Mobile Split Units: Quiet Cooling With the Compressor Outside
The compressor sits outside and the room stays quiet. Why a mobile split AC needs no drilling and no permit in almost any rented flat.
5 min read · Updated July 2026
Want to cool your flat without tearing open a wall or asking for a permit? A mobile split air conditioner is built for exactly that. The loudest part, the compressor, sits outside on your balcony or windowsill, and only a quiet indoor unit stays in the room. No drilling, no fixed refrigerant lines, no technician. Here is how it works and what is actually allowed in a rented flat.
What is a mobile split unit?
A split system has two parts: an indoor unit that cools the air, and an outdoor unit that holds the compressor. In a classic fixed split, both parts are mounted permanently and connected by refrigerant lines that run through a drilled wall. A mobile split keeps everything movable. The two units are joined by a flexible, factory-filled line that you run outside through a gap in the window.
- The indoor unit stands freely in the room, with no wall mounting needed
- You place the outdoor unit with the compressor on a balcony, terrace or windowsill
- The refrigerant circuit is hermetically sealed and pre-charged, so nothing has to be filled on site
- A window sealing kit closes the gap so warm air stays out
Why mobile split units are so quiet
The compressor is the noisiest part of any air conditioner. In a monoblock it sits inside the same housing and hums right next to you in the room. In a mobile split it sits outside, so most of the noise stays out of the flat. Indoors you usually hear only the soft sound of the fan.
Good models run at roughly 40 to 50 decibels at the indoor unit, about the level of a calm conversation. That makes them far more bedroom-friendly than a monoblock, which often reaches 55 to 65 decibels.
No drilling and no permit: the legal side
A mobile split does not change the building fabric. You drill no holes, mount nothing to the facade and lay no fixed lines. That is the legal difference from a permanently installed split system.
- No technician required: because the refrigerant circuit is factory-sealed and pre-charged, you do not need a certified refrigeration engineer. Installing a fixed split, by contrast, is reserved for firms certified under the EU F-Gas Regulation (in Germany via the Chemikalien-Klimaschutzverordnung).
- No structural change: a freestanding device you can remove at any time normally does not count as a permit-requiring modification of your rented flat.
- Fixed splits almost always need the landlord's consent and often a building permit, because a wall opening and an outdoor unit alter the substance and appearance of the building.
What to watch out for
- Window seal: without a proper seal, warm air flows back in, so a sealing kit is a must.
- Outdoor unit placement: a balcony or a stable windowsill works well. Make sure nothing can fall down and that escape routes stay clear.
- Consideration for neighbours: the compressor makes noise outside. Do not put it directly under a neighbour's bedroom window, and respect quiet hours at night.
- Refrigerant: modern units often use R290 (propane) with a very low global warming potential, which is greener than older refrigerants.
- Power use: check the energy label. A split usually runs more efficiently than a monoblock.
Mobile split, monoblock or window unit?
All three cool without a permit as long as you do not drill. They differ in noise, efficiency and where they fit.
- Monoblock: everything in one housing with a single exhaust hose out the window. The most flexible option, but louder and a little less efficient.
- Window unit: sits in the window frame and saves floor space, but does not fit every window.
- Mobile split: quieter and more efficient because the compressor stands outside, but it needs a spot for the outdoor unit.
A mobile split sits between a simple monoblock and a fixed split system. It is quieter than the monoblock and far less hassle than a permanent install. For many renters in Berlin and Brandenburg it is the most practical way to get through a hot summer without asking the landlord. If you would rather rent one seasonally than buy, these units are now also available as a summer subscription with delivery and pickup.
FAQ
Does a mobile split unit need the landlord's permission?
Usually not. As long as you drill nothing, fix nothing permanently to the facade and can remove the device again without a trace, it does not count as a structural change. It is different for fixed split systems, which almost always need the landlord's consent.
Does a refrigeration technician have to install a mobile split?
No. The refrigerant circuit is hermetically sealed and pre-charged at the factory, so you may set it up yourself. Only fixed splits, where the lines are filled on site, must be installed by a firm certified under the F-Gas Regulation.
How loud is a mobile split unit indoors?
Because the compressor sits outside, indoors you mostly hear only the fan. Good models run at about 40 to 50 decibels, which is noticeably quieter than a monoblock.
Where do I put the outdoor unit?
On a balcony, terrace or a stable windowsill. Make sure it stands safely, nothing can fall down, escape routes stay clear and neighbours are not disturbed at night.
Want cool without the hassle?
KlimaLegal rents you a permit-free AC for the summer — delivered, set up, and collected in September. No deposit.
Secure your spot