Fermax systems explained: 4+N, VDS and DUOX Plus — which one do I have?

Fermax has been manufacturing door entry systems in Valencia since 1949, and three generations of its technology coexist in the city's buildings today. Knowing them is the key to understanding what can be repaired, what can be replaced and what is worth upgrading:

4+N: the veteran that keeps going

If your building dates from the 60s, 70s or 80s and the intercom has never been replaced, you almost certainly have 4+N. It works well until the wiring ages: since every flat needs its own call wire, faults from crossed wires or oxidised joints are common in older buildings in Cabanyal, Benimaclet or central Valencia.

VDS: the digital middle generation

In the 90s Fermax went digital with VDS. The call signal travels encoded, each handset carries a programmed address and the installation gains reliability. Many Valencia buildings constructed between 1995 and 2015 run VDS, often with video monitors.

DUOX Plus: today's standard

DUOX Plus simplifies everything to the extreme: just 2 wires with no polarity for power, audio, video and data. That makes it possible to renovate old installations reusing the existing wiring, add WIFI monitors and open the door from your phone.

You don't need to dismantle anything: three clues — the age of the building, the wires reaching the handset and the look of the unit — get it right in the vast majority of cases:

• Look for the model reference printed on the handset or monitor casing (for example TELEFONO 80447, LOFT VDS, VEO XS DUOX): with that reference the system is identified instantly.
• If the lobby has been recently refurbished and the monitor has a colour screen and menu, it is most likely DUOX Plus.
• If you are torn between VDS and DUOX, check the connection terminals: VDS usually marks +, -, L; DUOX Plus only B, B (a line with no polarity).

Here lies the most expensive mistake we see over and over: buying a handset online "because it's Fermax" and discovering it doesn't work. Two units sharing a brand does not make them compatible, because each system speaks a different "language":

• Within the same system, yes: a modern 4+N phone (Citymax series or a compatible universal handset) can replace an old 4+N one as long as the wiring is respected.
• Across systems, no: a VDS terminal won't work in a 4+N or DUOX Plus installation, and vice versa. Connecting it usually won't damage it, but it won't get a signal.
• VDS and DUOX Plus need programming: connecting the new terminal isn't enough; it must be assigned the flat's address, something we do in the same visit.
• Discontinued models: for many old units there is no longer an original spare, but a current compatible substitute almost always exists. We cover this in depth in what to do with an old Fermax intercom with no spare parts.

If your community is considering renewing the door entry system, the short answer in 2026 is: DUOX Plus. It is the platform Fermax's current catalogue is built around and the one that offers the most value per euro:

• Reuses the wiring: running on 2 non-polarised wires, in most older buildings only the panel, power supply and terminals are replaced — no chasing walls, no building work.
• Scalable: from a single-family home to hundreds of flats on the same technology, including concierge units and multiple entrance panels.
• Video and mobile: VEO and VEO XS monitors and, with WIFI or the right module, call forwarding to your phone so you can see who's at the door wherever you are.
• Stronger warranty: 5 years of Fermax warranty on DUOX Plus equipment installed in communities since 2023.

Does that mean every 4+N or VDS system should be retired? Not necessarily. If your installation works and only one element fails — for example, a handset that has stopped ringing, as we explain in this article about intercoms that don't ring — the sensible move is to repair or replace that part. A full upgrade pays off when faults keep recurring, spares are scarce or the community wants video and smartphone door release.