What you need
- A gauge manifold, digital or analog, set to the refrigerant on the rating plate.
- Two pipe-clamp or strap-on temperature sensors, one for each line.
- For fixed-orifice systems: a wet bulb reading of the return air (wetted wick or digital hygrometer) and a dry bulb sensor for the air entering the condenser.
California's Title 24 charge test is a useful benchmark for instrument quality. It calls for pipe and air temperature sensors accurate to 2°F, digital refrigerant gauges accurate to 3.5 psi on the suction side and 7 psi on the liquid side, a monthly ice-bath check of temperature sensors (32°F, within 1°F), and a gauge check against a refrigerant cylinder that has sat at a steady temperature for at least 4 hours, compared with the PT chart pressure for that temperature.
Setting up
- Connect the gauges to the service ports without letting air into the system.
- Clamp one sensor on the suction line near the suction (low side) service valve and one on the liquid line near the liquid (high side) service valve, positioned to make good contact with the line. That is the Title 24 placement. Carrier's R-410A instructions put the suction sensor at the service valve and the liquid sensor near the outdoor coil, and Trane reads liquid line temperature at the outdoor unit's service valve. Some manuals differ: one Daikin manual has the suction thermometer 4 to 6 in from the compressor on piston systems, insulated for the best reading. Follow the manual for the unit.
- Shade the condenser air sensor so it reads the air entering the coil, not the sun.
- Put all cabinet panels back on. Open panels change airflow and the readings.
- Run the system in cooling for at least 15 minutes before reading, as Title 24 and Carrier specify. Payne's heat pump instructions also check charge after at least 15 minutes in cooling, and Trane calls subcooling in cooling the only recommended charging method above 55°F outdoors.
Measuring superheat
- Read the suction pressure.
- Convert it to the saturation temperature on the refrigerant's PT chart. For a blend, use the dew point.
- Read the suction line temperature.
- Superheat = suction line temperature - saturation temperature.
Example, R-410A: suction 123.0 psig is a 42°F dew point. A 54°F suction line gives 12°F of superheat. The superheat calculator does the lookup.
Measuring subcooling
- Read the liquid line pressure.
- Convert it to the saturation temperature. For a blend, use the bubble point.
- Read the liquid line temperature.
- Subcooling = saturation temperature - liquid line temperature.
Example, R-410A: liquid 366.4 psig is a 110°F bubble point. A 100°F liquid line gives 10°F of subcooling. The subcooling calculator does the lookup.
Which number to charge by
Carrier's instructions and the Title 24 procedure charge fixed-orifice (piston or capillary) systems by superheat, against a target from the manufacturer's superheat charging table that depends on indoor wet bulb and outdoor dry bulb. TXV and EEV systems are charged by subcooling to the target on the rating plate or charging label, and Title 24 then checks superheat as a test of the metering device. The DOE Building America diagnostics guide reads the pair together: low superheat with high subcooling typically means overcharge, high superheat with low subcooling undercharge, and high-high or low-low points to some other fault.
Readings that mislead
- Poor probe contact. Title 24 calls for pipe sensors positioned to make good contact with the line, and Daikin says to insulate the suction thermometer. Superheat and subcooling are only as good as the line temperature.
- Not settled. Readings in the first minutes after start-up or after a charge change are not steady. Title 24 runs the system 15 minutes after the final adjustment before the final measurements; Trane waits 20 minutes between adjustments.
- Bad airflow. Carrier's superheat procedure is valid only with indoor airflow within 21% of rated, Daikin says to verify airflow before charging, and Title 24 requires a minimum airflow across the cooling coil at the time of charge verification.
- Wrong conditions. Charging tables have limits. Payne's instructions describe favorable conditions as 70 to 100°F outdoors and 70 to 80°F indoors; Title 24 requires return air at 70°F or warmer. Below 55°F outdoors, Trane calls for weighing in the charge and returning when the weather allows a subcooling check.
- Wrong chart or column. A manifold set to the wrong refrigerant, or the bubble column used for superheat on a blend, shifts the result by degrees. See how to read a PT chart and bubble point, dew point and glide.
- Uncalibrated gauges. Title 24's monthly field check zeroes the gauges open to air, then compares a cylinder's pressure with the PT chart pressure for its measured temperature; a gauge that does not match needs recalibrating.