How Many KNX IP Routers Are Required in One Project?

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Introduction

One of the most common design questions in KNX projects is deceptively simple:

“How many KNX IP routers do we actually need?”

The wrong answer leads to overloaded lines, slow communication, and systems that are difficult to expand. The right answer depends on project size, topology, traffic flow, and future growth, not on a fixed device count.

This article explains how to decide the correct number of KNX IP routers in a project, using real design logic and site experience, not rules of thumb copied from brochures.

What a KNX IP Router Is Responsible For

A KNX IP router:

  • Connects one KNX TP line to an IP backbone
  • Routes group telegrams using multicast
  • Applies filtering between lines and areas
  • Acts as a backbone component, not just a programming tool

Because of this role, the number of IP routers directly affects:

  • System stability
  • Telegram traffic distribution
  • Commissioning speed
  • Future scalability

There Is No “One-Size-Fits-All” Number

There is no universal rule like:

  • “One router per X devices”
  • “One router per building”

These rules ignore how KNX systems actually behave.

Instead, IP routers should be planned based on:

  • Line segmentation
  • Physical layout
  • Traffic density
  • Operational independence

Start With the KNX Line Structure

A fundamental principle:

One KNX IP router serves one KNX line.

So the first question is not how many routers, but:

How many KNX lines does this project need?

When Does a Project Need Multiple KNX Lines?

You should split a project into multiple lines when:

  • Device count approaches practical limits
  • Cable distances become long
  • Floors or zones need isolation
  • Future expansion is expected
  • Traffic load becomes significant

Each of these situations naturally increases the need for additional IP routers.

Typical Scenarios and Recommended Approach

1. Small Apartment or Compact Villa

Characteristics

  • One electrical panel
  • Limited device count
  • Minimal future expansion

Design

  • One KNX line
  • One IP router (or even none, if only local ETS access is needed)

Note
In such cases, an IP router may be optional, not mandatory.

2. Large Villa (Multiple Floors)

Characteristics

  • Separate panels per floor
  • Distributed loads
  • Future extensions likely

Design

  • One KNX line per floor
  • One IP router per floor
  • IP backbone between floors

Result

  • Clean segmentation
  • Easier troubleshooting
  • Safe expansion

This is one of the most common real-world designs.

3. Apartment Building or Residential Tower

Characteristics

  • Repeating floor layouts
  • Common services
  • Long-term operation

Design

  • One KNX line per floor or zone
  • One IP router per line
  • Central IP backbone

Key Benefit
A fault on one floor does not affect others.

4. Hotel Project

Characteristics

  • Continuous operation
  • Commissioning during live use
  • Maintenance without shutdown

Design

  • One KNX line per floor
  • Separate lines for common areas
  • One IP router per line

Why This Matters
Hotels designed with too few routers often suffer from “random” issues during peak usage.

5. Office or Commercial Building

Characteristics

  • Departments change over time
  • Layout reconfiguration is common
  • Integration with BMS

Design

  • Lines based on functional zones
  • IP routers per zone
  • Backbone integration via IP

This allows IT-style scalability.

Traffic Matters More Than Device Count

Two projects with the same number of devices may need different numbers of IP routers.

Why?

  • Motion sensors
  • Scene logic
  • Frequent feedback updates
  • Central monitoring

High telegram traffic benefits from distributed routing, not centralisation.

Common Design Mistake: “One Router Is Enough”

Seen on many sites:

  • One IP router for the whole building
  • Multiple lines connected via TP couplers
  • Heavy backbone load

Problems

  • Slow response
  • Difficult troubleshooting
  • No redundancy
  • Hard expansion

Saving on routers often increases long-term cost.

IP Routers vs Backbone TP Couplers

In older designs, TP backbone couplers were used.

Today:

  • IP routing is faster
  • Distance is no longer a limitation
  • Commissioning is easier
  • Network diagnostics are better

For modern projects, IP routers are the preferred backbone solution.

Future Expansion: The Most Ignored Factor

Clients almost always say:

“This is final.”

Reality:

  • New rooms
  • New integrations
  • New expectations

Adding one IP router during design is cheap.
Adding it later is disruptive.

Always design with spare routing capacity.

A Simple Design Checklist

Ask yourself:

✔ Do different floors/zones need isolation?
✔ Will the project expand?
✔ Is telegram traffic expected to be high?
✔ Is IP backbone already available?
✔ Is long-term maintenance important?

If yes to most → more IP routers are justified.

Integrator’s Rule of Thumb (Practical)

  • One IP router per KNX line
  • One KNX line per logical zone or floor
  • Avoid centralising everything

This approach scales cleanly from villas to campuses.

Conclusion

The correct number of KNX IP routers is not about cost or convenience — it is about system architecture.

Well-distributed IP routing:

  • Improves stability
  • Simplifies commissioning
  • Reduces troubleshooting
  • Protects future expansion

In professional KNX projects, IP routers are structural components, not optional accessories.

Design them early, design them properly, and the system will reward you for years.

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