The Castair PSH1MIX is a 145-175 PSI pressure switch with an integral unloader valve and on/off control. At $40.73, it is the direct upgrade from the PSH1 (no-unloader version) and is the correct choice when the compressor motor needs the line pressure vented at shutdown to enable reliable restart. The unloader valve in the PSH1MIX opens when the compressor reaches cut-out pressure (175 PSI), releasing the pressure trapped in the pump head and outlet line — so when the motor restarts at cut-in (145 PSI), it starts against much lower back-pressure and has an easier time getting up to speed.
Note: Cast Air did not provide a dedicated manufacturer product page for this switch. Specifications are drawn from the product listing. Contact us to confirm electrical rating and full compatibility with your compressor model before ordering.
Why the Unloader Valve Matters
When a piston compressor motor stops at cut-out pressure, compressed air remains in the cylinder head and the outlet line between the check valve and the motor outlet. This trapped air creates back-pressure that the motor must overcome when it restarts. For motors with high starting torque (capacitor-start designs with strong start windings), this is manageable. For motors with weaker starting characteristics — or motors running near their nameplate amperage limit — starting against residual head pressure causes:
- Slow or failed start (motor hums, does not reach operating speed)
- Overload protector trips on the motor
- Excessive inrush current that trips breakers or damages motor windings over time
- Premature motor failure from repeated locked-rotor current events
The unloader valve in the PSH1MIX solves this by bleeding off the head pressure the moment the compressor stops. At the next start cycle, the motor starts against essentially atmospheric pressure — its easiest possible load condition.
Switch Specification Details
- Part Number: PSH1MIX
- Pressure Range: 145-175 PSI (cut-in 145 PSI / cut-out 175 PSI)
- Control Type: On/Off
- Unloader: Yes — integral unloader valve included
- Application: Cast Air single-stage compressors where unloader is required for reliable motor restart
- Price: $40.73
How the Integral Unloader Works
The PSH1MIX combines the pressure switch contacts and the unloader valve in a single assembly. Here is the operating sequence:
- Tank pressure reaches 175 PSI cut-out → switch contacts open → motor stops → unloader valve opens → head pressure vents to atmosphere
- Tank pressure drops to 145 PSI cut-in → switch contacts close → motor starts against vented (low back-pressure) head → compressor pumps air into tank
- Tank pressure reaches 175 PSI cut-out → cycle repeats
The unloader vent is typically plumbed to the pump head or the outlet line between the pump and the check valve. Confirm the unloader port connection for your specific Cast Air compressor before installing.
PSH1MIX vs. PSH1 — When to Use Each
- Use PSH1MIX (this switch) when: motor struggles to restart, overload protector trips repeatedly at restart, breaker trips on startup, or when replacing a failed switch that originally had an unloader
- Use PSH1 (no unloader) when: the compressor uses a separate unloader mechanism, the motor restarts reliably without an unloader, or the system design specifically calls for no unloader in the pressure switch
- If you are not sure which is correct for your compressor, call us — we can identify the right switch based on the compressor model and motor type
When Does a Shop Need the PSH1MIX?
- Pressure switch has failed and the original switch had an integral unloader
- Motor is struggling to restart, tripping overload, or slow to start after stopping at pressure
- Upgrading from PSH1 (no unloader) because of motor restart issues
- Replacing a corroded or mechanically failed pressure switch/unloader assembly on a Cast Air high-pressure unit
Technical Specifications
- Part Number: PSH1MIX
- Manufacturer: Cast Air (Castair)
- Cut-In Pressure: 145 PSI
- Cut-Out Pressure: 175 PSI
- Differential: 30 PSI
- Unloader: Yes — integral
- Control: On/Off
- Electrical Rating: Not confirmed in listing — call to verify voltage and amperage rating for your motor
- Price: $40.73
- Manufacturer Data: Product page not available — specifications from product listing data
What Is Included
- 1x Castair PSH1MIX pressure switch with integral unloader valve
Unloader tubing and fittings for plumbing to the pump head are not included. Confirm the unloader port connection type before installing. Always de-energize and depressurize the system completely before replacing a pressure switch.
Why Buy From Air Compressor Factory Direct?
We stock both the PSH1 and PSH1MIX and can help you determine which is correct for your specific Cast Air compressor. If your motor is struggling to restart, the PSH1MIX is almost always the right answer — and at only $4.42 more than the PSH1, the unloader is a minimal cost for the protection it provides. Free shipping on all orders, free tech support, full manufacturer warranty since 2016.
- Cast Air specialist since 2016
- Free shipping on all orders
- Free tech support — we determine whether PSH1 or PSH1MIX is correct for your application
- Full manufacturer warranty
- Honest guidance — we will tell you if the no-unloader PSH1 is actually sufficient
Call 866-217-0063 for quick answers and help!
