5/20/2023 0 Comments Heating diagramsUnder 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. Swing check valves should never be mounted in vertical piping. There is a swing check valve mounted in a vertical pipe coming from the boiler.There are no purging valves in the load circuits.There are no check valves in the load circuits to prevent reverse flow when some loads are active while others are not.Aside from the “morphed” piping layout, there are several other details that should be of concern: The system shown in Figure 1 accurately represents a drawing that I received. These tees isolate the pressure dynamics of each circulator from the other circulators in the system. If you are 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. 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? 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. 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 flows (which is NOT true), then there won’t be recirculation mixing. The latter depends on the flow rate in the primary loop versus the flow rates in the load circuits. 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. Flow would occur in any load circuit where the forward opening resistance of any check valve (which is typically 0.3-0.5 psi) is smaller than the developed pressure differential between the supply and return sides of that circuit. It is conceivable that all the zone circuits could have some flow through them when only one zone is actually calling for heat. It is not supposed to occur, and customers have every right to complain when it does. Call it heat migration, ghost flow, or whatever you want. The result is heat delivery into a circuit in which the zone circulator is off, and there is no need of heat. And, if nothing blocks its path, the water will flow from A to B. If the pressure at point A is higher than the pressure at point B, the water “wants” to move from A to B. Still, it is possible that the pressure differential between points where a load circuit begins and ends could be several pounds per square inch (psi). However, the pressure differential across any given load circuit at any given time will also be influenced by the on / off status of the load circulators, and thus highly variable. It would decrease to some minimum value between points C and D. If just 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. There is 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. 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. The fact that the “headers” are connected at their ends doesn’t seem to matter. With that in mind, the designer connects the supply side of each zone circuit to the upper part of the loop (thinking it is a header), and the return side to the lower portion of the loop (again viewing it as a header). This is where the designer’s memory flashes back to neatly aligned zone circulators all lined up on a wall. Next it is time to add some load circuits. The designer proceeds to sketch out the loop, and puts in a primary loop circulator. Related: When to use a three-pipe buffer tank configuration The heat source(s) will inject heat into this loop, and the load circuits will extract heat from it. My guess as to how this rogue piping layout repeatedly manifests itself is that the designer begins thinking about primary/secondary piping, and therefore thinks they needs a primary loop. It is undefined among proven hydronic piping designs. This piping layout is neither primary/secondary, nor a “header-type” multi-zone syste. The piping error that I am referring to is represented by Figure 1.
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