This war story begins with a hospital having issues with subcooling. Receiving a phone call from the facility engineer I had built a relationship with a great time back when performing TAB services with the hospital, I was asked to review issues with some pneumatic variable air volume (VAV) boxes serving an area of the hospital. Calibrating pneumatic volume regulators and thermostats was outside his range of expertise. Stopping by to talk with the facility engineer, I investigated what might be the issue. The facility engineer had asked if we could review a few volume regulators that he had adjusted.
As most of us with hospital experience know, some office areas are often VAV while other areas are constant air volume (CAV). The existing install was VAV primary air boxes with hot water reheat coils. The hot water reheat consisted of two- and three-way control valves going back to a steam bundle in a nearby mechanical room.
We started by reviewing the office and administration areas. The facility engineer had been going through the area in question, confirming the CFM values set were what was spelled out on the information tag attached to the side of the VAV boxes. The areas in question had not had any recent modification or changes made with walls or duct systems and were still set as previous mechanical drawings had shown. Knowing this, we felt good verifying that values set based on the information on the sides of the VAV boxes were indeed correct.
After going through a few boxes, I watched the facility engineer go through the calibration procedure to confirm the adjustments he was making were indeed correct. After making these adjustments, I started to ask him more questions. He stated that although the area was currently comfortable, it was slightly cool in the mornings before the personnel staff arrived at work, resulting in complaints to the facility director.
With this in mind, I asked if we could do a few additional checks regarding the pneumatic boxes. I also asked if we could review the entering and exiting air and water temperatures of the system to compare to the mechanical schedules. I told the facility engineer that I had a good feeling that everything he had accomplished with adjustments to the minimum and maximum airflows of the volume regulators were correct as to the design documents.
We started by testing the last box that was verified for airflow adjustments. When taking a temperature of the discharge, we observed the leaving temperature was 65 to 64 degrees Fahrenheit. At this point, we thought that the control valve was not 100% commanded or we had a restriction of GPM flow through a y-strainer. Commanding the thermostat to full heat, we verified the heating valve was 100% open with no branch air pressure from the thermostat being that it was a direct acting.
Prior to pulling the y-strainer, we reviewed the inlet water temperature to the heating coil. We saw that the inlet water temperature was only 90 degrees Fahrenheit. At this point, I explained to the facility engineer that this had most likely been the problem the entire time. I asked if we could go to the mechanical room where the steam heat exchanger was located to determine if we had issues with the steam heat exchanger pneumatic control valve. The steam valves for these devices are typically closed, so I considered that we might have an issue with the diaphragm or output from the controller to open the valve.
The reason these valves are normally closed is to help prevent issues with the heating loop if the valve should fail. If installed open, this could cause the heating loop to get way too warm and possibly cause the water loop to go to steam and flash if not caught in a sufficient amount of time. The controls engineer designs the valves to fail closed whether it is pneumatic or DDC.
Looking into this issue further, we discovered that the steam valve was indeed controlling to the set-point value. After reaching out to others in management, we determined that the set point was dropped down to help save energy and the actual set point that was given was indeed 90 degrees.
After seeing this, I recommended that the setting be adjusted to a minimum of 140 degrees as a starting point and adjust from there. I had explained to both the management and facility engineer that the original design was 180 degrees for an entering water temperature, but 9 times out of 10 the heating coils were more than enough capacity to meet the designs from original buildout documents. I suggested adjusting the set point while monitoring area space temperatures to confirm areas were indeed comfortable–rather than subcooling to try and save as much energy as possible–based also on perimeter offices and outdoor ambient temperatures.
In summary, we needed sufficient knowledge of pneumatic control devices on this call–they are not going to go away anytime soon. I also recommend that any business having energy audits and reviews ask their energy consultant to review the original design documents before making decisions that could affect overall operational conditions for various areas of the hospital, especially areas that would have CAV boxes installed.
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