One System 4 or Two Smaller Systems? How Many Condensers Your Home Needs

Once you want four rooms cooled, most quotations arrive with the same answer: a System 4. One outdoor condenser on the ledge, four indoor units, one job. It is often the right answer. But it is not the only way to cool four rooms, and the choice between one big system and two smaller ones is one of the few aircon decisions you will live with for ten years or more.

The alternatives are ordinary: two System 2s, or a System 3 plus a single split. Same rooms, same number of indoor units. What changes is how many outdoor units you have and how many refrigerant circuits your home depends on. This guide is about what that difference means in practice, and how to decide before the survey rather than after the brackets are up.

The photo above is from one of our jobs: two newly mounted wall units, still in their protective wrap, in rooms next to each other. It shows the indoor side only. The decision this article is about happens outside, at the condenser.

What you are actually choosing

A System 4 is not four aircons. It is one outdoor condenser connected to four indoor units through one refrigerant circuit. All four rooms share the same compressor and the same gas. We covered what that means when a single indoor unit fails in replacing one indoor unit on a System 3, and what it means for day-to-day cooling in one room that never gets cold on a System 3 or 4.

Two System 2s are two separate outdoor units, each with its own compressor, its own circuit and its own two rooms. A System 3 plus a single split is the same idea with a different grouping. So the real question is not "System 4 or not". It is: how many rooms are you willing to have depend on one outdoor unit?

The published starting prices

These are the "from" figures on our installation cost page:

Read the table honestly. On starting price alone, one System 4 begins lower than two System 2s, each of which starts from $2,200 on its own. A second outdoor unit is a second machine, a second set of brackets and a second electrical connection, and that is not free.

But a "from" price is the starting point for a straightforward job, not your figure. Extra copper piping beyond the standard run, concealed trunking and a new electrical point are all quoted upfront at the free site survey. On some layouts those items move differently depending on where the outdoor units can sit, which is why the survey, not the table, settles the comparison.

When something goes wrong, one condenser means every room

This is the part people do not think about on installation day.

On a System 4, a compressor fault, a refrigerant leak or a failed outdoor fan takes all four rooms off at once. So does any repair that opens the circuit: the gas has to be recovered or pumped down before anything is disconnected, so every room on that circuit is down for the day, not just the one with the problem.

With two systems, the same fault takes out half the home. The other half keeps running while it is diagnosed and fixed.

Whether that matters depends on the household. For a couple who are out all day, probably not much. For a home with a newborn, an elderly parent, someone who works nights and sleeps in the day, or someone who works from a bedroom, one room that must stay cold can decide the whole layout.

There is also a pricing detail worth knowing. Gas top-up on our price list is priced per outdoor circuit, not per indoor unit: R22 $115, R410A $130, R32 $170. A leak on a System 4 is one circuit. With two systems, a leak is usually on one of them. The per-circuit price does not make either layout cheaper to own. It just means the number of outdoor units, not the number of rooms, is what a gas job is counted against. Before anyone tops up anything, the leak itself is the real question, as covered in where refrigerant leaks actually happen.

Group rooms by when they run, not by where they are

A multi-split condenser shares its capacity across the indoor units connected to it. That works best when the rooms on it are used in a similar way.

So before the survey, write down how your home actually uses its aircon on a normal day:

• Which rooms run together in the evening, typically the living room and one or two bedrooms
• Which room runs alone all night, typically the master bedroom
• Which room is rarely used, such as a study or a guest room
• Whether anyone needs a room cooled during the day while the rest of the flat is empty

If your household runs one bedroom on its own for eight hours every night, it is fair to ask the installer how the specific outdoor unit behaves when only one small indoor unit is running. The answer varies by model, and it is better given to you in writing at the quotation stage than assumed. It is also a reason some owners put that bedroom on its own system.

Where the outdoor units can go

Two systems only work if two outdoor units can be placed properly. That is a site question, not a preference.

• HDB: the outdoor condenser must sit on the flat's designated aircon ledge, on proper brackets, installed by a BCA-licensed contractor. Whether your ledge can properly take a second outdoor unit is something to confirm at the survey. Do not assume it from a neighbour's flat. What HDB allows is set out in what HDB allows on an aircon ledge.
• Condo: most MCSTs specify where condenser units may sit and how piping is routed. If the management only permits one position, the decision may already be made for you. Check before you plan around two.
• Landed: the most flexible, and the place two systems most often make sense. Rooms can sit far apart, and a second outdoor unit placed nearer the far rooms can shorten the piping run to them.

That last point applies anywhere the outdoor space allows it. A standard length of insulated copper piping is included in the installation price, and extra piping beyond that is quoted at the survey. A single condenser serving a room at the far end of the home means a long run to that room. Whether splitting the layout shortens the runs or just adds a second outdoor unit to the same spot depends entirely on where the outdoor units can physically go.

The electrical side

Every installation includes electrical wiring and the isolator connection. Older properties sometimes need an isolator or a dedicated point added, and we check that at the survey. With two outdoor units, that check covers both, so ask plainly whether your existing supply already covers the layout you are considering, or whether new electrical work is part of the quote. The background is in what electrical work a new aircon install needs.

What does not change: servicing

Servicing is counted by indoor unit, and the number of outdoor units does not change the price. Four indoor units cost $115 for a general service whether they hang off one condenser or two. A chemical wash for four units is $425 either way. So the running cost of cleaning is not a reason to pick one layout over the other.

Ten years from now

The decision also shapes what replacement looks like later.

A System 4 ages as one machine. When its compressor is at the end of its life, the practical choice is usually the whole system, because the four indoor units, the drainage and the piping are all the same age. We set out that decision in aircon compressor failure: replace or repair.

Two systems can be replaced at different times, which spreads the cost. They can also end up on different refrigerants a decade apart, which is fine as long as each circuit stays on its own gas. R22, R410A and R32 cannot be mixed on one circuit.

And plan for rooms you do not cool yet. A condenser is rated for a set combination of indoor units, so a System 4 is not a system you can hang a fifth room on later. If a room is likely to need cooling in a few years, a nursery or a converted study, say so at the survey. It can change which grouping makes sense now.

A simple way to decide

One System 4 usually makes sense when:

• The outdoor space properly takes one condenser and not two
• The rooms are close together and the piping runs are short
• No single room has to stay cold if the system is down for a day
• The lowest starting price is the priority

Two smaller systems usually make sense when:

• One room must keep running even when the rest of the home is down
• One bedroom runs on its own every night while the others are off
• The rooms are far apart and a second outdoor position is available near them
• You would rather replace the home's aircon in two stages than all at once

If most of your answers land in the first list, take the System 4 and do not let anyone talk you out of it. If several land in the second, ask for both layouts to be quoted at the survey so you can see the actual difference rather than guess it.

Questions to ask at the site survey

1. Can my ledge or outdoor area properly take a second outdoor unit, and where exactly would it go?
2. For each layout, how long is the piping run to the furthest room, and is any of it beyond the standard length?
3. Does my existing electrical supply cover two outdoor units, or is new electrical work part of the quote?
4. How does the outdoor unit you are proposing behave when only one indoor unit is running?
5. If a room may need cooling later, which layout leaves room for it?
6. Can you quote both layouts so I can compare them line by line?

How many BTU each room needs is a separate question from how the rooms are grouped. That is covered in how aircon BTU sizing works in Singapore, and the overall sizing picture is in System 3 vs System 4 vs System 5.

The short version

A System 4 is one outdoor unit that every room depends on. Two smaller systems cost more to start with, but a fault takes out half the home instead of all of it. Decide by how your rooms are actually used, where the outdoor units can properly sit, and whether any room must never go down. Servicing costs the same either way. Ask for both layouts at the free site survey if you are unsure, and compare the real quotes.

Planning a new installation and not sure how to group the rooms? WhatsApp us on 9107 2601 with the rooms you want cooled and a floor plan if you have one, and we will arrange a free site survey. Every published installation price is on the installation cost page, and HDB specifics are on our HDB installation page.