Castair 1/2"NPT-F 1/2" NPT-F Outlet Vent Outlet 3/8" NPT 25 Max. CFM Electric Dual 3 Way Pilot Valve NDC-1

Castair NDC-1 Three-Way Pilot Valve — 1/2" NPT Electric Dual, 25 CFM

The Castair NDC-1 is a three-way pilot valve designed for electric dual-control air compressor systems. It handles the load/unload cycle on compressors drawing up to 25 CFM, using 1/2" female NPT connections on both inlet and outlet, with a 3/8" NPT vent port. If your shop runs a mid-size reciprocating compressor with a pressure switch and you need the unloader valve to work in sync with motor start/stop, the NDC-1 is the component that makes that happen. It is solenoid-operated, meaning it responds to the electrical signal from the pressure switch rather than requiring a separate mechanical pilot line.

Before you buy a replacement or upgrade valve, confirm two things: your compressor draws 25 CFM or less at the valve, and your control system is electric dual (pressure switch with motor starter) rather than a gas engine drive. If your compressor exceeds 25 CFM or uses a gas engine, you need a different valve — the lineup section below covers your options.

What the NDC-1 Does in Your Control System

• Load/Unload Control: The NDC-1 opens and closes the compressor unloader port based on the pressure switch signal. When tank pressure hits the cut-out setting, the valve unloads the compressor head so the motor can restart without fighting full line pressure.
• Three-Way Function: Unlike a simple two-way valve that only opens or closes a single path, a three-way valve routes flow between inlet, outlet, and vent. On unload, it vents trapped pressure through the 3/8" NPT vent port rather than leaving it locked in the line.
• Electric Dual Operation: Solenoid-operated, triggered directly by the pressure switch electrical signal. No separate pneumatic pilot line is needed. The solenoid coil energizes when the switch signals, shifting the valve spool to unload position.
• Check Valve Integration: The three-way design incorporates check valve function to prevent backflow through the pilot circuit during the loaded condition.
• 25 CFM Rating: Sized for mid-range compressors. This rating applies to the valve's flow capacity, not the compressor's displacement — confirm your compressor's actual flow demand at the control circuit before ordering.

Port Configuration and Plumbing

• Inlet: 1/2" female NPT — connects to the compressor discharge or control line from the tank
• Outlet: 1/2" female NPT — connects downstream to the unloader mechanism or control device
• Vent Outlet: 3/8" NPT — exhausts trapped pressure on unload cycle; connect to a muffler (Castair M38, 3/8" muffler) to reduce exhaust noise in the shop
• Pilot Ports: 1/8" female NPT — used for the solenoid control signal connection
• PSI Range Options: Order NDC-1100 for 100–120 PSI systems, or NDC-1140 for 140–170 PSI systems. Specify the correct pressure range for your application — the internal spring and seat configuration differs between the two versions.

Electric Dual vs. Gas Engine — Why It Matters for Valve Selection

• Electric Dual System: The pressure switch both starts/stops the motor and signals the unloader valve. The NDC-1's solenoid receives this electrical signal directly. This is the correct valve for electric motor-driven compressors with dual-pressure-switch control.
• Gas Engine Systems: Gas engine drives do not use a solenoid — the engine runs continuously and the valve must mechanically unload the compressor head. The NG-1 (same port sizes, same CFM, but mechanically operated) is the correct valve for those applications.
• Mixing Types is a Failure Point: Installing a gas-engine valve on an electric dual system — or vice versa — will cause the unloader to malfunction, leading to hard-start motor failures or compressor head damage. Confirm your drive type before ordering.

How a Three-Way Pilot Valve Works

A three-way pilot valve has three ports: inlet, outlet, and exhaust (vent). In the loaded position, the valve connects inlet to outlet, allowing the control signal to pass through and hold the compressor in the operating (loaded) condition. When the pressure switch reaches cut-out and signals the valve, the spool shifts: inlet is blocked, and outlet is connected to the vent port. This exhausts the trapped pressure from the unloader line, allowing the unloader mechanism to open and unload the compressor head. When the switch signals to reload, the spool shifts back, re-pressurizing the control line and closing the unloader.

The vent function is critical. Without proper venting on unload, residual pressure holds the unloader partially open or causes erratic cycling. The NDC-1's 3/8" NPT vent port handles the exhaust volume for compressors up to 25 CFM. Blocking or restricting this port — including connecting it to a line with back-pressure — will cause control failures.

Pilot Valve Selection Guide — Complete Castair Lineup

Castair makes 1-way, 2-way, and 3-way pilot valves across a range of port sizes and flow ratings. Here is the complete lineup with part numbers to help you identify the correct replacement or upgrade:

PSI Suffix: Add 100 (for 100–120 PSI) or 140 (for 140–170 PSI) to the part number when ordering. Example: NDC-1100 or NDC-1140. Muffler: Castair M38 fits the 3/8" vent port on the NDC-1, NG-1, and NG-18.

When Does a Shop Need the NDC-1 vs. Alternatives

• NDC-1 is right when: You have an electric motor-driven reciprocating compressor, a dual-pressure-switch control system, 1/2" NPT control lines, and the compressor runs 25 CFM or less through the valve. It is also the correct replacement when the original NDC-1 has failed and the original plumbing is intact.
• Step up to NLDC-1 when: Your compressor exceeds 25 CFM at the valve, or you have 3/4" NPT control lines. The NLDC-1 handles 50 CFM with 3/4" fem. NPT inlet and outlet. It costs more ($324.94 vs. $110.11), but undersizing the valve on a larger compressor causes pressure drop and failed unload cycles.
• Use NG-1 instead when: Your compressor is gas engine-driven. The port sizes are identical to the NDC-1 (1/2" NPT inlet and outlet, 3/8" vent), but the NG-1 is mechanically operated rather than solenoid-operated. Gas engines do not produce the electrical switching signal the NDC-1's solenoid needs.
• Step down to BE2-M when: Your control system only needs a 2-way function with 1/4" NPT connections at 20 CFM or less. The BE2-M is a simpler, lower-cost valve for smaller electric dual systems.
• Do not use on gas engine compressors: The NDC-1 is electrically operated. It will not function correctly without the solenoid trigger signal that an electric dual control system provides.

Technical Specifications

• Part Number: NDC-1
• Valve Type: 3-way pilot valve
• Application: Electric dual air compressor control systems
• Operation: Solenoid-operated
• Inlet Port: 1/2" female NPT
• Outlet Port: 1/2" female NPT
• Vent Outlet Port: 3/8" NPT
• Pilot Ports: 1/8" female NPT
• Maximum Flow Capacity: 25 CFM
• Pressure Range Options: 100–120 PSI (order as NDC-1100) or 140–170 PSI (order as NDC-1140)
• Manufacturer: Castair
• Price: $110.11
• Compatible Muffler: Castair M38 (3/8" vent port)

Why Buy From Air Compressor Factory Direct?

Air Compressor Factory Direct has specialized in compressor parts and controls since 2016. We stock Castair pilot valves including the complete NDC-1 lineup across both pressure ranges. When you call, you talk to someone who understands how pilot valves fit into the load/unload circuit — not a general parts counter that will guess at a substitute.

• Free Shipping: All orders ship free. No minimum.
• Free Tech Support: Call 866-217-0063 before you order if you are not sure whether the NDC-1 or a different valve fits your application. We will confirm the correct part number and pressure range.
• Full Manufacturer Warranty: Castair warranty applies to every valve we sell. No grey-market parts.
• Honest Guidance: If you need the NLDC-1 instead of the NDC-1, we will tell you. We would rather lose a $15 upsell than have you install the wrong valve and call back with a failed compressor.

Call 866-217-0063 for quick answers and help!