Standard + Heavy-Duty
Pressure Switch
MPS / MPSHD | Installation Guide
Confirm the model and electrical ratings on the cover label.
Warning | Qualified installer requiredELECTRIC SHOCK, ARC FLASH, EXPLOSION, AND FIRE HAZARD. Incorrect wiring or installation can cause severe injury, death, or property damage. Install and wire only by a qualified person (licensed electrician or trained pump technician) in accordance with NEC (NFPA 70), all applicable local codes, and Merrill instructions. If you are not qualified, contact a professional.
Which switch do you have?
Select either image to open it at full size. Replace the cover before startup.
Before you start
MPS and MPSHD share the same basic installation and wiring procedure. Verify the model, voltage and horsepower ratings, pressure range, tank air charge, and piping connection. Follow the wiring diagram for the exact switch.
Installation | 4 simple steps
Disconnect power. Relieve pressure.
A qualified installer locks out the correct breaker, verifies zero voltage, then opens a drain until the gauge reads 0 PSI.
Check the tank and pressure connection.
Check precharge with the water side drained. Remove sediment from the pressure sensing port; replace damaged or restricted fittings.
Install the pressure switch.
Use suitable thread sealant on compatible male pipe threads. Keep the pressure opening clear. Place the wrench on the hex of the pipe connection to avoid twisting or damaging the switch housing.
Connect wiring. Replace the cover.
A qualified installer follows the exact switch markings and model wiring diagram, verifies grounding and ratings, then secures the cover before restoring power.
Operation + safe startup
How the pressure switch works
MPS + MPSHD | Automatic cut-in / cut-out
As system pressure falls to the cut-in setting, the switch closes its contacts to start the pump. When pressure rises to the cut-out setting, the contacts open to stop the pump.
Open contacts are not an electrical disconnect.Always isolate power at the breaker before service.
Pressure settings
MPS and MPSHD switches operate at the cut-in and cut-out settings shown on their labels. Examples include 20/40, 30/50, and 40/60 PSI. Confirm the settings for the exact model. Only a qualified installer should make adjustments, following the switch instructions.
Startup check | Qualified installer
1 | Restore
Confirm cover is secured. Restore power under qualified supervision.
2 | Observe
Use a working gauge; verify pump starts and stops at the correct pressures.
3 | Stop if
Stop for unsafe pressure, repeat trips or abnormal cycling.
Bladder tank precharge (empty)
| Switch setting | Precharge |
|---|---|
| 20/40 PSI | 18 PSI |
| 30/50 PSI | 28 PSI |
| 40/60 PSI | 38 PSI |
Stop + call a qualified professionalPump will not stop, no water, rapid cycling, leaks, tripped breaker, overheated wiring or relief-valve discharge. Disconnect power at the breaker and seek qualified service.
Important safety + legal notice
BEFORE ANY WORK: Disconnect and lock out power, verify zero voltage with a properly rated tester, and relieve system pressure. A pressure switch is not a service disconnect. Never rely on open contacts to make wiring safe.
QUALIFIED INSTALLATION: Only a qualified electrician or trained pump technician should install, wire, adjust or service this switch. Follow the model wiring diagram, nameplate ratings, NEC and local codes. Ground correctly and secure the cover before restoring power.
PRODUCT LIMITATIONS: Standard MPS and MPSHD models do not provide a manual OFF lever or low-water/dry-run protection. Never bypass safety devices or alter the switch. Improper wiring, installation or use may cause shock, fire, injury, death or property damage.
RESPONSIBILITY: Installers and users must follow these instructions and applicable codes. To the extent permitted by law, Merrill disclaims responsibility for losses caused by misuse, unauthorized changes or installation contrary to these instructions. This notice does not exclude liability that law prohibits limiting.