Why there is no single right pressure
This page covers home central air conditioners and heat pumps. It does not cover car air conditioning. Inside a running AC, refrigerant boils in the indoor coil (evaporator) and condenses in the outdoor coil (condenser). Wherever it is boiling or condensing, its pressure is fixed by its temperature, as a PT chart shows, so the U.S. Department of Energy's Building America diagnostics guide reads evaporator and condenser saturation temperatures straight from the low and high side gauges. The low side tracks how cold the indoor coil is. The high side tracks the condensing temperature, which the guide measures as a split above the temperature of the air entering the condenser. A correct pressure on a 75°F day is wrong on a 100°F day.
The tables on this page are estimates built from the two rules below. They show a range to compare against, not a target. Equipment manufacturers publish their own pressure curves and check charts, and those govern. Even those are checks rather than charging tools: Trane's pressure curves are for verifying typical performance after charging by subcooling, and Payne says not to use its heating check chart to adjust refrigerant charge.
Low side: the evaporator temperature
The DOE guide's Table 3 gives minimum evaporator saturation temperatures of 35°F at 62 to 63°F indoor wet bulb, 40°F at 64 to 65°F and 45°F at 66 to 67°F. It flags readings below 28°F as a sign of undercharge, a liquid line restriction or a fouled evaporator coil, and says readings above 45°F are seen at outdoor temperatures above 100°F, with an overcharge or with an expansion device overfeed. The tables use a 35 to 45°F coil:
- R-410A: 107.3 psig to 130.1 psig
- R-454B (dew point): 96.9 psig to 117.8 psig
- R-32: 109.7 psig to 133.0 psig
- R-22: 61.5 psig to 76.1 psig
A healthy system can sit outside that band. Trane's sample pressure curve for one R-410A heat pump shows a suction pressure at 82°F outdoors and 67°F indoor wet bulb that corresponds to a 47.0°F coil.
High side: outdoor temperature plus the condenser split
The DOE guide calls the split condensing temperature over ambient (COA): the condensing saturation temperature minus the temperature of the air entering the condenser. Its Table 3 lists maximum COA values of 25°F for equipment above 12 SEER, 30°F for 10 to 12 SEER and 35°F below 10 SEER. A high COA can indicate an overcharge, a liquid line restriction or non-condensable gas; a low COA suggests an undercharge or an expansion device overfeed. For a working example, Trane's sample pressure curve for the same heat pump shows a discharge pressure at 82°F outdoors that corresponds to condensing at 99.2°F, about 17°F over ambient.
The tables use 17 to 30°F over outdoor air: the low end is that Trane example and the high end is the DOE maximum for 10 to 12 SEER equipment. Equipment above 12 SEER should stay at or below the DOE's 25°F maximum. On an 85°F day:
- R-410A: 327.7 psig to 392.3 psig
- R-454B (bubble point): 309.5 psig to 370.1 psig
- R-32: 335.1 psig to 401.4 psig
- R-22: 201.8 psig to 242.8 psig
Reading your gauges against the chart
- Let the system run in cooling for at least 15 minutes before reading, as the DOE guide and Carrier's instructions both specify.
- Measure outdoor temperature at the air entering the condenser, with the sensor shaded from direct sun (DOE guide).
- Compare the low side with the low side range and the high side with the row for your outdoor temperature.
A reading outside the range is a reason to look further, not to add refrigerant. Daikin's instructions say not to adjust charge based only on suction pressure except for general charging in a gross undercharge. The DOE guide lists faults that move pressures besides charge: liquid line restrictions, defective expansion devices, non-condensable gases and fouled condenser or evaporator coils. It also says airflow below 350 cfm per ton makes standard charge tests invalid. Charge on home systems is set by superheat or subcooling against the manufacturer's target.
Safety and the law
- High pressure. On a 100°F day at the DOE guide's 25°F maximum split for equipment above 12 SEER, an R-410A high side reaches 448.0 psig. Carrier calls for R-410A manifold sets rated 700 psig on the high side and hoses with a 700 psig service rating. Lennox calls for R-454B gauges rated 0 to 800 psig on the high side and hoses rated for 800 psig with a 4000 psig burst rating. Daikin warns to connect and disconnect gauge hoses carefully because escaping liquid refrigerant can cause burns.
- Cold liquid. Chemours warns that liquid refrigerant can freeze skin or eyes on contact and calls for gloves, goggles and a face shield where liquid exposure is possible.
- Flammable refrigerants. R-454B and R-32 are A2L refrigerants, which Chemours describes as low toxicity and mildly flammable. See A2L refrigerants.
- Home and stationary equipment. EPA rules under Clean Air Act Section 608 require technicians who maintain, service, repair or dispose of equipment that could release refrigerant to be certified, and EPA counts attaching hoses and gauges to measure pressure as technician work. The rules also prohibit intentionally venting refrigerant and restrict refrigerant sales to certified technicians, with limited exceptions.