The Inverter Compressor

the inverter compressor

The Inverter Compressor – Comments from the Duct Free Zone

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

I have repeatedly made that statement in this column and in my TOSOT mini-split and APEX training events.

I remain committed to it and offer the following as further evidence to support it.

There has been a feature of the TOSOT multi-zone mini split that I had been apprehensive about using myself in a residential application when I was still contracting and one that I had difficulty describing objectively as a trainer.

Features and Components of the Inverter Mini Split

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.

Looking at the chart of approved TOSOT Standard Multi-Zone combinations, you will notice that 73 of the 123 approved combinations actually have more indoor capacity than outdoor capacity … how can that be?

AC Ductless Capacity Charts

Well, clearly, you are always limited to the capacity of the outdoor unit.

Still, when you look closely at the capacities of the TOSOT Standard Multi-Zone outdoor units, you see that the 18K, 24K and 30K outdoor unit capacity range exceeds the capacity stated in their model number for both cooling & heating.

In addition, you will also notice that the 36K and 42K exceed the capacity stated in their model number for heating.

So, when we look at the approved combination of 9+9+9 for the TM24H4-O, at first glance, it appears we have 3K more indoor capacity than the 24K outdoor unit.

However, a closer examination reveals that the TM24H4-O has a capacity for cooling to 33K and heating to 28K … more than enough to handle the 27K of indoor units!

AC Ductless Capacity Charts

Now, 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, which may result in the indoor units de-rating.

This feature is yet another example of the power of the inverter compressor as a modulating compressor.

My example of the 9+9+9 with the TM24H4-O exemplifies how instead of going to the TM30H4-O outdoor unit, you can keep your equipment cost down yet still maintain the necessary capacity for any weather event … ultimately helping you get the job!

I used the Standard Multi-Zone outdoor unit in my example, but the same applies to the UltraHeat Multi-Zone as well …

AC Ductless Capacity Charts

Final Thoughts

I love that even after all my years with the TOSOT product, I still find revelations like this to excite me and cause me to rethink previous concerns.

The inverter compressor IS the greatest invention in the HVAC industry in my lifetime.

It continues to impress with its ability to make what was once unthinkable now possible!

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Gerry Wagner

Gerry Wagner is the Vice President of Business Development for Bathica, Quebec, Canada. He has forty one years in the HVACR industry, working in manufacturing, contracting and training. You can contact Gerry by email: [email protected] and also please visit our website:

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About Our Blog

HVAC Know It All was created by Gary McCreadie, a licensed refrigeration and gas technician, with three goals in mind:
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