Should I get Aircon system 4 or 3+ 1?

Introduction to aircon system 4 or 3 + 1

Aircon system 4 or system 3 + 1? For those of you buying air-conditioners for the first time, especially those that are installing in their newly purchased HDB units, you may have a lot of questions. What is Aircon system 4? What is BTU? You can take a look at this full walkthrough on Aircon installation in HDB for a simple guide. If you are thinking of installing air-conditioners in all of your bedrooms and living room also, you may wonder, “is it better if I just install a system 4 ac unit, or would it be better if I Install 2 condensing units for separate systems?”. So should you get Aircon system 4 or 3+ 1? We will explore this question in this article. 

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Understanding “Aircon system 4 or 3 + 1”

To better understand this question, we first have to define Aircon system 4. System 4 refers to the condenser unit connecting to 4 outlets. This means it has the ability to connect to 4 blower units (or indoor units) at the same time. You can technically use a system 4 to connect to 3 indoor units, leaving one of the outlets untouched. Whereas 3 + 1 basically means having 2 condenser for 2 separate systems – system 3 and system 1. This combination also allows for a total of 4 indoor units. This article helps you decide which option to choose between having 1 system or 2 instead.

BTU for Aircon

abbreviations BTU

BTU stands for British Thermal Unit. It is a form of measurement that measures energy. One BTU refers the amount of energy needed to raise the temperature of a pound of water by 1° F. It is common terminology when talking about  aircon units. The higher the BTU rating of an aircon, the more powerful it is. The primary method of knowing how much BTU you need is by using square footage of an area. Multiplying the area in square foot by 50 – 60 gives you a good gauge. There are many online charts that can help in this calculation. This guide gives you all the information you need on BTU.

Aircon System 4 BTU


For a system 4 condenser unit, we need to know the model number and the maximum capacity of it. The Daikin system 4 Ismile Series consists of 2 models: MKM-85VVMG and MKM-100VVMG for example. Between the two, the more commonly used model will be MKM-85VVMG. This particular model has a maximum BTU of 36100.

Next, you will need to know how much BTU you need for your living room. For a living room in 5-room flats, you will typically need 21000 – 24000 worth of BTU. It’s 18000 BTU for a 4-room flat living room and 12000 for a 3-room flat’s. Nevertheless there could always be exceptions to these estimates and thus it would be best if an experienced aircon technician heads to your place for a better assessment. For HDB units, each bedroom will need about 9000 BTU. For a 4-room flat, you will need about 45000 BTU in total (18000 + 9000 x 3). Hence, if you were to use a Daikin system 4 MKM-85VVMG you will not be able to ensure that every room will be cold if the indoor units are all turned on simultaneously. This is because the maximum BTU for that model is only 36100 BTU as mentioned previously.

Aircon system 3 + 1 BTU


As mentioned above, you will need 45000 BTU in total to ensure that all your rooms can be cooled down simultaneously (including living room). This is why Aircon System 3 + 1 is a popular option as well. With 2 condenser units, the total BTU can be higher. Let’s use Daikin as an example again. Daikin system 3 MKM-75VVMG has BTU of about 32000. Daikin single split condenser unit RKM50PVMG has a BTU of 18000. If you add up these 2 together, it has a combined BTU of 50000, it is definitely more than sufficient to ensure all of the fan coil units are cold when it’s turned on together. Nevertheless, if that is the case, why does everyone not install 2 condensing units instead of a single system 4 condensing unit?

Want to know more about


  • Do you know the properties you are installing in matters?


  • What kind of air-con do you plan to install at each location?


  • Do you know what aircon BTU and how to calculate it?

Comparison between Aircon System 4 or 3+1



A system 4 aircon is definitely cheaper than that of 2 condensing units combined. This is because the compressor (in condenser unit) is the most expensive component in an aircon. Therefore, having 2 condensing units will be a lot more expensive than having just one big one. Let’s use Daikin as an example again. A Daikin system 4 MKM-85VVMG/ CTKM25VVMG x 3 + CTKM50VVMG (9000 BTU x 3 + 18000 BTU) costs about $4300 for HDB. Whereas Daikin system 3 MKM-75VVMG/ CTKM25VVMG x 3 and a Daikin system 1 RKM50PVMG / FTKM50PVM cost about $4700 combined. That’s a cost saving of $400. Hence based on cost alone, most consumers would opt to get a System 4 instead of a system 3+1.

Electricity consumption


Having 2 sets of condensing unit will definitely consume more electricity than just one system 4 aircon unit . That is because 2 compressor running at the same time will consume more power than just one. So if you were to use a system 4 aircon, you will see a cheaper electricity bill at the end of the month.



aircon system 4 or 3+1
condenser unit on aircon ledge


For HDB units without an aircon ledge, not every house can install 2 sets of bracket. You will need a metal bracket to place your Aircon condensing unit on. You will also have to call up HDB or go through HDB electrical upgrading and check before you can install 2 condensers. This additional paperwork turns many HDB home owners away from installing an additional set of condensing unit in their house.


As for HDB units with an existing aircon ledge, the feasibility of installing 2 sets of condensing units will depend on the window height and the size of the ledge. Not every ledge is big enough to contain 2 sets of compressor. For example, for some of the smaller ledge, the condensing unit actually has to be placed diagonally. This is to ensure that the hot air blowing out from the condenser unit will not be obstructed. If it is obstructed, it will cause overheating for the compressor.


Regarding HDB 5 room flat, only 24000 BTU is suitable. However the condensing unit for a 24000 single split is very lengthy in size. There might be some house that have window height constraints for this particular condenser unit. Hence, before you make any decision on whether to get 2 compressors, it might be best to call (96515152) us up and check first. We provide over-the-phone consultation and offer site visits if you still have any enquiries.


Here at Silverback Aircon, our friendly staff and technicians provide you with the most efficient service for the best price. All of our spare parts are tested, refurbished and commissioned before selling it to you. Hence, you can trust us with all your aircon needs! For enquiries regarding system 4 aircon, don’t hesitate to contact us and our friendly staff will provide you with the information you need. If needed, you can even book an appointment with us for a site visit so we can provide you with an accurate diagnosis on whether you should use a system 4 or 3 + 1. We also provide other aircon services as well! You should also check out this aircon installation guide if you require any assistance with installation.

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How about system 2 + system 2 instead?

mitsubishi system 2

2 sets of system 2 aircon is also a viable choice. However in terms of electricity consumption, it is usually better to get a system 3 + 1. Most of the time, the air-conditioner in the living hall will not be turned on when everyone in the household goes to sleep. This means only one compressor will be running if you were to use a system 3. If you were to use two sets of system 2 there will be 2 condensing unit running at the same time if you turn the aircon on for all 3 bedrooms. This will result in unnecessary additional electrical cost.

Conclusion for choosing aircon system 4 or 3 + 1


We hope you have a better understand of whether you should get Aircon system 4 or 3+ 1 after reading this article. You now know the pros and cons on both options which should help a ton in your decision-making process. Approach Silverback Aircon for any of your aircon needs!

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