Refrigerant Charge Calculator
Evaluate refrigerant charge status using subcooling and superheat measurements for TXV and piston metering devices.
About this calculator
This calculator evaluates whether an air conditioning or heat pump system's refrigerant charge is correct, using the diagnostic method appropriate to its metering device. Metering Device (1 = TXV, 2 = Piston/Cap tube) is a coded mode switch, not a continuous quantity: for a TXV system, Charge Status and Estimated Adjustment are computed entirely from the subcooling deviation (Measured Subcooling minus Target Subcooling), and Measured Superheat has zero effect on either output. For a piston or capillary-tube system, the roles reverse completely -- Charge Status and Estimated Adjustment come entirely from the superheat deviation, and Measured Subcooling has zero effect.
This flips fully at the coded boundary: with subcooling 3 degrees F low and superheat 8 degrees F high, a TXV system reads Charge Status as Low with a 12 oz suggested addition, while a piston system with subcooling still 3 degrees F low but superheat exactly on target reads Charge Status as Correct with no adjustment at all, because the piston branch never looks at the subcooling number. Lineset Length beyond the 15 ft standard adds an estimated 0.6 oz per extra foot, but only when the diagnosed status is already Low. This is a field diagnostic aid, not a substitute for manifold gauge readings and manufacturer charging charts -- always verify against the equipment's actual charging chart before adding or removing refrigerant.
Inputs
ASHRAE Refrigeration Handbook: TXV systems target subcooling 8–12°F; verify against manufacturer charging chart
Results
Charge Status
2 (1=Low, 2=Correct, 3=High)
Estimated Adjustment
0 oz
How to Use This Calculator
- Enter Measured Subcooling, Target Subcooling, and Measured Superheat.
- Set Target Superheat, Lineset Length, and Metering Device.
- Review Charge Status ((1=Low, 2=Correct, 3=High)) and Estimated Adjustment (oz).
- Use Subcooling Deviation (°F) and Superheat Deviation (°F) to inform your decision.
What each input means
- Measured Subcooling
- Measured liquid line subcooling (condenser saturation temp minus liquid line temp).
- Target Subcooling
- Manufacturer's specified target subcooling for TXV systems per ASHRAE Refrigeration Handbook and manufacturer charging charts. Typically 8–12°F for TXV; shorter linesets tend toward lower values. Always use nameplate or charging chart.
- Measured Superheat
- Measured suction line superheat (suction line temp minus evaporator saturation temp).
- Target Superheat
- Target superheat per manufacturer or charging chart. Typically 10–15°F.
- Lineset Length
- Total length of refrigerant lineset. Standard is 15 ft; additional length may require extra charge.
- Metering Device
- Metering device type: 1 = TXV (use subcooling method), 2 = Piston/Cap tube (use superheat method).
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersMeasured Subcooling = 10, Target Subcooling = 10, Measured Superheat = 12, Target Superheat = 12 = 6 input(s) provided
- Calculate Charge StatusCharge Status2 = 2
- Calculate Estimated AdjustmentEstimated Adjustment0 = 0
- Calculate Subcooling DeviationSubcooling Deviation0 = 0
- Calculate Superheat DeviationSuperheat Deviation0 = 0
Engine last updated . Checked against 3 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Does the metering device selection actually change which measurements matter?
Yes, completely -- selecting Metering Device = 1 (TXV) makes Charge Status and Estimated Adjustment depend entirely on the subcooling deviation and ignore superheat, while Metering Device = 2 (Piston/Cap tube) reverses that: those same outputs then depend entirely on the superheat deviation and ignore subcooling. Neither measurement set is blended between the two modes.
Why did the calculator say the charge was correct even though my subcooling was off?
If Metering Device is set to 2 (Piston/Cap tube), Charge Status and Estimated Adjustment are computed only from the superheat deviation -- Measured Subcooling and Target Subcooling have no effect on those two outputs in that mode, so a subcooling reading that's off target won't change the diagnosis unless the superheat reading is also off.
How does lineset length affect the recommended charge adjustment?
Lineset Length only adds to Estimated Adjustment when the system is already diagnosed as Low: every foot of lineset beyond the 15 ft standard adds an estimated 0.6 oz to the suggested addition, but that lineset allowance doesn't apply at all when the charge is already correct or high.
What subcooling or superheat deviation counts as low or high charge?
For a TXV system, subcooling more than 2 degrees F below target reads as Low charge and more than 2 degrees F above target reads as High. For a piston or cap-tube system, superheat more than 3 degrees F above target reads as Low charge (too little refrigerant) and more than 3 degrees F below target reads as High -- readings inside those bands are reported as Correct.
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