Control Trends by Stromquist

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Even with all the great new technology in HVAC controls, there are still many pneumatic buildings out there.  If you happen to be someone who has to work on pneumatic controls, you know it can be frustrating when you have a problem with the system.  Here’s a good troubleshooting procedure from Jerry Condiff, the instructor who teaches a great pneumatics class for us here at Stromquist every year.  Jerry has been in the HVAC controls field for over 30 years and really knows pneumatics.

Pneumatics Troubleshooting Procedure

Start at the thermostat or controller.  Check to see if the thermostat will put out a high pressure by turning the set point to its minimum setting.  The pressure should be high on the output gauge.  If not, try adjusting the calibration screw. 

If no air noise is distinguished, check to see if air is present in the supply line into the thermostat (this will require removing the stat from the mounting bracket). If supply air is present but there is no air bleed noise or output pressure, the restrictor is plugged or the nozzle at the bleed port is dirty.  Check and clean the nozzle or clean out the restrictor.

If there is no pressure output but air bleed noise is present, check for an air leak behind the mounting bracket.  Check the air hoses that are connected to the mounting base for deterioration.  Check Honeywell stats for broken plastic air nipples in the back of the mounting base.

If no air leaks are found, plug the branch line of the stat with a jumper hose connected from the supply line to the main or supply to the stat.  If pressure builds on the branch gauge, the branch line of the controlled device (valve, damper or pressure switch) has a leak.  You may also make up a tool consisting of two 5/32 lines and a barbed gauge tee and a 30# pressure gauge.  Connect one end of the hose to supply and the other to the branch port on the mounting base. If you do not read any pressure on the gauge, the branch line has a leak.  If pressure builds, the stat is bad and will need to be replaced.  Use a squeeze bulb and gauge to check controlled device diaphragm for leaks.  If no leaks are present, the branch line hose has a leak and needs to be replaced. 

This procedure may not cover every potential issue you run into with your pneumatic system, but it should at least be a good start, and I hope you find it helpful.  Happy troubleshooting!

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1. Determine the throttling range for your control system.  Let’s use 3-13#’s as an example.
2. Next measure the ambient temperature at the thermostat with a precision temp meter.
3. Adjust the setpoint knob on the T Stat to the current ambient temperature.
4. Pull the branch line off the T-Stat and T tap a gauge between the T-stat and branch line.
5. Adjust the calibration screw in the T-Stat until the branch PSI reads zero.
6. Now adjust the calibration screw in the opposite direction until the branch pressure reads mid range. In this case our differential  is 10#’s of pressure, so (13-3)= 10(diff) /2 or 3+5 = 8 psi.
7. Verify the box damper and reheat valve(if applicable) are both closed. Fine tune if needed.
8. Remove your calibration gauge and replace the branch line back onto the T-Stat.
9. Adjust the T-Stat to the desired setpoint.

Your T-Stat is now calibrated…

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Take a look at the tool kit we’ve designed for commercial and industrial control field professionals.

Visit Stromquist.com to order your kit.

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