Updated: Jun 26
A question that I get asked frequently is “Can I check the charge of a mini-split ductless unit with superheat and subcooling the same way I do on a conventional air conditioner or heat pump?". The answer is not exactly, but you can do some troubleshooting.
A mini-split differs from a standard central air conditioning system in a variety of ways. Conventional split systems usually have a TXV (Thermostatic Expansion Valve) or a piston metering device, a single or two speed compressor, a single or two speed condenser fan, and a multi-speed indoor blower motor. The speed of the condenser fan and the indoor blower match the heat removal capacity of the compressor speed during air conditioning mode. The same goes for during heating mode. In most cases, on standard split systems, we are dealing with a single speed compressor with a single speed condenser fan and a fixed speed on the indoor blower motor. This means that the system is always running at a constant known speed and capacity.
Most current mini-split systems are VRF (Variable Refrigerant Flow). These systems include an EEV (Electronic Expansion Valve), a DC inverter compressor along with a DC powered condenser fan and DC powered indoor blower motor. Because all these components are powered with DC voltage, they can all be ramped up and down in capacity by the main control board. The control board uses temperature and sometimes pressure sensors in a variety of locations in order to control the operation and safeguard the system.
A mini-split system may be equipped with one indoor head unit or multiple indoor head units. When there are multiple indoor head units and the piping is directly mounted from each of the indoor head units to the outdoor unit, there is an EEV for each head unit in the outdoor unit. Regardless whether there is one EEV for a single indoor head system or there are two EEV’s or more for a multi head indoor unit, there is usually a temp sensor mounted by the manufacturer on both sides of each EEV, even if the sensor is a little distance away. The system will also have other temp sensors as well. The temp sensors are monitoring the compressor discharge temp, the saturated refrigerant temp in both coils, and the temp on the refrigerant lines exiting both the condenser and the evaporator coil. The EEV will not only adjust for an efficient superheat but it will also maintain a steady range of vapor pressure depending on the conditions. This means that if you hook your pressure gauge onto the available vapor line port, it will give you a vapor pressure that you can’t do much with because you don’t know exactly what vapor pressure the system is targeting at that moment in time. However, if the pressure converted to saturated temperature is below 32°F, this would signal a problem and eventually a frozen evaporator coil.
Now that we know that a VRF mini-split ramps up and down in refrigerant flow and heat removal capacity, let’s move back a bit and talk about a fixed speed system. A properly charged system in air conditioning mode with a standard single speed compressor and a TXV which has correct airflow and no other problems, will usually operate with roughly 8°F to 15°F of subcooling and roughly 8°F to 15°F of superheat. You can determine the exact target subcooling based on the target subcooling posted on the outdoor unit rating plate or under the outdoor unit shroud. The TXV will usually hold the superheat around 10°F to 14°F but it may fluctuate to around 8°F to 17°F depending on the conditions. We know this to be the case in systems that have a single speed compressor with a fixed airflow speed at the indoor and outdoor unit. (To learn more about subcooling and target subcooling, and measurement locations, read this article. To learn more about superheat, total superheat, and target superheat, and measurement locations, read this article.) However, a VRF mini-split in air conditioning mode will ramp up and down the speed of the compressor, the indoor and outdoor fans, as well as the volume of refrigerant moving through the EEV. To get a useful measurement from a mini-split, we must lock it in full speed air conditioning mode. Sometimes this is called emergency operation or full air conditioning speed.
You will notice on most mini-splits, there is only one vapor port and no liquid port on the outdoor unit service valves. Sometimes there will be one main vapor port even if there are multiple indoor head units. On other outdoor units, there will be one vapor service port for every indoor head unit. The biggest reason that you don’t see a liquid service port on the outdoor unit is because on a mini-split, the EEV is mounted inside the outdoor unit. Therefore, if you had a port on the liquid service valve, you would be measuring the low pressure exiting the EEV. This will be roughly same pressure as the saturated refrigerant in the middle of the evaporator coil. (This is also why you see the liquid line is insulated as well as the vapor line on mini-split units. Both low temperature lines will attract humidity and absorb heat if there is no insulation on this line set tubing.) A liquid pressure measurement at the liquid line service valve wouldn’t be useful to calculate subcooling and no manufacturer wants you to get confused so no port is mounted there. Some outdoor mini-split units have a liquid service port mounted inside the outdoor unit, underneath the outdoor unit shroud, prior to the EEV(s). This can be used for troubleshooting to determine if there is at least some subcooling prior to the refrigerant entering the EEV(s) but because the manufacturer does not provide us with a target subcooling, this port is not necessarily used to check the charge.
Now we are getting to the good stuff. A VRF mini-split is locked in emergency cooling via a button on the indoor head unit, a dip switch or control program, or when the temperature on the thermostat is turned down very low in temp along with the high fan speed setting and after a time requirement, the unit will run at full capacity. Each unit is a little different.
You will notice that a VRF mini-split running at full capacity during cooling mode will generally have a higher Delta T than a conventional single speed split unit. Delta T is the change in temp across the evaporator coil. Delta T will also depend on the heat load in the building, including the humidity. Keeping that in mind, usually during average humidity and temperatures, the Delta T of a mini-split may average 20°F to 24°F instead of 18°F to 21°F on a single speed unit. (For more information on Delta T, make sure to check out our article on it!) This is because a mini-split will run at a lower superheat across the evaporator coil. This means that there is more saturated refrigerant in the coil to absorb the heat load. Usually on a single speed system, we are worried about maintaining a good total superheat so that the compressor does not have saturated refrigerant entering it. However, on most mini-split units running at full cooling capacity, the total superheat measured at the outdoor unit will typically be between 0°F to 5°F. Often times it is right in the middle at 2°F or 3°F. Remember that if superheat is present on the vapor line where the refrigerant enters the compressor, then we know that the refrigerant is no longer saturated but is fully in the vapor form. The vapor compressor must only have vapor refrigerant enter it or it will become damaged. (To learn more about superheat, read this article.)
How can a mini-split system run with 0°F of superheat and not kill the compressor you ask? A mini-split has a rotary style compressor which always has a type of accumulator mounted to it’s side. This would be considered an accumulator with a vaporizing screen inside. Prior to that in the refrigerant circuit, may be another accumulator. This second larger accumulator’s job is to store extra refrigerant when it is not needed. Remember that the system will ramp up and down in capacity depending on the need. Also remember that unlike a single speed system, a mini-split comes with enough refrigerant for a range of line set length. Because there is a range of line set length, this equates to a range of total refrigerant weight that is allowed in the system while still being able to operate correctly. The amount of refrigerant is usually not an exact amount like in a single speed system due to this range.
Getting back to our point though, an accumulator’s job is to store saturated refrigerant and it only allows vapor refrigerant along with a little bit of oil to exit the accumulator and to enter the compressor. (To learn more about the accumulator, check out this video.) Because an accumulator will only allow vapor refrigerant to exit the tank, the vapor compressor will be safe from liquid or saturated refrigerant entering in. Because most larger mini-splits have two accumulators and the last one right before the compressor inlet includes a vaporizing screen, the compressor is very safe as long as the system is not severely overcharged. Smaller mini-split units may just have the one single accumulator with the vaporizing screen which is mounted on the side of the compressor. If there is only one accumulator present, this single accumulator will typically be larger in size and will be enough to protect the compressor except if the system is severely overcharged.
During emergency cooling mode, remember that it must be above 70°F indoors and outdoors before taking any measurements and the system must be running for at least 10 minutes. A total superheat of 0°F to 5°F, a saturated temperature above 36°F, and a Delta T of 20°F to 24°F is a very good indication that the system is charged properly. However, this does not mean that we should charge a system that is low on refrigerant to these numbers. These are merely common indicators that are seen on systems with an accurate charge and this should not be mistaken as a charging method. Remember that targeting a certain vapor pressure is futile because the system is monitoring the vapor saturated temperature constantly and is adjusting the EEV for an optimal vapor pressure and superheat. Of course, if you notice a vapor pressure converted to saturated temperature that is below 32°F, then there is likely a problem such as low refrigerant charge, liquid line restriction or low indoor airflow. You can notice low airflow right away by measuring the CFM (Cubic Feet Per Minute) using a rotating vane anemometer at the indoor head units and comparing that to the BTU/HR capacity of each head unit. You are looking for roughly 400 CFM per 12,000 BTU/HR of capacity but 350 to 425 CFM will still work. If you have an available liquid port inside the outdoor unit, you can check subcooling. If the subcooling is very low such as 1°F or 2°F, the system is low on refrigerant if you are running at full capacity cooling mode.
Remember that you cannot see the amount of refrigerant that is in each accumulator and you don’t want to overcharge the system to predetermined pressures or temperatures that you have in your mind. Some manufacturers may provide certain pressure and temperature specifications along with an average compressor current, but this is usually a rare occurrence. Remember that if your superheat is a little higher than 5 degrees, maybe 5 to 10 degrees, it may not mean that the system has a problem such as a low refrigerant charge. There are many sensors and settings on these mini-split units and you don’t want to take the approach of just adding more refrigerant to see if the system will work better or be closer to certain standards that you would like to see. If you overcharge the unit a bit, the unit may not allow itself to run at full cooling capacity because of a high compressor current or high discharge temp. Just because you see an EEV hunting by reading the total superheat, it doesn’t mean that the unit is low on refrigerant. It could mean that the unit is overcharged so don’t just add more refrigerant to the system! However, if during emergency cooling mode, the superheat is 15°F, 20°F, or higher, the unit may be low on refrigerant or have a liquid line restriction. An example of a liquid line restriction could be an EEV that is not opening up enough maybe due to corrosion between the valve coil and the valve body affecting the magnetic field and therefore not allowing the inner stem to turn to open the inside of the valve. If the system has a liquid line restriction, it would not be wise to add refrigerant to the system thinking it was low on refrigerant.
In the example below, we see an R-410A saturated temperature of 27°F and a vapor line temp of 56°F.
Total Superheat = Actual Vapor Line Temp - Sat Temp
Total Superheat = 29°F so there is a problem such as one of the following:
low refrigerant charge, low airflow, or liquid line restriction
The best thing to do on a mini split system that is not working properly or is thought to have a low refrigerant charge is to search for leaks. Look for oil stains where refrigerant and oil have seeped out. Apply bubble leak detector at the joints, and/or use a refrigerant leak detector such as the ultrasonic leak detector to find the leaks. (Here are links to these products: bubble leak detector, ultrasonic leak detector) Then recover the refrigerant, fix the leaks, pressure test, vacuum, and weigh in the total amount of refrigerant needed for the system. Breaking the vacuum with the correct amount of refrigerant needed for the system is referred to as the “Total Weight Method”. The total weight of refrigerant needed is usually posted on the outdoor unit rating plate and is listed as the “factory charge”. This includes the amount of refrigerant needed for the outdoor unit, the indoor unit(s) and a specified length and size of line set. Additional refrigerant weight may need to be added for line set that exceeds the total cumulative length stated on the outdoor unit rating plate.
Remember that if you think the system has an electrical based problem, it may just be undercharged or overcharged and the system is compensating for that incorrect charge. Once you know that the correct amount of refrigerant is inside the unit, you can confidently troubleshoot the problem but often, the problem goes away because it was an incorrect refrigerant charge to begin with. If you notice a severe electrical problem or communication issue instead of a capacity issue, then yes, look at and troubleshoot the electrical components.
The total weight method is the recommended charging procedure for verifying the refrigerant level in a unit. If the unit is new, it will have the factory charge already inside and the refrigerant will be held back by the service valves. You can open the service valves after pressure testing, vacuuming, and performing the standing vacuum test. If additional refrigerant needs to be added to the system due to an extra long line set run, break the vacuum with the correct amount of refrigerant from the bottle by using an electronic scale. (Here is a link to an electronic scale.) After breaking the vacuum with refrigerant from the bottle, open the service valves.
Here is a Total Weight Method example:
On the rating plate, the R-410A system lists the factory charge as 4lbs 4oz. The rated cumulative line set length for this factory charge is 25’ to 90’. (Cumulative means that if there is more than one head unit, add the line set lengths for each head unit together to equal the cumulative length.) If the actual cumulative length was measured to be 125’, you would need to add enough refrigerant for 35’ worth of line set.
125 - 90 = 35’
The unit says to add 1.08 oz for every 5’ of additional line set added.
35 / 5= 7
7 x 1.08 oz = 7.56 oz additional refrigerant needed
Check out our Total Weight Method article including examples and the refrigerant weight chart by clicking here: https://www.acservicetech.com/post/the-total-weight-method
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Published: 5/6/2020 Author: Craig Migliaccio
About the Author: Craig is the owner of AC Service Tech LLC and the Author of the book “Refrigerant Charging and Service Procedures for Air Conditioning”. Craig is a licensed Teacher of HVACR, Sheet Metal, and Building Maintenance in the State of New Jersey of the USA. He is also an HVACR Contracting Business owner of 15 years and holds an NJ HVACR Master License. Craig creates educational HVACR articles and videos which are posted at https://www.acservicetech.com & https://www.youtube.com/acservicetechchannel & https://www.facebook.com/acservicetech/