Danfoss Pressure Switch FAQ: Answers From a Field Technician With 200+ Emergency Callouts

I've been a field service technician for 12 years, and I've lost count of how many Danfoss pressure switches I've replaced or reset — especially during after-hours emergency callouts. The truth is, most questions I get are variations of the same handful. So here's a direct set of answers, no fluff. I've linked official Danfoss references where I could. Prices and specs are accurate as of early 2025; things change, so verify before you buy.

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

Short version: KP switches are the workhorses for refrigeration and HVAC — compact, adjustable, and used for high/low pressure control on compressors. RT series are more industrial — higher current ratings, often used in oil pressure differential applications and larger systems. MBC is the newest line, designed for mobile hydraulics and off-road equipment, with greater vibration resistance.

In my experience, if you're working on a commercial fridge or AC unit, it's almost always KP (KP1, KP15, KP35). If you see a pressure switch on a big industrial chiller or an oil separator, it's probably RT (like RT116). MBC I see mostly on excavators and agricultural sprayers.

One thing I learned the hard way: never assume a KP and RT are interchangeable just because the pressure range looks similar. The contact loads are different, and the switch housings have different ingress protection. Check the datasheet (Danfoss.com > Products > Pressure Switches) before swapping. (Source: Danfoss technical leaflets for KP 060-1086-12 and RT 060-1088-01, accessed Jan 2025.)

2. How do I wire a Danfoss pressure switch? I've seen four terminals and only need two.

Standard Danfoss pressure switches come with two sets of terminals: one for normally open (NO) and one for normally closed (NC). They also have a common (COM) terminal. So you'll see three or four terminals depending on whether it's single-pole or double-pole.

The most common wiring for compressor control: Use the NC terminal and COM. That way, when pressure rises to the cut-out setpoint, the switch opens and stops the compressor. For an alarm signal, you'd use the NO terminal.

I still kick myself for a time in 2022 when I wired a KP36 backwards — used NO instead of NC — and the compressor ran until the pressure safety valve blew. Cost us a $700 valve replacement and a lost day of production. Now I always double-check with a multimeter before hooking up power. Danfoss provides wiring diagrams in every box (and online), but they're small print — I keep a magnifier in my tool bag.

Tip: For differential pressure switches (like the MP 55 or KPI series), wiring is the same but you need to confirm that the low‑pressure and high‑pressure ports are connected correctly. Swapping them will give you reverse action.

3. How do I adjust the setpoint on a KP pressure switch? The range plate is confusing.

The KP series has a central adjustment screw under the cap. Turning it clockwise increases the cut-out pressure — that's the pressure at which the switch opens. The cut-in pressure (where it closes again) is set by a differential adjustment screw (usually a smaller one nearby).

I've never fully understood why Danfoss uses such small range plates — the numbers are tiny and sometimes in bar and psi simultaneously. My hack: use a pressure gauge and a small portable compressor to test the actual trip point after adjustment. Set the screw a quarter turn, then pressurize until you hear the click. It's slower but error‑proof.

Here's the thing: many technicians try to adjust the setpoint without a gauge, guessing by the spring tension. That's how you end up with a switch that trips at 8 bar instead of 12 bar. Don't do it. (Source: Danfoss KP selection guide — 'Always verify setpoints with a calibrated gauge.')

4. Why does my Danfoss pressure switch keep tripping? It started happening last week.

Most of the time, the switch itself isn't the problem. It's doing its job. The real issue is the system pressure is fluctuating outside the setpoint window.

Common causes I've seen (more than 50 times):

• A bad expansion valve causing liquid slugging and pressure spikes.
• A dirty condenser or evaporator coil reducing heat exchange — pressure skyrockets.
• A failing compressor that can't maintain discharge pressure.
• Low refrigerant charge — the low-pressure switch keeps cutting out.

I assumed a switch was bad in 2023 — changed it, same problem. Then I found the condenser fan was dead. That cost the client $350 for a service call that should have been $150. Now I always do a system pressure test before blaming the switch. (Source: Danfoss troubleshooting guide for KP switches — diagnostic flowchart.)

5. Can a Danfoss pressure switch be used for differential pressure? I see MP 55 and KPI models listed.

Absolutely. The Danfoss MP 55 (part of the KPI family) is a dedicated differential pressure switch. It has two pressure ports — high and low — and compares them. It's used to protect filter driers, oil pumps, and airflow across coils.

However, a standard KP or RT switch is not a differential switch — it's a single‑port pressure switch that measures gauge pressure. If you try to use it for differential by connecting both ports, you'll either get false readings or no trip at all. I've seen a refrigeration contractor put a KP1 on an oil differential application — it never tripped, and the compressor seized. That was an expensive lesson.

So, if you need differential, get the KPI or MP 55. Don't jury‑rig. (Source: Danfoss product program for KPI switches, January 2025.)

6. How do I test if a Danfoss pressure switch is faulty with just a multimeter?

With the switch disconnected from power:

1. Set your meter to continuity (or ohms).
2. Connect probes to COM and the NC terminal. You should read near zero ohms (closed circuit).
3. Apply pressure using a hand pump or system pressure that's above the switch's setpoint. You should hear a click and the continuity should break (open circuit).
4. Then check the NO terminal — it should be open in the rest state and close when pressure is applied.

If the contacts don't change state at the expected pressure — or if they bounce or chatter — the switch is faulty. I've found that about 10% of 'faulty' switches I replace are actually fine; the problem is the system or the wiring. (Not sure why the percentage is so consistent — maybe confirmation bias.)

7. What do the IP ratings (IP54, IP65) mean for these switches? Can I use them outdoors?

Most Danfoss KP and RT switches have IP54 rating (splash water and dust protected). That's fine for indoor industrial environments or sheltered outdoors. For direct rain or washdown areas, you want at least IP65. The MBC series often comes with IP67 or IP69K for mobile equipment.

I'm glad I made the mistake once — installed a KP35 in an outdoor walk‑in cooler's condensing unit without a weather cover. Water got into the switch after a heavy rain, shorted the contacts, and the compressor ran non‑stop. Now I spec an IP65 enclosure or use an RT series with an optional weatherproof kit. (Danfoss technical data lists IP ratings per model – check the latest version.)

8. My Danfoss pressure switch is leaking oil around the pressure connection — what's going on?

Usually it's one of three things:

• Loose connection — the fitting isn't tightened enough, or the thread sealant is missing. Danfoss switches come with a Schrader valve or 1/4″ flare; use Teflon tape on NPT threads, but keep it off the flare seats.
• Damaged diaphragm — if the switch's pressure sensor diaphragm has ruptured, oil or refrigerant will seep out through the electrical connection (that's a big red flag). Replace the switch immediately.
• O‑ring failure — on some models (RT 116, MBC 5100), there's an internal O‑ring that can degrade in high‑temperature or high‑pressure applications. Danfoss sells rebuild kits for some RT models, but honestly, I just replace the whole unit — rebuilding in the field is messy and the cost is similar.

If you see oil at the electrical terminals, shut the system down. That's a fire hazard. I saw a small oil fire once from a leaking RT switch on a compressor control panel — not something I want to repeat. (Source: Danfoss safety bulletin, 2022 – 'Oil leaks in pressure switches can cause short circuits.')