HVAC MEP Thumb Rules & Formulas (With Examples)

 HVAC MEP Thumb Rules & Formulas (With Examples)

1. Heat Load Calculation Formula:

Q = Area (sq.ft) x Heat Load Factor (BTU/hr per sq.ft)

Example:

500 sq.ft office:

Q = 500 x 30 = 15,000 BTU/hr TR = 1.25

2. CFM Calculation Formula:

CFM = Sensible Heat (BTU/hr) / (1.08 x Delta T)

Example:

12,000 BTU/hr, Delta T = 20°F CFM = 556

3. AHU/FCU Sizing Rule:

1 TR = 400 CFM

2 TR Airflow = 800 CFM

4. Duct Sizing Velocity Limits:

Main: 1400-1800 FPM

800 CFM @ 1000 FPM 0.8 sq.ft 14"x10"

5. Chilled Water Flow Rate Formula:

GPM = BTU/hr / (500 x Delta T)

Example:

24,000 BTU/hr GPM = 4.8

6. Pipe Sizing

1" pipe: 8-12 GPM

2" pipe: 30-40 GPM

35 GPM Use 2"

7. Chiller Sizing Formula:

TR = BTU/hr / 12,000

Example:

60,000 BTU/hr → 5 TR

8. Cooling Tower Sizing Rule:

Heat Rejection = 1.25 x Load

10 TR → Tower = 12.5 TR

9. Pump Head Calculation Formula:

Power (kW) = (Q x H x 9.81) / (Efficiency x 1000)

Example:

Q = 5 L/s, H = 20 m, Efficiency = 0.75

Power 1.31 kW

10. Fresh Air Requirement

Office: 15-20 CFM/person

300 CFM 20 people

11. Electrical Load

1 TR 1.25 kW

10 TR 12.5 kW

12. Condenser Water Flow

3 GPM per TR

15 TR 45 GPM

13. Return Air Duct

2 sq.in. per CFM

600 CFM 1200 sq.in. ≈ 10"x12"

14. VRV/VRF Capacity

1 HP = 0.8 TR

COP 3.5-4.5

15. Chilled Water Pipe Velocity

Chilled Water: 3-12 ft/s

Condenser: 6-9 ft/s