What is the SEER2 rating of the Mainline ML18HP230NEX-O?

The Mainline ML18HP230NEX-O is rated at up to 24.0 SEER2 and 13.5 EER2 for cooling and up to 13.5 HSPF2 for heating. The exact figure depends on the indoor unit it is paired with; across its 16 certified matchups SEER2 ranges from 18.5-24.0.

Is the Mainline ML18HP230NEX-O good for cold climates?

Yes. At least one certified matchup meets the ENERGY STAR cold-climate criteria, delivering up to 14,200 BTU/h at 5°F with a COP of 2.40, so it is still 2.4 times as efficient as electric resistance heat at that temperature.

How much does the Mainline ML18HP230NEX-O cost to run per year?

At its rated efficiency it costs about $351-$861 a year in electricity in large U.S. cities, depending on climate and local rates - for example $422 in Phoenix and $711 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 ML18HP230NEX-O?

ENERGY STAR lists 16 certified combinations for this outdoor unit. The highest-rated ones use MAINLINE ML18HP230NEX-I, MAINLINE ML16HP230CON-I, MAINLINE ML18HP230FLO-I.

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

Mainline ML18HP230NEX-O

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

Ranks #64 of 465 18,000 BTU mini-splits for cooling efficiency, and costs about $368 a year to run in a mixed climate like Atlanta.

SEER2: 24.0 Cooling efficiency over a season
Very good · beats 75%
HSPF2: 13.5 Heating efficiency over a season
Excellent · beats 96%
EER2: 13.5 Cooling efficiency on a 95°F day
Very good · beats 74%
Heat at 5°F: 14,200 BTU/h72% of its 47°F output
Excellent · beats 93%

A good fit if…

You want low cooling bills - its SEER2 beats 75% of mini-splits.
Your winters get properly cold - it is ENERGY STAR cold-climate certified, with 14,200 BTU/h at 5°F.
It will heat most of the year - its HSPF2 beats 96% of mini-splits.
You want steady temperatures and quiet running - the compressor varies its speed.

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	ML18HP230NEX-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	24.0 (range 18.5-24.0)
EER2Efficiency at 95°F outdoors	12.1-13.5
Cooling capacity	16,000-18,000 BTU/h
Heating
HSPF2Seasonal efficiency, higher is better	9.8-13.5
Capacity at 47°F	17,000-20,000 BTU/h
Capacity at 17°F	13,200-15,000 BTU/h
Capacity at 5°F	12,800-14,200 BTU/h
COP at 5°F2.0 = twice as efficient as a space heater	2.05-2.40
ENERGY STAR cold climate	Yes

Heating output as it gets colder72% 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	24.0	13.5	11.1	13,000	217607249
ML18HP230NEX-I MAINLINE · cold climate	24.0	13.5	11.1	13,000	215864336
ML16HP230CON-I MAINLINE · cold climate	23.8	12.5	13.5	13,100	217607276
ML16HP230CON-I MAINLINE · cold climate	23.8	12.5	13.5	13,100	215864363
ML18HP230FLO-I MAINLINE · cold climate	22.3	12.5	10.4	12,800	217607277
ML18HP230FLO-I MAINLINE · cold climate	22.3	12.5	10.4	12,800	215864364
ML18HP2301WYCAS-I MAINLINE · cold climate	22.2	12.5	12.2	13,100	217607281
ML18HP2301WYCAS-I MAINLINE · cold climate	22.2	12.5	12.2	13,100	215864368
ML18HP230STDCAS-I MAINLINE · cold climate	20.7	12.5	11.4	12,800	217607265
ML18HP230STDCAS-I MAINLINE · cold climate	20.7	12.5	11.4	12,800	215864352
ML18HP230HSDUC-I MAINLINE · cold climate	20.0	13.3	11.3	14,200	217607272
ML18HP230HSDUC-I MAINLINE · cold climate	20.0	13.3	11.3	14,200	215864359
ML18HP230LSDUC-I MAINLINE · cold climate	20.0	12.5	10.5	14,000	217607269
ML18HP230LSDUC-I MAINLINE · cold climate	20.0	12.5	10.5	14,000	215864356
ML18HP230AHU-I · cold climate	18.5	12.1	9.8	13,000	217607158
ML18HP230AHU-I · cold climate	18.5	12.1	9.8	13,000	215864291

Yearly running cost in 10 large cities$351-$861

Phoenix, AZ$422/yr
Miami, FL$351/yr
Houston, TX$378/yr
Atlanta, GA$368/yr
Los Angeles, CA$355/yr
New York, NY$861/yr
Chicago, IL$696/yr
Denver, CO$576/yr
Seattle, WA$352/yr
Minneapolis, MN$711/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 ML18HP230NEX-O?: The Mainline ML18HP230NEX-O is rated at up to 24.0 SEER2 and 13.5 EER2 for cooling and up to 13.5 HSPF2 for heating. The exact figure depends on the indoor unit it is paired with; across its 16 certified matchups SEER2 ranges from 18.5-24.0.
Is the Mainline ML18HP230NEX-O good for cold climates?: Yes. At least one certified matchup meets the ENERGY STAR cold-climate criteria, delivering up to 14,200 BTU/h at 5°F with a COP of 2.40, so it is still 2.4 times as efficient as electric resistance heat at that temperature.
How much does the Mainline ML18HP230NEX-O cost to run per year?: At its rated efficiency it costs about $351-$861 a year in electricity in large U.S. cities, depending on climate and local rates - for example $422 in Phoenix and $711 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 ML18HP230NEX-O?: ENERGY STAR lists 16 certified combinations for this outdoor unit. The highest-rated ones use MAINLINE ML18HP230NEX-I, MAINLINE ML16HP230CON-I, MAINLINE ML18HP230FLO-I.
What refrigerant does the Mainline ML18HP230NEX-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 $413/yrYou save $515/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 ML18HP230NEX-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: $413
Current system, per year: $928
You save per year: $515
Payback: 13.6 years
Net after 10 years: -$1,852

Cooling $264Heating $149Electricity used: 2,583 kWhSaved vs. a minimum-efficiency heat pump: $299

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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MDV MOX430-18HFN10-M1 · cold climate24.0 SEER2 · 13.5 HSPF2
CanAir 4C25SCH18H21 · cold climate24.0 SEER2 · 13.5 HSPF2
Napoleon NDHAS22BA-18-O · cold climate24.0 SEER2 · 13.5 HSPF2
AI AIR CONDITIONING AO24-18N2-M1 · cold climate24.0 SEER2 · 13.5 HSPF2
NOVAIR LEA18SZ26-O · cold climate24.0 SEER2 · 13.5 HSPF2
TOSOT TWH18PR32DO · cold climate24.5 SEER2 · 9.0 HSPF2
TRUST AIR CONDITIONERS OUS18H/O6S4P · cold climate24.0 SEER2 · 13.5 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.