A Common Pitfall with Reheat Actuators and How Analytics Can Help

A Common Pitfall with Reheat Actuators and How Analytics Can Help

One of the most common value propositions for a new VAV system is to reduce the component cost by utilizing floating reheat actuators in place of the more expensive proportional motors. The floating actuators rely on valve stroke time for the building automation’s controller to understand when the valve is fully open or fully closed. An inevitable truth is that everything with moving pieces eventually breaks down. In this case, the amount of breakdown will depend on control stability, building load, and the load variability. There is no way to predict actuator reliability over time. As such, preventive maintenance efforts rarely cover reheat actuators. What facility managers are left with is a building full of ticking time bombs without any insight into reheat maintenance needs outside of hot/cold calls from unsatisfied customers. There’s either not enough time in the day or it’s too expensive to add to a maintenance contract.  This is a perfect application for advanced equipment analytics.

Let me walk you through a typical scenario; a 90 second actuator with an ON command for 10 seconds is known to be open 11.11%. When the valve is closed for 3 seconds it is closed by 3.33%. No actuator is precise enough to achieve the exact position calculated by the known valve stroke time. Under normal circumstances this isn’t an issue. The valve opens more when it needs more and closes more when it needs less. It stops when it knows it is open or when it knows it is closed.

The problem is when the valve timing has been increased by worn mechanics internal to the actuator. When the zone needs more heat, it will energize the ON command on for what used to be fully open. At this point a spot check by the facility manager would see that the zone is too cool, the valve is commanded open as indicated by the estimated valve position, the VAV discharge air temperature isn’t as high as expected. A typical response is to increase the hot water temperature. Now the facility manager can move on to item 2 of 50 for the morning having fixed the issue quickly and efficiently. The lesson learned is to override the hot water system for the VAVs to operate correctly.

The reality is more sinister. The VAV, now having satisfied the zone temperature will close the reheat valve. In reality, the actuator will only partially close, leaving the VAV with a 3-8 degree temperature rise from the air handler discharge air to the VAV discharge air. On a spot check now it could be explained away as any combination as issues with duct insulation, or temperature sensor calibration drift. Only viewing the data and FacilityConneX analytic results over time does it become apparent what an underlying problem exists and what the cause is.

FacilityConneX Mobile Equipment Performance View

You could send your facilities team into the field to investigate 100’s of VAVs, only some of which could need preventative maintenance to remedy this issue… or you could deploy FacilityConneX streaming analytics and let those analytic findings pair those 100s of VAVs into a concise list of 15 VAVs with a specific recommendation for troubleshooting the valve timing. The facility management team can work the corrective action into their backlog. The days of scheduling unnecessary fleet maintenance has always been a common practice. By using FacilityConneX’ intelligent analytics to schedule maintenance for the actual issues, those days can finally be left in the past.

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