Tankless Size Chart

Flow rate and temperature rise of small tankless water heaters for ADUs, cabins and off-grid buildings

Section 12

Minimum activation flow rate by tankless model

This table lists the minimum flow that turns on each of 18 small tankless water heaters, from 0.26 to 0.77 GPM. A tankless heater stays off until the flow trips its flow sensor. A unit that needs 0.6 GPM does not fire at a 0.5 GPM bathroom aerator alone.

Table 12.1 Activation flow by model
ManufacturerModelActivation flow
Stiebel EltronDHC-E 8/10 Classic (set to 9.6 kW)0.264 GPM
Stiebel EltronDHC-E 8/10 Classic (set to 7.2 kW)0.264 GPM
Stiebel EltronDHC-E 12 Classic0.264 GPM
EcoSmartECO 80.3 GPM
EcoSmartECO 110.3 GPM
EcoSmartECO 180.3 GPM
EcoSmartECO 240.3 GPM
EcoSmartECO 270.3 GPM
EcoSmartECO 360.3 GPM
Stiebel EltronTempra 12 Trend / Plus0.37 GPM
RinnaiV65eP (propane) / V65eN0.4 GPM
RinnaiV75eP (propane) / V75eN0.4 GPM
Stiebel EltronTempra 15 Trend / Plus0.5 GPM
Stiebel EltronTempra 20 Trend / Plus0.5 GPM
Stiebel EltronTempra 24 Trend / Plus0.5 GPM
RinnaiV53DeP (propane) / V53DeN0.6 GPM
Stiebel EltronTempra 29 Trend / Plus0.77 GPM
Stiebel EltronTempra 36 Trend / Plus0.77 GPM

Sources: Stiebel Eltron Tempra[1] and DHC-E[2] sheets, EcoSmart product pages[3] and Rinnai spec sheets[4][5][6]. Eccotemp's SmartHome product pages list no activation flow. Rinnai's V53De sheet lists "0.6 GPM (1.5 L/min)". Calculated: 1.5 L/min is 0.4 GPM. One of the two numbers is a misprint, and the higher one is shown here.

12.2 Low-flow fixtures

WaterSense bathroom faucets run at up to 1.5 GPM[7], and many ship with 1.0 or 0.5 GPM aerators. Every unit in the table turns on below 1.0 GPM. At 0.5 GPM, the Tempra 29 and 36 (0.77 GPM) and the Rinnai V53De (0.6 GPM as listed) stay off. The larger Tempra units fit a building where the smallest hot water draw is a kitchen sink or a shower.

Notes and sources

  1. Stiebel Eltron, Tempra Trend and Plus technical specifications, rev. 6.2025 (PDF)
  2. Stiebel Eltron, DHC-E Classic technical specifications, rev. 6.2025 (PDF)
  3. EcoSmart, electric tankless product list (ECO 8 to ECO 36 pages)
  4. Rinnai, non-condensing tankless brochure with United States Ground Water Temperature Map and V53De table, 06/2022 (PDF hosted by Woodstove Pro)
  5. Rinnai, V65e specification sheet with water flow curve (PDF)
  6. Rinnai, V75e specification sheet with water flow curve (PDF)
  7. EPA WaterSense, Bathroom Faucets