Is a System 4 enough for a BTO, or do you need a System 3 + System 2?
For a BTO with an open living, dining and kitchen area, a System 3 for the bedrooms plus a System 2 for the living and dining usually cools better than one System 4. A single living room aircon leaves warm blind spots in an open space. A System 4 suits a closed living room that is used lightly.
Key takeaways
- One unit cannot reach everywhere: in our video, a 24,000 BTU aircon cooled only the living room stretch, not the dining and kitchen.
- Cabinets make blind spots: a tall cabinet blocked the airflow, and the dining end stayed warm even with a fan.
- 3 + 2 gives the living area its own outdoor unit: two indoor units cover the open space, and the bedrooms share the other system.
- Some call it System 5: it means a System 3 plus a System 2, five indoor units on two outdoor units.
In the video above, Uncle Eric visits an open-concept flat whose owner says the living room is not cold enough, especially in April, May and June, Singapore's hottest months. One 24,000 BTU wall unit sits high above a tall cabinet. It cools the stretch in front of it, but the dining area and the kitchen behind it stay warm, even with the yard door closed and a fan running. His verdict: one horse cannot pull a hundred people, and an aircon should have gone above the dining area too.
How to decide between a System 4 and a 3 + 2
- Is it one open space? If the living, dining and kitchen run into each other with no doors, a single indoor unit has to cool all of it.
- Compare it with the master bedroom. If living plus dining is at least twice the size of the master bedroom, give it its own system.
- How often will it run? If the living room aircon will run most days, give it its own system so it does not share an outdoor unit with the bedrooms.
- Plan where each indoor unit goes. Keep tall cabinets and partitions out of the airflow, or the far end becomes a blind spot.
- Run the numbers. Our aircon BTU calculator uses these rules to work out the sizes and the system split for your rooms.
If steps 2 or 3 point to a separate living room system, ask about installation before the carpentry is built.
System 4 vs System 3 + System 2
| System 4 | System 3 + System 2 |
|---|---|---|
Outdoor units | 1 | 2 |
Indoor units | 4: usually 3 bedrooms + 1 living room | 5: 3 bedrooms + 2 for living and dining |
Open living and dining | One unit, blind spots likely | Two units, one for each end |
Living room shares with bedrooms | Yes | No, it has its own outdoor unit |
Cost to buy and install | Lower | Usually higher: two outdoor units |
Suits | Closed living room, light daytime use | Open concept, living room used most days |
Why one living room aircon falls behind
An open-concept flat turns the living room, dining area and often the kitchen into one big space. The kitchen adds heat when you cook, the windows let in the afternoon sun, and the air from one wall unit only travels so far before it warms up. Furniture finishes the job: in the video, the cabinet stands right in the path of the air, so the dining end never gets the cold air directly. Eric calls that a blind spot.
The hottest months make it obvious. An aircon that copes in December can run flat out in May and still not reach the far end of the room. A fan helps push some cold air along, but it cannot add cooling that is not there.
How the calculator splits a big living area
Our calculator follows Uncle Eric's rules of thumb. When the living and dining together need more than 18,000 BTU, it splits them into two indoor units, first 18K + 12K, then 24K + 12K, up to 18K + 18K. A two-unit outdoor unit runs about 38,000 BTU in total, so it never puts 24K + 18K on one. Want the reasons behind the sizes? See our guide to buying a new aircon.
Sharing has another side too. On a multi-split, the indoor units share one outdoor unit, so a fault there can stop every room at once, as we explain in all aircons not cold at once. With a 3 + 2, the living area and the bedrooms do not depend on the same outdoor unit.
What it costs
Job | Price | What drives it |
|---|---|---|
New installation (labour) | From S$500, quoted per job | Number of systems and indoor units, piping length, access to the ledge |
Aircon supply | Added as needed | The brand, model and sizes you choose |
When to call a technician
Talk to us before the carpentry and ceiling works are built, while the indoor units can still go where the air needs them. If your living room already runs flat out and the dining end stays warm, we can check whether more cooling is needed. You can book online in under a minute, or ask Luna on WhatsApp at any hour.
Frequently asked questions
What is a System 5 aircon?
When Uncle Eric and many installers talk about a System 5 for a BTO, they mean two systems: a System 3 for the bedrooms and a System 2 for the living and dining area. That is five indoor units on two outdoor units.
Can I add a second living room aircon to my System 4 later?
Usually not to the same system. A System 4's outdoor unit is made for four indoor units. Adding one later normally means another outdoor unit, which is why it pays to plan the living and dining before the renovation.
Will a fan help if my living room aircon cannot keep up?
It helps a little. A fan placed between the living and dining areas can push some cold air into the warm end, as Eric shows in the video, but it cannot add cooling the aircon does not have.
What size aircon does a BTO living room need?
It depends on the floor area, the sun, the ceiling and whether the kitchen is open. Our BTU calculator works it out from your room sizes, using the same rules Uncle Eric uses on site.
Is a System 3 + System 2 more expensive to install?
It has two outdoor units instead of one, so it usually costs more to buy and install. In return, the living area gets its own outdoor unit and enough cooling for an open space.
Eric Lim, aka Uncle EricFounder of iAircon.Repair, now in its 10th year servicing aircons in Singapore. Some of our jobs are filmed, and you can watch the work on YouTube.
