Mitsubishi 2 Zone Outdoor Unit Sizing Question (Cooling only)

HVAC_P

Hello,

I am currently in the process of sizing a 2 zone cool only Mitsubishi ductless split. The two sizes I need according to load calculations are a 9K and a 12K. This puts me at 21K total capacity.

My question is do I upsize to a 24K outdoor unit or go down to 18K outdoor unit? I should note that I only install a few ductless systems each summer as I am more forced air on the side of things. Does Mitsubishi even make a 2 zone Cool only unit?

Thank you in advance for any input. I greatly appreciate it.

ratio

Do both heads need to operate at 100% at the same time?

EdTheHeaterMan

If your calculation is accurate and both of the zones are on the same side of the building, then you should get a 24,000. There may be a time when you need 21,000 BTU and the 18,000 will not be sufficient.


If on the other hand the rooms are oriented where one zone is on the East side of the building and the other zone is on the West side of the building, then the solar gain will be of the East in the morning and the West in the afternoon. the 18,000 may be sufficient. This is because the different zones will not need peak performance at the same time. Likewise, if the zones are used differently, as in the sleeping area and the living area. If you don't need peak performance at the same time of the day, then you may not need 21,000 simultaneously. the 18,000 may be adequate.

You can be at ease with either choice because the variable speed compressor and fan will modulate to the BTU requirements in real-time. So the 24,000 will not be a 24,000 but will actually be a 9,500 total at times and 21,000+ at other times and will modulate between the two extremes as needed.

The beauty of inverter technology.

Yours truly,
Mr.Ed

HVAC_P

EdTheHeaterMan said:

If your calculation is accurate and both of the zones are on the same side of the building, then you should get a 24,000. There may be a time when you need 21,000 BTU and the 18,000 will not be sufficient.


If on the other hand the rooms are oriented where one zone is on the East side of the building and the other zone is on the West side of the building, then the solar gain will be of the East in the morning and the West in the afternoon. the 18,000 may be sufficient. This is because the different zones will not need peak performance at the same time. Likewise, if the zones are used differently, as in the sleeping area and the living area. If you don't need peak performance at the same time of the day, then you may not need 21,000 simultaneously. the 18,000 may be adequate.

You can be at ease with either choice because the variable speed compressor and fan will modulate to the BTU requirements in real-time. So the 24,000 will not be a 24,000 but will actually be a 3,000 at times and 21,000 at other times and will modulate between the two extremes as needed.

The beauty of inverter technology.

Yours truly,
Mr.Ed

Thank you very much for the informative response. I called my local Mitsu supply house today and they gave me a very similar answer. Thanks again

EdTheHeaterMan

HVAC_P said:

Thank you very much for the informative response. I called my local Mitsu supply house today and they gave me a very similar answer. Thanks again

Well... at least one of us has to be right!

pecmsg

A word of warning avoid over sizing minis. Mold growth is a problem running low speeds. 

Poor dehumidification at low loads. 

TAG

Have you looked at the Mitsubishi rating tables ?

There are tables that will tell you the outputs of each head with a given outdoor unit ...... the duel and 3 head units don't work the same as the individual units. Both output and turndown

.... also, the output of the compressor is capable of above 100 % for those odd days. .. again. It's in the table.

You don't want an oversized compressor as the turn down will exceed what you need .... IE at 24k and 25% ... the 9k has to take around 6k. There is also some waste as the lines to any head can never be off.

Have two three head system in two different houses ..... 30k compressor -- 3x 12k heads. The multi don't work the way many think ... including many of the people at supply houses.

I had to explain this to my current installers -- who called Mitsubishi and got the word. You have to look at the tables

GW

What did I miss- a Mitsu 24 can ramp down to 3000 BTUs? Specs say 9500 for the non hyper 

EdTheHeaterMan

I did not look at the specs. You are correct. I should change my post. I was thinking 15% from a different appliance.

Fixed!

GW

Ed which multi can go to 15%?