Can you oversize an inverter ductless mini split system?
This is a question that I pose to the attendees of each mini split class I conduct. Before you start writing your reply to what I’m sure you think is an obvious answer, 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 hydronics are 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 fires the boiler accordingly.
Now, you can take that old oversized, non-modulating boiler, i.e., 150 MBH in a home with a heat loss of 75 MBH, and stop it from short cycling by adding ‘indoor / outdoor reset’ to the control system. Instead of heating the water up to 180 degrees even when the outdoor temp is 50 degrees and you ‘just need to take the chill off’, the control will fire the boiler only up to the temperature required to meet the heat loss of the home based on the outdoor air temp. There would be some mechanical changes required, specifically a bypass from the boiler supply to the return to make sure a minimum return temp came back to the boiler to prevent thermal shock…new condensing boilers don’t require this as the lower the return temp the more efficient they run.
Why the Hydronics 101…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…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, (the compressor is the burner if you will).
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 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 controls programmed into the equipment will keep the compressor off for a minimum of three minutes and then it takes another three minutes for the compressor to comeback 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 will provide. Unlike a non-inverter conventional system that only knows two performance points, on and off, the inverter offers a range of performance.
An 9K inverter evaporator will give you an operating performance range of approximately
2 MBUH – 11 MBUH of cooling.
An 12K inverter evaporator will give you an operating performance range of approximately
3 MBUH – 13 MBUH of cooling.
An 18K inverter evaporator will give you an operating performance range of approximately
7 MBUH – 23 MBUH of cooling.
Let’s look at an example;
If you install a 9K inverter evaporator in a room that has a heat gain of 5 MBUH will it ever produce 9 MBUH? NO! It will ramp up to 5 MBUH and satisfy the setpoint programmed by the homeowner and then 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 MS…the first is the obvious capacity oversize.
Let’s use the same example of a room with a 5 MBUH heat gain. ..if you install a 18K inverter evaporator in that room are you oversized…hell yeah! The low end of the performance range of an 18K inverter evaporator is 7 MBUH…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 MBUH heat gain but now we have an installer who decides to install a 12K evaporator in the room saying to himself, ‘it’s an inverter, it won’t ever go up to 12K because the room only needs 5 MBUH… so what’s the harm?’.
Well, the harm is that his competitor who makes the more accurate choice of a 9K evaporator 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 to make sure we are choosing the right equipment for each application.
This is not your father’s electric baseboard!
I had the pleasure of meeting Don Brickner, USA Sales Manager of Dimplex, at a recent trade event. Don was displaying the Dimplex LPC Series of electric baseboard.
Now, I have to be honest, I was thinking exactly what you are thinking right now… ‘electric baseboard makes the meter spin off the wall’!
Electric baseboard became somewhat taboo, did it not? The cost of operating electric baseboard has always been prohibitive based on the high cost of a kilowatt…unless of course you live near Hoover Dam or Niagara Falls.
There is no more efficient form of heat…for every kilowatt in we get a BTU out but again, the cost of the kilowatt has always been the obstacle.
I am pleased that I listened to Don because he enlightened me as to where the technology has taken this once forgotten form of heat. Much like the modulating boiler and the inverter compressor we just discussed in the earlier article, the Dimplex LPC Series modulates or ‘proportions’ the heat output to maintain the temperature with a 33% savings on energy use over traditional electric baseboard products.
What drew me to the Dimplex product was it’s appearance…it looks really nice! It is very slim and has a low profile. The LPC Series are up to 42% smaller than conventional baseboard heaters.
The LPC Series is available in lengths from 20” to 60” and wattages from 500W – 2500W.
A 500W element will give you about 1024 BTU / foot. A 2500W element will give you 1706 BTU / foot and there are five further wattage increments between 500 and 2500 to satisfy any need and application.
The baseboards have an on-board electronic baseboard that can communicate with others in the same zone or a wireless remote is available as an option.
Dimplex claims their LPC series will maintain a consistent room temperature to within .9 degrees Fahrenheit…very impressive!
You can check out Dimplex’s entire product offering at their website:
Follow up to an article in the May & June Newsletter about the ‘HydroClaw’
Great news from my friends the Perry brothers who invented the expansion tank mounting device they call the ‘HydroClaw’. Our mutual good friend, Dan Holohan of HeatingHelp.com featured the HydroClaw in his recent newsletter and the Perry’s established a website as well. You can see the site at this address:
You can purchase the HydroClaw at:
1175 Erickson Ave.
Harrisonburg, VA. 22801
New classes posted on the website
Please keep checking my website for an upcoming class near you. I have scheduled classes in Florida, Western Canada, New York and New Jersey and I believe I will be posting classes for the Chicago area as well as more classes in Ontario Canada very soon. Subjects include mini splits, geothermal, commercial water source heat pumps and hydronic pumping…sign up for a class today!
It has been a while since I had the space to share with you some musical insights and discoveries I have made recently…I decided in this newsletter to make the space because I just had to turn you on to a new artist.
Jason Casterlin has released his debut CD entitled ‘Old Stone Church’. If I were to label Jason’s genre I would say he has a heavy ‘country’ influence. Now, usually that is not my own taste, but I find I can’t keep this CD out of my player in my pickup truck, (appropriate I guess…country music in a pickup truck).
I have always been a sucker for a pedal steel and Jason has a fellow named Jay Ganz who plays a crazy pedal steel and enriches each track he plays on.
I know Jason because his day job is at the local guitar shop that I frequent in Newburgh, NY. Imperial Guitar & Soundworks. I think Bill Imperial may need some help soon as I predict Jason will be quitting his ‘day gig’ to hit the road to the big time as a result of this incredible CD.
You can buy Jason’s CD at iTunes and Amazon and go to his website at:
Need a great luthier?
Those of you obsessed with guitars like I am need a good luthier to keep your axes in good shape. I recently got turned on to a great luthier named Scott Kopec right here in Orange County, NY.
Scott was the guy who established and ran the guitar shop at Alto Music in Middletown, NY for many years and he is now out on his own. I recently brought Scott one of my 1930’s Kay Kraft venetian archtops which was really unplayable and in need of some serious work. Scott worked his magic on this very unique guitar and I couldn’t be happier with the result. I left Scott my other Kay Kraft as well as two other guitars…as a collector, finding a great local luthier is invaluable.
You can reach Scott through his website at:
See you all soon!