What is the SEER2 rating of the Mainline ML18HP230NEXH-O?

The Mainline ML18HP230NEXH-O is rated at up to 21.7 SEER2 and 13.0 EER2 for cooling and up to 12.3 HSPF2 for heating. The exact figure depends on the indoor unit it is paired with; across its 16 certified matchups SEER2 ranges from 18.0-21.7.

Is the Mainline ML18HP230NEXH-O good for cold climates?

Yes. At least one certified matchup meets the ENERGY STAR cold-climate criteria, delivering up to 18,000 BTU/h at 5°F with a COP of 2.20, so it is still 2.2 times as efficient as electric resistance heat at that temperature.

How much does the Mainline ML18HP230NEXH-O cost to run per year?

At its rated efficiency it costs about $368-$977 a year in electricity in large U.S. cities, depending on climate and local rates - for example $453 in Phoenix and $816 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 Mainline ML18HP230NEXH-O?

ENERGY STAR lists 16 certified combinations for this outdoor unit. The highest-rated ones use MAINLINE ML18HP230NEX-I, MAINLINE ML18HP2301WYCAS-I, MAINLINE ML18HP230FLO-I.

18,000 BTU mini-split heat pump · ML Series

Mainline ML18HP230NEXH-O

Average cooling efficiency and excellent heating efficiency for a 18,000 BTU mini-split heat pump, and certified for cold climates.

Ranks #247 of 465 18,000 BTU mini-splits for cooling efficiency, and costs about $410 a year to run in a mixed climate like Atlanta.

SEER2: 21.7 Cooling efficiency over a season
Average · beats 42%
HSPF2: 12.3 Heating efficiency over a season
Excellent · beats 92%
EER2: 13.0 Cooling efficiency on a 95°F day
Average · beats 62%
Heat at 5°F: 18,000 BTU/h100% of its 47°F output
Very good · beats 78%

A good fit if…

Your winters get properly cold - it is ENERGY STAR cold-climate certified, with 18,000 BTU/h at 5°F.
It will heat most of the year - its HSPF2 beats 92% of mini-splits.
You want steady temperatures and quiet running - the compressor varies its speed.
You want a choice of indoor units - 16 certified pairings.

Think twice if…

Nothing stands out - its ratings have no weak spot for its class.

Calculate my running cost

Details

Specifications18,000 BTU, R-454B

General
Brand	Mainline
Outdoor unit model	ML18HP230NEXH-O
Series	ML Series
Type	Mini-split heat pump
Nominal size	18,000 BTU
CompressorVariable-speed units run longer at low power	Continuously variable
Refrigerant	R-454B
Connected (smart) capable	No
On the market since	May 20, 2024
Last certified	August 19, 2026
Cooling
SEER2Seasonal efficiency, higher is better	21.7 (range 18.0-21.7)
EER2Efficiency at 95°F outdoors	12.0-13.0
Cooling capacity	16,000-18,000 BTU/h
Heating
HSPF2Seasonal efficiency, higher is better	10.0-12.3
Capacity at 47°F	17,000-20,000 BTU/h
Capacity at 17°F	14,000-16,600 BTU/h
Capacity at 5°F	17,000-18,000 BTU/h
COP at 5°F2.0 = twice as efficient as a space heater	1.86-2.20
ENERGY STAR cold climate	Yes

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 ML18HP230NEX-I at the standard test temperatures, in BTU/h:

Certified matchups16 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 unit	SEER2	EER2	HSPF2	Heat @5°F	AHRI ref.
ML18HP230NEX-I MAINLINE · cold climate	21.7	13.0	10.8	18,000	217607254
ML18HP230NEX-I MAINLINE · cold climate	21.7	13.0	10.8	18,000	215864341
ML18HP2301WYCAS-I MAINLINE · cold climate	20.6	12.5	12.2	17,500	217607302
ML18HP2301WYCAS-I MAINLINE · cold climate	20.6	12.5	12.2	17,500	215864389
ML18HP230FLO-I MAINLINE · cold climate	20.4	12.5	10.3	17,000	217607297
ML18HP230FLO-I MAINLINE · cold climate	20.4	12.5	10.3	17,000	215864384
ML18HP230STDCAS-I MAINLINE · cold climate	20.0	12.5	12.3	17,000	217607284
ML18HP230STDCAS-I MAINLINE · cold climate	20.0	12.5	12.3	17,000	215864371
ML16HP230CON-I MAINLINE · cold climate	20.0	12.5	10.3	17,500	217607296
ML16HP230CON-I MAINLINE · cold climate	20.0	12.5	10.3	17,500	215864383
ML18HP230HSDUC-I MAINLINE · cold climate	18.4	12.0	11.1	18,000	217607292
ML18HP230HSDUC-I MAINLINE · cold climate	18.4	12.0	11.1	18,000	215864379
ML18HP230LSDUC-I MAINLINE · cold climate	18.0	12.0	10.5	18,000	217607289
ML18HP230LSDUC-I MAINLINE · cold climate	18.0	12.0	10.5	18,000	215864376
ML18HP230AHU-I · cold climate	18.0	12.0	10.0	18,000	217607161
ML18HP230AHU-I · cold climate	18.0	12.0	10.0	18,000	215864295

Yearly running cost in 10 large cities$368-$977

Phoenix, AZ$453/yr
Miami, FL$368/yr
Houston, TX$406/yr
Atlanta, GA$410/yr
Los Angeles, CA$398/yr
New York, NY$977/yr
Chicago, IL$793/yr
Denver, CO$659/yr
Seattle, WA$405/yr
Minneapolis, MN$816/yr

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

Questions5

What is the SEER2 rating of the Mainline ML18HP230NEXH-O?: The Mainline ML18HP230NEXH-O is rated at up to 21.7 SEER2 and 13.0 EER2 for cooling and up to 12.3 HSPF2 for heating. The exact figure depends on the indoor unit it is paired with; across its 16 certified matchups SEER2 ranges from 18.0-21.7.
Is the Mainline ML18HP230NEXH-O good for cold climates?: Yes. At least one certified matchup meets the ENERGY STAR cold-climate criteria, delivering up to 18,000 BTU/h at 5°F with a COP of 2.20, so it is still 2.2 times as efficient as electric resistance heat at that temperature.
How much does the Mainline ML18HP230NEXH-O cost to run per year?: At its rated efficiency it costs about $368-$977 a year in electricity in large U.S. cities, depending on climate and local rates - for example $453 in Phoenix and $816 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 Mainline ML18HP230NEXH-O?: ENERGY STAR lists 16 certified combinations for this outdoor unit. The highest-rated ones use MAINLINE ML18HP230NEX-I, MAINLINE ML18HP2301WYCAS-I, MAINLINE ML18HP230FLO-I.
What refrigerant does the Mainline ML18HP230NEXH-O use?: It uses R-454B.

Running cost and savings

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

Heat pump $449/yrYou save $461/yrSee details ↓

1. Where you live

StateCityElectricity ¢ per kWh

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.

Cooling hours full-load hours per yearHeating hours full-load hours per year

2. The heat pump Mainline ML18HP230NEXH-O

Find a model (fills in the ratings below)

SEER2 cooling efficiencyHSPF2 heating efficiencyCooling capacity BTU/hHeating capacity BTU/h

3. What it replaces

Current heatingFurnace efficiency (AFUE %)Gas price, $/thermCurrent coolingOld AC rating SEER2 (10 ≈ 15+ years old)

4. Price replace the example with your quote

Installed price, $Rebates and incentives, $ utility, state

Heat pump, per year: $449
Current system, per year: $909
You save per year: $461
Payback: 14.1 years
Net after 10 years: -$1,895

Cooling $276Heating $173Electricity used: 2,805 kWhSaved vs. a minimum-efficiency heat pump: $253

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 18,000 BTU models

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NOVAIR LEA18SZ36-O · cold climate21.7 SEER2 · 12.3 HSPF2
CanAir 4C28SCH18H21 · cold climate21.7 SEER2 · 12.3 HSPF2
SLIMAIR SHC18AH2AG · cold climate21.7 SEER2 · 12.3 HSPF2
AI AIR CONDITIONING AO25-18N2-M1H · cold climate21.7 SEER2 · 12.3 HSPF2
TRUST AIR CONDITIONERS OAH18H/O6S4P · cold climate21.7 SEER2 · 12.3 HSPF2
ACIQ ACIQ-18ZPL-HP230C · cold climate21.7 SEER2 · 12.3 HSPF2
Wells WOS-18PRE/230 · cold climate21.7 SEER2 · 12.3 HSPF2

See all 465 18,000 BTU mini-splits, ranked

Source: ENERGY STAR Certified Heat Pumps (U.S. EPA), certification of August 19, 2026. How we use the data.