Do you charge refigerant on the high or low side

So, you have decided to do a repair on your AC or HVAC system. Maybe it was blowing lukewarm air, or you are just the type that loves to tinker around, knowing where and how to add refrigerant is the difference between a chilly living room and a destroyed compressor. Do You Charge on the High or Low Side? The short answer is that it depends on the state of the system (whether it is running or empty) and the type of refrigerant you are using. 1). System is running/ refrigerant top up If the system is running and you are just topping up a bit of refrigerant, you charge through the Low-Pressure Side. The logic is that the pressure of the tank is usually higher than the low side pressure, so it literally sucks the refrigerant out of the bottle into the system, secondly once the liquid refrigerant leaves the bottle into the low side, it turns to vapor and the low side is mostly vapor(gas) which is safe for the compressor. Safety Warning: Never, ever charge though the High Side valve while the compressor is running. The pressure inside the system AC is much higher than in your canister; if you open that valve, the AC will "charge" your canister instead, which can cause the tank to burst. 2). System is empty/ fresh start If you have evacuated the system, and the unit is off, you should start by charging into the High-Pressure Side (Liquid Line) because the system is empty and you want to get as much refrigerant in as fast as possible. This also allows you can dump liquid refrigerant into the high side safely because the compressor isn't moving. You are supposed to stop once the pressure in your tank matches the high side pressure. How to tell which is the low side and which is the high side? The suction or low side is always the larger tube going back to the compressor. The compressor discharge or the high side has the smaller tubing, and it is hot or warmer to the touch compared to the low side. Charging Tip: Since you can't send liquid into a running compressor (it will "slug" and break the valves), you must throttle the valve on your gauges. Open it just a tiny bit so the liquid turns into a gas inside the hose before it actually enters the AC unit. What Should Your AC pressured on the HP and LP side be? One very common question is what the optimum pressure in an AC system that is required to cool a space. The answer is that the optimum pressure in an AC system for proper cooling is not a single number, but rather a range that depends on the refrigerant type, outdoor ambient temperature, and indoor humidity. For R-410 the operating pressure typically ranges from 102 to 145 PSIG on the LP side For R-R22 the operating pressure typically ranges from 58 to 85 PSIG on the LP side To eliminate guess, work the best way is to use the actual indoor conditions and make your calculation. Step 1 - Measure Indoor Wet Bulb Temperature (using a psychrometer or wet bulb thermometer) Step 2 - Subtract 35°F(1.7°C) from the indoor wet bulb to get your target evaporator temperature Step 3 - Convert that evaporator temperature to pressure using a refrigerant pressure-temperature (PT) chart for your specific refrigerant Your answer gives you a specific target based on YOUR conditions, not a generic range. Remember to use the PT chart for the refrigerant you are using.