Short answer
Modern two-wire systems send power, speech, video and door release over two cores. If there is a two-core cable running from the entrance to each home, replacement without cutting into walls is usually possible. With systems that have one core per function or with damaged cabling, new cable does have to be pulled, and then the riser shaft decides what is possible.
Who this article is for For homes and buildings with an intercom that is due for replacement. Testing the cabling is work for the electrician.
Why the cable is the story
Replacing the handsets is an hour's work. Pulling cable through an existing building is days of work and causes disruption. That is why replacement always comes down to the question: what is there and is it usable?
What is usually there
| What you find | Usable for a new system |
|---|---|
| Two-core cable per home | yes, even for video with a two-wire system |
| Bundle with four to eight cores | yes, two good cores are enough |
| Coax for video plus cores for audio | often yes, depending on the brand |
| Network cable | yes, and then much more is possible |
| Cores with too small a cross-section over a long distance | sometimes not, voltage drop |
What I measure before anything is ordered
- continuity of each core, from the entrance to the home
- insulation between the cores, because old damage causes noise later
- length and cross-section, with a view to voltage drop
- whether the cores are free, so not looped halfway to something else
An hour of measuring prevents a wrong order worth thousands of euros.
That is an hour of measuring that prevents a wrong order.
Video over old wires
It sounds unlikely but it works: modern two-wire systems modulate the video signal onto the same two cores as the power supply. That is exactly why replacement is so much less disruptive today than ten years ago.
The condition is that the cores are clean and that the length stays within the range of the system. In large blocks an amplifier or a distributor per floor is sometimes added.
If new cable has to be pulled after all
Then the riser shaft decides how it turns out. If there is an empty conduit, it is a day's work. If everything is full or there is no shaft, it becomes surface trunking in the stairwell or a wireless solution per home.
Wireless is possible these days, but bear in mind that each home then depends on its own wifi. In an owners' association (VvE) that causes hassle with faults: who is responsible if it does not work for one resident.
What I advise
When replacing, choose a system whose parts you can still get in ten years, and record what is there. At handover I supply a diagram of the cabling and connections, so the next engineer, me or someone else, does not have to measure everything again.
From my own work
In a building in Maastricht there was an old bundle with seven cores to each home. Six of them were no longer needed, but two of them were still fine. The new system could work with those. The rest of the bundle has been neatly terminated and stays in place as a spare, because one day someone will have a use for it.
When to call an electrician
- You do not know which cable is there and whether it is still good
- The old installation is partly faulty and the supplier no longer exists
- You want video while there is only audio now
I test the existing cabling before anything is ordered. Request a quote.
Frequently asked questions
How do I know which cable is there?
By testing it: continuity, insulation between the cores, length and whether the cores are free. That is an hour's work and prevents a wrong order.
Can part of an old installation stay?
Sometimes. A usable riser cable almost always stays in place; handsets and control unit come out.
What if there is no usable cable?
Then I look at an empty conduit in the shaft, surface trunking in the stairwell or a wireless solution per home.
Further reading
Doorbell transformer: what it is for and why your doorbell is not working
If your doorbell is not working, it is usually the push button and not the transformer. Search in this order and you will usually have it within a quarter of an hour.
The intercom is not working: troubleshooting in four steps
If it does not work in one home, the problem is in that home. If it works for nobody, the problem is at the entrance or the power supply.
Intercom in a block of flats: how it is put together
In a block of flats the work is in the riser cable and the door entry panel, not in the handsets. That decides whether replacement turns out easier or harder than expected.
Questions and comments
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