Trades Math Basics: Temperature Conversions (Fahrenheit ↔ Celsius, ΔT, Superheat/Subcooling)

Trades Math – Temperature Conversions (Fahrenheit ↔ Celsius, ΔT, Superheat/Subcooling)

On-the-Job Study Guide for Apprentices & Journeymen

What This Is

Temperature conversions are non-negotiable in HVAC, refrigeration, plumbing, and electrical work. You’ll use them to: - Set thermostats (e.g., converting a customer’s 72°F request to Celsius for a smart thermostat).
- Charge refrigerant (calculating superheat/subcooling to avoid compressor failure).
- Size hydronic systems (ΔT across a boiler or chiller to ensure efficiency).
- Troubleshoot (e.g., "Why is my heat pump freezing up?" → Check subcooling temps).

Real-world scenario:
An HVAC tech is charging a new R-410A system. The manufacturer’s specs call for 10°F of subcooling at 95°F outdoor temp. The liquid line temp reads 85°F, and the high-side pressure is 340 PSI (which converts to 105°F saturation temp). Is the system overcharged, undercharged, or just right? (Answer: Overcharged—subcooling is 20°F, not 10°F.)

Key Terms & Formulas

Conversions

• Fahrenheit to Celsius:
  °C = (°F – 32) × 5/9
  Example: Convert 75°F to Celsius: (75 – 32) × 5/9 = 23.9°C.

• Celsius to Fahrenheit:
  °F = (°C × 9/5) + 32
  Example: Convert 20°C to Fahrenheit: (20 × 9/5) + 32 = 68°F.

• Quick Estimate (Field Trick):
  For rough conversions, use °F ≈ (°C × 2) + 30.
  Example: 25°C × 2 + 30 ≈ 80°F (actual: 77°F—close enough for a quick check).

ΔT (Delta T)

• ΔT = T_hot – T_cold
  T_hot = Higher temperature (e.g., supply air, hot water).
  T_cold = Lower temperature (e.g., return air, cold water).
  Example: A hydronic boiler has a supply temp of 180°F and return temp of 160°F. ΔT = 20°F (used to size pumps and radiators).

Superheat (Vapor)

• Superheat = Suction Line Temp – Saturation Temp (at suction pressure)
  Measures how much the refrigerant vapor is heated above its boiling point. Example: Suction pressure = 120 PSI (R-410A) → Saturation temp = 45°F. Suction line temp = 55°F. Superheat = 10°F (correct for a TXV system).

Subcooling (Liquid)

• Subcooling = Saturation Temp (at liquid pressure) – Liquid Line Temp
  Measures how much the refrigerant liquid is cooled below its condensation point. Example: Liquid pressure = 340 PSI (R-410A) → Saturation temp = 105°F. Liquid line temp = 95°F. Subcooling = 10°F (target for most systems).

Step-by-Step / Process Flow

1. Converting Between Fahrenheit and Celsius

When to use: Setting thermostats, reading international equipment specs, or troubleshooting.
Steps:
1. Identify the known temp (e.g., "The manual says 25°C, but my gauge reads in °F").
2. Pick the right formula:
- °F → °C: Subtract 32, multiply by 5/9.
- °C → °F: Multiply by 9/5, add 32.
3. Plug in the numbers (use a calculator—no shame!).
4. Double-check with the quick estimate (e.g., 25°C × 2 + 30 ≈ 80°F → actual is 77°F, so you’re close).

Example:
Convert 95°F (outdoor temp) to Celsius for a heat pump sizing calculation.
- 95 – 32 = 63
- 63 × 5/9 = 35°C

2. Calculating ΔT (Temperature Difference)

When to use: Sizing HVAC systems, checking boiler efficiency, or verifying airflow.
Steps:
1. Measure the two temps (e.g., supply and return air, hot and cold water).
- Pro tip: Use a digital thermometer with a probe (not an infrared gun—it’s not accurate for air/water temps).
2. Subtract the smaller temp from the larger temp (ΔT = T_hot – T_cold).
3. Compare to target ΔT:
- Airflow: 15–20°F ΔT across an evaporator coil (e.g., 55°F supply, 75°F return → 20°F ΔT = good).
- Hydronic systems: 20°F ΔT for baseboard, 10°F for radiant floors.

Example:
A boiler’s supply temp is 170°F, return temp is 150°F. ΔT = 20°F → Correct for a standard baseboard system.

3. Measuring Superheat (For Refrigeration/HVAC)

When to use: Charging a system, diagnosing compressor issues, or verifying TXV operation.
Steps:
1. Attach gauges to the suction line (low side).
2. Read suction pressure (e.g., 120 PSI for R-410A).
3. Convert pressure to saturation temp (use a PT chart or app):
- 120 PSI (R-410A) = 45°F saturation temp.
4. Measure suction line temp (e.g., 55°F).
5. Calculate superheat: 55°F – 45°F = 10°F.
6. Compare to target:
- Fixed orifice: 8–12°F superheat.
- TXV system: 10–15°F superheat.

Example:
A walk-in cooler with a TXV has a suction pressure of 30 PSI (R-134a) → Saturation temp = 20°F. Suction line temp = 30°F. Superheat = 10°F → Correct for a TXV.

4. Measuring Subcooling (For Refrigeration/HVAC)

When to use: Charging a system, checking for overcharge, or verifying condenser performance.
Steps:
1. Attach gauges to the liquid line (high side).
2. Read liquid pressure (e.g., 340 PSI for R-410A).
3. Convert pressure to saturation temp (340 PSI = 105°F).
4. Measure liquid line temp (e.g., 95°F).
5. Calculate subcooling: 105°F – 95°F = 10°F.
6. Compare to target:
- Standard AC: 10–15°F subcooling.
- Heat pump (heating mode): 5–10°F subcooling.

Example:
A mini-split system has a liquid pressure of 320 PSI (R-410A) → Saturation temp = 100°F. Liquid line temp = 85°F. Subcooling = 15°F → Slightly overcharged (target is 10°F).

Common Mistakes

Trade-Specific Insights

HVAC/Refrigeration

• Superheat vs. Subcooling Balance:
• High superheat + low subcooling = Undercharged (not enough refrigerant).
• Low superheat + high subcooling = Overcharged (too much refrigerant).

• High superheat + high subcooling = Restricted metering device (e.g., clogged TXV).

• Field Trick for Quick Checks:

• If you don’t have gauges, feel the suction line—it should be cold but not frosted (frost = low superheat = risk of liquid floodback).

Plumbing/Hydronics

• ΔT for Boiler Efficiency:
• A 20°F ΔT is standard for baseboard, but 10°F ΔT is better for radiant floors (lower ΔT = more even heat).

• If ΔT is too high, the pump may be undersized or the system is air-bound.

• Code Note (IPC/IMC):

• Hot water supply temp must be ≤ 140°F for safety (scalding risk). Use a mixing valve to blend hot/cold water if needed.

Electrical (Thermal Overloads)

• Motor Overloads:
• If a motor’s ambient temp + ΔT exceeds its rating, it will trip. Example: A 100°F motor in a 90°F room with a 20°F ΔT = 110°F → Overload risk.

Quick Check Questions

1. A customer’s thermostat is set to 22°C, but they complain it feels too cold. What’s the equivalent in °F?
2. Answer: 71.6°F (22 × 9/5 + 32).

3. Why? Most people in the U.S. think in °F—22°C is a cool setting.

4. An AC system has a suction pressure of 70 PSI (R-410A) and a suction line temp of 50°F. What’s the superheat? Is this acceptable for a TXV system?

5. Answer: 70 PSI = 35°F saturation temp. Superheat = 50°F – 35°F = 15°F → Acceptable (10–15°F target for TXV).

6. A boiler’s supply temp is 180°F, and the return temp is 150°F. What’s the ΔT? Is this correct for baseboard heating?

7. Answer: ΔT = 180°F – 150°F = 30°F → Too high (target is 20°F). Check for air in the system or undersized pump.

Last-Minute Cram Sheet

1. °F → °C: (°F – 32) × 5/9
2. °C → °F: (°C × 9/5) + 32
3. Quick °C → °F estimate: (°C × 2) + 30
4. ΔT = T_hot – T_cold (e.g., supply – return air/water).
5. Superheat = Suction line temp – Saturation temp (at suction pressure).
6. Subcooling = Saturation temp (at liquid pressure) – Liquid line temp.
7. ⚠️ Always use the correct PT chart for the refrigerant (R-410A ≠ R-22).
8. ⚠️ Measure superheat/subcooling 6–12" from the compressor on a straight pipe.
9. Target superheat (TXV): 10–15°F | Target subcooling (AC): 10–15°F.
10. ⚠️ Outdoor temp affects superheat/subcooling targets—check manufacturer specs!