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      4-ton central heat pump · FLEXX High-efficiency Series

      Willis FLEX48/60KUH

      Below average cooling efficiency and average heating efficiency for a 4-ton central heat pump, and certified for cold climates.

      Ranks #226 of 319 4 ton central heat pumps for cooling efficiency, and costs about $1,656 a year to run in a mixed climate like Atlanta.

      SEER2
      16.0 Cooling efficiency over a season
      Below average · beats 26%
      HSPF2
      9.0 Heating efficiency over a season
      Average · beats 54%
      EER2
      10.0 Cooling efficiency on a 95°F day
      Below average · beats 10%
      Heat at 5°F
      41,000 BTU/h76% of its 47°F output
      Below average · beats 5%

      A good fit if…

      • Your winters get properly cold - it is ENERGY STAR cold-climate certified, with 41,000 BTU/h at 5°F.
      • 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 central heat pumps.
      • You want the newest refrigerant - it uses R-410A, which is being phased down.

      Calculate my running cost

      Details

      Specifications4 ton, R-410A
      General
      BrandWillis
      Outdoor unit modelFLEX48/60KUH
      SeriesFLEXX High-efficiency Series
      TypeCentral heat pump
      Nominal size4 ton
      CompressorVariable-speed units run longer at low powerContinuously variable
      RefrigerantR-410A
      Connected (smart) capableNo
      On the market sinceJune 5, 2024
      Last certifiedJuly 7, 2026
      Cooling
      SEER2Seasonal efficiency, higher is better16.0 (range 16.0)
      EER2Efficiency at 95°F outdoors10.0
      Cooling capacity54,000 BTU/h
      Heating
      HSPF2Seasonal efficiency, higher is better9.0
      Capacity at 47°F54,000 BTU/h
      Capacity at 17°F44,000 BTU/h
      Capacity at 5°F41,000 BTU/h
      COP at 5°F2.0 = twice as efficient as a space heater1.80
      ENERGY STAR cold climateYes
      Heating output as it gets colder76% kept at 5°F

      Every heat pump delivers less heat as the outdoor temperature drops. Certified output of its matchup with FLEXAH48/60K at the standard test temperatures, in BTU/h:

      Certified matchups1 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.
      FLEXAH48/60K · cold climate16.010.09.041,000214797686
      Yearly running cost in 10 large cities$1,580-$3,876

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

      Questions5
      What is the SEER2 rating of the Willis FLEX48/60KUH?
      The Willis FLEX48/60KUH is rated at up to 16.0 SEER2 and 10.0 EER2 for cooling and up to 9.0 HSPF2 for heating. The exact figure depends on the indoor unit it is paired with.
      Is the Willis FLEX48/60KUH good for cold climates?
      Yes. At least one certified matchup meets the ENERGY STAR cold-climate criteria, delivering up to 41,000 BTU/h at 5°F with a COP of 1.80, so it is still 1.8 times as efficient as electric resistance heat at that temperature.
      How much does the Willis FLEX48/60KUH cost to run per year?
      At its rated efficiency it costs about $1,580-$3,876 a year in electricity in large U.S. cities, depending on climate and local rates - for example $1,900 in Phoenix and $3,201 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 Willis FLEX48/60KUH?
      ENERGY STAR lists 1 certified combination for this outdoor unit. The highest-rated one uses FLEXAH48/60K.
      What refrigerant does the Willis FLEX48/60KUH 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 $1,860/yrYou save $924/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 Willis FLEX48/60KUH
      3. What it replaces
      4. Price replace the example with your quote
      Heat pump, per year
      $1,860
      Current system, per year
      $2,784
      You save per year
      $924
      Payback
      22.2 years
      Net after 10 years
      -$11,256

      Cooling $1,188Heating $672Electricity used: 11,625 kWhSaved vs. a minimum-efficiency heat pump: $276

      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.

      Similar 4 ton models

      See all 319 4 ton central heat pumps, ranked

      Source: ENERGY STAR Certified Heat Pumps (U.S. EPA), certification of July 7, 2026. How we use the data.