HVAC Guide
How to Check Subcooling on a Heat Pump
A heat pump charges exactly like an air conditioner in cooling mode, but the reversing valve changes the rules in heating. Here is where to measure, what to target, and when to weigh the charge in instead.
Intry Verified. The calculator's PT interpolation is computed from its cited manufacturer pressure-temperature data, and the target ranges come from the same locked refrigerant module. Those figures are computed by a locked engine straight from the source it cites, not hand-typed, and that computation re-runs inside a deterministic test on every deploy. Source-checked. Locked. This is our own deterministic gate, not a third-party audit. What Intry Verified means
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Connect Intry to Claude once, then just say “use Intry to…” to run the superheat & subcooling calculator from a plain-English question, on your phone, on the job. You get the number (computed live, not guessed), what it means, and the locked modules each figure is traced to: the manufacturer PT tables behind the saturation temperatures, superheat and subcooling, and the transcribed code and service-literature constants behind the charge, metering, airflow, approach and discharge line figures, each quoted with its own source in the panel it appears in. Works on every Claude plan.
Cooling vs Heating Mode: Which Coil Condenses
A heat pump is an air conditioner with a reversing valve. That valve decides which coil rejects heat (the condenser) and which absorbs it (the evaporator). Subcooling only means something at the condenser outlet, so the moment the valve flips, the place you measure subcooling flips with it. This is the one fact that makes heat-pump charging different from straight-cool AC.
| Mode | Condenser (where subcooling matters) | Charging indicator | Target or method |
|---|---|---|---|
| Cooling | Outdoor coil | Subcooling at the outdoor liquid line (TXV) | 8 to 14°F subcooling |
| Heating | Indoor coil | Your maker’s heating rule: a heating procedure inside its window, or weigh-in | Per maker (quoted below) |
Makers disagree about which mode to check in. Trane and Nortek both say to come back and verify the charge in cooling once it is warm; Rheem and Lennox each print a heating window of their own. The table below quotes what each maker we hold prints.
Charging in Heating Mode: What Each Maker Prints
Each row quotes the maker’s own heating-mode rule and its weigh-in rule, with the document and page. We publish no heating subcooling figure of our own. Type your outdoor model into the calculator above and, for a model it can place, it names your maker’s rule for you.
Carrier, Bryant, Payne
For Carrier-family R-454B single-stage heat pumps (the one book we hold). The heating chart on the outdoor unit is a check chart, and the book forbids charging from it; its line-set rule is quoted.
“To check system operation during heating cycle, refer to the Heating Check Chart on outdoor unit.” (PH5SAN5-01SI, the R-454B single-stage heat pump installation instructions (Carrier family), page 10)
“Do not use chart to adjust refrigerant charge.” (PH5SAN5-01SI, the R-454B single-stage heat pump installation instructions (Carrier family), page 10)
Weigh-in: “Unit is factory charged for 15ft (5 m) of lineset. Adjust charge by adding or removing 0.6 oz/ft (.018 kg/m) of 3/8 liquid line above or below 15ft (5 m) respectively.” (PH5SAN5-01SI, the R-454B single-stage heat pump installation instructions (Carrier family), page 9)
ICP (Heil, Tempstar, Comfortmaker, Arcoaire, Day & Night, KeepRite)
For ICP R-410A heat pumps (the one book we hold). The Tech Label on the outdoor unit is a check chart, and the book forbids charging from it.
“To check system operation during heating cycle, refer to the Tech Label on outdoor unit.” (ICP 428 01 5106 00, R-410A split system heat pump installation instructions, page 16)
“Do not use chart to adjust refrigerant charge.” (ICP 428 01 5106 00, R-410A split system heat pump installation instructions, page 16)
Weigh-in: “NOTE: When charging is necessary during heating season, charge must be weighed in accordance with unit rating plate ±0.6 ounces per foot of 3/8 inch liquid line above or below 15 feet respectively” (ICP 428 01 5106 00, R-410A split system heat pump installation instructions, page 16)
Lennox
For Lennox's EL16KP1 and ML14KP1 heat pumps (unit information 100195). The heating subcooling levels are printed per indoor unit match, which this tool does not know, so it names no value: read the one for your indoor unit. Other Lennox lines print their own rule and their own line-set basis.
“Heating Mode – When the outdoor ambient temperature is below 60°F (15°C), use the heating mode to adjust the charge using the subcooling charge levels” (Lennox 100195, EL16KP1 unit information (service literature), page 39)
“Cooling Mode – When the outdoor ambient temperature is 60°F (15°C) and above, use the cooling mode to adjust the charge using the subcooling method.” (Lennox 100195, EL16KP1 unit information (service literature), page 39)
Weigh-in: “Charge levels on the unit nameplate are based on installations with 30ft. (9.1m) line sets” (Lennox 100195, EL16KP1 unit information (service literature), page 39)
For Lennox's EL18KSLV heat pump (installation instructions 508700-01). The heating subcooling levels are printed per indoor unit match, which this tool does not know, so it names no value: read the one for your indoor unit. Other Lennox lines print their own rule and their own line-set basis.
“Charge Mode DIP Operation in the Heat Pump Heating Mode” (Lennox 508700-01, EL18KSLV R-454B heat pump installation instructions, page 22)
“To test the supercooling degree in heating mode, an external pressure gauge and thermometer need to be connected as shown in figure 10. The saturation temperature of the refrigerant is checked using table 4, and the current supercooling degree is calcutated by using the saturation temperature minus the thermometer temperature. The target supercooling degree is compared with that in table 5.” (Lennox 508700-01, EL18KSLV R-454B heat pump installation instructions, page 22)
“Indoor Unit Matches and Subcooling Charge Levels (TXV System) and Additional Charge (15 ft. Line set)” (Lennox 508700-01, EL18KSLV R-454B heat pump installation instructions, page 25)
“TABLE 6. Charge Mode Operating Pressures – Liquid ±10 and Vapor ±5 PSIG” (Lennox 508700-01, EL18KSLV R-454B heat pump installation instructions, page 26)
“NOTE – Pressures shown are under Charge mode, not normal operating pressures.” (Lennox 508700-01, EL18KSLV R-454B heat pump installation instructions, page 26)
“System charging should be performed with the unit operating at the maximum cooling capacity (100% capacity).” (Lennox 508700-01, EL18KSLV R-454B heat pump installation instructions, page 29)
“Initiate a call for cooling and allow the refrigerant pressures and temperatures to stabilize.” (Lennox 508700-01, EL18KSLV R-454B heat pump installation instructions, page 29)
Weigh-in: “EL18KSLV unit is factory-charged with enough R454B refrigerant to accommodate a 15-foot length of refrigerant piping. For line lengths over 15 feet, add 2.75 oz of refrigerant for every 5 feet of piping beyond 15 feet.” (Lennox 508700-01, EL18KSLV R-454B heat pump installation instructions, page 29)
Weigh-in: “ADDING REFRIGERANT FOR LONG LINE SETS WEIGH IN CHARGING METHOD FOR LONG LINE SETS 64ºF (17.7ºC) and Below” (Lennox 508700-01, EL18KSLV R-454B heat pump installation instructions, page 10)
Allied (Armstrong Air, AirEase, Ducane, Concord)
For the Allied 7HP19V heat pump. Neither Allied book prints heating subcooling targets or a heating charging method (its charge mode can run in heating, page 21): each prints a weigh-in method for long line sets at 64°F and below and a subcooling check on a call for cooling against the unit's charging label. Where the label prints a different line-set baseline than the book, the label governs. Lennox's heating rules are Lennox books' and do not carry over.
“ADDING REFRIGERANT FOR LONG LINE SETS WEIGH IN CHARGING METHOD FOR LONG LINE SETS 64ºF (17.7ºC) and Below” (Allied 508529-02, 7HP19V R-454B heat pump installation instructions, page 8)
“Initiate a call for cooling and allow the refrigerant pressures and temperatures to stabilize.” (Allied 508529-02, 7HP19V R-454B heat pump installation instructions, page 26)
Weigh-in: “7HP19V unit is factory charged with R454B. Refer to unit Charging Label for baseline line set length for factory unit charge and charge adder guidance.” (Allied 508529-02, 7HP19V R-454B heat pump installation instructions, page 26)
Weigh-in: “OUNCES PER 5 FEET (G PER 1.5 M) ADJUST FROM 15 FEET (4.6 M) LINE SET* 3/8" (9.5 MM) 3 OUNCES PER 5' (85 G PER 1.5 M)” (Allied 508529-02, 7HP19V R-454B heat pump installation instructions, page 26)
Weigh-in: “If line length is greater than 15 ft. (4.6 m), add this amount. If line length is less than 15 ft. (4.6 m), subtract this amount.” (Allied 508529-02, 7HP19V R-454B heat pump installation instructions, page 26)
For the Allied A7HP19V heat pump (its own book, 508530-02). Neither Allied book prints heating subcooling targets or a heating charging method (its charge mode can run in heating, page 21): each prints a weigh-in method for long line sets at 64°F and below and a subcooling check on a call for cooling against the unit's charging label. Where the label prints a different line-set baseline than the book, the label governs. Lennox's heating rules are Lennox books' and do not carry over.
“ADDING REFRIGERANT FOR LONG LINE SETS WEIGH IN CHARGING METHOD FOR LONG LINE SETS 64ºF (17.7ºC) and Below” (Allied 508530-02, A7HP19V R-454B heat pump installation instructions, page 7)
“Initiate a call for cooling and allow the refrigerant pressures and temperatures to stabilize.” (Allied 508530-02, A7HP19V R-454B heat pump installation instructions, page 27)
Weigh-in: “A7HP19V unit is factory charged with R454B. Refer to unit Charging Label for baseline line set length for factory unit charge and charge adder guidance.” (Allied 508530-02, A7HP19V R-454B heat pump installation instructions, page 27)
Weigh-in: This book prints two line-set bases: the example in its weigh-in figure says the units are factory-charged for 15 feet (page 7), and its charge table adjusts from 30 feet (page 27). It sends you to the charging label on the unit for the baseline, so this tool publishes neither: read the label.
Goodman, Amana, Daikin
For Goodman R-32 heat pumps (the one reference we hold). The book prints no heating chart.
“The proper method of charging a heat pump in the heat mode is by weight with the additional charge adjustments for line size, line length, and other system components.” (Goodman IOG-4064A (November 2024), heat pump installation and service reference; superseded by IOG-4064B, which we do not hold, page 11)
“Make final charge adjustments in the cooling cycle.” (Goodman IOG-4064A (November 2024), heat pump installation and service reference; superseded by IOG-4064B, which we do not hold, page 11)
Weigh-in: The per-foot addition is a table in the same book, keyed by both the liquid and the suction line size. That book is superseded, so we publish none of its values: read the table in the book for your unit.
Nortek (Frigidaire, Gibson, Maytag, Intertherm, Broan)
For the iQ Drive inverter heat pump. The iQ Drive heating procedure is drawn as curves, and this tool never reads a number off a curve. The T4BE heating charts are for verifying the charge, not setting it.
“Use this method when the outdoor temperature is 60°F or lower to estimate the correct charge in heating mode.” (Nortek form 708921E, iQ Drive inverter split system heat pump installation instructions (printed 2012), page 6)
“The preferred method of charging is by weighing in the additional refrigerant required.” (Nortek form 708921E, iQ Drive inverter split system heat pump installation instructions (printed 2012), page 6)
“Refrigerant can be stored in the receiver and accumulators and result in an over-charged system.” (Nortek form 708921E, iQ Drive inverter split system heat pump installation instructions (printed 2012), page 6)
“If it is necessary to use this method, follow up service should be scheduled when the temperature is above 60°F, and then charge should be verified in cooling mode.” (Nortek form 708921E, iQ Drive inverter split system heat pump installation instructions (printed 2012), page 6)
Weigh-in: The added charge for longer line sets is a table in the same book. Its currency is unconfirmed, so we publish none of its values.
For the T4BE conventional heat pump. The iQ Drive heating procedure is drawn as curves, and this tool never reads a number off a curve. The T4BE heating charts are for verifying the charge, not setting it.
“The most reliable way of verifying the system is at the correct charge is to evacuate the system and weigh in the charge to the amount shown on the rating label.” (Nortek form 7094080, T4BE split system heat pump installation instructions (printed 2012), page 15)
“if an inspection with these verification charts does not line up with the values shown and the ambient temperature is above 50° F, then a more accurate way to inspect the system for proper charge is with the cooling mode charging charts.” (Nortek form 7094080, T4BE split system heat pump installation instructions (printed 2012), page 15)
Weigh-in: “weigh in the charge to the amount shown on the rating label” (Nortek form 7094080, T4BE split system heat pump installation instructions (printed 2012), page 15)
Trane, American Standard
For the Trane 4TWX6060E two-stage heat pump (the American Standard book we hold has no heating section). The heating figures in this book are for that one two-stage heat pump and that procedure, not a target for other Trane or American Standard units.
“The only recommended method of charging at outdoor ambients below 55°F, is to weigh in the charge in the heating mode.” (Trane 4TWX6060E-SF-1A, Service Facts for the two-stage R-410A heat pump, page 2)
“Use Nameplate charge plus standard charge adders for line length.” (Trane 4TWX6060E-SF-1A, Service Facts for the two-stage R-410A heat pump, page 2)
“Add charge if a minimum of 10°F subcooling is not obtained with the nameplate charge plus line length correction.” (Trane 4TWX6060E-SF-1A, Service Facts for the two-stage R-410A heat pump, page 2)
“It is important to return in the spring or summer to accurately charge the system in the cooling mode at outdoor ambients above 55°F.” (Trane 4TWX6060E-SF-1A, Service Facts for the two-stage R-410A heat pump, page 2)
Rheem, Ruud
For the Rheem and Ruud (-)P14AY and (-)P15AY R-454B heat pumps. Rheem's charging chart is inside the unit's access panel cover and we do not hold it, so this tool names no subcooling value: read it off your unit.
“Heating Mode: Between 40°F [4.4°C] and 60°F [15.6°C] outdoor dry bulb” (Rheem 92-104921-51-03, R-454B heat pump outdoor units installation instructions, page 33)
“CHARGING BY LIQUID SUBCOOLING PER THE SYSTEM CHARGING CHART MUST BE USED FOR FINAL CHARGE ADJUSTMENT.” (Rheem 92-104921-51-03, R-454B heat pump outdoor units installation instructions, page 32)
“NOTICE: Systems should not be charged below 40°F [4.4°C] outdoor temperature.” (Rheem 92-104921-51-03, R-454B heat pump outdoor units installation instructions, page 33)
“Cooling Mode: 55°F [12.8°C] outdoor dry bulb and above” (Rheem 92-104921-51-03, R-454B heat pump outdoor units installation instructions, page 33)
Weigh-in: “Charging by weight is considered a starting point ONLY.” (Rheem 92-104921-51-03, R-454B heat pump outdoor units installation instructions, page 32)
Weigh-in: “3/8" [9.5 mm] O.D. = .6 oz./ft. [56.7 g/m]” (Rheem 92-104921-51-03, R-454B heat pump outdoor units installation instructions, page 32)
Weigh-in: “Add 6 oz. [170 g] for field-installed filter drier.” (Rheem 92-104921-51-03, R-454B heat pump outdoor units installation instructions, page 32)
York, Luxaire, Coleman
For York's R-454B heat pumps (YH4, XH4, RH4). York picks a heating chart by the installed indoor unit, its airflow direction and its airflow, which this tool does not know, so it picks none. We do not hold the Tabular Data Sheet that carries the line-set amount.
“If the system is being installed during seasonally cold weather of 55°F or below, the preferred method is to weigh in the charge. For charging or checking the system charge at 55°F or below, see the Heating charging charts.” (York 6694740-UIM-B-1025, R-454B split-system heat pump installation manual (YH4, XH4, RH4), page 18)
Weigh-in: “The factory charge in the outdoor unit includes enough charge for the unit, 15 ft (4.6 m) of refrigerant piping, and the smallest indoor coil or air handler match-up.” (York 6694740-UIM-B-1025, R-454B split-system heat pump installation manual (YH4, XH4, RH4), page 40)
Weigh-in: “Calculate the additional charge for the refrigerant piping using the Tabular Data Sheet if the line length is greater than 15 ft (4.6 m)” (York 6694740-UIM-B-1025, R-454B split-system heat pump installation manual (YH4, XH4, RH4), page 41)
Ductless mini-splits (and the brands this tool reads with them)
For ductless mini-splits. None of the ductless books we hold prints a heating-mode charging procedure.
Weigh-in: Mini-splits are charged by weight plus a per-length adder that is specific to each model. Use your unit's own book.
Bard wall-mount packaged units
For Bard wall-mount units. Factory-sealed packaged equipment is outside this check.
Weigh-in: These are factory sealed with no field line set, so there is no line-set weigh-in.
How to Measure Subcooling on a Heat Pump (Cooling Mode)
- Check it is warm enough: Charge by cooling subcooling only when it is warm enough: below 55F outdoor every cooling-mode book we hold says to weigh the charge in instead (or follow your maker's heating rule), and between 55 and 70F they disagree, so use the unit's own instructions. Then run the system in cooling mode until it stabilizes (about 15 minutes on R-410A).
- Read the liquid-line pressure at the outdoor unit service port.
- Convert that pressure to a saturation (condensing) temperature with the PT chart for your refrigerant, or enter it in the calculator above.
- Clamp a pipe thermometer to the liquid line at the outdoor coil outlet and read the actual temperature.
- Subtract: subcooling = saturation liquid temperature minus measured liquid temperature.
- Compare against the target (8 to 14°F with a TXV) or the nameplate value, and adjust charge only after ruling out low or restricted condenser airflow.
Worked Example: R-410A Heat Pump in Cooling Mode
An R-410A heat pump running in cooling with a TXV. Liquid-line pressure 340 psig, liquid-line temperature 93°F.
Saturated liquid temp at 340 psig = 105°F
Subcooling = 105°F − 93°F = 12°F
Diagnosis: 12°F sits inside the 8–14°F TXV target, so the charge is good. Pair it with superheat in the 8–15°F window to confirm. If subcooling read low with normal superheat, suspect low charge; if it read high, suspect overcharge or a condenser airflow problem.
Heat Pump Subcooling FAQs
How do I calculate subcooling on a heat pump?
Subcooling equals the saturation temperature at the liquid-line pressure minus the measured liquid-line temperature. Read the liquid-line pressure at the service port, convert it to a saturation (condensing) temperature with the PT chart for your refrigerant, clamp a thermometer to the liquid line at the condenser outlet, and subtract. In cooling mode the outdoor coil is the condenser and the target with a TXV is 8 to 14F, the same as an air conditioner. In heating mode the reversing valve makes the indoor coil the condenser, so the "liquid line" you measure is different. Whether a heating reading may set the charge is each maker's own rule: Rheem's (-)P14AY and (-)P15AY R-454B book recommends charging in heating only between 40 and 60F outdoor, with subcooling per the chart inside the unit as the final adjustment, and never below 40F; Lennox's EL16KP1 literature says to use heating mode below 60F with the heating subcooling level for your indoor unit; Lennox's EL18KSLV book prints a heating charge mode, set on a DIP switch, with a heating subcooling target for each indoor unit and heating charge-mode rows up to 60F; Allied's 7HP19V and A7HP19V books print no heating subcooling targets or heating charging method and weigh the charge in for long line sets at 64F and below; Trane's two-stage 4TWX6060E Service Facts says to weigh the charge in below 55F; York's R-454B book prefers weighing in at 55F or below; Carrier and ICP print "Do not use chart to adjust refrigerant charge" for their heating charts, and ICP adds that heating-season charge is weighed in.
What is a good subcooling for a heat pump?
In cooling mode, a good subcooling for a heat pump with a TXV is 8 to 14F, measured at the liquid line near the outdoor coil, exactly like an AC. Many nameplates print a specific subcooling target; the nameplate value always wins over a generic range. In heating mode there is no single good subcooling number across makers, and we publish none: Rheem's (-)P14AY and (-)P15AY R-454B book recommends charging in heating only between 40 and 60F outdoor, with subcooling per the chart inside the unit as the final adjustment, and never below 40F; Lennox's EL16KP1 literature says to use heating mode below 60F with the heating subcooling level for your indoor unit; Lennox's EL18KSLV book prints a heating charge mode, set on a DIP switch, with a heating subcooling target for each indoor unit and heating charge-mode rows up to 60F; Allied's 7HP19V and A7HP19V books print no heating subcooling targets or heating charging method and weigh the charge in for long line sets at 64F and below; Trane's two-stage 4TWX6060E Service Facts says to weigh the charge in below 55F; York's R-454B book prefers weighing in at 55F or below; Carrier and ICP print "Do not use chart to adjust refrigerant charge" for their heating charts, and ICP adds that heating-season charge is weighed in.
Can you check subcooling in heating mode?
Yes, and on some units the maker says to set the charge by it, inside an outdoor-temperature window. In heating mode the reversing valve sends hot discharge gas to the indoor coil, which becomes the condenser, so subcooling is measured at the indoor coil outlet and shifts with outdoor temperature. The rule is the maker's, not a general one: Rheem's (-)P14AY and (-)P15AY R-454B book recommends charging in heating only between 40 and 60F outdoor, with subcooling per the chart inside the unit as the final adjustment, and never below 40F; Lennox's EL16KP1 literature says to use heating mode below 60F with the heating subcooling level for your indoor unit; Lennox's EL18KSLV book prints a heating charge mode, set on a DIP switch, with a heating subcooling target for each indoor unit and heating charge-mode rows up to 60F; Allied's 7HP19V and A7HP19V books print no heating subcooling targets or heating charging method and weigh the charge in for long line sets at 64F and below; Trane's two-stage 4TWX6060E Service Facts says to weigh the charge in below 55F; York's R-454B book prefers weighing in at 55F or below; Carrier and ICP print "Do not use chart to adjust refrigerant charge" for their heating charts, and ICP adds that heating-season charge is weighed in. Where a maker prints no heating procedure, weigh the charge in by that maker's own line-set rule.
Where do you measure subcooling on a heat pump?
Measure subcooling on the liquid line at the outlet of whichever coil is acting as the condenser. In cooling mode that is the outdoor coil, so you read the liquid-line pressure and temperature at the outdoor unit, the same place as an AC. In heating mode the indoor coil is the condenser, so the relevant liquid line is at the indoor coil. Clamp the pipe thermometer to clean, bare copper near the coil outlet and insulate it from ambient air for an accurate reading.
Why is subcooling different on a heat pump than an air conditioner?
In cooling mode there is no difference: a heat pump charges exactly like an AC, so target subcooling with a TXV is the same 8 to 14F. The difference appears in heating mode. A heat pump has a reversing valve that swaps the roles of the two coils, so the coil that condenses refrigerant (and therefore where subcooling matters) flips from outdoor in cooling to indoor in heating. That is why the charging rule in heating is each maker's own: some print a heating-mode procedure within an outdoor-temperature window and others say to weigh the charge in.
What subcooling should a heat pump have in cooling mode?
With a TXV in cooling mode, target 8 to 14F of subcooling and 8 to 15F of superheat, the same windows as an air conditioner on the same refrigerant. Charge by cooling subcooling only when it is warm enough: below 55F outdoor every cooling-mode book we hold says to weigh the charge in instead (or follow your maker's heating rule), and between 55 and 70F they disagree, so use the unit's own instructions. Low subcooling with normal superheat usually means low charge; high subcooling with normal superheat usually means overcharge. Confirm against the nameplate target before adding or recovering refrigerant.
Do R-454B and R-32 heat pumps use the same subcooling target?
Yes. R-454B and R-32 use the same 8 to 14F TXV subcooling target as R-410A in cooling mode, because their pressure-temperature behavior is close. What changes is the PT scale you convert with: use the correct refrigerant's chart to turn liquid pressure into saturation temperature, and charge A2L blends like R-454B as a liquid. In heating mode, follow the maker's own rule for that unit; the table on this page quotes each maker we hold.
Related Calculators & Charts
Charge verified. Was the heat pump sized for the house?
Chronic charge and comfort complaints on a heat pump often trace back to sizing. Run the load by climate zone, insulation, and windows before you trust the nameplate tonnage.