Commercial HVAC Diagnosis – Seasons 4 Reheat Issue
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Commercial technician Eric Mele diagnoses improper dehumidification on a Seasons 4 system with reheat. He goes through the diagnostic process on the Seasons 4 RTU in the field.
Eric starts by tracing the wiring so that he can understand exactly how the system is wired. He checks his control board to see where all the wires are hooked up and how they’re configured. While checking the system, he notices that a three-way valve is closed, so he opens it. He feels water moving through the hot water coil, but he needs to be sure that the flow rate is correct.
When Eric finds the analog output for the modulating damper, he measures it with his voltmeter and reads 6.1 amps DC, which falls within the 1-10-volt throttling range.
The choice to open the valve affected the dew point, and the hot water coil became warmer. Eric wonders if the system didn’t have enough water flow to retain the heat in the water until he opened the valve. He also notices that the “Compressor 1” switch has been set to “Module Reset,” which seems odd. We can verify if that is correct or incorrect by checking the manual. Compressor 1 also plays a role in dehumidification. Only two steps of cooling are on at the moment, too.
The dew point comes back up, and the supply temperature is a bit high. However, the AHU status is in “Reclaim Cool Dehumidify.” When Eric attempts to change the Compressor 1 switch, it flips back to “Module Reset.”
Eric checks out the discharge and notices that the hot water reclaim is using up all of his discharge gas. So, none of the discharge gas is going to the air handler!
When Eric returns to the control boards, he notices that the dew point is nearly back down to normal, and the DC voltage on the valve continues to drop but is still in range. So, the valve is trying to open a little more. After reviewing the dew points over the past few days, Eric becomes more confident that the system is not getting enough hot water from the rack.
The discharge gas going into the heat exchanger is extremely hot, but the pipe leading out of the heat exchanger is cool enough to touch for a long period of time without Eric burning his hand or feeling uncomfortable.
When Eric returns to the control boards again, his voltmeter reads 2 VDC at the analog output, meaning that the valve is entirely open. All compressors are running, and we have a 5-degree temperature rise, which is optimal. So, it appears as though the issue was the closed three-way valve all along.
The three-way valve is a bypass valve. In this video, it is near the return side, so it is a mixing valve. (A valve on the supply side is called a diverting valve.) When that bypass valve was closed before Eric came, the system got very little water flow from the return pipe, as the water wasn’t mixing with the supply water. So, the valve did not properly respond to a call for dehumidification because the coil never got warm enough.
Since this application only has one coil, it must have a three-way valve. You can use some two-way valves on systems with multiple coils.
When Eric goes back to check the discharge after fixing the problem, the pipe leading out of the heat exchanger is finally very warm to the touch. The shaft seal is leaking, but that is easy enough to repair without an extensive diagnostic process.
When the coil is inactive, Eric’s voltmeter reads 10 VDC, which indicates that the valve is closed. However, dehumidification is active. It turns out that the dehumidification signal to the valve is flipped. Eric overrides the dehumidification to “inactive.”
Eric comes back and discovers that once the space dew point is satisfied, the controls display the outdoor air dew point. That indicates that the system has a flexible combiner that shows a new relevant measurement when one is satisfied. (In our case, the space dew point was satisfied, so the controls show the outdoor air dew point afterward.) The drive was also not initially programmed for the correct application.
Overall, Eric fixed the three-way valve (water flow to the hydronic loop), corrected the analog output setting to the valve, and communicated with Seasons 4 to configure the drive properly and confirm that the system was working properly.
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