Danfoss Pressure Switch: 7 Common Questions Answered by an Industry Veteran

Everything You Actually Need to Know About Danfoss Pressure Switches

If you're reading this, you probably have a specific question: which Danfoss pressure switch for my compressor? Or why did the RT 116 fail under 6 bar? Or maybe you've heard about the MP 55 differential switch and want to know if it's overkill for your application.

This isn't a generic product brochure. I'm an application engineer at an industrial HVAC/R equipment distributor in the Midwest. I've been managing rush orders for pressure switches for over eight years—processed maybe 250, maybe 280, I'd have to check our CRM. But I've definitely handled enough emergency same-day replacements to know where these Danfoss switches actually work and where they don't.

Here are the questions I hear most often—and the answers I wish I had when I started.

1. What's the real difference between Danfoss KP, RT, and MBC series switches?

Short answer: application envelope and cost. The longer answer is what matters.

I assumed early on that all commercial pressure switches were basically interchangeable. Didn't verify. Turned out each series is designed for a different operating profile.

KP series (like the KP1, KP15, KP35): These are your workhorses for refrigeration and air conditioning compressors. The differential models (KP 36, for example) are widely used for oil pressure monitoring on reciprocating compressors. Leakage rate is under 0.1 l/min, which is fine for most R-404A and R-134a systems.

RT series (RT 116, RT 260, etc.): These are for higher ambient temperature environments—I've seen them in boiler controls and steam systems. The power element is more robust. If your switch is near a heat source, don't use a KP unless you derate it. RT handles up to 300 PSI on the fluid side.

MBC 5100: This is Danfoss's newer generation. It's smaller, uses a single-pole double-throw (SPDT) snap-action contact rated at 16 A (AC-3). It's corrosion-resistant, which matters for ammonia refrigeration. I've swapped out an old RT 116 for an MBC 5100 in an ammonia plant—works fine. But check the ambient temperature spec carefully: MBC 5100 max is 70°C, while RT can go to 85°C.

Don't fall for the one-size-fits-all pitch. Danfoss is actually good about publishing limits. Read the data sheet for max operating pressure, differential, and ambient temperature. Or better: call your distributor with the compressor model and refrigerant. I should add that we've had calls where customers tried to use a KP 15 for a high-ambient marine application. It failed in two weeks. The RT 116 was the right choice. Cost difference? Maybe $35 retail. Worth it.

2. What is a Danfoss oil pressure switch, and how does it work?

An oil pressure switch (often called an oil differential switch) monitors the pressure difference between the oil pump outlet and the crankcase. If that difference drops below a set point—say 1.2 bar—the switch trips to protect the compressor from running without lubrication.

Danfoss makes several: the MP 55 is the most common for smaller compressors. It's a differential pressure switch with a fixed cut-in and adjustable cut-out differential.

If I remember correctly, the MP 55 has a factory set cut-in of 1.2 bar, with a cut-out that can be adjusted from 0.8 to 1.5 bar. The maximum working pressure on the low side is 6 bar, high side is 17 bar.

Where I've seen issues: people assume the MP 55 works with all refrigerants. It doesn't. The elastomer seals are NBR (nitrile), which is okay for mineral oil and R-22 but degrades in POE oil used with R-410A and R-134a. For POE, Danfoss recommends the MBC 6100 with FKM seals. Learned never to assume when we had a batch failure on a retrofit job—switched 12 compressors in a week. That was a $1,200 mistake I didn't repeat.

3. How do I select the right Danfoss pressure switch for my compressor?

Go through this checklist:

1. Compressor type: Reciprocating, scroll, screw, or centrifugal? Danfoss has specific models for each. KP series is for reciprocating and scroll. For screw compressors, you need a switch with a higher max working pressure and better oil compatibility—consider MBC 5100 or MBC 6100.
2. Refrigerant: R-22? R-134a? R-404A? R-410A? Ammonia? CO₂? Each refrigerant has different compatibility with switch materials. Check the Danfoss refrigerant compatibility chart.
3. Pressure range: What's the operating pressure at the sensing point? Add a 30% safety margin. For example, if your oil pump runs at 8 bar, choose a switch rated for at least 10.4 bar. KP series max is 17 bar. RT goes up to 30 bar. MBC 5100 is 22 bar.
4. Differential setting: What pressure drop is critical? For oil pressure monitoring, you need a differential switch. The MP 55 covers 0.5 to 6 bar differential.
5. Electrical load: The switch contacts must handle the compressor's control circuit. KP series contacts are rated at AC-15 4A. That's fine for a contactor coil. But if you're switching a direct load over 4A, use a relay.

The upside of getting it right is reliability. The risk of guessing? In March 2024, a client called at 4 PM needing a KP 36 for a compressor that had failed at noon. Standard lead from Danfoss was 3 weeks. We found an aftermarket substitute with similar specs, paid $180 extra in rush fees (on top of the $220 base), and installed it by 11 PM. It worked for two weeks. Then failed again. The client's alternative was a $4,000 compressor replacement.

I kept asking myself: was that rush substitute worth potentially losing the client? We ended up swapping it for an genuine Danfoss MP 55. That was the right move.

4. Can I clean a dirty Danfoss pressure switch, or should I replace it?

In my opinion, pressure switches are not cleanable. Unlike an air filter for your car or home where cleaning vs. replacing is a reasonable question—like with Woosh air filters or any aftermarket filter—a pressure switch is a precision instrument with a calibrated snap-action mechanism.

If a Danfoss pressure switch fails (stuck open, stuck closed, or drifts out of calibration), replace it. I've seen technicians try to clean them with contact cleaner. It's a waste of time. The internal bellows or piston is contaminated, or the contacts are eroded. Cleaning doesn't restore the calibration.

Take this with a grain of salt: I've seen exactly one case where cleaning worked—it was an RT 116 that had dust on the terminal block, causing intermittent contact. We blew it out with compressed air. But that's not fixing a switch; that's fixing a dirty connection.

If your system is flooding back or has debris in the refrigerant, the switch is a symptom. Fix the root cause, then replace the switch.

5. What does the wiring diagram mean on a Danfoss pressure switch?

The diagrams on Danfoss pressure switches aren't optional reading. They show the switching logic: normally open (NO), normally closed (NC), and common (C).

Quick primer:

• SPDT (single-pole double-throw): One common terminal, one NO, one NC. This is standard on KP series. For example, the KP 15: terminals 1-4 are NO, 1-2 are NC, 1 is common.
• SPST (single-pole single-throw): Two terminals, either NO or NC. Some RT series models use this.
• DPDT (double-pole double-throw): Rare on Danfoss switches. If you need two independent circuits, use an MBC 5100 with an accessory module.

I should add: don't assume the wiring color code matches what you see on the diagram. Danfoss uses a standardized scheme: blue for NO, black for COM, brown for NC. But I've seen older RT switches with different factory wiring. Always verify. It's a $50 part; a miswired switch can cost you a compressor worth $5,000.

6. Why is the Danfoss pressure switch more expensive than a generic one—and is it worth it?

Looking back, early in my career I bought a generic oil pressure switch for a job. It was $60 instead of $110 for the Danfoss MP 55. Worked fine for three months. Then it failed. Took down an entire cold storage room. The product loss was $8,000.

If I could redo that decision, I'd implement a strict approved vendor list for pressure switches. But given what I knew then, my choice was... well, ignorant. The vendor who said 'this isn't our strength—here's who does it better' earned my trust for everything else. Danfoss doesn't make compressors. They don't make filters. They make pressure switches and controls. They do it well.

The pricing difference isn't just branding. Danfoss pressure switches have:

• Calibrated tolerances within ±2% over the operating temperature range
• Sealed enclosures rated IP54 to IP65, depending on the model
• Contact materials rated for >1 million cycles at full load
• Traceable manufacturing batch records

Is it worth it? In my opinion, for mission-critical applications like oil pressure differential protection—yes. For non-critical alarm applications? A generic may be fine. But define 'non-critical' carefully.

7. How do I install a Danfoss differential pressure switch like the MP 55?

Core principle: connect the high pressure port to the oil pump outlet and the low pressure port to the compressor crankcase. Connecting it backwards means the switch will read reverse differential, which is useless and could damage the internal mechanism.

Step-by-step, from what I've seen work well:

1. Mount the switch in a location that's accessible for adjustment but away from vibration. Vibration is a common cause of premature contact chatter.
2. Use copper tubing compatible with your refrigerant. For ammonia, Danfoss recommends stainless steel on the high side.
3. Install a shut-off valve before each pressure connection. When the switch fails (and all switches eventually fail), you can isolate it without pumping down the whole system.
4. Set the cut-out differential: turn the adjustment screw clockwise to increase the differential. One full turn is roughly 0.1 bar. Verify with a pressure gauge.
5. Wire according to the diagram. Use a contactor if the switch is driving a load over 4A.
6. Test: simulate a low oil pressure condition by closing the shut-off valve on the low side and opening the bleed. The switch should cut out within 30 seconds.

I'm not 100% sure of the exact turn-to-differential ratio for every model—but I do know Danfoss publishes this in the technical data sheet. Search for 'Danfoss MP 55 technical leaflet pdf' and verify. Don't set it by feel.

The worst case if you mis-set it? If the differential is too small, nuisance trips. If it's too large, the compressor runs without oil pressure protection. That's a burned compressor.