What is the definition of “critical charge”? The American Society of Heating, Refrigeration and Air Conditioning Engineers ASHRAE, defines critical charge as follows:

“Refrigerant quantity required by a system to maximize performance when a capillary or fixed restriction
expansion device is used.”

The inverter mini split world has been using the term critical charge in describing the critical nature of the charge in their systems but if we look at the literal definition of critical charge, it is an inaccurate use of the term…I have been guilty of this myself.

Most inverter mini splits utilize an Electronic Expansion Valve EEV as the refrigerant metering device. The EEV by its anatomy is NOT a “fixed restriction expansion device” like a traditional capillary tube or Thermostatic Expansion Valve TXV.

The following comes from an article written by John Tomczyk, a professor of HVACR at Ferris State University and published in the August 2, 2004 issue of The ACHR News:

“EEVs control the flow of refrigerant entering a direct expansion evaporator. They do this in response to signals sent to them by an electronic controller. A small motor is used to open and close the valve port. The motor is called a step or stepper motor. Step motors do not rotate continuously. They are controlled by an electronic controller and rotate a fraction of a revolution for each signal sent to them by the electronic controller. The step motor is driven by a gear train, which positions a pin in a port in which
refrigerant flows.”

In my GREE mini split troubleshooting training, I dissect a GREE EEV and can actually operate it while holding it in my hand with a simple magnet for demonstration purposes. A GREE EEV has a total of 480 “steps” from totally open to totally closed…and yes, a GREE EEV does give you 100% shut-off.

So, by the strict interpretation of the ASHRAE definition of critical charge and the nature of the anatomy and function of an EEV, the term critical charge does not apply to most inverter mini splits…so what do we call it?

Well, here is my proposal…Non-Fixed Orifice Critical Charge NFOCC.

Please, don’t try to make an acronym out of NFOCC!

Are we splitting hairs here? Maybe…but this misuse of the term critical charge was brought to my attention in a mini split class I conducted in Canada some years back and I had to admit to the faux pas. The mistake is that we have been using critical charge as a noun and we should have been using it as an adjective.

Another area where the literal definition of critical charge does not apply to some inverter mini splits,
i.e. GREE, is the fact that it is NOT necessary for an installer to remove refrigerant should he or she not run a lineset equal to the factory pre-charge of refrigerant. Where a receiver or second accumulator is

present in these systems, any unused refrigerant will be stored in the receiver or accumulator. In smaller GREE systems that do not have a second accumulator; unused refrigerant will be stored in the single, purposely oversized accumulator. If the charge was truly defined as critical charge, refrigerant would need to be removed in this scenario.

This all said…the charge in inverter mini splits is…here I go again….CRITICAL, but now I am using the word critical as an adjective. These systems can be a hot mess (pun intended) when the charge is not correct. I have experienced this first hand…you can convince yourself that the problem is in a control board (either indoor unit or outdoor or both) and in fact your problem is being under charged.

In my August, 2018 edition of this column, I spoke about not putting all your trust in the onboard
diagnostic systems that many inverter mini split systems offer. I stress to always “put a gauge on it” to
prove or disprove what the diagnostics are telling you.

I express this same sentiment in every mini split troubleshooting training event that I conduct and recently an attendee challenged me about putting a manifold gauge set on an inverter mini split. He said that “other manufacturers don’t want you to put manifold gauges on their mini splits because you will lose too much refrigerant in the hoses and manifold.” He used the term, “critical charge” when describing the charge of an inverter mini split.

I know there are inverter mini split manufacturers out there who indeed discourage the use of traditional manifold gauges and I have always been baffled by this. Inverter or not…mini split or unitary…basic diagnostics in all refrigeration systems are rooted in temperature and pressure. The
concern about loss of refrigerant in the process is an exaggeration at best and just not based in fact at worst.

Let me digress for a moment…

I have done a lot of work with water source heat pumps over the years…very popular in low – midrise buildings here in New York and the surrounding area. A 9,000 BTUH water source heat pump can
contain just 18.6 ounces of refrigerant…that’s it! The use of a traditional manifold gauge set and hoses is mechanical suicide because the loss of any refrigerant in a system that contains just 18.6 ounces will have a distinct, negative effect.

Gauge manufacturers like Appion, have developed digital “stub” gauges, (no hoses and no manifold) just for use in systems such as Water Source Heat Pumps WSHP.

In contrast, a 9,000 BTUH GREE SAPPHIRE model SAP09HP230V1A contains a minimum of 45.9 ounces, or 2.87 pounds of R410A refrigerant when used with the minimum 10’ lineset, but could have as much as 50.7 ounces, or 3.17 pounds when the maximum lineset length of 49’ is reached.

Any theoretical loss of refrigerant as a result of utilizing a traditional manifold gauge set and hoses in this scenario will be negligible, and if you are still concerned, the use of digital “stub” gauges on inverter mini splits would negate any concern.

So, in conclusion, remembering your anniversary is CRITICAL. Remembering Valentine’s Day is CRITICAL.
The correct refrigerant charge in an inverter mini split is CRITICAL but it is not critical charge.

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