Danfoss KP35 Pressure Switch: The Installation Mistakes I Learned the Hard Way (and How to Avoid Them)

Why I'm Writing This (and What It Cost Me)

I've been handling industrial control component orders for a bit over six years now. My title is 'Procurement & Application Specialist' for a mid-sized HVAC/R service company. But my real qualification for this article? I've personally made (and meticulously documented) a dozen significant mistakes with pressure switch orders and installations, totaling roughly $4,200 in wasted budget and countless hours of rework.

My experience is primarily with the Danfoss KP series—mainly the KP1, KP15, and KP35 models—on commercial refrigeration racks and industrial compressor units. I can't speak to how this applies to the RT series or MBC 5100 for marine applications, as I've only dealt with those a handful of times. But the KP35 is a staple for us, and it's where I've bled the most.

The most frustrating part: the mistakes were almost always the same things—simple, preventable oversights. After the third rejection of a commissioned unit in Q1 2024, I created our team's pre-check list. This article is that list, wrapped in the stories of how I learned each point.

Frequently Asked (and Should-Be-Asked) Questions About the Danfoss KP35

1. What's the single most common mistake with a Danfoss KP35 pressure switch?

Hands down, it's messing up the differential setting. The KP35 is a single-pole, double-throw (SPDT) switch with a fixed or adjustable differential, depending on the variant. I once ordered 50 units for a new chiller line—50 units, each one set to the wrong differential.

I had specified a standard KP35 060-117366 (with a 0.2 to 1.5 bar differential), but the application required a tighter 0.4 bar differential for a specific screw compressor oil pressure monitoring. The switches arrived, I checked the cut-out pressure (fine), but I completely glossed over the differential. Every single one was set to the factory default of ~0.7 bar. That error cost us $890 in re-switching labor plus a one-week project delay. (Ugh.)

Lesson: Always confirm the differential setting, not just the cut-in/cut-out. Write it on the packing slip.

2. Can I swap a KP1 for a KP35 if they look similar?

On paper? Sometimes. In practice, I'd recommend against it unless you're really sure. I went back and forth on this for a whole afternoon once. The KP1 and KP35 look nearly identical and share many mechanical parts. The difference is in the contact system.

The KP35 has a silver-tipped contact system designed for higher electrical loads and inductive circuits (like contactor coils). The KP1 uses gold-plated contacts for low-voltage/low-current applications (like PLC inputs). If you put a KP1 on a circuit with a high inrush current, the contacts will pit and fail prematurely. If you put a KP35 on a sensitive control circuit, the higher contact force could cause signal bounce.

Lesson: 'Looks similar' is not a valid engineering decision. Check the contact rating on the datasheet. (I keep a printed copy of the Danfoss KP contact comparison chart in my toolbox now—note to self: I need to laminate that).

3. Why does my new KP35 keep getting 'oil logged' or failing?

This one hurt. In September 2022, I installed a KP35 as a differential oil pressure switch on a large ammonia compressor. The switch was mounted directly to the compressor housing, as per the 'standard' location. It failed within two months.

The issue wasn't the switch; it was the mounting location and orientation. The KP35 has a mechanical diaphragm. Vibration from the compressor, combined with temperature cycling, caused the internal mechanism to fatigue. On top of that, oil was draining down the capillary tube and collecting in the switch housing, causing the diaphragm to become sluggish.

Per Danfoss technical guidelines (and I should have read this more carefully), the KP series should be mounted with the diaphragm in a vertical plane to prevent oil from settling. It should also be isolated from high-frequency vibration using a flexible capillary or a mounting bracket. Since I didn't, $320 replacement cost + credibility damaged with the customer.

Lesson: Mount it vertical, keep it vibration-free. Don't let gravity and oil work against you.

4. How do I get the wiring correct on a KP35? The diagram is confusing.

The wiring diagram on the KP35 can be a bit of a puzzle, especially if you're used to standard industrial switches. (I really should create a cheat sheet for this.) The KP35 is SPDT, but the terminals are labeled 1 (common), 2 (normally open), and 4 (normally closed). My first time, I wired it backwards, thinking 2 was NC and 4 was NO. The result: the compressor alarm went off immediately because the pressure switch was signaling the opposite state.

The actual standard wiring, as per the Danfoss KP wiring diagram, is:

• Terminal 1: Common (COM)
• Terminal 2: Normally Open (NO) – closes on rising pressure (e.g., for a high-pressure safety with manual reset).
• Terminal 4: Normally Closed (NC) – opens on rising pressure (e.g., for an alarm circuit).

The key is to determine your fail-safe state. For a high-pressure cut-out, you want the switch to trip (open) when pressure gets too high. That means you use terminal 2 (NO) to power a contactor, so when pressure rises, the circuit closes and the contactor stays engaged until the safety point. Confusing, right? That's why I now have a laminated cheat sheet taped to my toolbox.

5. Do I need a manual reset version for safety applications?

Not always, but I'd argue yes for critical applications. The KP35 comes in auto-reset and manual-reset (with a 'D' designation, like KP35 060-113766) versions. The choice isn't just a cost decision; it's a safety and serviceability decision.

I once automated a small walk-in cooler with auto-reset KP5s. When the condenser fan failed, the high-pressure switch tripped, the contactor dropped out, and then when the gas cooled and the pressure dropped a bit, the switch reset itself immediately. This caused the compressor to start up directly against a high head pressure, which is bad for the motor. The compressor short-cycled six times before the breaker tripped.

For any application where an operator should investigate the cause of the trip (high pressure, oil failure), use a manual reset version. Period.

6. This is probably obvious, but... Can I use a KP35 for refrigerant R-32?

This is a question a lot of people are asking now, and it's a good one to ask. The KP35 is designed for R-22, R-134a, R-404A, R-407C, R-410A, R-507, and others. But R-32? The answer is not in all applications.

Danfoss explicitly states in its technical literature that the KP35 is not recommended for R-32 (or R-454B) due to the specific pressure differentials and material compatibility requirements. The standard KP35 has a maximum working pressure (MWP) of 22 bar/320 PSI and a maximum test pressure of 33 bar/480 PSI. While R-32 systems can operate within those numbers, the material compatibility with the internal O-rings and the switch's ability to handle the specific pressure/temperature curve isn't validated for anything outside the listed refrigerants.

If you're working with R-32 or other mildly flammable (A2L) refrigerants, you need a specifically approved pressure switch. (I've only worked with standard refrigerants; I can't speak to how the MBC series handles A2Ls, but I know they have their own selection guide.) Don't assume compatibility.

Final Checklist (Based on My Mistakes)

1. Check the differential setting – Don't just read the cut-out. Calculate the cut-in (= cut-out - differential) and verify it.
2. Verify the contact type – KP35 is for inductive loads. KP1 is for dry circuits. Don't cross them.
3. Mount it vertically – With the capillary point downwards (if applicable to your model) to prevent oil logging.
4. Wire it correctly for fail-safe – Terminal 2 is NO, Terminal 4 is NC. Confirm with the wiring diagram for your specific model variant.
5. Choose manual reset for safety – Avoid auto-reset on compressors and oil differential applications.
6. Check refrigerant compatibility – Don't assume R-32 or new refrigerants are OK without checking the official compatibility list.

That's it. I've caught 47 potential errors using this checklist in the past 18 months. It's saved us a lot of money and embarrassment. Your mileage may vary, but this is what works for me.