How they are different:
Mini splits, like their bigger cousins the VRF machines, have a lot of mystique that surrounds them that is unjustly warranted. They still operate under the same rules of thermodynamics that standard unitary equipment does, they just do it in a little different way. Let’s dive into what’s different about them.
Inverter driven compressors
Right off the start, these systems use an inverter driven compressor (some have an ac compressor, others have a dc compressor, refer to manufacturers literature or data plate) to control capacity. A circuit board either rectifies incoming power to dc then inverts it back to ac to drive the compressor or simply controls the dc output to produce a variable speed effect. Compressor speeds are a function of meeting either the target evaporator temp (cooling mode) or the target condensing temp (heating mode). TE (target evap) and TC (target condenser) calculation comes from how far off setpoint the return air temp/room controller temp/remote controller temp is. The farther off setpoint that it is, the faster the compressor will run (the lower the TE or the higher the TC). Run speed is published typically in rps (revolutions per second). The typical range on most units is 25-120 rps.
Electronic expansion valves are electronically controlled devices that meter the flow of refrigerant through the system. 1 to 1 units will have 1 expansion valve while multi splits will have 1 per port that it has. If it’s a 4 port unit, there will be 4 EEVs in the machine. Each EEV controls refrigerant flow to each indoor coil attached. EEVs are either controlling the superheat (ac mode) or the subcooling (heating mode). Superheat/subcooling targets (the number that it’s aiming to get to) are anywhere from 8-15 and typically vary depending on how far off setpoint you are. This is important information to remember as it will help you evaluate the charge without weighing it out just yet. We will touch on EEVs later.
4 Way Valve
Commonly called the reversing valve in unitary equipment, is a device used to divert refrigerant either to the indoor coil for heating mode or outdoor coil for cooling mode. Most manufacturers’ valves default to cooling and energize in heating mode.
Thermistors are devices that change resistance based upon temperature. Common thermistor types are: discharge pipe temp, gas pipe temp, liquid pipe temp, room temp, return air temp, inverter temp, and compressor body temp to name a few. There are many different thermistor ohm ratings, refer to your specific manufacturer’s literature for your temperature to resistance charts. Common ones are a 10k either ptc or ntc thermistors.
Unlike traditional unitary systems, mini splits use many forms of protection control to prevent serious damage from occurring. Different manufacturers have different limits but generally speaking they’re all looking at the same things. Some examples are:
Low Pressure Protection
Once the unit gets below a certain suction pressure, the compressor will be commanded to slow down or maintain its current speed. There’s a possibility you can get hung up in a low pressure step-down where it doesn’t get high enough to allow it to try to ramp up again, so it will look to you as if the machine is just running at a low rps.
High Pressure Protection
Once the unit hits a certain head pressure, compressor speeds will be limited and slowed down if needed to lower the discharge pressure.
Discharge Temperature Stepdown
Discharge pipe temperature step down occurs if the thermistor off of the discharge line of the compressor is too hot. This could mean any number of things but one of which could mean it’s low on refrigerant. The machine is going to limit the compressor speed in an effort to save the compressor from being damaged.
Inverter Temperature Step
If the temperature of the inverter board heatsink becomes too hot, inverter operation could be stopped, or compressor speed will slow down to prevent damage to the board. You could run into this if a board has recently been replaced and the heatsink paste was not applied during replacement. It’s also imperative that all screws get replaced in the proper position or you will not get good contact on the heatsink and likely overheating the board will occur.
This is a typical piping diagram shown in a service manual. These can be helpful tools in troubleshooting an issue. Note the red arrows indicate flow direction during heating operation and blue for cooling operation. Note that the upper line coming into the 4 way valve is always discharge gas, and the middle line coming out of the 4 way valve is always suction gas. Also note that this unit’s EEV is located in the outdoor unit, meaning the lines leaving the outdoor unit are the expansion line and the gas pipe line.
If you are going to work on mini splits, there are a couple of items that you MUST HAVE. A good voltmeter that can read a high resistance is an ABSOLUTE MUST HAVE. Most thermistor values are 5-75K ohms. If you do not have a meter that’s capable of reading that high of a resistance value, it will show you an OL even though the thermistor isn’t actually open, rather it’s just a value that’s higher than what your meter is capable of reading. If you’re unsure, check the spec sheet for your respective meter. Make sure you have a nitrogen regulator that is capable of 600 psig, that way the proper pressure test can be performed. Also having a good thermometer that’s capable of reading pipe temperature is recommended.
Let’s touch on a couple of things that could very well help you out if you are on a mini split call.
1.) Leaking Flares.
Leaking flares are likely the most common cause for a service call and one of the easiest to avoid. Using a good sharp blade on your tubing cutter, deburring the copper, making a proper flare (using an orbital flaring tool with clutch), checking it with a flare gauge, and tightening it to the proper torque settings are the keys to avoid this from happening. Expansion and contraction put the flare under a huge amount of stress changing between modes. See Bryan’s tech tip for more info. https://www.hvacrschool.com/making-flare-quick-tips/
2.) Communication Problems.
Communication problems can stem from any number of things. Hit the basics first. Make sure your wiring terminations are correct as most use a 1, 2, 3 configuration. Ensure any external safety circuit wiring like a condensate overflow switch is wired properly, drains are clear, and condensate pumps are in working order. Generally speaking, most manufacturers use the 3rd wire plus one of the other high voltage conductors as a traveler wire (or common if you want to look at it that way). If you check between either 1->3 or 2-3 (again depending on manufacturer, refer to the manufacturer for specs) but typically you are going to be looking for a fluctuating DC voltage signal from 30-100 VDC. If you remove the 3rd wire from the outdoor unit and check for VDC coming from the outdoor unit and also from the indoor unit, you could narrow your work by seeing if there is one voltage that is not fluctuating. Continue to narrow your work until you can identify the issue or the failed component.
3.) Refrigerant Leaks.
See #1 for the most common places to find refrigerant leaks. Improperly made and improperly tightened flares seem to account for a large majority of the refrigerant leaks.
How do I know if the machine is low on refrigerant? Good question. While superheat and subcooling on these can’t tell you the whole story, don’t discount the data you can gather from it. Find in the manual how the machine’s eevs are being controlled (see above explanation on how target superheat/target subcooling is achieved). Once you have established that, it is possible to at least establish whether you are low or not. I will typically run the machine in full heat or full cool (whichever the conditions allow) then observe what the superheat is doing. Most machines seem to run between 4-10 degrees of suction superheat in both heating and cooling mode. Remember that in heating mode, the big line leaving the unit is actually discharge gas (so please don’t call it the suction line cause it isn’t), so you need to take your pipe temp exiting the outdoor coil to see what the suction superheat is. Once you’ve established that it is low, it’s best practice to pull the entire charge, pressure test, evacuate, and recharge. If a flare is “loose”, please just don’t tighten it. Take it apart and remake it, it only takes a few minutes compared to it leaking again and getting a call back.
4.) EEV Issues.
Troubleshooting EEV issues can be very complicated. First you need to understand where to find the information on how to check them out. Refer to manufacturers published documents for the ratings of each type of EEV coil. Make sure you bring your needle leads or you’ll break the plug. If the motor ohms out ok, then you should be able to check the voltage coming out of the board (typically 12 VDC) and verify the board is outputting the voltage. As the valve is driving, you’ll see a pulsing of voltage so keep that in mind. If you don’t see the pulsing voltage when the valve should be driving, consider looking at the board. There are numerous EEV donut magnets that can allow you to operate the valve body by hand (https://weatherallreps.com/shop/ols/categories/specialized-tools) . *******NOTE—-The board and the valve have no way of knowing if they are out of sequence so if you go moving the valve, you need to return it to a spot that the board will know where it is at startup. I suggest running it all the way closed then cycling the power to the machine. If this is not done, compressor failure can and will occur.************* Along with the EEV donut, there is also a hand held EEV driver that fits most manufacturers EEV plugs. This allows you to operate the EEV by hand. (https://youtu.be/xUVV80BcU1M email firstname.lastname@example.org for more info) EEV issues can show up in a starved coil (typical) or a flooded coil. Knowing how to effectively measure superheat will help you determine what you’re dealing with.
5.) Incoming Power Issues.
With all of the electronics in the outdoor unit, incoming power is less forgiving than their standard equipment counterparts. They are sensitive to voltage not within their specs in addition to surges. Check and see if the manufacturer has a recommended surge suppressor. Corrections to incoming power issues and surge protection are a must. Circuit boards do not like these items and they will not last if left uncorrected.
6.) Condensate Pump Wiring.
Please follow manufacturers recommended wiring for the auxiliary contacts to shut down the unit in the event of an overflow situation. Most will break the communication wire, which in most instances is ok, but some indoor units have designated terminals and a plug for condensate overflow that should be used instead. Make sure if wiring to auxiliary terminals that you change the appropriate settings in the controls so the unit will operate correctly.
7.) Use Appropriate Connectors.
Wire nuts for communication lines is not the best practice for joining communication wiring together. Butt splices, bell connector, terminal blocks, and soldered connections are superior methods. The use of wire nuts is a common problem waiting to happen and I would not recommend using them unless you like troubleshooting common problems.
8.) Crossed Wiring/Piping.
This is a very common issue in multi splits. Installers, it is of the utmost importance that you color code/mark your line sets as it’s really easy to get them crossed up. If you happen to get them crossed up, any number of complaints can happen. The biggest one is overcooling or overheating of a space. Another one could be getting a condensate fault (especially if the unit has a condensate pump) as the condensate pump is only active when the unit is calling for cooling. If you suspect crossed wiring/piping, run each head individually one by one verifying refrigerant is flowing to the unit that you want it to. Pick up some phase tape from your local hardware store or electrical supply in a number of colors, it could really save you down the road.
In closing, stick to the basics. They really aren’t as complicated as they appear to be. Remember that all circuit boards do is take inputs, apply logic, then produce an output. Remember that poor inputs will create poor output control. For those of you who are old school programming types will remember GIGO (garbage in and garbage out). If the input from say a thermistor is reading incorrectly, the output that it is responsible for controlling (EEV) will not modulate or control correctly.