How to Calibrate a Danfoss KP35 Pressure Switch: Setting Cut-In, Cut-Out & Differential

Walk into the pump room of any commercial building, beverage plant, or hospital in Kenya, and you will almost certainly find a familiar blue rectangular box with a transparent plastic cover mounted above the discharge manifold: the Danfoss KP35 pressure switch.

You will also frequently hear one of the most destructive sounds in industrial maintenance: a 7.5 kW motor contactor clattering like a machine gun—click-clack-click-clack-click-clack—cycling three times a second until the thermal overload trips, the contact tips melt together, or the pump motor burns out.

When a pump chatters, maintenance teams often curse the pressure switch and replace it with a new one. But the switch was not defective. It was simply calibrated by someone who did not understand the relationship between the Range screw and the Differential screw.

The Danfoss KP series is an industry benchmark for mechanical reliability, but its two adjustment scales work differently from generic mechanical pressure switches. Master the calibration math once, and your pumps will cycle smoothly for years.

Working Principle of the KP35 Bellows Mechanism

Inside the bottom of the Danfoss KP35 sits a precision-formed phosphor-bronze bellows connected directly to the 1/4" BSP male or female process thread. Fluid pressure enters the bellows, expanding it against a calibrated internal main range spring.

As line pressure rises, the expanding bellows drives an internal lever mechanism upward. At the pre-set cut-in threshold, a snap-action microswitch toggles with an audible click, transferring electrical continuity across its Single-Pole Double-Throw (SPDT) contact points.

The switch features two separate top adjustment screws:

1. 1Range Spindle (Left): Adjusts the main spring tension and sets the upper operational limit.
2. 2Differential Spindle (Right): Adjusts the deadband (the gap between cut-in and cut-out).

The Universal Calibration Formula

The most common error made by technicians is assuming that the Range scale sets the cut-in pressure and the Differential scale sets the cut-out pressure directly. That is incorrect.

On the standard KP35 with automatic reset, the relationship is defined by a simple subtraction formula:

$$\text{Cut-Out Pressure} = \text{Range Setting} - \text{Differential Setting}$$

• •Cut-In Pressure: The pressure at which the electrical contact between terminals 1 and 4 opens on rising pressure (stopping the booster pump).
• •Cut-Out Pressure: The pressure at which the contact between terminals 1 and 4 snaps closed on falling pressure (starting the booster pump).

               DANFOSS KP35 PRESSURE CYCLING DYNAMICS:
  Pressure
     ▲
     │     ────────────────────────────────── Range Setting (Cut-In / Pump Stops)
     │     ▲
     │     │  DIFF SCALE (Deadband)
     │     ▼
     │     ────────────────────────────────── Cut-Out = Range - Diff (Pump Starts)
     │
     └────────────────────────────────────────────────────────► Time

Practical Calibration Examples

Example 1: Factory Water Booster Pump

• •Requirement: The plant requires water pressure between 2.5 bar and 4.0 bar. When line pressure drops to 2.5 bar, the booster pump must start. When line pressure reaches 4.0 bar, the pump must shut down.
• •Calculation:
• •Target Cut-In (Upper / Stop): 4.0 bar
• •Target Cut-Out (Lower / Start): 2.5 bar
• •Required Differential: $4.0 - 2.5 = \mathbf{1.5\text{ bar}}$
• •Adjustment:

1. 1Turn the Range screw until the left scale pointer aligns with 4.0 bar.
2. 2Turn the Differential screw until the right scale pointer aligns with 1.5 bar.

Example 2: Low-Pressure Steam Boiler Limiter

• •Requirement: A low-pressure steam boiler operating at 3.0 bar must trip the burner if steam pressure rises above 3.5 bar, and allow reset once pressure drops back to 2.8 bar.
• •Calculation:
• •Upper Trip: 3.5 bar
• •Lower Re-arm: 2.8 bar
• •Required Differential: $3.5 - 2.8 = \mathbf{0.7\text{ bar}}$
• •Adjustment: Set Range to 3.5 bar, set Differential to 0.7 bar.

Quick Calibration Matrix Table

Use this lookup table to quickly set your KP35 for standard factory water systems:

SPDT Electrical Wiring Pinout

The electrical terminal block inside the KP35 contains three numbered screw clamps:

                    KP35 ELECTRICAL CONTACT SCHEMATIC:
                            [ Terminal 1 ]
                             (Common Line)
                                  o
                                 /
                                /  <-- Snap-Action Contact
                               v
                   o──────────────────────o
             [ Terminal 2 ]         [ Terminal 4 ]
          (Opens on Fall /       (Closes on Fall /
           High Pressure Alarm)   Starts Pump Motor)

• •Terminal 1 (Common Line): Connect the 230VAC live control circuit phase from the control circuit breaker.
• •Terminal 4 (Closes on Pressure Drop): Connect to the A1 coil terminal of the motor starter contactor. When system pressure drops below the cut-out threshold, the contact between 1 and 4 snaps closed, energizing the contactor coil and starting the pump.
• •Terminal 2 (Opens on Pressure Drop / Closes on Pressure Rise): Use this terminal for a high-pressure warning beacon, an alarm buzzer, or a remote BMS indicator.

The Root Causes of Contact Chatter

If your pump contactor is chattering violently, do not replace the switch until you have verified these two mechanical issues:

1. The Expansion Vessel (Accumulator) Has Lost Its Air Cushion

Water is practically incompressible. In a closed pipe network without an expansion tank, opening a 1/2" sink faucet vents the hydraulic pressure from 4.0 bar to 0 bar in a fraction of a second. The KP35 senses the pressure collapse, snaps contact 1–4 closed, and starts the pump.

The pump discharges water into the pipe; because water cannot compress, pressure spikes from 0 to 4.5 bar in 50 milliseconds. The KP35 senses 4.5 bar, snaps open, and cuts the pump off. The moment the pump stops, pressure drops again. This cycle repeats multiple times per second.

The Fix: Inspect the pressure tank on the manifold. Depress the Schrader air valve stem on the tank top. If water squirts out, the internal rubber diaphragm is ruptured and the tank is waterlogged; replace the bladder or tank. If air hisses out, measure pre-charge pressure with a tire gauge. Pre-charge pressure should be set to 0.2 bar below the pump cut-out pressure when the system is drained.

2. Differential Set Too Narrow

The minimum adjustable differential on a KP35 is 0.7 bar. If an operator forces the differential pointer down to 0.2 or 0.3 bar, hydraulic pressure pulses caused by closing non-return check valves will bounce the switch mechanism across its deadband.

The Fix: Maintain a minimum differential of at least 1.2 to 1.5 bar on standard factory booster pumps.

Steam Installations: Why You Must Use a Pigtail Siphon

While the Danfoss KP35 is widely used for boiler feedwater and low-pressure steam regulation, bronze bellows cannot survive direct contact with live steam.

Live saturated steam at 5 bar has a temperature of over 158°C. If live steam reaches the internal bellows, the phosphor-bronze elastomer tempers, loses its spring elasticity, and ruptures. Furthermore, hydraulic steam water hammer can deform the internal mechanism.

Always install a steel or copper pigtail siphon loop between the boiler steam header and the KP35. Before commissioning, fill the siphon loop with cold water. When steam enters the loop, it pushes against the water seal, ensuring that only cool condensate contacts the KP35 bellows.

What Information to Provide STARNEX for Replacements

STARNEX supplies genuine Danfoss KP pressure switches from our central Nairobi warehouse for same-day delivery across East Africa. When ordering a replacement, confirm:

• •Exact Part Number: The standard Danfoss KP35 auto-reset unit carries code 060-113366 (1/4" male thread) or 060-538666 with enclosure rating IP44.
• •Reset Function: Confirm whether your application requires Automatic Reset (standard for booster pumps) or Manual Minimum/Maximum Reset (mandatory for boiler safety low-pressure or high-pressure limiters).
• •Pressure Range: Verify whether you require a KP35 (0.2 to 7.5 bar) or a KP36 (2.0 to 14.0 bar).

For technical assistance or to verify stock availability, contact the STARNEX technical team at Enterprise Road, Nairobi via WhatsApp at +254 740 459 672.