Split, multi-split or VRF: choosing a system for the building

The three architectures

Single split

One indoor unit, one outdoor unit, one pair of refrigerant pipes. The simplest arrangement and frequently the right one.

• Best for a single room that genuinely needs cooling – a west-facing bedroom, a home office with equipment, a conservatory.
• Completely independent. A failure affects one room and nothing else.
• Simplest to install and to fault-find.
• Cheapest per room for one or two rooms.
• Small refrigerant charge, usually below the threshold that triggers mandatory F-Gas leak checking – though the handling and certification requirements apply regardless of size.
• The disadvantage is outdoor units. Three rooms means three condensers, which is three positions, three sets of brackets and three things on an elevation.

Multi-split

Several indoor units connected to one outdoor unit. Each indoor unit has its own pipe pair back to the outdoor unit, or to a branch box.

Why choose one

• One outdoor position instead of several – frequently the deciding factor on a constrained site.
• Better appearance on the elevation.
• Lower total noise, with one fan and one compressor rather than several.
• Efficient at part load, because a single inverter outdoor unit serving four rooms of which two are running gently modulates down efficiently.

The diversity question

This is the whole design job. The outdoor unit’s capacity is usually less than the sum of the connected indoor units – deliberately, on the assumption that not everything runs at full output at once. That assumption is generally sound in a home.

Where it fails is on the hottest evening, when every bedroom wants cooling simultaneously. The outdoor unit then runs at its limit and every room gets a share rather than what it asked for. Getting the diversity assumption right is the difference between a system that satisfies and one that disappoints on exactly the days it is needed.

The limitations

• Single point of failure. Outdoor unit down means every connected room down.
• All indoor units in the same mode. Most domestic multi-splits cannot cool one room while heating another.
• Cannot be extended beyond the outdoor unit’s connected capacity, and only if the pipework was planned for it.
• More pipework than separate splits, all of it routed back to one point.

VRF and VRV

VRF varies the refrigerant flow to each indoor unit independently, with the compressor modulating to match the total demand. VRV is one manufacturer’s trade name for the same technology.

Heat pump versus heat recovery

A heat pump VRF system runs all indoor units in the same mode. Cheaper, and suits buildings whose zones all want the same thing.

A heat recovery VRF system runs some units cooling and others heating simultaneously, using branch controllers to route refrigerant. The heat taken from the zones being cooled is delivered to the zones being heated rather than rejected outside. In a multi-aspect office in spring – south side overheating, north side cold – that is genuinely efficient.

The honest test for heat recovery: does your building actually have zones wanting opposite things at the same time? A single-aspect unit does not, and a heat pump VRF or even a multi-split is better value.

What comes with it

• A large refrigerant charge, which means a high CO2e and therefore statutory leak checking obligations, and often mandatory automatic leak detection. This is a real ongoing cost and it should be understood before choosing – see F-Gas compliance.
• Connected capacity limits set by the manufacturer.
• Pipe length and height difference limits, which are genuine constraints in a multi-storey building.
• Branch controllers that need to remain accessible.
• BMS integration decisions, which are far cheaper made at design than retrofitted.

Choosing

1. How many rooms actually need it? One or two – single splits. Three to five in a house – multi-split. A multi-zone commercial building – VRF.
2. How many outdoor positions are available? One available position pushes you straight to multi-split or VRF regardless of anything else.
3. Will rooms want opposite modes at the same time? If yes, heat recovery VRF. If no, do not pay for it.
4. How critical is it? Where cooling failing genuinely matters – a comms room, a server – separate splits give resilience that shared systems do not.
5. Will it be extended later? Say so at design. Retrofitting indoor units onto a fully loaded outdoor unit is not possible.
6. What compliance burden is acceptable? A large charge brings statutory obligations that a small one does not.

The most common good answer for a Norfolk house wanting cooling in two or three bedrooms and a home office is a multi-split, sized with an honest view of which rooms will genuinely run together. The most common good answer for one hot west-facing room is a single split.