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      42,000 BTU mini-split heat pump · KUSXB421A

      LG KUSXB421A

      Below average cooling efficiency and very good heating efficiency for a 42,000 BTU mini-split heat pump, and certified for cold climates.

      Ranks #33 of 61 42,000 BTU mini-splits for cooling efficiency, and costs about $1,098 a year to run in a mixed climate like Atlanta.

      SEER2
      21.0 Cooling efficiency over a season
      Below average · beats 30%
      HSPF2
      10.8 Heating efficiency over a season
      Very good · beats 71%
      EER2
      13.2 Cooling efficiency on a 95°F day
      Average · beats 69%
      Heat at 5°F
      40,000 BTU/h83% of its 47°F output
      Below average · beats 36%

      A good fit if…

      • Your winters get properly cold - it is ENERGY STAR cold-climate certified, with 40,000 BTU/h at 5°F.
      • It will heat most of the year - its HSPF2 beats 71% of mini-splits.
      • You want steady temperatures and quiet running - the compressor varies its speed.

      Think twice if…

      • Cooling cost matters most - its SEER2 is below average for mini-splits.

      Calculate my running cost

      Details

      Specifications42,000 BTU, R-32
      General
      BrandLG
      Outdoor unit modelKUSXB421A
      SeriesKUSXB421A
      TypeMini-split heat pump
      Nominal size42,000 BTU
      CompressorVariable-speed units run longer at low powerContinuously variable
      RefrigerantR-32
      Connected (smart) capableNo
      On the market sinceDecember 2, 2024
      Last certifiedApril 22, 2026
      Cooling
      SEER2Seasonal efficiency, higher is better21.0 (range 15.2-21.0)
      EER2Efficiency at 95°F outdoors11.0-13.2
      Cooling capacity41,000-42,000 BTU/h
      Heating
      HSPF2Seasonal efficiency, higher is better8.5-10.8
      Capacity at 47°F43,000-48,000 BTU/h
      Capacity at 17°F28,600-31,000 BTU/h
      Capacity at 5°F36,000-40,000 BTU/h
      COP at 5°F2.0 = twice as efficient as a space heater1.80-1.96
      ENERGY STAR cold climateYes
      Heating output as it gets colder83% kept at 5°F

      Every heat pump delivers less heat as the outdoor temperature drops. Certified output of its matchup with KNSCB421A at the standard test temperatures, in BTU/h:

      Certified matchups6 indoor units

      The same outdoor unit scores differently with each indoor unit and furnace. Ask your installer for the AHRI reference number of the exact system you are quoted.

      Indoor unitFurnaceSEER2EER2HSPF2Heat @5°FAHRI ref.
      KNSCB421A · cold climate-21.013.210.840,000214837927
      KNSJB421A · cold climate-18.312.09.039,000214825958
      KNSLB421A · cold climate-18.011.79.538,500215588573
      KNSLB421A · cold climate-18.011.79.538,500217284915
      LKMMA42C1 · cold climateLNA**100C117.211.09.537,000217284966
      LKMMA42C1 · cold climate-15.211.78.536,000216445249
      Yearly running cost in 10 large cities$942-$2,656

      At each state's average electricity price and the city's typical heating and cooling hours.

      Questions5
      What is the SEER2 rating of the LG KUSXB421A?
      The LG KUSXB421A is rated at up to 21.0 SEER2 and 13.2 EER2 for cooling and up to 10.8 HSPF2 for heating. The exact figure depends on the indoor unit it is paired with; across its 6 certified matchups SEER2 ranges from 15.2-21.0.
      Is the LG KUSXB421A good for cold climates?
      Yes. At least one certified matchup meets the ENERGY STAR cold-climate criteria, delivering up to 40,000 BTU/h at 5°F with a COP of 1.96, so it is still 2.0 times as efficient as electric resistance heat at that temperature.
      How much does the LG KUSXB421A cost to run per year?
      At its rated efficiency it costs about $942-$2,656 a year in electricity in large U.S. cities, depending on climate and local rates - for example $1,180 in Phoenix and $2,234 in Minneapolis. The calculator on this page works it out for your city and compares it with your current system.
      Which indoor units work with the LG KUSXB421A?
      ENERGY STAR lists 6 certified combinations for this outdoor unit. The highest-rated ones use KNSCB421A, KNSJB421A, KNSLB421A.
      What refrigerant does the LG KUSXB421A use?
      It uses R-32.

      Running cost and savings

      Pre-filled with this model. Pick your city and what you heat with today; everything else is optional.

      Heat pump $1,181/yrYou save $1,050/yrSee details ↓
      1. Where you live
      Adjust heating and cooling hours

      Filled in from your city's climate. Change them if you run the system much more or less than usual.

      2. The heat pump LG KUSXB421A
      3. What it replaces
      4. Price replace the example with your quote
      Heat pump, per year
      $1,181
      Current system, per year
      $2,231
      You save per year
      $1,050
      Payback
      15.2 years
      Net after 10 years
      -$5,500

      Cooling $704Heating $477Electricity used: 7,381 kWhSaved vs. a minimum-efficiency heat pump: $540

      Estimates use the unit's rated SEER2 and HSPF2, full-load hours derived from NOAA 1991-2020 degree days, average residential electricity prices from the U.S. EIA (12 months to July 2026) and fuel prices from EIA (2025 for natural gas, week of 2026-10-05 for propane and heating oil). Very cold days that need backup heat are not modelled. Federal minimum for comparison: 14.3 SEER2 / 7.5 HSPF2.

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      Source: ENERGY STAR Certified Heat Pumps (U.S. EPA), certification of April 22, 2026. How we use the data.