## HEAT AND FLUID FLOW CALCULATIONS FOR HEATING SYSTEM SAMPLE AND TUTORIALS

Calculate gpm of water flow if the heating coil load is 243,810 Btuh and TD is 20°F (200°F EWT - 180°F LWT). Btuh = gpm × 500 × TD

Where:
Btuh = Btu per hour
gpm = volume of water flow, gallons per minute
500 = constant
60 min/hr × 8.33 lb/gal × 1 Btu/lb/°F
TD = temperature difference of the water entering (EWT) and leaving (LWT) the coil. ΔT may be used substituted for TD.

Then:
gpm = Btuh ÷ (500 × TD)
gpm = 243,810 ÷ (500 × 20)

24.4 gpm of water flows through the heating coil.
Now calculate the air TD across the heating coil if:

198,450 Btuh is the Sensible Room Heating Load.

The math is: 198,450 = 5250 cfm × 1.08 × 35 TD (105-70)
243,810 Btuh is the Sensible Coil Heating Load.

The difference of 45,360 Btuh (243,810–198,450) is the additional heat required for the outside air.
The math is: 45,360 = 1050 cfm × 1.08 × 40 TD (70-30)
LAT - leaving air temperature (coil). (Also called SAT, supply air temperature) (105°F)
EAT - entering air temperature (coil), (also called RAT, room air temperature) (70°F)
OAT - outside air temperature (30°F)

Then:
TD = Btuh ÷ (1.08 × cfm)
TD = 243,810 ÷ (1.08 × 5250)
43°F TD (62 EAT + 43 TD = 105 LAT)
243,810 Btuh is the Sensible Coil Heating Load.
The math is: 243,810 = 5250 cfm × 1.08 × 43 TD (105-62)
The mixed air temperature (MAT also called EAT) was calculated
using this equation:
MAT = (%OA × OAT) + (%RA × RAT)

Where:
MAT = mixed air temperature

OAT = outside air temperature
RAT = return air temperature, also called room air temperature

Then:
MAT = (20% × 30°F) + (80% × 70°F)
MAT = (6) + (56)