Troubleshooting Basic Pneumatics

Skylar Kaczanowski

Apr 03, 2025

Troubleshooting Basic Pneumatics

Pneumatic control is a mystery to many technicians today. No one is training on pneumatic systems any more perhaps because there have been few advancements in the past five decades. Still pneumatic controls are effective if the system receives regular maintenance. Step one is a clean dry air supply. Coalescing filters equipped with auto drain systems require new filter elements at least annually. Compressor oil or water do not belong in the air lines. This is not a hydraulic system. If water squirts out of the tubes upon removing a thermostat from its base, the system needs flushing. Water mixed with oil makes a slime that plugs the bleed orifice in a thermostat. Devices downstream from the thermostat do not work properly if the thermostat is not able to bleed air.

A typical heating application will be direct acting. When the temperature goes up the pressure goes up. When someone crosses the goal line with the football the referee signals direct acting. Temperature up with one hand, pressure up with the other hand, is a touchdown. The bi-metal strip inside the thermostat usually is marked with a DA. What this means is that when a direct acting thermostat is satisfied the bleed port is closed and all the air pressure is going to the valve actuator. A direct acting valve closes when pressure is present on the top of the diaphragm. Such valves are said to be push down to close. If the bleed port opens to relieve the pressure from the valve, its spring pulls the stem up and the valve is open.

A preliminary check for this kind of system is to pull the tube off the back of the thermostat. The one marked S is the supply line. It should have 15-20 psi of air pressure. Obviously, a gauge is a good tool for that but the presence of air pressure at all starts the process. If there is air present on the supply line the next check is to pull the air line off from the valve actuator. If the valve springs open with the tube removed and closes when the air is restored the valve is working properly. If the valve doesn’t move regardless of air pressure the diaphragm is bad. Replacement diaphragms are not expensive and are relatively easy to install. Sometimes stem leaks have corroded the valve bonnet so severely the stem would not move under any circumstance. A replacement of the whole assembly solves the problem.

Most manufacturers make both direct and reverse acting valves. The actuators from these manufacturers are always the same. Pressure pushes the diaphragm down. Whether the valve pushes down to open or pushes down to close determines the action. Honeywell is the exception. Honeywell valve all are open when the stem is up and closed when it is down. Different actuators determine the action.

Pneumatic thermostats all have service ports for reading the internal pressure. Each manufacturer has its own version of pressure probe for connecting with a gauge. Calibration of thermostats requires the proper probe for each brand. All brands use these probes to connect with 1/8” NPT gauges. The pressure scale for these is 0-30 psi. Each brand also has a unique wrench or screwdriver for calibrating thermostats.

Another indispensable tool for pneumatic service is a squeeze bulb assembly. Connecting the squeeze bulb to an actuator demonstrates whether air pressure applied to a diaphragm moves the plunger. If it does not it indicates a ruptured diaphragm. If the actuator moves, the technician can determine what pressure equals full stroke and what pressure is closed. That is the spring range. Typical heating ranges are 3-6 psi, 2-7 psi, 3-8 psi. Cooling ranges are more like 8-13 psi. The hardware for the assembly includes the squeeze bulb itself, 0ne 3/8”x1/4” barbed coupling, a 1/4” barbed gauge tee, a 0-30 psi gauge, and about a foot each of 3/8” and ¼” soft vinyl tubing. Soft tubing is important because it has to go on and off barbed fittings. Sometimes the tubing is surgical tube that is even more flexible. Big box home supply stores carry suitable tubing and they sell it by the foot. To get the proper sizes of barbed fittings one would need to shop at a distributor of pneumatic controls.

Here is a Summary for a faster read:

Troubleshooting Basic Pneumatics

Key Points:

Pneumatic systems require regular maintenance to function properly.
The most important factor is a clean, dry air supply.
Water or oil in air lines can cause failures by clogging components.

Step 1: Check the Air Supply

✅ Ensure air is clean and dry

Use coalescing filters with auto drains
Replace filter elements annually
No oil or water should be in the air lines

🚨 Signs of Contamination

If water squirts from tubing when removing a thermostat, the system needs flushing.
Water mixed with oil forms slime, which clogs thermostats and prevents proper function.

Step 2: Understanding Direct-Acting Thermostats

🟢 Direct Acting (DA) Thermostat Basics

When temperature increases, pressure increases.
When the thermostat is satisfied, it closes the bleed port, sending air pressure to the valve actuator.
The valve closes when air pressure pushes down on the diaphragm.

📌 Quick Analogy: A referee signals touchdown—one hand up for temperature, the other for pressure.

Step 3: Basic System Check

1️⃣ Check the thermostat’s supply line (S port)

Disconnect the tube from the back of the thermostat.
The supply line should have 15-20 psi of air pressure.
A pressure gauge is helpful but not required.

2️⃣ Check the valve actuator

Remove the air line from the actuator.
If the valve springs open when the tube is removed and closes when reconnected, the valve is working.
If the valve does not move, the diaphragm is likely damaged and needs replacement.

3️⃣ Inspect for corrosion

Severe stem leaks can corrode the valve bonnet, preventing movement.
If corrosion is present, replace the entire valve assembly.

Step 4: Direct vs. Reverse Acting Valves

Most manufacturers produce both direct and reverse acting valves.
Pressure always pushes the diaphragm down.
Whether the valve opens or closes depends on the design.

⚠ Honeywell Exception:

All Honeywell valves are open when the stem is up and closed when the stem is down.
The actuator determines the action, not the valve itself.

Step 5: Tools for Pneumatic Service

✅ Essential Tools:

Pressure gauge (0-30 psi) – Required for calibration.
Squeeze bulb assembly – Used to test actuator response.
Calibration probe – Each brand has a unique probe for reading internal thermostat pressure.
Soft vinyl tubing (3/8” and 1/4”) – Connects components easily.

🛠 Building a Squeeze Bulb Assembly:

Parts required:
- 1 squeeze bulb
- 1 3/8” x 1/4” barbed coupling
- 1 1/4” barbed gauge tee
- 1 0-30 psi gauge
- 1 foot each of 3/8” and 1/4” soft vinyl tubing

📍 Where to Buy:

Soft tubing: Home improvement stores (sold by the foot).
Barbed fittings: Pneumatic control distributors.

Common Pressure Ranges

Application	Pressure Range (psi)
Heating Systems	3-6, 2-7, or 3-8 psi
Cooling Systems	8-13 psi

Dallas Strong - Johnstone Supply Muskegon Group