Section 0

Tankless water heater flow rate by temperature rise

This table lists the flow rate of 27 small electric and propane tankless water heaters at a 43 F and a 63 F temperature rise. The models come from Stiebel Eltron, EcoSmart, Rinnai and Eccotemp and are sized for ADUs, cabins and off-grid buildings. All ratings come from the manufacturers.

A 43 F rise takes 62 F water to 105 F. A 63 F rise takes 42 F water to 105 F. "Not rated" means the manufacturer lists no temperature rise. "Not confirmed" means the rating did not pass the heat balance in section 9.

0.2 Main sections

1. Flow and rise sizer. Fixtures and a state give the required flow and the rise. The 21 rated models are sorted by margin.
2. Rise tables. Every rated model at 30 to 80 F rise.
3. Ground water by state. The temperature range on Rinnai's map for each state.
4. Heat balance. Each rating compared with the burner or element input. One rating is not confirmed.

0.3 Flow and rise formula

Water takes about 500 BTU per hour to rise 1 F at a flow of 1 GPM (8.34 lb per gallon times 60 minutes). A heater's heat output therefore fixes the product of GPM and rise. At twice the rise, the flow is half. Each table on this site applies that formula to the manufacturer's rated points.

Stiebel Eltron rates the 24 kW Tempra 24 at 2.52 GPM with a 66 F rise^[1]. By the formula that is about 3.6 GPM at a 46 F rise. Stiebel Eltron's table lists 3.63 GPM at 46 F.

0.4 Coverage

The site covers 15 electric configurations from Stiebel Eltron and EcoSmart, 7 propane wall-mounted models from Rinnai and Eccotemp, and 5 portable propane models from Eccotemp. Natural gas sizing for large homes and recirculation loops are not covered. Installation is not covered. It belongs to the manufacturer's manual and a licensed installer.