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      18,000 BTU mini-split heat pump · MMLD Mini-Split

      Lennox MMLD018S6S-1P

      Very good cooling efficiency and below average heating efficiency for a 18,000 BTU mini-split heat pump, and certified for cold climates.

      Ranks #101 of 465 18,000 BTU mini-splits for cooling efficiency, and costs about $494 a year to run in a mixed climate like Atlanta.

      SEER2
      23.8 Cooling efficiency over a season
      Very good · beats 72%
      HSPF2
      9.6 Heating efficiency over a season
      Below average · beats 30%
      EER2
      14.5 Cooling efficiency on a 95°F day
      Very good · beats 88%
      Heat at 5°F
      20,000 BTU/h100% of its 47°F output
      Below average · beats 9%

      A good fit if…

      • You want low cooling bills - its SEER2 beats 72% of mini-splits.
      • Your winters get properly cold - it is ENERGY STAR cold-climate certified, with 20,000 BTU/h at 5°F.
      • You want steady temperatures and quiet running - the compressor varies its speed.

      Think twice if…

      • You mainly need heat - its HSPF2 is below average for mini-splits.

      Calculate my running cost

      Details

      Specifications18,000 BTU, R-32
      General
      BrandLennox
      Outdoor unit modelMMLD018S6S-1P
      SeriesMMLD Mini-Split
      TypeMini-split heat pump
      Nominal size18,000 BTU
      CompressorVariable-speed units run longer at low powerContinuously variable
      RefrigerantR-32
      Connected (smart) capableNo
      On the market sinceAugust 2, 2024
      Last certifiedApril 27, 2026
      Cooling
      SEER2Seasonal efficiency, higher is better23.8 (range 17.0-23.8)
      EER2Efficiency at 95°F outdoors11.7-14.5
      Cooling capacity16,000-18,000 BTU/h
      Heating
      HSPF2Seasonal efficiency, higher is better8.5-9.6
      Capacity at 47°F19,000-20,000 BTU/h
      Capacity at 17°F11,900-16,400 BTU/h
      Capacity at 5°F18,100-20,000 BTU/h
      COP at 5°F2.0 = twice as efficient as a space heater1.80
      ENERGY STAR cold climateYes
      Heating output as it gets colder100% kept at 5°F

      Every heat pump delivers less heat as the outdoor temperature drops. Certified output of its matchup with M33D018S6-1P at the standard test temperatures, in BTU/h:

      Certified matchups8 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.
      M33D018S6-1P LENNOX · cold climate23.814.59.620,000215810024
      M22D018S6-1P LENNOX · cold climate20.512.59.520,000215810027
      MWXD018S6-1P LENNOX · cold climate20.512.29.519,000218214255
      M1WD018S6-1P LENNOX · cold climate20.012.09.520,000215810023
      MFMD018S6-1P LENNOX · cold climate20.012.29.518,100216581330
      MDDD018S6-1P LENNOX · cold climate18.312.08.920,000215810025
      MMD018S6-1P LENNOX · cold climate18.012.08.519,000216581331
      MMDD018S6-1P LENNOX · cold climate17.011.79.020,000215810026
      Yearly running cost in 10 large cities$360-$1,246

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

      Questions5
      What is the SEER2 rating of the Lennox MMLD018S6S-1P?
      The Lennox MMLD018S6S-1P is rated at up to 23.8 SEER2 and 14.5 EER2 for cooling and up to 9.6 HSPF2 for heating. The exact figure depends on the indoor unit it is paired with; across its 8 certified matchups SEER2 ranges from 17.0-23.8.
      Is the Lennox MMLD018S6S-1P good for cold climates?
      Yes. At least one certified matchup meets the ENERGY STAR cold-climate criteria, delivering up to 20,000 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 Lennox MMLD018S6S-1P cost to run per year?
      At its rated efficiency it costs about $360-$1,246 a year in electricity in large U.S. cities, depending on climate and local rates - for example $483 in Phoenix and $1,071 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 Lennox MMLD018S6S-1P?
      ENERGY STAR lists 8 certified combinations for this outdoor unit. The highest-rated ones use LENNOX M33D018S6-1P, LENNOX M22D018S6-1P, LENNOX MWXD018S6-1P.
      What refrigerant does the Lennox MMLD018S6S-1P 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 $500/yrYou save $461/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 Lennox MMLD018S6S-1P
      3. What it replaces
      4. Price replace the example with your quote
      Heat pump, per year
      $500
      Current system, per year
      $961
      You save per year
      $461
      Payback
      15.2 years
      Net after 10 years
      -$2,387

      Cooling $266Heating $233Electricity used: 3,122 kWhSaved vs. a minimum-efficiency heat pump: $242

      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 27, 2026. How we use the data.