HPAC Magazine

Inverters Improve Capacity

July 25, 2023 | By Gerry Wagner

When considering installation of multi-zone mini-split systems, inverter compressors often allow more indoor capacity than you might think.

“The inverter compressor is the greatest invention in the HVAC industry in my lifetime.”

I have made that statement many times in this column as well in my mini-split and central-ducted training events. I remain committed to it and offer the following as further evidence to support it.

There is a feature of the multi-zone mini split system I represent that I had been apprehensive to use myself in a residential application when I was still contracting, and it’s one that as a trainer I had a difficult time describing objectively.

The feature is the ability to install more indoor capacity than outdoor capacity.

In my training events I almost have a hard time completing that sentence because on the surface, it seems so wrong.

However, like most features and components of the inverter mini split, we need to scratch below the surface a little bit to uncover the true advantage.

If you look at the chart of approved standard multi-zone combinations I’ve included below (Figure 1, outdoor unit models in the top bar and capacity of the multiple indoor units below), you will notice that 73 of the 123 approved combinations actually add up to more indoor capacity than outdoor capacity. How can that be?

Figure 1. Chart of approved standard multi-zone combinations, outdoor unit models in the top bar and capacity of multiple indoor units below.

Well, clearly you are always limited to the capacity of the outdoor unit, but when you look closely at the capacities of these standard multi-zone outdoor units (Figure 2), you see that the 18K, 24K and 30K outdoor unit capacity range in brackets exceeds their capacity stated in their model number for both cooling and heating.

Figure 2. Capacities of standard multi-zone outdoor units, variable range in brackets.

You will also notice that the 36K and 42K exceeds their capacity stated in their model number for heating.

So, when we look at the approved combination of 9+9+9 for the 24K unit, at first glance it appears we have 3K more indoor capacity than the 24K outdoor unit. However, a closer examination reveals that the 24K unit has a capacity in cooling to 33K and in heating to 28K, more than enough to handle the 27K of indoor units!

Please note that not all of the approved combinations work out this way. Many still represent more indoor capacity than outdoor, and in these instances, should all indoor units call simultaneously the system is limited to the overall capacity of the outdoor unit and may result in the indoor units de-rating.

My example of the 9+9+9 with the 24K exemplifies how instead of moving up to the 34K outdoor unit, you can keep your equipment cost down yet still maintain necessary capacity.

Figure 3. Capacities of enhanced (cold climate) multi-zone outdoor units.

Note this also applies to enhanced multi-zone units as well (Figure 3).

I love that even after all my years with these products, I still find revelations like this to excite me and cause me to rethink previous concerns. <>

Gerry Wagner is vice president business development for Bathica, and he can be contacted at



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