Multi-split branch box: the valves above your ceiling

A branch box is a steel case hung inside a ceiling void, with one refrigerant pipe pair arriving and several leaving. Mitsubishi Electric builds a residential version as the PAC-MK series, catalogued in a three-branch type and a five-branch type. Its installation manual is explicit that the box is for indoor use.

Indoor use is a hard line in that document. The box is specified for inside the building, and Mitsubishi sells a separate cover, the PAC-AH350CVR-E, for the case where one has to go outdoors. An aircon ledge is not the default home for this equipment, and treating it as one puts the box outside what it was built for.

Where it goes indoors is settled by the manual as well. The instruction is to install it above the ceiling of a corridor or bath room, where persons are not regularly there, and to avoid installing at around the centre of the room. The same page tells the installer not to put it near bedrooms.

Sound is the stated reason. Refrigerant travelling through the pipework may sometimes be audible, so the siting rule is there to keep that away from the rooms people sleep in. A box positioned only for a tidy pipe route can end up over a bed, and the manual is trying to prevent exactly that.

The rule has a second effect that nobody plans for. It puts aircon equipment in the part of the home least associated with aircon. A corridor ceiling carries no grille, no controller and no visible sign of what is above it, so the household has nothing to learn the box from.

Each branch gets a valve and a sensor of its own. Mitsubishi's technical and service manual covers the MXZ outdoor units alongside the PAC-MKA branch boxes. It lists a linear expansion valve and a gas pipe thermistor against every lettered port. Both wire back to the branch box controller board, at connectors labelled LEV-A through LEV-E.

That valve is driven, not sprung. The manual describes it opening and closing through a stepping motor after receiving a pulse signal from the board, with valve position changing in proportion to the number of pulses. The metering for a bedroom can therefore sit above a corridor, several metres from the bedroom itself.

Shared parts sit beside the per-branch ones. There is a single controller board, fuses, rotary switches for setting the address in ones and tens, and LEDs the manual describes as being for service. Those shared parts are why a box in a ceiling needs an electrical supply at all.

How that supply arrives is the detail most owners find surprising. Wiring connecting branch box and outdoor unit, and branch box and indoor units, functions as both power supply and signal cable, the manual says. Power reaches the box either off the outdoor unit or from a separate supply with an isolator of its own.

• •A linear expansion valve on each lettered port, driven by pulses from the board.
• •A gas pipe thermistor on each port, so every branch is sensed separately.
• •One branch box controller board, with fuses and service LEDs on it.
• •Rotary address switches, which means the ports are commissioned and not self-configuring.
• •Terminal blocks carrying power and signal on the same wiring to each indoor unit.

The alternative is what most Singapore flats already have. A bank of connections sits at the outdoor unit, with a dedicated pipe pair running from there to each indoor head, sharing nothing along the way. How a multi-split aircon is put together, and why one outdoor unit ends up serving several rooms, is settled ground on its own page.

Mitsubishi makes the case for the other arrangement in its own words. The service manual says the branch box, used with a compact outdoor unit, can successfully realize a long distance piping for large houses. One long run leaves the outdoor unit, and the short runs start at the box instead of at the ledge.

Appearance is part of the same argument. The manual's typical combination example shows the box installed indoors and notes that only piping is then required between the outdoor unit and the indoor units, offering a fine external view. A single pair leaving a ledge reads very differently from five.

Control is the third reason, and the one owners never hear stated. Equipped with a microcomputer, the manual says, the branch box can translate the transmission signal of indoor units to achieve the optimum control. So the box is interpreting instructions, not only dividing a pipe.

Access is written into the requirement, not left to the trade. The installation manual instructs that the branch box go in a location which facilitates servicing and maintenance, then adds a parenthesis: ensure that the required maintenance hole or service space is available. A later edition puts it more bluntly, saying not to install where maintenance cannot be carried out.

What earns an owner's attention is that the manual grades the result. It does not only ask for an opening. It states what remains possible at each clearance, which turns a decision made on a building site into a permanent limit on what any future repair can reach.

The figures below belong to one model series. They are not an industry rule. For the PAC-MK30BC and PAC-MK50BC, the recommendation on the piping side is a minimum of 200 mm. Go under that and the manual says one thing plainly. Exchanging the branch box itself through a maintenance hole becomes difficult, and only a PCB, linear expansion valve coils and sensors can still be exchanged. Where that piping-side dimension is 450 mm, an opening is wanted at the board side instead, with a minimum of 300 mm. Below that figure, only the PCB can be exchanged.

Everything after that follows from one afternoon's work. The clearance built in on the day the ceiling closed decides which repairs stay available for as long as that ceiling stands, and no later decision recovers it. Who provides the opening, and who pays to make good around it, is a separate argument that the false ceiling access panel page works through properly.

Three questions settle it, and not one of them sends anybody near a ceiling. Whether the system has a box of this kind, which ceiling it sits above, and what was left around it when that ceiling was closed.

Whether a system has one at all is settled by reading the model designation off the outdoor unit, together with the indoor units it serves. This architecture turns up on larger homes, on long pipe routes, and where ceiling-concealed indoor units are in the mix. A supplier working from that model number can say whether a branch box belongs to the combination being quoted. Larger premises running a vrf system are a different class again, and that has its own page.

Location is meant to be recorded, in a box most installers leave empty. Mitsubishi's service manual prints a blank table for the combination of indoor units. It instructs that their locations and model names be entered, because it is necessary for service and maintenance. Filled in, that table names which room hangs off which lettered port.

Port lettering is a commissioning question, and it gets cheaper the earlier anybody asks it. The manual instructs that indoor units be connected in the order A through E, and that refrigerant pipes and connection wires go to ports marked with matching letters. A room wired to one letter and piped to another is an error worth finding early.

• •Whether the quoted combination includes a branch box, and how many branches it carries.
• •Which ceiling it is going above, and whether that ceiling is over a corridor or a bedroom.
• •What clearance is being left around it, checked against that model's own manual.
• •Which trade provides the opening, and whether it is sized to take the box out or only its board.
• •Which room is connected to which lettered port, written down and handed over.
• •Where the box takes its power from, and whether it has an isolator of its own.