circuit where the forward opening resistance of any check valve or weighted plug-flow check (which typically is 0.3-0.5 psi) is smaller than the developed pressure differential between the supply and return sides of that circuit. The rate of undesirable heat migration depends on the pressure differential between the supply and return of each zone circuit and the amount of recirculation mixing that occurs. The latter depends on the flow rate in the primary loop vs. the flow rates in the load circuits. If flow is rocketing through the primary loop — because someone thinks the primary loop flow has to be at least equal to the sum of the load circuit flow rates (which is not true), then there won’t be recirculation mixing. However, if the flow in the primary loop is less than the sum of the active load circuit flow rates, there is sure to be recirculation somewhere. Think like water. Why should the water travel all the way back to where the boiler(s) are connected to the primary loop if it can just take a shorter detour and end up back at the inlet of a zone circulator? If you’re going to build a true primary/secondary system, each load circuit and each heat source needs to connect to the primary loop using a pair of closely spaced tees. These tees isolate the pressure dynamics of each circulator from that of the other circulators in the system. This is called hydraulic separation. More problems: The system shown in Figure 1 accurately represents a drawing I received. Aside from the “morphed” piping layout, there are several other details that should be of concern: 1. There are no check valves in the load circuits to prevent reverse flow when some loads are active while others are not. 2. There are no purging valves in the load circuits. 3. There is a swing check valve mounted in a vertical pipe coming from the boiler. Swing check valves should never be mounted in vertical piping. Under some conditions the flapper within the check valve can “hang” in the open position when flow stops and slam shut when sufficient reverse flow develops. This can create a strong water hammer effect. 4. The tees connecting the boilers to the “primary loop” should be as close together as possible. The pressure drop between the more widely separated tees connecting each boiler to the “primary loop” in Figure 1 will induce some flow through an inactive boiler. This increases heat loss from the boiler jacket and creates convective air currents that suck heat up the chimney. Lose the loop: Properly designed primary/secondary systems work. Still, in my opinion there are better options that provide the hydraulic separation benefits of primary/secondary piping, but with simpler and less expensive piping configurations. One example is shown in Figure 3. This system connects the boilers to a header system that leads to a hydraulic separator. The load circuits connect to short/generously sized headers leaving the right side of the hydraulic separator. High-performance air and dirt separation is provided by the coalescing media inside the hydraulic separator. This eliminates the need for air and dirt separators as individual components. By keeping the headers short and generously sized, the pressure drop along them is very low. This, in combination with the very low pressure drop through the hydraulic separator, provides excellent hydraulic separation of all the circulators in the system. My suggestion is to size the headers so the flow velocity within them does not exceed 2 ft. per second when the headers are operating at maximum flow rate. This piping layout eliminates the “ghost flow” and possible recirculation issues previously described. It also provides equal supply temperatures to each of the load circuits. It does away with the primary loop circulator, and perhaps most importantly, it eliminates the operating cost of a primary loop circulator over the life of the system. So please, don’t repeatedly prove that Einstein was right about insanity. If you’re intent on building a primary/secondary system, be sure to connect the load circuits to the primary loop with closely spaced tees and a properly sized primary circulator. If you’re open to approaches that provide better value and better overall performance than classic primary/secondary systems, consider the hydraulic separator configuration shown in Figure 3. Einstein’s definition of insanity was doing the same thing over and over and expecting different results. Some North American system designers cling to certain system piping configurations, even though they produce problems.
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