Takagi 110U Indoor (T-KJr2U-IN) Installation Manual And Owner's Manual page 19

Based on energy content of 1,000 BTU/Cubic ft.: The water heater requires 140 Cubic ft./hr for the 110U (T-KJr2U)
models, 190 Cubic ft./hr for the 310U (T-K4U) models, and 199 Cubic ft./hr for the 510U (T-D2U) models.
Pipe size
Diameter 10' 20'
3/4"
1"
1 1/4"
1/2"
1
2"
Gas sizing example
(Natural Gas)
Gas meter
30' 40' 50' 60'
Water
heater
3/4" Pipe size
A
Length
70' 80' 90' 100'
B
C
Unit: Cubic feet per hour
125' 150' 200'