The 100-gallon water heater is a true commercial-residential capacity — sized for light-commercial applications and estate-class residential where demand exceeds the standard 75-gallon residential maximum. 100G water heaters carry commercial warranties (typically 3 years) reflecting their higher duty cycle.
True commercial install — 1"+ gas line, 4"+ vent, 1-1/4" NPT plumbing, commercial-certified installer required, permit + commercial code inspection. Most residential buyers should not consider 100G unless peak demand truly exceeds 130 GPH FHD. For most light-commercial residential applications, the 75-gallon residential at 120–130 GPH FHD is sufficient and ships longer warranties (10–12 years vs 3 years on commercial).
For high-demand applications where 100G might apply, modern installs often prefer tankless cascade — dual Rheem RTGH-95DVLN or Rinnai RU199iN cascade delivers continuous 19–22 GPM with better operating efficiency than a single 100G tank.
The 100-gallon water heater is true light-commercial territory — sized for restaurants, multi-family residential, and estate-class residential with demonstrated high simultaneous demand. Commercial 3-year warranties reflect duty cycle. For residential single-family use, the 75-gallon residential is almost always correct sizing. For high-flow continuous demand, tankless cascade often beats 100G tank.