Danfoss Pressure Switch Adjustment: A Practical Field Guide for Air Compressors

I've been managing maintenance procurement for a mid-sized industrial plant for about eight years now. In that time, I've ordered, replaced, and adjusted more Danfoss pressure switches than I can count—KP, RT, MBC, the whole lineup. Most of the time, the job is straightforward. But the amount of confusion I see around how to actually set a Danfoss pressure switch is surprising, especially when it's going on an air compressor.

So here's a checklist-style guide. If you're fitting a Danfoss pressure switch for an air compressor application and need to get the adjustment right the first time, this is for you. I'm going to walk through the settings, the wiring realities, and a few things nobody tells you until you've already made the mistake.

Before You Start: Know Which Series You're Holding

Danfoss pressure switches aren't all adjusted the same way. The two big families you'll run into for air compressor work are the KP series (like the KP1, KP15, KP35) and the RT series (like the RT116). There's also the MBC 5100, which is more common in refrigeration but shows up in industrial air and gas applications too.

Quick rule of thumb:

• KP series uses a single scale for cut-out pressure and a separate differential scale (the difference between cut-out and cut-in).
• RT series usually has a main pressure scale and a differential scale, but the arrangement and mechanical feel are different. The RT settings are often more fiddly.
• MBC series is compact and often has fixed or limited differential, which makes it less flexible for compressor duty cycles.

If you look at the Danfoss catalogue PDF for the specific switch (yes, I've got a folder of them), the adjustment diagrams are clear. The challenge is translating that into the real switch in front of you.

Step 1: Set the Cut-Out Pressure First

This is the pressure at which the compressor stops. On a KP switch, check the nameplate for the setting range. For example, a KP1 has a cut-out range of about 0.2 to 1.5 bar, while a KP15 runs higher. Rotate the main adjusting spindle (usually the one with the larger scale) to your desired cut-out pressure. Most industrial compressor applications run somewhere between 8 and 12 bar, but you need to match the switch's range to the compressor's actual duty.

Checkpoint: Turn the spindle clockwise to increase the cut-out pressure, counterclockwise to decrease. I've seen technicians invert this more than once—write it on the cover sticker if you have to.

Step 2: Set the Differential (Cut-In Pressure)

The differential is the gap between cut-out and cut-in. For a standard compressor duty cycle, you want a differential of around 1.5 to 2 bar. That means if your cut-out is 10 bar, the compressor restarts at 8 to 8.5 bar.

The differential spindle is the smaller one on the side. On KP switches, it's marked with a range, usually in bar. The factory setting might be at the minimum, which causes short cycling—starting and stopping too frequently. A compressor that short cycles burns more electricity and wears out the motor contactor faster.

Here's what I learned the hard way: the differential scale is not the cut-in pressure. It's the gap. So for a 10 bar cut-out and a 1.5 bar differential, cut-in is 8.5 bar. Seems obvious, but I've seen panels wired up with the differential set backwards—cutting in at 10 bar and cutting out at 8.5, which is the opposite of what you want.

Checkpoint: If you can't remember the order, think about start/stop, not stop/start. The main spindle sets the stop; the differential sets the gap before the start.

Step 3: Adjust on the Rig, Not the Bench

If at all possible, set the switch with the compressor running and the system pressurized. Static bench testing is okay for verifying that the switch opens and closes, but the actual cut-in and cut-out values will shift slightly due to system volume, pipe resistance, and the hysteresis of the switch mechanism itself.

Here's what I do:

1. Close the compressor's discharge valve, or isolate the pressure switch circuit.
2. Let the compressor build pressure until it cuts out. Record the pressure.
3. Bleed the system slowly until it cuts in. Record the pressure.
4. Compare to your target values.
5. Fine-tune via the main spindle for cut-out, and the differential spindle for the gap.
6. Repeat the cycle at least twice, because the first adjustment tends to drift slightly as the mechanism seats.

This process takes maybe ten minutes. The time I skipped it and trusted a pre-set switch from the parts shelf, the compressor cut out at 9.2 bar instead of 10.5. That meant the system never fully pressurized. The operator blamed the compressor, not the switch. Ten minutes would have saved me a lot of explaining.

Step 4: Wire It Correctly—Phase and Contact Ratings Matter

Most Danfoss pressure switches are single-pole, double-throw (SPDT), meaning they have a common (C), a normally open (NO), and a normally closed (NC) terminal. For a compressor, you want the NO contact to close when the pressure drops, pulling in the motor contactor. That sounds obvious, but I've come across installations where the wiring was reversed, and the compressor ran only when the system was already at pressure.

Check the contact rating on the switch against your contactor coil current. A KP pressure switch is generally rated for a decent inductive load, but if your contactor coil is drawing more than the switch's rating—check the micro-switch data sheet—use the pressure switch to energize a larger relay or contactor coil instead.

A common mistake is using a pressure switch that's rated for resistive load as if it can handle the inrush current of a motor contactor. The switch will weld internally over time. I've replaced KCW and ACB series units that failed this way. Not necessarily Danfoss's fault—just a selection error.

Checkpoint: Look at the switch rating, then look at your contactor coil. If the contactor coil draws more than the switch's AC-rated current (typically 4A or 10A, but verify on the label), add an interposing relay.

A Step Most People Forget: Verify the Capillary Tube and Pressure Connection

If your Danfoss pressure switch has a capillary tube (RT series often does, and some KP variants with remote sensing), you need to mount the switch body where it won't vibrate excessively. Vibration is one of the biggest killers of pressure switches on reciprocating compressors. The internal mechanism is sensitive; constant vibration can cause premature wear and setting drift.

Also, the pressure connection itself. Make sure you're using a proper sealant on the thread. PTFE tape is fine, but don't over-torque the switch body—the housing is often die-cast zinc or aluminium and can crack. I've seen replacements that were supposed to be plug-and-play fail because the technician cranked the fitting down with a wrench. The thread should seat firmly, not gorilla-tight.

Check for leaks at the connection with soapy water. A tiny leak at the switch port will cause the compressor to short cycle, because it bleeds off pressure slowly even when the system is shut off.

Air Filter Flow Direction: The Odd Connection No One Expects

Now, here's the weird part—and why I'm including it. When we're talking about compressors, the intake air filter matters just as much as the pressure switch setting. If an air filter is installed backwards, it restricts airflow, causing pressure drop, which can affect the duty cycle and the pressure readings you see at the switch.

I know the target keyword list for this article includes air filter flow direction and which way to install air filter, so let me say something clearly: the arrow on an air filter indicates the direction of airflow. It's that simple. If the element has a reinforced mesh side and a softer media side, the mesh side is usually the intake side—but the arrow is the authoritative guide. I've pulled filters out of housings installed backwards, and they always look distorted, sometimes collapsed.

For a Mann filter like the C 37 153 (which is a common cross-reference for some compressor intake elements), the flow direction is printed on the frame. Install it so the arrow points toward the intake manifold, away from the outside air inlet. If you ignore this, you'll get higher differential pressure across the filter, which doesn't directly change the pressure switch cut-in/cut-out (the switch reads downstream pressure), but it does make the compressor work harder and can lead to oil carry-over and higher discharge temperatures.

So when you're doing your compressor maintenance checklist, check the intake filter direction at the same time you verify the pressure switch settings. They're not connected electrically, but they're connected in terms of system performance.

How to Tell If You've Wired or Set It Wrong

Before you button everything up, run through these symptoms:

• Compressor doesn't stop at cut-out pressure: Most likely the main spindle is set too high, or the switch is wired in the wrong mode (using NC instead of NO, meaning it never opens the contactor).
• Compressor short cycles with no air demand: The differential is probably set too narrow, or there's a leak in the pressure switch capillary or connection.
• Compressor starts, but trips a breaker: Contact inrush current exceeds the switch rating, or the motor is bad—but also verify the switch doesn't chatter on start-up.

I have mixed feelings about the whole scenario of replacing a pressure switch on a compressor. On one hand, it's a cheap part, often under $100 depending on the series (a KP1 replacement is usually in the $60-90 range, an RT116 closer to $150—prices from typical suppliers as of early 2025, verify current costs). On the other hand, if it's set wrong, it can cost you a compressor head replacement or a burned-up motor. For a $400 compressor, that's a bad scene. For a $40,000 industrial compressor, it's a disaster.

Common Mistakes I've Seen (and Made)

• Adjusting with the compressor cover off and assuming the values from the switch label are exactly right. The label is a range, not a calibration certificate.
• Using the same pressure switch for high cut-out and low cut-in in a header system, without checking whether the switch's differential range is wide enough to cover both points.
• Skipping the pressure gauge calibration. If your test gauge is off by 0.5 bar, your precise adjustment is meaningless.
• Forgetting to close the drain valve on the separator before testing. You'll chase a drop in pressure that's not a switch problem at all.

One final note on trusting the adjustment scale. I've worked with about 200 pressure switch orders across different projects in the last six years, and the factory scale on the Danfoss switch is usually close—but not precise enough to skip a live adjustment test. The scale is for reference; the gauge is for truth.

If you're working with a different brand or an old switch that's been replaced before, your experience might differ. My experience is based mostly on KP and RT series in industrial compressor applications. For refrigeration or gas, the same fundamentals apply, but the setting ranges and safety requirements are stricter.

Prices I've cited are from my recent procurement orders—check with your local distributor for current numbers.