What is the SEER2 rating of the Willis FLEX24/36KUH?

The Willis FLEX24/36KUH 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 FLEX24/36KUH good for cold climates?

Yes. At least one certified matchup meets the ENERGY STAR cold-climate criteria, delivering up to 28,000 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 FLEX24/36KUH cost to run per year?

At its rated efficiency it costs about $996-$2,497 a year in electricity in large U.S. cities, depending on climate and local rates - for example $1,205 in Phoenix and $2,069 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 FLEX24/36KUH?

ENERGY STAR lists 1 certified combination for this outdoor unit. The highest-rated one uses FLEXAH24/36K.

What refrigerant does the Willis FLEX24/36KUH 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.

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

Willis FLEX24/36KUH

Below average cooling efficiency and average heating efficiency for a 3-ton central heat pump, and certified for cold climates.

Ranks #231 of 302 3 ton central heat pumps for cooling efficiency, and costs about $1,061 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: 28,000 BTU/h80% 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 28,000 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

Specifications3 ton, R-410A

General
Brand	Willis
Outdoor unit model	FLEX24/36KUH
Series	FLEXX High-efficiency Series
Type	Central heat pump
Nominal size	3 ton
CompressorVariable-speed units run longer at low power	Continuously variable
Refrigerant	R-410A
Connected (smart) capable	No
On the market since	September 25, 2023
Last certified	June 29, 2026
Cooling
SEER2Seasonal efficiency, higher is better	16.0 (range 16.0)
EER2Efficiency at 95°F outdoors	10.0
Cooling capacity	34,000 BTU/h
Heating
HSPF2Seasonal efficiency, higher is better	9.0
Capacity at 47°F	35,000 BTU/h
Capacity at 17°F	29,000 BTU/h
Capacity at 5°F	28,000 BTU/h
COP at 5°F2.0 = twice as efficient as a space heater	1.80
ENERGY STAR cold climate	Yes

Heating output as it gets colder80% kept at 5°F

Every heat pump delivers less heat as the outdoor temperature drops. Certified output of its matchup with FLEXAH24/36K 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 unit	SEER2	EER2	HSPF2	Heat @5°F	AHRI ref.
FLEXAH24/36K · cold climate	16.0	10.0	9.0	28,000	214137765

Yearly running cost in 10 large cities$996-$2,497

Phoenix, AZ$1,205/yr
Miami, FL$996/yr
Houston, TX$1,079/yr
Atlanta, GA$1,061/yr
Los Angeles, CA$1,025/yr
New York, NY$2,497/yr
Chicago, IL$2,020/yr
Denver, CO$1,674/yr
Seattle, WA$1,024/yr
Minneapolis, MN$2,069/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 Willis FLEX24/36KUH?: The Willis FLEX24/36KUH 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 FLEX24/36KUH good for cold climates?: Yes. At least one certified matchup meets the ENERGY STAR cold-climate criteria, delivering up to 28,000 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 FLEX24/36KUH cost to run per year?: At its rated efficiency it costs about $996-$2,497 a year in electricity in large U.S. cities, depending on climate and local rates - for example $1,205 in Phoenix and $2,069 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 FLEX24/36KUH?: ENERGY STAR lists 1 certified combination for this outdoor unit. The highest-rated one uses FLEXAH24/36K.
What refrigerant does the Willis FLEX24/36KUH 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 $1,184/yrYou save $586/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 Willis FLEX24/36KUH

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: $1,184
Current system, per year: $1,769
You save per year: $586
Payback: 22.2 years
Net after 10 years: -$7,141

Cooling $748Heating $436Electricity used: 7,397 kWhSaved vs. a minimum-efficiency heat pump: $176

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 3 ton models

MRCOOL CENTRAL-36-HP-C-230A00 · cold climate16.0 SEER2 · 9.5 HSPF2
RunTru A5HP5036A116.0 SEER2 · 8.5 HSPF2
Mitsutomo UHO36-NB-Q · cold climate16.0 SEER2 · 9.5 HSPF2
Coleman XH536E2S1116.0 SEER2 · 8.1 HSPF2
1HVAC EZ-36-HP · cold climate16.0 SEER2 · 9.5 HSPF2
Luxaire THF2B36T21 · cold climate16.0 SEER2 · 8.1 HSPF2
TRUST AIR CONDITIONERS AA36H/O44P · cold climate16.0 SEER2 · 10.0 HSPF2
Trane 5HPL5036A1000AA · cold climate16.0 SEER2 · 8.1 HSPF2

See all 302 3 ton central heat pumps, ranked

Source: ENERGY STAR Certified Heat Pumps (U.S. EPA), certification of June 29, 2026. How we use the data.