Superheat and subcooling calculator

What the two numbers measure

Superheat is how many degrees above its boiling point the vapour leaving the evaporator has been heated. It tells you whether the evaporator is being fed enough refrigerant. Subcooling is how many degrees below its condensing point the liquid leaving the condenser has been cooled. It tells you how much liquid is stacked in the condenser — which is to say, how much refrigerant is in the system.

Both are the same subtraction: measure a pressure, convert it to a saturation temperature, and compare that to the actual line temperature at the same point. The calculator reads the saturation temperature off the refrigerant makers' published tables — the same numbers as the R-410A and R-454B pressure chart — using the dew column for superheat and the bubble column for subcooling — and it flags a reading that cannot be physically right instead of grading it. On a fixed-orifice system the target needs the indoor wet bulb; if you only have a dry bulb and a humidity reading, the wet bulb temperature calculator converts them.

Sources: Arkema, Forane 410A Saturation Pressure-Temperature Data (psig) (read 2026-10-18) · Chemours, Thermodynamic Properties of Opteon XL41 (R-454B), Engineering Units (read 2026-10-18)

Which one to charge by

Target superheat for a fixed orifice

On a fixed-orifice system the target moves with conditions. The standard field approximation is (3 × indoor wet bulb − 80 − outdoor dry bulb) ÷ 2. At 63 °F indoor wet bulb and 95 °F outdoor that is 7 °F; on a mild 75 °F day with the same indoor conditions it rises to 17 °F. Charging to a fixed "10 degrees" on a cool day overcharges the system every time.

Before you touch the gauges

Most charge complaints are not charge problems. Confirm airflow first: a dirty filter, a collapsed flex duct or a blocked return will drag suction pressure down and look exactly like an undercharge. Take the static pressure reading with the static pressure calculator before you connect a manifold, and check the coil and condenser are clean.

Section 608 of the Clean Air Act (40 CFR part 82, subpart F) bans intentional venting of ODS refrigerants and their substitutes, including HFCs, during service, repair or disposal. R-454B adds its own A2L handling requirements on top, and the reason new equipment carries it is regulatory: EPA's Technology Transitions programme under the AIM Act sets sector and subsector limits on HFC use in refrigeration, air conditioning and heat pumps. The R-454B guide covers what changes in the van and on the roof.

Sources: US EPA, Stationary Refrigeration and Air Conditioning (read 2026-09-04) · US EPA, Technology Transitions (page last updated 2025-12-03) (read 2026-09-04)