Should you get a System 3 or System 4 aircon?

System 3 vs system 4 aircon: the short answer

Choose System 3 to cool a living room and two bedrooms, and System 4 if a study or third bedroom also needs its own fan coil. Both run several fan coils from one outdoor unit; the difference is the extra room and what it asks of your ledge and piping.

In practice, start from the rooms you actually use, not the system name. A fourth fan coil makes sense when the study or third bedroom is used often enough to need cooling of its own. If that room sits empty most days, System 3 covers the rooms that matter and keeps the outdoor unit smaller.

Each extra fan coil adds a piping run, so plan the pipe routes before you settle on four. If you only need one or two rooms cooled, compare multi-split or single-split first.

Sizing each room

The system name tells you how many fan coils you get, not how big each one should be. In practice, size each fan coil to the room it cools, based on that room's size and how you use it. A small study and a large living room should not get the same fan coil just because they share one system.

Once each room is sized, look at the outdoor unit. It must be able to cover the combined load of every fan coil connected to it. Whether you choose system 3 or system 4, the order is the same: size the rooms first, then match the outdoor unit to their total.

The aircon BTU calculator helps you work out each room's need, and the guide to BTU by room size explains how room size affects the figure. Share the results with the installers you contact.

Energy Label and running cost

The National Environment Agency (NEA) Energy Label shows 1 to 5 ticks, and more ticks means a more energy-efficient model. The label covers household aircons and other appliances, and only registered models may be supplied in Singapore.

NEA's tips on buying energy-efficient appliances advise comparing the life-cycle cost, which is the purchase price plus the running cost over the years you use the appliance, not only the price tag. The same tips suggest choosing the more efficient model with more ticks.

Since 2022, split-type household aircons supplied in Singapore must use a refrigerant with a global warming potential (GWP) of no more than 750, as NEA explains in its page on reducing emissions from refrigerants. Goods supplied before then are not affected.

In practice, a fourth fan coil you rarely use adds cost without saving energy. If the extra room is only used now and then, weigh that against the convenience before you choose System 4.

Installation rules in an HDB flat

The Building and Construction Authority (BCA) says only trained air-conditioning unit installers certified by BCA can install an aircon unit outside or projecting from a building. The trained installer submits the installation report no later than 14 days after completion, as set out on BCA's air-conditioning safety page.

HDB's air-conditioner installation guidelines say aircons must be installed by a BCA-trained air-conditioning condensing unit installer, and an installation report must be submitted to the HDB Branch within 14 days after installation.

HDB also says condensing units cannot be placed along the common corridor, and the aircon ledge is only for the condensing unit. All aircon piping must run internally, pipe joints must be insulated, condensate must discharge into a floor trap in the flat, and there must be no hacking through reinforced concrete (HDB guidelines).

Flats with a 40 amp main switch, and flats upgraded under the Main Upgrading Programme (MUP) or Home Improvement Programme (HIP), do not need a permit for aircon installation, though HDB says they should still follow its guidelines. Flats with a 30 amp main switch have separate requirements.

See the guide to HDB aircon installation rules for more detail.

Which one fits your flat

Run through this checklist before you ask for quotes:

• Rooms used daily. Count the rooms you cool on most days. Three suggests System 3; four suggests System 4.
• Ledge space. Measure your aircon ledge and check that it fits the larger outdoor unit a System 4 needs.
• Pipe routes. Trace how the pipes will reach each room. Every extra fan coil adds a piping run and more trunking to plan around.
• Plans for the study. If the study may become a bedroom or a home office, decide now whether it needs its own fan coil.

If neither system matches how you live, the page on aircon types explains the other options.

Questions