Switching from a 50-gallon storage tank to a whole-home tankless water heater requires engineering calculations based on peak simultaneous hot water flow and incoming groundwater temperature.
1. Calculating Required Flow Rate & Temperature Rise
The Delta-T (ΔT) Formula
Temperature Rise (ΔT) = Desired Output Temp (typically 120°F) - Incoming Ground Water Temp (35°F to 65°F depending on geography). A Northern winter with 40°F groundwater requires an 80°F rise, reducing maximum flow from 9.5 GPM to 5.0 GPM.
- Gas Supply Capacity: Tankless units demand 160,000 to 199,000 BTU/hr (compared to 40,000 BTU/hr for a tank). This frequently necessitates replacing a 1/2" gas branch with a dedicated 3/4" or 1" black iron pipe back to the gas meter.
- Direct Venting: High-efficiency condensing units (96%+ UEF) exhaust low-temperature acidic flue gas through Schedule 40 PVC or polypropylene pipe rather than metal chimneys.