Bedroom Air Conditioning Upgrade

Our current bedroom air conditioning system, a 16 year old Mitsubishi Electric ducted system, has been starting to have difficulties. For example, it has simply been refusing to start until I disconnect it from power and then reconnect it again. With summer fast approaching it was decided that it would be best to replace it. Originally, I thought the obvious solution would be to replace it with an inverter ducted unit, however after doing some research, it became apparent that there were other options that required careful consideration.

More of the Same Problems

The biggest problem with installing another ducted unit is how the unit is used most of the time. Some of the upstairs rooms are used as bedrooms, while another two are used as work/study rooms. During the day there is often only the need to air condition one, or two of the rooms. With the current ducted system, it was all of the rooms or nothing. This was wasting a lot of energy by air conditioning rooms that never had anyone in them. The opposite would then be true during the night; The work rooms would be cooled or heated when there was no one in them.

This led me to consider zoning the upstairs ducts so that I could select to only have one or two rooms running. This presented a new problem. The larger inverter units, 10KW and above, are not able to reduce their cooling (or heating) capacity much below 5KW. If I were to try and just run this 5KW into one room, it would lead to rapid temperature fluctuations, which would be inefficient and uncomfortable.

What I Really Wanted

At this stage, I had managed to eliminate most of the ducted options. After some thinking, I determined that the new system would have the following requirements:

  • Each room could operate on its own
    • Essentially to cool or heat one room, none of the other rooms should be cooled or heated.
  • Each room should be able to select its own temperature
    • Different members of the family prefer different temperatures.
    • With the old system, it was difficult to find a temperature that everyone liked.

The Options

After browsing the websites of many big air conditioning companies, I realised there were two main options for meeting my requirements.

Multi-Split System

  • Allows multiple indoor units, in this case one per room, to be serviced by a single outdoor unit.
    • Each of the indoor units could be operated independently and be set at their own temperature.
  • Advantages
    • Allows the system to remain centralised (Only one power connection required)
    • Only requires the placement of one outdoor condensing unit
    • Considerably more efficient than the current Mitsubishi unit
  • Disadvantages
    • Lack of redundency
      • If anything went wrong with the one outdoor unit, all of the rooms would be without heating or cooling
    • Due to the large capacity of the outdoor unit, about 10KW, the inverter compressor was only capable of  reducing its output to about 2KW. If there was only one room turned on, this would result in the unit having to cycle on and off to maintain the temperature.
      • While this problem is considerably smaller than with another ducted system, it was still present.

Four Mini-Split Systems

  • Basically four completely independent systems
  • Advantages
    • The most efficient solution
      • For 10KW of cooling in total, I calculated the power usage to be only 1960 watts.
      • Nearly 2000 watts more efficient than the current system.
    • Redundancy
      • If a fault was to occur, it would only affect the one room that unit was servicing.
    • As the smaller units are designed for small rooms, the compressor is capable of reducing its capacity down to 0.9KW, allowing the air conditioner to maintain the room at a much steadier temperature.
    • Cost
      • Despite my first thought, the installation quote for four mini-split systems was actually less than the quote for one multi-split system.
    • Allowed for the use of the new industry refrigerant R-32.
  • Disadvantages
    • Requires the placement of four outdoor units
      • In my situation this is not really a problem as the units can be mounted on the walls outside the bedrooms.
    • Each unit requires its own power supply.

 

After careful consideration of all the numbers involved, I decided to go for the four mini-splits.

Brand

After considerable research online and consulting with HVAC professionals, I narrowed the selection down to either Daikin or Panasonic. Due to the number of Daikins operating perfectly in other locations around the house, I decided to go for the Daikins.

Furthermore, the Daikin units were also the most efficient. They only used 490 watts of electrical energy to provide 2500 watts of thermal cooling. That’s an efficiency factor of over five times.

After the HVAC professional had appropriately sized the units it was determined that would would install four Daikin Cora series units (FTXM25QVMA).

Installation

A few pictures of the installed units

One unit connected to the vacuum pump
Two of the outdoor units mounted near each other.
Outdoor unit data sheet
One of the indoor units
The remote controller