Can you oversize an inverter ductless mini split system? This is a question that I pose to the attendees of each GREE mini split design and installation class I conduct. Before you start writing your reply to what I’m sure you think is a ridiculous question, let me save you the effort…YES…the answer is YES, of course you can!
Now, with that out of the way, let me address why I pose the question at all if it has such an obvious answer.
We need to start with the anatomy of an inverter system; an inverter compressor can run at 3600 RPM like every other compressor in HVACR equipment but the difference is that it doesn’t have to.
Let me digress a moment;
You guys know my background…I’m a wet-head by birth…being from NJ & NY, hydronic heat is the first HVACR subject you learn and air comes later. When I started in the boiler biz back in the 70’s, we made a lot of different boilers…50 MBH, 75 MBH, 100 MBH, 125 MBH, 150 MBH…and on and on. Now boiler companies for the most part make one boiler…a modulating boiler that can down-fire to as low as 50 MBH and modulate up all the way to 150 MBH. If you install this boiler in a home with a heat loss of 100 MBH it doesn’t short-cycle because it will never produce anything over 100 MBH. Why…because it measures indoor & outdoor ambient air temperature and modulates the BTUH output of the boiler accordingly. Why the Hydronics 101 lesson…well, because the SAME principal applies to an inverter compressor. The inverter compressor does not start at 3600 RPM and then ramp down as the system finds the setpoint programmed by the homeowner, rather it starts at the lowest RPM and ramps up. If the actual room temp and the setpoint are only a few degrees apart the compressor will never come anywhere near full capacity. Like the modulating boiler, the inverter system senses both indoor & outdoor temperatures and applies power to the compressor accordingly.
I can prove it very easily…my test lab here in New York is a 10′ X 20′ prefabricated garage. A recent equipment test had me install a single GREE VIREO VIR12HP230V1B 12K inverter ductless system in the lab. Rather than short cycle as you would expect, what happens is the system rarely shuts off…the compressor will initially, (over a three minute period if needed), ramp up to full capacity and get the room to the programmed setpoint and then you will see the compressor ramp down to a minimum running RPM, and if the fan is on AUTO, it will do the same. The compressor may shut down completely and the system algorithm will keep the compressor off for a minimum of three minutes. It takes another three minutes for the compressor to come back up to full capacity, so a minimum cycle, (if it existed), could be no shorter than 3 minutes.
You can further increase the run cycles by placing the fan of the evaporator in the manual mode and operate it at the lowest speed.
What always has to be considered when dealing with inverter ductless mini splits is the range of operation that the system can provide. Unlike a non-inverter, conventional system that only knows two performance points, on and off, the inverter offers a range of performance.
A GREE VIREO VIR09HP230V1B 9K inverter system will give you an operating performance range of
3.1 MBH – 9.6 MBH of cooling.
A GREE VIREO VIR12HP230V1B 12K inverter system will give you an operating performance range of
3.1 MBH – 13 MBH of cooling.
A GREE VIREO VIR18HP230V1B 18K inverter system will give you an operating performance range of
6.8 MBH – 20 MBH of cooling.
Let’s look at an example;
If you install a GREE VIREO VIR09HP230V1B 9K inverter system in a room that has a heat gain of 5 MBH, will it ever produce anything more than the 5 MBH heat gain of the room?
NO!
It will ramp up to 5 MBH, satisfy the setpoint, and maintain that setpoint thereafter.
So, with this in mind, back to my original question…can you oversize an inverter ductless mini split system?
Well, I believe there are two ways to oversize utilizing an inverter mini split…the first is the obvious capacity oversize.
Let’s use the same example of a room with a 5 MBH heat gain. ..if you install a GREE VIREO VIR18HP230V1B 18K inverter system in that room are you oversized?
HELL YEAH!
The low end of the performance range of the VIREO VIR18HP230V1B is 6.8 MBH…more than the total heat gain of the room.
Where I see installers make a less obvious mistake in sizing inverter mini splits is with this scenario…
Again, the room with the 5 MBH heat gain, but now we have an installer who decides to install a GREE VIREO VIR12HP230V1B 12K system in the room saying to him or herself, “it’s an inverter, it won’t ever go up to 12K because the room only needs 5 MBH… so what’s the harm?”
Well, the harm is that his or her competitor who makes the more accurate choice of a GREE VIREO VIR09HP230V1B 9K system for the same room will get the job based on equipment cost alone!
I’m a believer in inverter mini splits for so many reasons, in so many applications, but we still need to use our skills as professionals to make sure we are choosing the right equipment for each application.
…and before you pick up the phone or start typing an email to me to challenge my previous statement of, “If you install a GREE VIREO VIR09HP230V1B 9K inverter system in a room that has a heat gain of
5 MBH, will it ever produce anything more than the 5 MBH heat gain of the room? NO!”…let me suggest a scenario where the system will produce more than 5 MBH.
Our customer, Mrs. Gillacuddy, has a party every 4th of July when she invites her entire pinochle club over to her house…20 club members who play pinochle until the wee hours of the night. Mrs. Gillacuddy won’t be happy that I’m divulging this but, at midnight they put on the Bee Gee’s, Saturday Night Fever album and the dirty dancing begins…and as a result, the heat gain of the room goes up!
Can you see that even in this scenario, based on the performance range of the GREE VIREO VIR09HP230V1B, you still have an additional 4.6 MBH of cooling for just such a scenario?
This is a win, win, win!
It’s a win for you as the system designer…its’s a win for you as the system installer…and it’s a win for Mrs. Gillacuddy and she doesn’t even know she won! She just knows when the pinochle club is at her house and everyone is having a great time, they are all comfortable as well.
…and as Barry Gibb would say, this ain’t no “Jive Talkin.”
ABOUT THE AUTHOR: Gerry Wagner is the Vice President of HVAC Technical Training for Tradewinds Climate Systems. He has 38 years in the HVACR industry working in manufacturing, contracting and now training. You can contact Gerry by email: gwagner@twclimate.com and also please visit our website: www.twclimate.com
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