\u003c/strong>\u003cstrong >Lag pump staging\u003c/strong>\u003c/p>\u003cul >\u003cli >Paragraph 1.c. discusses staging logic for the lag pump. Typically, the logic should be tested by adjustments to the various set points in lieu of overrides to the differential pressure input reading or manipulation to downstream valves to drive up or down the system flow. This approach will verify set points shown on the graphic are correctly mapped to the set points used in the programming logic. On-site testing should focus on verifying the logic is thoroughly tested. Optimization of the staging set points is typically reserved for a later trend review, as discussed below. \u003c/li>\u003cli >\u003cp >Paragraph 1.c.ii. is intended to ensure both operating pumps operate at the same speed. Several issues can prevent this from happening.\u003c/p>\u003cul >\u003cli >Ensure communication between the building automation system (BAS) and the variable frequency drive (VFD) serving each pump is consistent. Both VFDs need to be configured with the same input range and reference frequency parameters. For example, a 60% speed signal sent to each pump should result in the same displayed frequency in Hz on both VFD’s display panels. Additionally, the minimum speed on each VFD needs to match, so that one VFD does not bottom out before the other when the control signal decreases.\u003c/li>\u003cli >\u003cp >The programming needs to have a single control loop whose output is sent to both pumps’ VFDs as the control signal, not two identical control loops. Additionally, the same control loop should be utilized regardless of the number of pumps in operation. This can be confirmed by diligently watching the control signal on the BAS graphic as the system stages from one to two pumps. \u003c/p>\u003cul >\u003cli >On a recent project, trend review prior to testing showed instability at times when the lag pump first enabled. The controls contractor was unable to diagnose the root cause prior to testing. During on-site testing, it was observed that when the lag pump was enabled, the control signal would immediately drop significantly, well below the \u003cem >Lag Pump Stage Off Set Point.\u003c/em> The\u003cem > Lag Pump Stage Off\u003c/em> \u003cem >Delay\u003c/em> discussed in paragraph 1.c.iii was not included in the approved sequence, thus this sudden drop in control signal would immediately disable the lag pump, creating instability with the lag pump repeatedly enabling and disabling. The root cause was that two identical control loops (one for single pump operation, and one for parallel pump operation) were programmed, but, for whatever reason, their outputs did not match. The second control loop was removed, and the first was instead used for control in all instances, regardless of the number of active pumps. This immediately eliminated the instability previously seen. \u003c/li>\u003c/ul>\u003c/li>\u003c/ul>\u003c/li>\u003cli >Paragraph 1.c.iv is a safety net to prevent someone from unknowingly making set point adjustments that would promote instability. Attempt to raise the \u003cem >Lag Pump Stage Off Set Point\u003c/em> above the \u003cem >Lag Pump Stage On Set Point,\u003c/em> and vice versa. This is a provision in the programming that is often skipped. \u003c/li>\u003c/ul>\u003ch2 >\u003cstrong >Optimizing Set Points\u003c/strong>\u003c/h2>\u003cp >For any given system, there will be a range of flow rates that could be stably maintained by operating either one or two pumps. There will be point where it is more energy efficient to operate two pumps instead of one. An optimized \u003cem >Lag Pump Stage On Set Point\u003c/em> should be at that time. What that value is will vary between different systems. However, often there is enough data available for the building to inform engineers what that optimized value is. For example, if pump power consumption is an available point on the BAS, set up a multiple point trend that displays both the number of operational pumps as well as the collective power consumption (see Figure 1). A trend review, once the system is fully operational, can identify if collective pump power consumption is reduced or increased when the lag pump is staged on. An observed reduction in power consumed would indicate the \u003cem >Lag Pump Stage On Set Point\u003c/em> being utilized was too high, thus the lag pump was essentially being staged on too late. An observed increase in power consumption would indicate too low of a value, thus the lag pump is being prematurely enabled. Iterations of this trend analysis, followed by minor adjustments to the \u003cem >Lag Pump Stage On Set Point,\u003c/em> will quickly zero in on the optimized set point value that results in negligible changes in pump power consumption.\u003c/p>\u003cp >If the \u003cem >Lag Pump Stage On Set Point\u003c/em> is optimized for energy savings, then the \u003cem >Lag Pump Stage Off Set Point\u003c/em> should be optimized for stability. The control signal will naturally begin to drop when the lag pump is enabled. You do not want the difference between the set points to be so small as to promote cycling of the lag pump. Conversely, you do not want the\u003cem > Lag Pump Stage Off Set Point\u003c/em> to be a value so low that the control signal never drops low enough to stage off the lag pump. ASHRAE Guideline 36\u003csup >1\u003c/sup> refers to this as the system getting “stuck” in parallel pump operation. Such a scenario is common when the \u003cem >Lag Pump Stage Off Set Point\u003c/em> is hardcoded, as it seems the default value of choice for both designers and controls technicians is 40%, which is often too low. Review the trends after the system if fully operational to see how much the control signal initially drops when the lag pump enables. For example, if it was reduced by 18 percentage points, then maybe the \u003cem >Lag Pump Stage Off Set Point\u003c/em> should be made 22 percentage points lower than the \u003cem >Lag Pump Stage On Set Point.\u003c/em> \u003c/p>\u003cp >It may not be possible for the commissioning provider to perform such post-testing optimization in every project. If not, provide instructions to the building operator to allow for him or her to carry out such tasks. \u003c/p>\u003ch2 >\u003cstrong >Conclusion\u003c/strong>\u003c/h2>\u003cp >This concludes a three-part series of commissioning lead/lag pumping operations. It’s a common control sequence that is littered with nuances, preventing it from being properly implemented. \u003c/p>\u003ch2 >\u003cstrong >Reference\u003c/strong>\u003c/h2>\u003cp >\u003csup >1\u003c/sup>ASHRAE Guideline 36-2021, \u003cem >High-Performance Sequences of Operation for HVAC Systems.\u003c/em>\u003c/p>\u003cp >\u003c/p>"},"type":"text","cl":"text p-_Js1xJSc"},{"key":"eacIXEgx-wP","type":"section","children":["eHy8cXYcKFJ"],"cl":"section"},{"key":"eNexQfHeaPL","content":{"text":"\u003ch3 >Miles Ryan, P.E.\u003c/h3>\u003cp >Miles Ryan is a commissioning engineer at Questions & Solutions Engineering. As a major in the Air Force Reserves, he serves as a mechanical systems instructor at the Air Force Institute of Technology. Contact him at \u003ca target=\"_blank\" href=\"mailto:miles.ryan@qseng.com\">miles.ryan@qseng.com\u003c/a>. \u003c/p>"},"type":"text","cl":"text 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