Temperature lift is condensing temperature minus evaporating temperature. A small lift is like pumping water up one floor. A big lift is like pumping it up to the tenth floor. The higher the climb, the more work per litre.

Physics sets a ceiling called the Carnot COP. It uses absolute temperatures in kelvin (°C + 273.15). Real heat pumps typically reach about 40–60% of Carnot COP, because of compressor losses, heat exchanger temperature differences and pressure drops.

So COP rises when:

• the source is warmer (summer air in Chennai beats winter air in Delhi);
• the delivery water is cooler (45 °C needs less lift than 65 °C);
• heat exchangers are clean and correctly sized (smaller approach temperatures);
• water and air flows are correct.

All rows assume an evaporating temperature of 0 °C, so condensing temperature equals the lift.

Worked example — same unit, Chennai summer vs Delhi winter, water at 55 °C.

• Assume condensing at 60 °C (5 K above water) = 333.15 K.
• Chennai, air 32 °C, evaporating about 24 °C = 297.15 K. Lift = 36 K. Carnot COP = 333.15 ÷ 36 ≈ 9.3. Real at 50% ≈ 4.6.
• Delhi winter, air 8 °C, evaporating about 0 °C = 273.15 K. Lift = 60 K. Carnot COP = 333.15 ÷ 60 ≈ 5.6. Real at 50% ≈ 2.8.
• Daily 17.4 kWh of heat (from lesson 2.1): Chennai 17.4 ÷ 4.6 ≈ 3.8 kWh ≈ ₹34; Delhi 17.4 ÷ 2.8 ≈ 6.2 kWh ≈ ₹56 (₹9/kWh example rate).

Remember

• Lift is the main driver of COP.
• Lower the delivery temperature whenever the application allows.
• Carnot COP is a ceiling; real units reach about half of it (typical).
• Always quote COP with its test conditions (air and water temperatures).