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System 3 and System 4 are the two most popular aircon configurations in Singapore homes. Both use a single outdoor condensing unit to serve multiple indoor fan coil units. The difference is how many indoor units the system supports. But that one-unit difference affects cost, cooling capacity, outdoor unit size, and electricity consumption. This guide helps you decide which is right for your home.
| System 3 | System 4 | |
| Indoor units | 3 fan coil units | 4 fan coil units |
| Outdoor units | 1 condensing unit | 1 condensing unit (larger) |
| Installation cost (labour only) | $2,200 to $3,800 | $2,800 to $4,500 |
| Full package (units + install) | $3,900 to $7,000 | $5,000 to $9,000 |
| Common BTU config | 9k + 9k + 12k or 9k + 9k + 18k | 9k + 9k + 9k + 12k |
| Typical property | 3 to 4-room HDB, 2-bed condo | 5-room HDB, 3-bed condo, terrace |
| Outdoor unit size | Smaller, fits most standard HDB ledges | Larger, check ledge dimensions first |
| Electricity consumption | Lower baseline | Higher due to larger compressor |
System 3 is the right setup when you have three rooms that need cooling and no realistic need for a fourth indoor unit. This covers the majority of 3 to 4-room HDB flats and most 2-bedroom condominiums in Singapore.
A typical System 3 configuration for a 4-room HDB flat:
If you only need aircon in 2 bedrooms and the living room, System 3 is sufficient and the most cost-efficient option. Do not install System 4 just because the flat has 4 rooms. If the fourth room is a study, storeroom or rarely used guest room, the added cost is not justified.
System 4 makes sense when you have four rooms that genuinely need cooling on a regular basis. This is most common in 5-room HDB flats, 3-bedroom condos and terrace houses.
A typical System 4 configuration for a 5-room HDB flat:
The key question is not ‘how many rooms do I have?’ but ‘how many rooms will realistically have the aircon running at the same time?’ If three rooms are always in use and the fourth is only cooled occasionally, System 3 plus a portable unit for the fourth room may be cheaper and more practical.
System 4 outdoor units are physically larger than System 3 units. Not all HDB aircon ledges can accommodate the larger unit while maintaining the required 30cm clearance on all sides. Ask your installer to confirm the ledge dimensions during the site survey before committing to System 4.
A System 4 draws more power than a System 3. Older HDB flats may need an electrical capacity upgrade to support the larger system safely. Your installer should check the distribution board during the site survey.
A System 4 has a larger compressor that consumes more electricity, even when not all 4 indoor units are running simultaneously. If you genuinely only cool 3 rooms most of the time, a System 3 will cost less to run every month for the life of the system.
For full installation pricing across all system types and what is included in a standard quote, read our complete aircon installation guide for Singapore.
Cool Channels Engineering is a trusted aircon service provider in Singapore with over 25 years of experience installing System 1 through System 5 configurations across HDB, condo and landed properties.
See our aircon installation service page for current availability and packages, or WhatsApp us at +65 9003 9098 for a free site survey.
Not easily. It requires replacing the outdoor unit, adding piping for the fourth indoor unit, and potentially upgrading the electrical supply. It is significantly cheaper to install the right system from the start.
System 3 is the standard and most cost-effective choice for a 4-room HDB flat. System 4 is only worth it if the fourth room will have aircon running regularly.
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