A branch box is a small distribution unit on some multi-split systems. One outdoor unit feeds the box, and the box feeds several indoor units, each with its own pipes. Not every multi-split has one. Some outdoors simply offer several pairs of ports. The box needs a reachable place, pipe lengths inside the manual, and sometimes a drain. It is a piece of the refrigerant system, installed by an F-gas certified company, not a loft accessory you add later.
Ports on the outdoor unit, or a box nearer the rooms
A single split is one indoor and one outdoor. A multi-split serves several rooms from one outdoor. On some ranges the refrigerant is split at the outdoor unit. On others it is split in a branch box or branch selector closer to the rooms. The box lets several modest pipe runs start from a cupboard, a ceiling void or a loft, which can look tidier than five pairs leaving one condenser. The indoors still have their own fans, filters and setpoints. The box does not make two rooms one zone. It only shares the outdoor capacity under rules the manufacturer prints for simultaneous use. Energy Saving Trust's air-to-air guidance is the domestic context for that outdoor unit.
Lofts, landings and bedroom cupboards
Lofts, landing cupboards and ceiling voids are the usual candidates. The spot needs safe access for service, enough clearance, a sound level you can live with, and a drain if that model produces condensate. Burying the box above a plasterboard ceiling with no hatch is how a simple fault becomes a builder's job. Noise is a low hum or a valve movement, not a second condenser, but it is still noise. A bedroom cupboard is often a poor choice. A landing cupboard beside the airing cupboard is a more common yes, if the pipe limits work. The box must stay accessible, and pipe lengths on every leg must suit the manual. An F-gas certified company installs it. GOV.UK publishes that company certification.
Simultaneous capacity and a domestic charge
Ask which indoors connect, whether a branch box is required for that outdoor, where it will sit, and the pipe length of each leg. Ask for simultaneous capacity: the outdoor cannot deliver every indoor's maximum at the same second if the combination exceeds its limit. A house branch box is still a residential multi-split. The quote should say multi-split and the outdoor model, or name the commercial system if that is what you are buying. Where cooling output is large enough, the GOV.UK air conditioning inspections page is a separate duty from the domestic combination chart. As a 2026 illustration, specifying a branch-box architecture instead of two simple single splits can add equipment and labour from the high hundreds into the low thousands of pounds including VAT, depending on access and how many rooms you connect. Compare that with two outdoor units only after both drawings exist.
Frequently asked questions
Do all multi-splits have a branch box?
No. Some outdoor units provide a pair of ports per room and need no box. The manufacturer's combination chart decides. Ask the quote to say which architecture you are buying.
Can the branch box go in the loft?
Often, if there is a boarded route, a hatch, working clearance, and a drain where the model needs one. An unboarded loft with the box laid on insulation is not a service position.
Is a branch box the same as VRV?
No. A branch box on a house multi-split is a residential distributor. VRV is Daikin's commercial variable-refrigerant system. Keep the model names straight on the quote.
Does the box make two rooms one zone?
No. The indoors still have their own fans, filters and setpoints. The box only shares the outdoor capacity under the manufacturer's rules for simultaneous use.
Does a domestic branch box automatically create a commercial leak-check regime?
No. Larger charges bring operator duties that depend on carbon-dioxide equivalent. A small domestic multi-split may sit in a different band from a rooftop VRF. Check the current GOV.UK guidance for the system you actually install.
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How this guide was prepared
This guide was written by the Local AC Installers Editorial Team. We compare official guidance, legislation and established consumer information, separate general information from project-specific advice, and show publication dates and sources. We do not accept installer accreditation claims at face value.