Arkansas HVACR NewsMagazine March 2021

S tate, National, Chapter News Tech News

control the superheat (AC mode) or the subcooling (heating mode). We aim to get these values to specific numbers, called superheat/subcooling targets. Superheat/subcooling targets are anywhere from 8-15° F 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 again later. Four-Way Valve : Commonly called the reversing valve in unitary equipment, a four-way valve diverts refrigerant 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: 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 10k thermistors, which come in PTC or NTC varieties. Protection Modes : 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 all look 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 stepdown 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 on the compressor’s discharge line is too hot. This could mean any number of things, but one of which could mean it’s low on refrigerant. The machine will limit the compressor speed to save the compressor from being damaged. • Inverter Temperature Step: If the temperature of the inverter board heat sync becomes too hot, inverter operation could stop, or compressor speed will slow down to prevent damage to the board. You could run into this if someone recently replaced the board without applying heat sync paste. It’s also imperative that all screws get replaced in the proper position, or you will not get good contact on the heat sync, and the board will likely overheat.