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      24,000 BTU mini-split heat pump · X series

      SMARTFLEX DHX24CSB21S

      Below average cooling and heating efficiency for a 24,000 BTU mini-split heat pump, but not rated for cold climates.

      Ranks #357 of 430 24,000 BTU mini-splits for cooling efficiency, and costs about $684 a year to run in a mixed climate like Atlanta.

      SEER2
      20.0 Cooling efficiency over a season
      Below average · beats 18%
      HSPF2
      8.5 Heating efficiency over a season
      Below average · beats 2%
      EER2
      12.5 Cooling efficiency on a 95°F day
      Average · beats 44%
      Heat at 5°F
      19,800 BTU/h83% of its 47°F output
      Below average · beats 1%

      A good fit if…

      • You want steady temperatures and quiet running - the compressor varies its speed.

      Think twice if…

      • You often see nights below 0°F - it is not cold-climate certified.
      • Cooling cost matters most - its SEER2 is below average for mini-splits.
      • You mainly need heat - its HSPF2 is below average for mini-splits.
      • You want the newest refrigerant - it uses R-410A, which is being phased down.

      Calculate my running cost

      Details

      Specifications24,000 BTU, R-410A
      General
      BrandSMARTFLEX
      Also sold asCOLEMAN BY JOHNSON CONTROLS, GUARDIAN BY JOHNSON CONTROLS, Champion, LUXAIRE BY JOHNSON CONTROLS
      Outdoor unit modelDHX24CSB21S
      SeriesX series
      TypeMini-split heat pump
      Nominal size24,000 BTU
      CompressorVariable-speed units run longer at low powerContinuously variable
      RefrigerantR-410A
      Connected (smart) capableNo
      On the market sinceJanuary 1, 2023
      Last certifiedFebruary 17, 2025
      Cooling
      SEER2Seasonal efficiency, higher is better20.0 (range 20.0)
      EER2Efficiency at 95°F outdoors12.5
      Cooling capacity22,000 BTU/h
      Heating
      HSPF2Seasonal efficiency, higher is better8.5
      Capacity at 47°F24,000 BTU/h
      Capacity at 17°F20,000 BTU/h
      Capacity at 5°F19,800 BTU/h
      COP at 5°F2.0 = twice as efficient as a space heater1.75
      ENERGY STAR cold climateNo
      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 DHX24NWB21S at the standard test temperatures, in BTU/h:

      Certified matchups5 indoor units

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

      Indoor unitSEER2EER2HSPF2Heat @5°FAHRI ref.
      DHX24NWB21S20.012.58.519,800202208608
      DHX24NWB21S COLEMAN BY JOHNSON CONTROLS20.012.58.519,8008578582
      DHX24NWB21S GUARDIAN BY JOHNSON CONTROLS20.012.58.519,8008578584
      DHX24NWB21S CHAMPION20.012.58.519,8008657818
      DHX24NWB21S LUXAIRE BY JOHNSON CONTROLS20.012.58.519,8008578583
      Yearly running cost in 10 large cities$522-$1,707

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

      Questions5
      What is the SEER2 rating of the SMARTFLEX DHX24CSB21S?
      The SMARTFLEX DHX24CSB21S is rated at up to 20.0 SEER2 and 12.5 EER2 for cooling and up to 8.5 HSPF2 for heating. The exact figure depends on the indoor unit it is paired with.
      Is the SMARTFLEX DHX24CSB21S good for cold climates?
      It is not cold-climate certified: none of its matchups meet the ENERGY STAR cold-climate criteria (a COP of at least 1.75 at 5°F among other limits). In places with frequent sub-zero nights, pick a cold-climate 24,000 BTU model.
      How much does the SMARTFLEX DHX24CSB21S cost to run per year?
      At its rated efficiency it costs about $522-$1,707 a year in electricity in large U.S. cities, depending on climate and local rates - for example $686 in Phoenix and $1,459 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 SMARTFLEX DHX24CSB21S?
      ENERGY STAR lists 5 certified combinations for this outdoor unit. The highest-rated ones use DHX24NWB21S, COLEMAN BY JOHNSON CONTROLS DHX24NWB21S, GUARDIAN BY JOHNSON CONTROLS DHX24NWB21S.
      What refrigerant does the SMARTFLEX DHX24CSB21S use?
      It uses R-410A. R-410A is being phased down under the EPA AIM Act, and new systems are moving to R-454B and 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 $703/yrYou save $464/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 SMARTFLEX DHX24CSB21S
      3. What it replaces
      4. Price replace the example with your quote
      Heat pump, per year
      $703
      Current system, per year
      $1,167
      You save per year
      $464
      Payback
      18.3 years
      Net after 10 years
      -$3,863

      Cooling $387Heating $316Electricity used: 4,396 kWhSaved vs. a minimum-efficiency heat pump: $197

      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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      See all 430 24,000 BTU mini-splits, ranked

      Source: ENERGY STAR Certified Heat Pumps (U.S. EPA), certification of February 17, 2025. How we use the data.