Danfoss Pressure Switch FAQ: What I've Learned From Buying Them for a Living

I'm not an engineer, and I'm not going to pretend otherwise. I'm the office administrator who orders MRO parts for a 40-person HVAC and refrigeration service company. That means I'm the one on the phone when a technician needs a Danfoss pressure switch before the end of the day. When I took over purchasing in 2021, I had to learn the hard way that a pressure switch isn't just 'the thing with the wires sticking out.' It's a safety component, and the part number matters.

If you've landed here from a search like 'why is there oil in my air filter' or 'changing air filter in house,' I'll get to those too. Here are the questions that show up in my inbox, roughly in the order I'd want them if I were you:

• What's the difference between the Danfoss KP series and the RT series?
• What should I order when someone asks for a Danfoss KP1 pressure switch?
• Are Danfoss RT pressure switches really heavy duty?
• Why is there oil in my air filter?
• What does 'changing air filter in house' have to do with pressure switches?
• What about that 'oil filter suppressor adapter legal' search?
• Should I buy a genuine Danfoss switch or a cheaper substitute?
• Where do I find a wiring diagram or installation manual?

1. What's the difference between the Danfoss KP and RT pressure switches?

The KP series is the refrigeration workhorse. The Danfoss KP1 pressure switch is one of the parts we order most, and it's a compact single-pole, double-throw (SPDT) switch used for pressure control in commercial refrigeration and HVAC equipment. The RT series is the heavy-duty line. It's built for industrial plants, marine engine rooms, and other places where the environment is harder than a clean rooftop unit.

From a purchasing seat, the difference is practical. The RT series costs more, it's physically bigger, and it's usually what our techs ask for when vibration or corrosion keeps killing a standard switch. If the switch is for a clean commercial unit, a KP model is usually fine. If it's for a hydraulic skid or a ship, I'd order an RT and not argue with the tech.

I can only speak to what our techs tell me. If you're designing a safety circuit, the Danfoss datasheet is the real authority, not a blog.

2. Danfoss KP1 pressure switch: what should I order?

Don't assume all KP1s are interchangeable. The full part number matters because pressure range, differential, connection thread, and electrical ratings vary. If you type 'danfoss kp1 pressure switch' into a supplier search and grab the first result, you might be back on the phone later explaining why the compressor is short-cycling.

When I place an order, I ask for the model code printed on the old switch and, if possible, a photo. The photo is not a substitute for the code, but it helps me catch obvious mismatches before the part goes on a truck. Danfoss includes the wiring diagram in the installation guide for the KP series. I keep those PDFs on my desktop because it saves all of us a phone call.

3. Are Danfoss RT pressure switches really heavy duty?

Yes. It's not just a label. In the Danfoss product literature at danfoss.com, the RT range is described as suitable for heavy-duty industrial and marine applications. The enclosure is more rugged, the switch handles vibration better, and the scale is easier to read in a dark panel.

One customer has an RT switch on a hydraulic oil system that has been running for, I don't know, 15 years. Maybe 12. I'd have to pull the maintenance records for the exact year. Nobody is in a hurry to replace it.

If you're searching for 'danfoss rt pressure switch heavy duty,' you'll get several models. They're not all interchangeable. Thread size, pressure range, and electrical ratings matter. Check the datasheet, or ask a distributor to confirm.

4. Why is there oil in my air filter?

This one usually comes from a car owner or a homeowner, not from someone servicing a refrigeration rack. In a car, oil in the air filter is often blow-by from the PCV system. In a furnace, oil around the filter area can point to a failing blower motor bearing or something more serious. Neither one is a pressure switch issue.

But there's a crossover. If a furnace pressure switch keeps tripping and there's oil near the filter, the switch is trying to tell you something about the system. Replacing the pressure switch without figuring out why it tripped is a good way to replace it again next month.

5. What does 'changing air filter in house' have to do with pressure switches?

Not much on the residential side, honestly. Most home HVAC systems don't use a pressure switch to monitor filters. Commercial air handlers sometimes do; that's where a differential pressure switch watches filter load and triggers an alarm when the filter is getting plugged.

So if your search history includes 'changing air filter in house,' here's the closest useful thing I can tell you: change it on a schedule. A clogged filter changes airflow, airflow changes pressure, and pressure changes make pressure switches open or close. Sometimes the switch is doing its job and the real problem is a $10 filter.

6. What about that 'oil filter suppressor adapter legal' search?

If that's what you typed, you're not the first to land here from a search that has nothing to do with pressure switches. I am not going to guess about something with legal consequences. That's not a cop-out; it's the responsible answer. Please talk to a real lawyer who handles that area.

For everyone else, yes, that query shows up in our analytics, and no, I don't have a better ending for that story. Above my pay grade. Moving on.

7. Should I buy a genuine Danfoss pressure switch or a cheaper substitute?

I've been the buyer who tried to save money on this. In 2022 I ordered a lookalike KP1 that was about 40% cheaper than the Danfoss part. It looked fine and arrived fast. The tech installed it on a small display case, and it failed six weeks later. Product loss plus the emergency service call erased the savings, and I had to explain to my operations manager why I'd risked a paying customer's equipment to save $38.

From the outside, a pressure switch looks like a commodity part. The reality is it's a safety component. If it fails, the system can fail. For a service company, every part we install carries our name. If you ask me, that's worth more than the price difference. Put another way: I'd rather explain to finance why I bought the OEM part than explain to a customer why their compressor died.

8. Where do I find a Danfoss pressure switch wiring diagram or manual?

Go to danfoss.com and search the product code. Current pressure switches have a datasheet and installation guide, and where relevant a Declaration of Conformity. Download the PDFs before you call a distributor. You'll get a faster answer if you already know the model code.

For an older switch, look for the type stamped on the housing, like RT 116. Search that type plus 'manual.' If that doesn't work, email a Danfoss distributor with a clear photo. In my experience, they can usually identify it from the stamp.

One last thing: if the part is going into a system handover, keep those PDFs in the project file. The commissioning engineer will ask for documentation. It's easier to click through a folder than to explain why the paperwork is missing.