HVAC Folks - Multi Split AC

[JHZR2]

Hello,

We live in an old home with radiator heat. We use window AC units where needed, and we have an in-wall unit downstairs (looks like a window unit but is 240V and 24k BTU).

For now it is just two folks for the most part in the home. We dont run the AC that much except to sleep.

That said, I am aware of these ductless split AC systems which just move refrigerant to the air handler in the room, and so installation is simple, no windows blocked, extremely quiet, etc. I know they work well as the in-laws have a bunch of them at their home which is designed by code to resist hurricanes (all concrete and rebar, no ducting).

I have a question though about the multi-unit split systems that I cannot find an answer to... Specifically the inverter type.

Say I get a system that is one outdoor unit and three air handlers. The smallest air handler they offer is 9k BTU, and say the outdoor unit is rated for up to 24k BTU. So I put a 9k unit in a room that a 5k window unit is known to cool well. I understand that the system can "throttle" down, but is there a minimum level before there is risk of liquid refrigerant returning to the compressor?

Can I regularly just run one zone at a low level without causing damage to the compressor or system?

Now, say I want to set the system up to use a 9k, a 9k and an 18k air handler, with a 24k outside unit. Is this OK? Will the three work only up to the amount of available refrigerant they have, or is there some sort of an interlock that prevents too large a total air handler rating to be installed with the outdoor unit?

Thanks for insight!

 

[brianl703]

Do they make chilled water systems for residential use? That would be ideal for your application.

 

[Brett Miller]

No sure how the multi indoor units work but I have a Mitsubishi Mr. Slim heat pump/AC in a sunroom and it works great. I would start with a good contractor and go from there.

 

[wulimaster]

The mini-splits use rotary or scroll compressors and are designed for a fluid and gas mixture. There is alway some fluid returning to the compressor. No problem on the inverter (DC motor) powered units. The inverter versions should be more comfortable and provide less short cycling of the compressor.

Most of the compressor units specify what combinations of air handlers you can use with them. No, you won't find 9+9+18 combo for a 24k btu unit. Also, they are sized for hooking up all the units at once. I don't believe you can add less air handlers than the outside unit requires.

I had the following in my house. 7+7+18 with 32 outside, 9+9 with 18 outside, and 9+9 with 18 outside for a total of 7 air handlers and 3 compressor units. With mine, there were actually 7 compressors total and no sharing of a compressor inside the units. The only thing they shared was the fan, single run of electrical wiring, and the enclosure. The condenser fan ran at different speeds depending on how many and what size of evaporators were in use.

There is a limit on how far you can run the freon lines, so if you are thinking of a sing outdoor unit and the runs to the different parts of the house exceed this run length, then you will have to buy multiple condenser units. The max run length is usually from 50ft to 75ft depending on model and manufacturer. You can probably go further with more freon added but the efficiency will go down.