Lesson HVAC fundamentals · Cycle and refrigerants
Temperature–pressure relationship
Boiling temperature follows pressure. Control vapor pressure above a liquid and you control its boiling point. At standard atmosphere water boils at 212°F; lower pressure → lower boil point. Refrigeration works by boiling refrigerant at low pressure in the evaporator and condensing at high pressure in the condenser. Memorize: control pressure → control temperature.
The sentence worth memorizing
The boiling point of a liquid can be changed by controlling the pressure on it.
This is not trivia—it is the control knob for the entire mechanical refrigeration cycle.
Water at standard conditions
At sea level with barometer 29.92 in Hg (14.696 psia, 0 psig on surface):
- water boils at 212°F
Freeze point at standard pressure: 32°F (same sea-level reference you already use for water).
Lower pressure → lower boiling point
On a mountain (~5000 ft), atmospheric pressure might be ~25 in Hg → water boils ~203°F. Potatoes cook slower—less heat delivered while steam stays at lower temperature.
In a bell jar with vacuum pump, reduce pressure until water boils at room temperature (~70°F)—the water feels cold because latent heat of vaporization is leaving the liquid.
Higher pressure → higher boiling point
Pressure cooker at ~15 psig above atmosphere (~30 psia) raises boiling point to about 250°F—faster cooking because higher temperature at higher pressure.
Same logic: compressor raises refrigerant pressure so it can condense hot enough to reject heat to outdoor air.
Reading T/P charts (and P/T charts)
Charts list saturation temperature vs pressure for a substance.
- Sometimes temperature on left, pressure in body → T/P chart
- Sometimes reversed → P/T chart
Same data—know which axis you are reading or calculations invert.
For refrigerants, saturation charts are your field “translator” between gauge pressure and boiling/condensing temperature.
Comfort cooling with water (thought experiment)
If water boiled at 40°F at low enough pressure (~0.122 psia / 0.248 in Hg), passing room air over coils of boiling water would absorb heat—air would leave cold.
That is exactly the refrigerant idea—except water is not practical for normal A/C (very low pressures, volume, freeze concerns). Hence synthetic refrigerants with useful T/P curves.
Refrigerant cylinder demo (concept only)
Partial liquid/vapor R-22 cylinder at equilibrium:
- in 75°F room → about 132 psig
- moved to 100°F room → pressure rises (~196 psig) as more liquid vaporizes
- moved to 35°F cooler → pressure falls (~61.5 psig) as vapor condenses
Temperature of the cylinder sets saturation pressure—not magic, not necessarily a leak.
Never vent refrigerant to atmosphere to “test” this—illegal and unsafe. Learn from charts and supervised training.
Connect to the cycle
| Component | Pressure role | Temperature result |
|---|---|---|
| Evaporator | Low pressure | Low saturation → boil → absorb heat |
| Compressor | Raises pressure | — |
| Condenser | High pressure | High saturation → condense → reject heat |
| Metering device | Drops pressure | Sets low-side boil point |
Control P → control T at saturation. Superheat and subcool are measured away from pure saturation—topics for later courses.
Field procedure mental loop
- Read psig (correct refrigerant scale)
- Convert to psia if needed for math
- Look up saturation temperature for that refrigerant
- Compare to measured line/coil temperature for superheat/subcool insight
Field case
Situation. Tech charges on a cool spring day. Outdoor ambient 55°F. Head pressure “looks low” vs summer chart memorized in head. Panic charge begins.
How to think with this lesson.
- Saturation pressure follows ambient on the condenser—lower outdoor temp → lower head pressure if system healthy
- Compare to T/P chart for that refrigerant at current conditions, not summer muscle memory alone
- Overcharge from misunderstanding T/P causes damage
Learning takeaway: “Low” or “high” gauge reading only means something against expected saturation for that T and refrigerant.
In the field
Symptom
Pressure “wrong” but system cools; or normal pressure dismissed as fault
Where to look
Correct P/T chart; refrigerant identity; coil temperatures; outdoor/indoor load
Likely causes
- Wrong chart
- ignoring ambient
- confusing psig with psia
- wrong refrigerant assumed
What to measure
- psig on high and low side
- corresponding saturation T
- actual line T
What not to do
- Vent refrigerant
- use another gas’s chart
- charge to “summer numbers” in winter without analysis
Checklist
- I state: boiling point changes with pressure
- I know water boils at 212°F at 29.92 in Hg / 14.696 psia
- I explain mountain vs pressure cooker examples
- I can read whether a chart is T/P or P/T
- I connect low-side P to evaporator temperature and high-side P to condensing temperature