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10 models to compare

What PSI does a tankless water heater need?

Minimum pressure varies. EcoSmart ECO models need 25 PSI, ECOS 27 needs 30 PSI; Eemax HomeAdvantage II needs 35 PSI. Tempra 20 Plus leaves it unknown.

All ten run on 240 V; wiring needs an electrician.

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With no preferences selected: EcoSmart ECO 18 is one option to compare.

No hands-on testingMethod and sources

EcoSmart ECO 18 · first with a product link

  • Heating input at stated voltage: 18 kW.
  • Width: 14 in.
What to note:

Activation flow, GPM: 0.3 gpm.

View at EcoSmart

If you choose At least 24 kW for heating input at stated voltage, the first option with a product link becomes EcoSmart ECO 24.

10 models compared; 10 have a product-page link.

The picks

5 options to compare

Compare the details that matter to your setup.

An option to compare

EcoSmart ECO 18

Heating input at stated voltage: 18 kW.

Key details

  • Width: 14 in.

What to note

Activation flow, GPM: 0.3 gpm.

Heating input at stated voltage: 18 kWMinimum operating pressure, PSI: 25 psiActivation flow, GPM: 0.3 gpm
See all 5 specs
Heating input at stated voltage18 kW
Minimum operating pressure, PSI25 psi
Activation flow, GPM0.3 gpm
Width14 in
Voltage240 V

5 of 5 specs available · source checked September 26, 2026 · Source ↗
EcoSmart · Product page checked September 26, 2026

View at EcoSmart
Heating input at stated voltage: At least 24 kW

EcoSmart ECO 24

Heating input at stated voltage: 24 kW.

Key details

  • Activation flow, GPM: 0.3 gpm.

What to note

Minimum operating pressure, PSI: 25 psi.

Heating input at stated voltage: 24 kWMinimum operating pressure, PSI: 25 psiActivation flow, GPM: 0.3 gpm
See all 5 specs
Heating input at stated voltage24 kW
Minimum operating pressure, PSI25 psi
Activation flow, GPM0.3 gpm
Width17 in
Voltage240 V

5 of 5 specs available · source checked September 26, 2026 · Source ↗
EcoSmart · Product page checked September 26, 2026

View at EcoSmart
Heating input at stated voltage: At least 36 kW

EcoSmart ECO 36

Heating input at stated voltage: 36 kW.

Key details

  • Minimum operating pressure, PSI: 25 psi.

What to note

Activation flow, GPM: 0.3 gpm.

Heating input at stated voltage: 36 kWMinimum operating pressure, PSI: 25 psiActivation flow, GPM: 0.3 gpm
See all 5 specs
Heating input at stated voltage36 kW
Minimum operating pressure, PSI25 psi
Activation flow, GPM0.3 gpm
Width21 in
Voltage240 V

5 of 5 specs available · source checked September 26, 2026 · Source ↗
EcoSmart · Product page checked September 26, 2026

View at EcoSmart
Also compare 4

EcoSmart ECO 27

Heating input at stated voltage: 27 kW.

Key details

  • Width: 17 in.

What to note

Activation flow, GPM: 0.3 gpm.

Heating input at stated voltage: 27 kWMinimum operating pressure, PSI: 25 psiActivation flow, GPM: 0.3 gpm
See all 5 specs
Heating input at stated voltage27 kW
Minimum operating pressure, PSI25 psi
Activation flow, GPM0.3 gpm
Width17 in
Voltage240 V

5 of 5 specs available · source checked September 26, 2026 · Source ↗
EcoSmart · Product page checked September 26, 2026

View at EcoSmart
Also compare 5

EcoSmart ECOS 27

Heating input at stated voltage: 27 kW.

Key details

  • Minimum operating pressure, PSI: 30 psi.

What to note

Activation flow, GPM: 0.3 gpm.

Heating input at stated voltage: 27 kWMinimum operating pressure, PSI: 30 psiActivation flow, GPM: 0.3 gpm
See all 5 specs
Heating input at stated voltage27 kW
Minimum operating pressure, PSI30 psi
Activation flow, GPM0.3 gpm
Width9 1/4 in
Voltage240 V

5 of 5 specs available · source checked September 26, 2026 · Source ↗
EcoSmart · Product page checked September 26, 2026

View at EcoSmart

Comparing EcoSmart ECO 18 · EcoSmart ECO 24

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The facts that differ

Choose up to three models to compare side by side.

3 identical specs shown. Details that do not apply to these models are omitted.

Published specification comparison
SpecEcoSmart ECO 18No preferences selected.EcoSmart ECO 24No preferences selected.
Heating input at stated voltage18 kW24 kW
Minimum operating pressure, PSI25 psi25 psi
Activation flow, GPM0.3 gpm0.3 gpm
Width14 in17 in
Voltage240 V240 V

EcoSmart ECO 18

No preferences selected.

Heating input at stated voltage
18 kW
Minimum operating pressure, PSI
25 psi
Activation flow, GPM
0.3 gpm
Width
14 in
Voltage
240 V

EcoSmart ECO 24

No preferences selected.

Heating input at stated voltage
24 kW
Minimum operating pressure, PSI
25 psi
Activation flow, GPM
0.3 gpm
Width
17 in
Voltage
240 V

EcoSmart ECO 18

View at EcoSmart

EcoSmart ECO 24

View at EcoSmart

Compare all 10 models

— = no verified listing yet. The published order is independent of links.

All models and their decision specs
ProductHow it comparesKey specCompareAction
EcoSmart ECO 18
Compared with EcoSmart ECO 24: heating input at stated voltage, 18 kW vs 24 kW.
Heating input at stated voltage: 18 kWView at EcoSmart
EcoSmart ECO 24
Compared with EcoSmart ECO 27: heating input at stated voltage, 24 kW vs 27 kW.
Heating input at stated voltage: 24 kWView at EcoSmart
EcoSmart ECO 27
Compared with EcoSmart ECO 36: heating input at stated voltage, 27 kW vs 36 kW.
Heating input at stated voltage: 27 kWView at EcoSmart
EcoSmart ECO 36
Compared with EcoSmart ECOS 27: heating input at stated voltage, 36 kW vs 27 kW.
Heating input at stated voltage: 36 kWView at EcoSmart
EcoSmart ECOS 27
Compared with Eemax HA018240: heating input at stated voltage, 27 kW vs 18 kW.
Heating input at stated voltage: 27 kWView at EcoSmart
Eemax HA018240
Compared with Eemax HA024240: heating input at stated voltage, 18 kW vs 24 kW.
Heating input at stated voltage: 18 kWView at Eemax
Eemax HA024240
Compared with Eemax HA027240: heating input at stated voltage, 24 kW vs 27 kW.
Heating input at stated voltage: 24 kWView at Eemax
EcoSmart ECO 18
Compared with EcoSmart ECO 24: heating input at stated voltage, 18 kW vs 24 kW.
Heating input at stated voltage: 18 kW
View at EcoSmart
EcoSmart ECO 24
Compared with EcoSmart ECO 27: heating input at stated voltage, 24 kW vs 27 kW.
Heating input at stated voltage: 24 kW
View at EcoSmart
EcoSmart ECO 27
Compared with EcoSmart ECO 36: heating input at stated voltage, 27 kW vs 36 kW.
Heating input at stated voltage: 27 kW
View at EcoSmart
EcoSmart ECO 36
Compared with EcoSmart ECOS 27: heating input at stated voltage, 36 kW vs 27 kW.
Heating input at stated voltage: 36 kW
View at EcoSmart
EcoSmart ECOS 27
Compared with Eemax HA018240: heating input at stated voltage, 27 kW vs 18 kW.
Heating input at stated voltage: 27 kW
View at EcoSmart
Eemax HA018240
Compared with Eemax HA024240: heating input at stated voltage, 18 kW vs 24 kW.
Heating input at stated voltage: 18 kW
View at Eemax
Eemax HA024240
Compared with Eemax HA027240: heating input at stated voltage, 24 kW vs 27 kW.
Heating input at stated voltage: 24 kW
View at Eemax
10 catalog models

How the comparison works

Your choices are compared with each model’s product details. A missing or conflicting value stays unknown and cannot satisfy a choice you select. The starting order helps you browse; it does not rate real-world performance.

Source dates appear with each link below. A source date does not confirm today's price or stock.

Finder methodologyReport a correction
Sources for all 10 models

Heating input and turn-on flow answer different questions

EcoSmart's ECO 18 has 18 kW heating input, a 0.3 gpm activation flow and 25 psi minimum operating pressure. Eemax's 18 kW option has the same activation flow but a 35 psi minimum pressure. Heating input alone misses that operating-pressure difference.

Actual hot-water draws were not measured, so the maker operating figures need to be read as separate limits. Activation flow tells you when a heater turns on; it does not describe delivery at your required temperature rise. The whole-house label cannot size a heater for incoming water temperature and simultaneous fixture demand.

A narrow enclosure can still need a substantial installation

EcoSmart's ECOS 27 is 9.25 in wide with 27 kW input, while its ECO 27 is 17 in wide at the same input. Enclosure widths with data span 9.25 to 21 in; the Stiebel Eltron Tempra has no width or activation-flow value available here. Missing figures should not be copied from another heater.

Have an installer assess electrical service, supply pressure, temperature rise and fixture demand using the exact manual. Enclosure width only addresses physical fit. With conflicting amperage and unsupported maximum-flow claims omitted, approval of hot-water sizing or electrical capacity remains outside this comparison.

Before you buy

Will one of these heat water for my whole home?

A spec sheet can't tell you. EcoSmart titles its pages Whole Home or Whole House, but sizing turns on incoming water temperature, how many fixtures run at once and your electrical service, so have an installer work from the maker's sizing guide. Heating input runs from 18 to 36 kW, and the Tempra 20 Plus is rated 19.2 kW maximum power output. The ECOS 27 is 9.25 in wide; other 27 kW units are 17 in.

What does activation flow tell me?

It's the flow that switches a heater on: 0.3 GPM for all nine that give it, with the Tempra 20 Plus unstated. It says nothing about how much hot water you'll get at a given temperature rise.

Why aren't breaker sizes or amperage here?

They aren't compared on this page. The Tempra 20 Plus page also gives no pressure, activation flow or width, so any pressure or width choice leaves it out. For breaker and wire sizing, use the exact model's installation manual and a licensed electrician.