Quick answer
Evaporation temperature is the temperature at which refrigerant boils inside the evaporator coil, fixed by the suction pressure, and is also called the saturated suction temperature.
It sets how cold the indoor coil runs in cooling. For R-32, a suction pressure of about 121 psig corresponds to a 40°F evaporation temperature, per NIST data.
Quick facts
The key facts about Evaporation Temperature, with sources:
- Also called
- Saturated suction temperature (SST), evaporating temperature
- Set by
- Suction pressure and the refrigerant's pressure-temperature curve 1
- R-32 at 121 psig (135.65 psia)
- 40°F 1
- R-32 at 99 psig (113.79 psia)
- 30°F, cold enough to freeze moisture on the coil 1
- Used for
- Superheat: line temperature − evaporation temperature 2
- R-454B normal boiling point
- −50.6°F at 1 atmosphere 3
Key takeaways
- Evaporation temperature is the refrigerant's boiling point in the indoor coil during cooling.
- Suction pressure sets it; each refrigerant has its own curve 1.
- It must be well below room air so heat flows in, and below the dew point so the coil dries the air.
- If it drops below about 32°F for long, the coil can ice over.
- Low airflow and low charge both pull it down.
What evaporation temperature is
Every liquid boils at a temperature that depends on pressure. Refrigerants are chosen because they boil far below room temperature at pressures a system can manage.
At normal air pressure, R-454B boils at −50.6°F 3. Inside a working evaporator, the pressure is much higher, so it boils at a useful temperature, often near 40°F.
While refrigerant is boiling, its temperature stays at that boiling point, no matter how much heat it absorbs. That steady temperature is the evaporation temperature. Technicians read it indirectly: they measure suction pressure and look up the matching saturation temperature.
It is one end of the refrigeration cycle. The other end is the condensation temperature in the outdoor coil.
What sets the evaporation temperature
- Expansion device meters refrigerant into the coil → sets how much liquid arrives
- Indoor airflow and air temperature → set how much heat the coil absorbs
- Compressor pulls vapor away → lowers suction pressure
- Balance of all three → suction pressure → evaporation temperature (from the PT curve)
How it behaves in a working system
The evaporation temperature floats to a balance. More heat from the room raises it; more compressor pull or less refrigerant feed lowers it. On a hot afternoon with lots of indoor heat, it sits higher. On a mild night with little load, it drops.
The coil runs a bit warmer than the boiling refrigerant inside it, and the air leaving runs warmer still. For moisture to condense, the coil surface must be below the air's dew point. That is why the evaporation temperature matters for humidity as much as for temperature.
If it drops below 32°F, water on the coil can freeze. A thin frost grows into a block of ice that blocks airflow, which lowers the temperature further. That spiral is a frozen coil.
Evaporation temperature in heat pump heating
In winter, a heat pump's outdoor coil becomes the evaporator. Its evaporation temperature must sit below the outdoor air so heat flows into the refrigerant. On a 35°F day, that means a boiling point in the 20s °F or lower. On a 5°F day, it must be colder still.
Two things follow. First, the coil runs below freezing in much of the winter, so moisture in the air frosts on it and the unit runs defrost cycles to clear it.
Second, the lower the evaporation temperature, the lower the suction pressure and the harder the compressor works. That is the main reason heat pump capacity and efficiency fall as it gets colder outside.
Cold-climate heat pumps use compressors and controls built to keep working at these low evaporating temperatures.
Reading the low side on a service ticket
- Suction pressure (psig): the raw reading.
- SST or saturated suction (°F): the evaporation temperature from that pressure.
- Suction line temperature (°F): measured on the pipe.
- Superheat (°F): the difference between the two.
- Refrigerant: the numbers only mean something for the refrigerant named on the data plate.
R-32 evaporation temperatures and pressures (NIST)
| Evaporation temperature | Pressure (psia) | Approx. gauge (psig) |
|---|---|---|
| 30°F | 113.79 | 99 |
| 35°F | 124.35 | 110 |
| 40°F | 135.65 | 121 |
| 45°F | 147.70 | 133 |
| 50°F | 160.54 | 146 |
Worked example: same system, three conditions
An R-32 system, with suction pressures assumed for three situations. Temperatures from NIST 1.
| Situation | Suction pressure (assumed) | Evaporation temperature | What it suggests |
|---|---|---|---|
| Normal day, clean filter | 121 psig | 40°F | Coil cold enough to cool and dry, above freezing |
| Heavy load, humid afternoon | 133 psig | 45°F | Higher load; still cooling, less drying margin |
| Clogged filter, low airflow | 99 psig | 30°F | Coil below freezing; ice likely to form |
The 30°F case is the classic frozen coil. Changing the filter and restoring airflow brings the evaporation temperature back up. If airflow is fine and it still runs low, a technician looks at charge and the metering device.
Where you see evaporation temperature
- On a digital gauge, shown as SST or saturated suction next to suction pressure.
- On a service ticket, often as a suction pressure only; ask for the saturation temperature.
- In superheat math: line temperature minus evaporation temperature 2.
- In maker performance data, where capacity depends on evaporating conditions.
Higher vs lower evaporation temperature
Higher (within range)
- Higher efficiency: the compressor lifts heat a shorter distance.
- Less risk of icing.
- More sensible cooling capacity.
Lower
- More dehumidification, up to a point.
- Lower efficiency.
- Risk of freezing below about 32°F.
Limits of the number
Evaporation temperature alone does not show charge. It moves with load and airflow, so a tech pairs it with superheat and subcooling. Two systems with the same suction pressure can be in very different health once those other readings are known.
Blends complicate it. R-454B has a temperature glide of about 1.5 K, so it starts boiling at one temperature and finishes slightly warmer 3. Gauges for blends show separate bubble and dew values; superheat uses the dew value.
Sources: [3]
What it means for running cost
A system run with too-low evaporation temperature, from a dirty filter or low charge, works harder for less cooling, and can freeze. Keeping airflow right is the cheapest way to hold the evaporation temperature where it belongs.
Repairs for low charge mean a leak fix; see refrigerant leak repair and the repair cost calculator.
How a technician uses it
- Check filter, blower and static pressure.
- Read suction pressure (EPA 608 certified work) 4.
- Convert to evaporation temperature with the refrigerant's data 1.
- Measure suction line temperature and calculate superheat 2.
- Compare with design values and diagnose airflow, charge or metering.
Owner tasks that keep it in range
- Change filters on time.
- Keep supply and return vents open.
- Have the indoor coil cleaned when dirty; a dirty coil passes less heat into the refrigerant and pulls its boiling point down.
- Keep interior doors open or provide return paths so the blower can move its full airflow.
- Do not set the thermostat fan to auto-off right after icing; run fan only to thaw, then call.
Warning signs of a too-low evaporation temperature
- Frost or ice on the large copper line or coil.
- Water overflowing as ice melts.
- Weak airflow from vents.
- Clammy rooms or poor cooling.
- Turn cooling off, run the fan, and call an EPA 608 certified tech if it returns.
Rules that apply
There is no code limit on evaporation temperature; it is an operating condition. Reading it means connecting gauges to the refrigerant circuit, which EPA treats as service: technicians must be Section 608 certified 4, and knowingly venting refrigerant is prohibited 5.
Makers set the targets that matter, such as design superheat and minimum airflow, in their installation manuals 2.
Evaporation temperature vs related terms
| Term | What it is | Relation |
|---|---|---|
| Evaporation temperature | Boiling point in the evaporator | Low-side saturation |
| Condensation temperature | Condensing point in the condenser | High-side saturation |
| Superheat | Vapor above boiling point | Line temp − evaporation temp |
| Dew point | Where air moisture condenses | Coil must be below it to dry air |
| Relative humidity | Moisture in room air | Sets the dew point |
Common misconceptions
- Myth Lower is better for cooling.
- Reality Too low wastes energy and freezes coils.
- Myth Suction pressure is the same as temperature.
- Reality Pressure maps to temperature only through the refrigerant's own curve 1.
- Myth A frozen coil means low refrigerant.
- Reality Often it is low airflow; check the filter first.
Humidity and the target
In humid places like Florida and Georgia, systems are often set up to run a slightly colder coil, for example with lower airflow, so more moisture condenses. In dry places like Arizona, a warmer coil gives more sensible cooling and better efficiency. The evaporation temperature is how that choice shows up on gauges.
When this matters to you
You will hear about evaporation temperature when a coil freezes or a tech diagnoses charge. Ask them to show the suction pressure, the saturation temperature and the superheat, not just one number.
Next step: if your coil keeps freezing, read about air filters and airflow first.
Questions about Evaporation Temperature
What is evaporation temperature in an AC?
It is the temperature at which the refrigerant boils inside the indoor coil. It is set by the suction pressure and is also called the saturated suction temperature. For R-32, about 121 psig matches 40°F.
What is a normal evaporating temperature?
It varies with the system and conditions. Many cooling systems run in the high 30s to mid 40s °F under normal load. The maker's data and the superheat target matter more than any single number.
Why does the coil freeze if evaporation temperature is too low?
If the refrigerant boils below 32°F, the coil surface can drop below freezing and water on it turns to ice. Ice blocks airflow, which lowers the temperature further. Low airflow and low charge are the usual causes.
How is evaporation temperature measured?
A technician reads suction pressure and converts it to temperature using the refrigerant's pressure-temperature data. Digital gauges do this automatically and display it as saturated suction temperature.
Is evaporation temperature the same as the coil temperature?
Close but not the same. The coil surface runs a little warmer than the boiling refrigerant, and the air leaving the coil warmer still. The refrigerant's boiling point is the coldest point.
Does evaporation temperature affect humidity?
Yes. The coil must be colder than the air's dew point to pull water out. A colder coil dries more air but lowers efficiency and raises freeze risk.
What raises evaporation temperature?
More heat reaching the coil, such as higher airflow or a hotter, more humid room, raises it. Overcharge or an overfeeding metering device can raise it too. A technician tells these apart with superheat.
Is the evaporation temperature the same for all refrigerants?
No. Each refrigerant has its own pressure-temperature curve. A reading on R-32 means a different temperature than the same pressure on R-410A or R-454B, so techs must use the right data.
Why does a heat pump outdoor coil frost in winter?
In heating, the outdoor coil is the evaporator and must run colder than the outdoor air. When that temperature is below freezing, moisture in the air turns to frost on the coil. The heat pump runs short defrost cycles to clear it.
What does SST mean on HVAC gauges?
SST stands for saturated suction temperature, another name for evaporation temperature. Digital gauges calculate it from suction pressure using the selected refrigerant's data, so the gauge must be set to the right refrigerant.
Sources
- NIST Chemistry WebBook, Thermophysical Properties of Fluid Systems: difluoromethane (R-32) saturation properties, 30°F–120°F, retrieved .
- Goodman / ACiQ, A/GPHM3 13.4 SEER2 M-Series packaged heat pump with R-32, installation instructions (IOG-3035B, 2025), retrieved .
- Chemours, Opteon XL41 (R-454B) Product Information (C-11001, 9/2016), retrieved .
- U.S. EPA, Section 608 Technician Certification, retrieved .
- U.S. EPA, Stationary Refrigeration: Prohibition on Venting Refrigerants, retrieved .
Expert review
Written by the HyreHVAC Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: R-32 pressure-temperature pairs copied from the NIST Chemistry WebBook (psig = psia − 14.7); superheat formula from the Goodman R-32 manual; R-454B boiling point and glide from Chemours; EPA rules read 10 October 2026. Suction pressures in the worked example are assumed.
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