Pneumatics Troubleshooting

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!


3 Responses

  1. I have a few Johnson Control RA thermostats that hiss non stop. I have found no leaks but I have found air pressure coming out of the tube leaving the thermostat when disconnected. This is not the norm for most all of my other thermostats. The others have no pressure when disconnected from the out going port of thermostat. Help!

  2. I have a dual duct hot and cold side vav. The hot side has a kmc velocity controller and a damper for the hot deck. The cold side simply has a Trane damper in a duct with no actuater. Does the cold side need it’s own velocity
    Controller? I hooked it up and tried it but the thermostat constantly bleeds air. Any ideas?
    Any help would sure be appreciated!
    Bill Murphy

  3. I have a pneumatic thermostat that the tubes are broken air coming out wont have kit to fix it till tomorrow can I jump one tube to other to stop air escape till tomorrow

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