Danfoss Pressure Switch FAQ: 7 Questions Worth Asking Before You Buy

This FAQ comes from someone who has paid for the mistakes so you don't have to.

I've been handling HVAC and refrigeration purchasing and maintenance for 11 years at a food processing plant, plus two rented cold storage units. I've made (and documented) five big enough mistakes to total roughly $4,300 in wasted budget. The 2018 reset-type order. The three-switch mess in 2017. A couple I'd rather leave off the record. Now I don't sign a purchase order without our team's pre-check list.

Below are the questions I get most from HVAC engineers, plant operators, and refrigeration integrators who've found the Danfoss pressure switch pages and haven't quite worked out which one to buy. Roughly ordered by what the mistake costs.

• What do I need to confirm before ordering?
• Danfoss KP vs the RT heavy-duty series — what's the difference?
• How do I set the switch without damaging it?
• Why do searches turn up oil filters, Sprint air filters, and Harbor Freight dual fuel generators?
• Where does a Danfoss pressure switch not belong?
• What documentation should be open before I wire anything?
• Why does the switch trip so often — and what do I check first?

What do I need to confirm before ordering a Danfoss pressure switch?

Four things: pressure range, reset type (auto or manual), connection size and material, and contact configuration. Confirm them against the code number on the old unit, not against a product name.

Pressure range isn't a single number. You size it on the setpoint plus the differential. A system running at 3.5 bar doesn't leave you margin on a 0–4 bar switch — setpoint plus differential pushes right up against the top and the switch trips on every start.

Reset type is where I lost money. In 2018 I ordered 38 switches for evaporators. Pressure range: correct. Reset: manual — and the reset button sat at the back of a cabinet nobody's arm reaches. Back to stock, re-order, $1,240 and a two-week delay on a line that was already behind.

Everyone told me to confirm the code number twice before approving. I didn't listen. Only believed it when the returns were sitting on my desk.

Contact configuration matters too — one compressor or two? Adding a relay later costs more than getting the contact block right on the purchase order. Write the code number on the PO. Not "KP 15." A code number.

What's the difference between the Danfoss KP series and the RT heavy-duty series?

Rule of thumb: KP for standard HVAC and refrigeration work. RT for when the equipment genuinely gets abused.

KP is the compact, cost-optimized line — case units, display cabinets, condensing units, small compressors on a steady cycle. This is what most people mean when they say "danfoss pressure switch."

RT is the heavy-duty series. Sturdier enclosure, wider adjustable differential, holds up better when something cycles constantly instead of a few times an hour. Industrial refrigeration, pump control where the pump hunts, large compressor cut-outs.

I've also made the mistake in the opposite direction. In 2019 I put an RT on a plain condensing unit because, well, it might come in handy. Cost about $60 more. It didn't earn an extra hour of service — the unit never needed the extra differential and it went back to a KP at the next maintenance cycle.

The question isn't "which is better." It's "how many times a month does this thing actually cycle?" If the answer is low, KP is usually the honest choice.

How do I set the pressure switch without damaging it?

Set the range screw first, then the differential screw, and watch a gauge rather than trusting the dial markings.

The differential is the smaller of the two adjustments, and it's the one people abuse. The minimum differential exists for a reason — go below it and the switch chatters around the setpoint. Sounds like a switch on its way out. It isn't.

In 2021 a tech set a differential to roughly 0.2 bar because the client kept asking for "tighter control." The switch cycled every few seconds and about two weeks later the contacts welded. We replaced the switch and I learned the actual lesson: if you need control that tight, you don't want a mechanical pressure switch. You want a transmitter and a controller.

Set it under real operating conditions, not with the system cold if the system runs hot. The setpoint drifts with temperature more than the datasheet suggests, and a setting done on a quiet Saturday doesn't survive Monday's production load.

Why do searches for Danfoss pressure switches turn up oil filters, Sprint air filters, and Harbor Freight generators?

Because search engines match on words, not meaning, and there's a lot of junk content fighting over every one of these queries.

Oil filter, air filter, pressure switch, generator — they share terms. Filter. Pressure. Switch. Load. Affiliate sites and content farms bundle them into the same page because it's cheaper than writing three honest pages. There are pages out there that claim to explain "danfoss pressure switch," "what is an oil filter," and "harbor freight dual fuel generator" all at once. None of it is trustworthy, and none of it is what you typed in.

To draw the line cleanly:

• Oil filter — filters metal debris and sludge out of engine oil as it circulates. Sits in the oil circuit. Never controls anything.
• Sprint air filter — a high-flow intake filter for engines, mostly bikes and cars. Filters air. Also never controls anything.
• Harbor Freight dual fuel generator — a consumer or temporary-power generator running on gasoline or propane. It has its own oil filter and air filter. Not one of those parts interchanges with an industrial pressure switch.

If you actually need an oil filter or a generator, this page isn't for you — go find a proper source. If you just got dragged in by a mixed search result, fair enough. Next question.

Where does a Danfoss pressure switch not belong?

I like these switches. Here's where I won't spec them.

• Safety-critical protection. Boiler protection, vessel cut-outs — anything where a failure to open means a burst or a fire — needs equipment built and certified for that job. The KP is a good industrial switch. It is not a safety instrument.
• Corrosive media against the bellows. Standard brass and stainless wetted parts have limits. If the media is ammonia or a specialty chemical, check wetted materials against the actual process, not against whatever was there before.
• High-pressure hydraulics. These are refrigeration and HVAC pressure switches. A 400-bar hydraulic system is not the place.
• Anything that needs an analog signal. Mechanical switches give you a dry contact. If the control system wants 4–20 mA, the problem isn't the model. It's the product category.

Around 80% of what I buy — condenser fan cycling, pump control, compressed air, cold storage — the switch does the job and does it for years. For the other 20%, the useful thing is to say so before the PO goes out.

What documentation should be open before I wire anything?

Three documents, pulled by code number: the datasheet, the wiring diagram, and the setting table for the pressure range and differential.

Danfoss keeps all of it on its product pages (store.danfoss.com), but you have to search the code number. Searching a series name like "KP 15" returns a dozen variants with different connection sizes, reset types, and contact blocks, and the first result is almost never yours.

The wiring diagram is not optional. We had an electrician wire a batch to the normally-open contact because that's the default in his head. The application needed normally-closed. Nothing broke. The compressor just wouldn't start for six hours while three of us traced the wrong fault.

Keep a folder on your phone with the PDF for every code number you install. Ours has been building for six years, and last month it saved a service call when the site team couldn't read the label on an eleven-year-old switch.

Why does my pressure switch trip so often — and what should I check first?

Two causes, and the order matters.

Check the differential first. If it's set below the manufacturer's minimum, the switch will chatter around the setpoint. Sounds like a failing switch. It isn't.

Then check the system for pulsation. Pump short-cycling, a worn compressor valve, a fast-acting valve near the sensing point — all of them make the gauge pointer shiver, and a mechanical switch has no way to ignore that. The fix isn't a new switch. It's a restrictor or a damping tube ahead of the sensing line.

I replaced three switches in 2017 before I worked that out. All three were fine. The pump was short-cycling, pressure was bouncing between roughly 4.5 and 6 bar, and the switches were doing exactly what they were told. A $35 damping restrictor fixed it in an afternoon. Between the three switches and two after-hours call-outs, I spent around $390 solving a $35 problem.

System first, device second. Reverse the order and you're not troubleshooting. You're just buying switches.