Danfoss Pressure Switch FAQ: KP vs RT vs KPI, RT116, Wiring, and Set Point Verification

I'm a quality and brand compliance manager at an industrial controls distributor. I review every pressure switch spec before it reaches customers—roughly 200 line items a year. In 2024, I rejected around 12% of first deliveries for things like set point drift outside datasheet tolerance, missing Declarations of Conformity, and contact ratings that didn't match the application. So when engineers ask me about Danfoss switches—especially the KP, RT, and KPI series—I tend to answer with the questions that actually matter on a purchase order. These are the ones I get asked most.

Which Danfoss pressure switch series should I use—KP, RT, or KPI?

The short answer: it depends on your environment and your electrical load. The KP series is the workhorse for HVAC and refrigeration—compact, reasonably priced, and heavily documented. KP1 and KP5 handle low-pressure and fan-cycling duties; KP15 and KP35 cover higher discharge pressures. The RT series is the heavier industrial line—metal enclosure, wider ambient tolerance, more terminal space—which is why it keeps showing up in marine panels, boiler protection, and compressor packages. The KPI series sits between the two: industrial-grade construction with a smaller footprint and easier wiring than an RT. Honestly, I've specified all three, sometimes on the same skid. If the switch lives in a clean electrical room and the media is refrigerant, KP is usually enough. If it's a harsh environment or you're switching significant inductive loads, step up to RT or KPI. My experience comes from about 200 specs across HVAC and refrigeration; if you're in offshore or hazardous areas, your requirements will differ.

How do I wire a Danfoss pressure switch?

People rarely fail at the switch itself—they fail at the circuit around it. Most KP models use a single-pole double-throw contact rated around 16 A at 250 V AC resistive. But if you're driving a contactor coil or a motor starter, the inrush current is a different animal, and that's where contacts weld. I had a customer send back a KP36 that failed after a week; the coil inrush was nearly triple the contact rating. The wiring convention on most of these is 1–2 for normally closed and 1–4 for normally open, but don't work from memory—pull the wiring diagram from the Danfoss website for your exact model code. The KPI series has its own wiring diagram, and it isn't identical to the KP. Also check the terminal wire size: I've rejected deliveries where a contractor used 0.75 mm² wire on terminals that clearly specified 1.5 mm². Vibration plus undersized wiring is a recipe for intermittent trips. Take the ten minutes to confirm before you terminate.

Is the RT116 still current, or has it been superseded?

The RT116 is still listed as far as I know—don't hold me to that, since product lifecycles vary by region. It's a long-serving high-pressure switch, common in marine hydraulics, boiler protection, and industrial compressor packages. Its repeatability is exactly why it keeps showing up in legacy specs even when newer models exist. The gotcha is the full type code: 'RT116' alone isn't enough. There are different pressure ranges, contact functions, and enclosure variants packed into that name. I once caught a purchase order where someone wrote 'RT116' and left the range blank. Even after I flagged it, I kept second-guessing—what if the vendor's system auto-selected the right variant? The two days until they confirmed were stressful. It ended fine, but the lesson stuck: always order with the complete Danfoss type code from the datasheet, not just the base model number.

What's the difference between a differential pressure switch and a regular pressure switch?

This trips up more buyers than any other question I get. A regular pressure switch monitors one pressure and changes state at the set point. A differential pressure switch measures the gap between two pressures—say, across a filter element or a pump. Danfoss makes both, but the marketing language gets kind of confusing because single-pressure switches like the KP1 have an adjustable differential (the spread between cut-in and cut-out). That's not the same thing as a two-port differential switch. The rule I use: one port = single-pressure switch with an adjustable differential; two ports = true differential switch. If you're trying to detect a clogged filter, you need the two-port type. I've had engineers order a standard KP for filter monitoring because they saw the word 'differential' in the datasheet and assumed it would work. It didn't. Check the connection diagram, not the headline specs.

How do I verify the set point before installation?

You don't need a lab, but you do need a calibrated gauge and a multimeter. I use a portable pressure test rig and verify every switch that goes into a safety-related application. Here's the protocol: apply pressure slowly, note where the contact changes state, then bleed down and note where it resets. Compare both values to the nameplate and the datasheet tolerance. For a typical KP model, set point tolerance is around ±0.1 to ±0.2 bar depending on the range—check the current datasheet for your variant. If the switch is more than a full tolerance band off, reject it. Also verify the differential while you're in there, because it's field-adjustable and misadjustment is the most common setup error I see. In our Q1 2024 audit, a batch of 50 switches came in with the differential set wrong on six units. The vendor redid them at their cost. That ten minutes per switch saved us a site visit later. Worth it.

Why are Danfoss pressure switches more expensive than generic ones?

Because the purchase price isn't the total cost. A generic switch might cost 60% of a Danfoss, but total cost of ownership is usually lower with the genuine part. Here's what I include in every vendor comparison: time to verify the datasheet, risk of set point drift causing false trips or missed alerts, the service call if it fails in the field, and the compliance paperwork if you're in a regulated environment. On one project, a low-cost switch failed in a refrigeration unit after 14 months. The replacement—with a service truck and refrigerant recovery—cost roughly four times the purchase price difference. Now, to be fair: if you're building a simple product with no safety claim and low cycle counts, a generic switch might be adequate. I get why people go that route. But when a warranty, a safety requirement, or an OEM logo is involved, Danfoss's documentation and traceability tend to pay for themselves. The lowest quote is rarely the cheapest.

How do I know I'm getting a genuine Danfoss switch, not a counterfeit?

Counterfeit control components are a real problem, and I've run into them in distribution. The first red flag is packaging: genuine Danfoss switches ship in branded boxes with a clear type code, serial number, and, on newer products, a QR code you can verify. Second is the terminal block—genuine units have clean stampings and proper torque markings; fakes look rough up close. Third is weight; a counterfeit with a thinner housing feels noticeably lighter. But the real test is documentation: genuine switches have an EU Declaration of Conformity tied to the type code, and the datasheet values should match the physical unit. I once received samples that passed visual checks but failed accuracy testing—set point off by nearly 15%. So glad I tested before approving that PO. It cost me two days and saved a recall. If a deal looks too good to be true, it usually is. Verify first.