Multi-split systems

Why choose one

• One outdoor unit instead of several. On a terraced house, a property with a small garden, or anywhere with a single viable condenser position, this is frequently the deciding factor.
• One set of services. One electrical supply, one set of foundations or brackets, one set of anti-vibration mounts.
• Better appearance. Four condensers on a rear elevation looks like plant. One looks like a unit.
• Efficiency at part load. An inverter outdoor unit serving four rooms, of which two are running gently, modulates down efficiently. Two separate small units both running at minimum are less efficient in aggregate.
• Lower total noise, because there is one fan and one compressor rather than several.

The trade-offs, honestly

A multi-split is a shared resource, and every characteristic of it follows from that.

Diversity and simultaneous demand

The outdoor unit’s capacity is usually less than the sum of the indoor units connected to it – that is deliberate and it is called diversity. The assumption is that not every room will demand full output simultaneously, which is generally true in a home. Where it is not true – four bedrooms all wanting cooling on the same hot night – the outdoor unit runs at its limit and every room gets a share rather than what it asked for.

Getting the diversity assumption right is the whole design job. Too aggressive and the system disappoints on precisely the hottest evenings. Too conservative and you have paid for capacity that never gets used.

Single point of failure

If the outdoor unit fails, every room loses cooling. With separate systems, one failure is one room. Worth weighing where cooling is genuinely important – a server or comms room should not share an outdoor unit with a bedroom.

Simultaneous heating and cooling

Most domestic multi-splits cannot cool one room while heating another – all indoor units must be in the same mode. If one person wants heat in October while another wants cooling, a multi-split will not do it. Systems that can are a different and more expensive class of equipment – see split, multi-split or VRF.

Pipework

Every indoor unit needs its own refrigerant pipe pair back to the outdoor unit, or to a branch box. That is a lot of pipework, and routing it through an occupied house is the practical constraint that most often determines what is achievable.

Designing one properly

1. Load per roomHeat gain calculated for each room individually – glazing, orientation, occupancy, equipment. Not floor area.
2. Realistic simultaneityAn honest conversation about which rooms will genuinely be used at the same time. This is what sets the outdoor unit size.
3. Indoor unit selectionType and capacity per room. They do not all have to be the same, and a bedroom and a living room usually should not be.
4. Outdoor unit selectionSized against the simultaneous load, with the connected-capacity limits checked – every manufacturer sets a maximum ratio of connected indoor capacity to outdoor capacity.
5. Pipe routingPlanned as a whole, including branch positions, pipe lengths and the height difference between indoor and outdoor units. Both have limits and both affect performance.
6. CondensateEvery indoor unit needs a drain. Gravity wherever possible. This frequently constrains indoor unit positions more than anything else.
7. ElectricalOne supply, correctly sized, with the interconnecting cabling between units run to the manufacturer’s specification.

Is it right for you?

For a typical house wanting cooling in two or three bedrooms and perhaps a home office, a multi-split is usually the right answer. For a single hot west-facing room, a single split is simpler, cheaper and more robust.