Under-Sink Water Heaters That Cut the Wait for Hot Water
A single sink is well served by the Bosch Tronic 3000 ES4 mini-tank, which runs on standard 120V power with no electrician needed. Two fixtures at once call for the Stiebel Eltron Tempra 12 Plus on a 240V circuit. EcoSmart’s ECO 11 balances flow rate and installed cost for one busy sink.
Voltage decides more here than brand does. The Bosch Tronic 3000 ES4 mini-tank works well for a single sink, since it plugs into a standard 120V outlet without an electrician. Two fixtures running together need the 240V circuit that Stiebel Eltron’s Tempra 12 Plus supplies. EcoSmart’s ECO 11 sits in between, matching one busy sink without the installation cost of a whole-fixture unit.
Choosing between a mini-tank and a tankless under-sink unit gets confusing once voltage, flow rate and cabinet space all factor in together. We compared published tank capacity or GPM ratings, power requirements and physical dimensions, then matched each model to an installation like a single vanity, a shared kitchen and bath line, or a tight cabinet.
The top picks, ranked
Bosch Electric Mini-Tank Water Heater Tronic 3000 US ES4 4-Gallon
This unit stores four gallons close to the point of use instead of heating water instantly. A standard 120V outlet powers it, so no dedicated 240V circuit is required. The compact tank fits inside a typical under-sink cabinet next to plumbing and cleaning supplies.
Pros
- Runs on a standard 120V outlet, no electrician needed
- Compact enough to share cabinet space with plumbing
- Cuts the wait for hot water at a distant sink
Cons
- 4-gallon tank runs out during back-to-back uses
- Not designed to supply a shower or bathtub
Stiebel Eltron Tempra 12 Plus Tankless Electric Water Heater
This unit heats water on demand for kitchen and bathroom sinks running at once. Advanced Flow Control automatically adjusts flow to hold a set output temperature. Wall mounting skips tank storage entirely, freeing cabinet space below the sink.
Pros
- Advanced Flow Control keeps output temperature stable
- No tank means no standby heat loss between uses
- Handles two fixtures running simultaneously at lower flow
Cons
- Requires a dedicated 240V circuit and professional install
- Draws more wattage than single-sink point-of-use units
EcoSmart ECO 11 Electric Tankless Water Heater
This unit modulates energy draw to match how much hot water is actually running. Self-modulating technology means it never heats more than current demand calls for. A digital control panel adjusts output temperature in one-degree increments.
Pros
- Self-modulating design avoids wasted standby energy
- Digital control adjusts temperature in one-degree steps
- Compact wall-mounted body saves under-sink space
Cons
- 1.5 GPM flow limits it to a single fixture at a time
- Needs a 240V circuit, which may require an electrician
Rheem RTEX-13 Tankless Electric Water Heater
Copper immersion heating elements bring water to temperature as it flows through this unit. A digital display on the front shows the current output temperature setting. Its 2 GPM rating covers a kitchen sink and a nearby bathroom fixture together.
Pros
- Copper immersion elements heat water directly in the flow
- Digital display shows exact temperature setting at a glance
- 2 GPM rating covers two nearby fixtures at once
Cons
- Still needs a 240V circuit like other tankless units here
- Flow drops further in colder climates with higher rise
Titan N-120 Tankless Water Heater
This unit’s housing stays under 12 inches tall, aimed at cabinets with limited vertical clearance. Copper and stainless steel components resist corrosion from hard water over time. A built-in high-temperature limit switch shuts the unit off if it overheats.
Pros
- Under 12 inches tall fits shallow or crowded cabinets
- Copper and stainless components resist hard water corrosion
- High-temperature limit switch adds a safety cutoff
Cons
- Flow rate drops noticeably in colder incoming water
- Still requires 240V wiring like full-size tankless units
EcoSmart ECO 8 Electric Tankless Water Heater
This unit draws less power than the ECO 11, matching a single bathroom vanity sink rather than a kitchen. Lower wattage means a lighter circuit requirement in some installations. The same self-modulating technology as its larger sibling avoids heating unused capacity.
Pros
- Lower 8 kW draw may allow a lighter dedicated circuit
- Self-modulating technology matches output to actual demand
- Compact size suits a single vanity sink installation
Cons
- 1.4 GPM flow runs too low for a kitchen sink alone
- Cannot supply two fixtures running at the same time
Rheem RTEX-06 Tankless Electric Water Heater
This unit targets a single small sink, like a kitchen prep sink or wet bar, rather than a full bathroom. Its 6.5 kW rating draws less current than the RTEX-13, which can simplify breaker panel planning. An LED indicator confirms power and heating status at a glance.
Pros
- Lower 6.5 kW draw simplifies breaker panel planning
- Compact size suits a single prep sink or wet bar
- LED indicator confirms heating status without a display panel
Cons
- 1.15 GPM flow limits it to one low-flow fixture
- Not sized for a kitchen sink with a dishwasher attached
How to choose
Tank vs tankless under-sink water heaters explained Mini-tank units like Bosch’s ES4 store a few gallons and reheat them steadily, similar to a small version of a whole-home tank. Tankless units heat water only as it flows through, using copper or stainless elements rated in kilowatts. Mini-tanks often run on a standard 120V outlet, while most tankless models need a dedicated 240V circuit. Recovery rate matters for a mini-tank, while GPM at a given temperature rise matters for a tankless model.
Best under-sink heater for a single bathroom or kitchen sink A single sink rarely needs more than 1 to 1.5 GPM of hot water flow at once. EcoSmart’s ECO 8 and Rheem’s RTEX-06 both sit in that range, aimed at a vanity or small prep sink. Bosch’s ES4 mini-tank works differently, storing four gallons rather than heating on demand. Any of these three skips the cost of running new 240V wiring for a full-size unit.
Sizing a unit for multiple fixtures or high-demand homes Homes running a kitchen sink and a bathroom faucet at the same time need higher flow than a single-fixture unit provides. Stiebel Eltron’s Tempra 12 Plus and Rheem’s RTEX-13 both draw 13 kW or more on a 240V circuit to cover two fixtures. Advanced Flow Control, built into the Tempra 12 Plus, holds a steady output temperature as demand shifts. Checking breaker panel capacity before adding a unit in this power range avoids a surprise upgrade.
Installing an under-sink water heater in a tight cabinet Cabinet depth and height often decide which unit fits before flow rate even enters the conversation. Titan’s N-120 keeps its housing under 12 inches tall for shallow cabinets. Wall-mounted tankless units generally take up less floor space than a mini-tank sitting on the cabinet floor. Local plumbing codes may require a licensed installer for any unit tied into a 240V circuit.
The lineup, side by side
| # | Model | Tank Capacity | Best for | Score | Price |
|---|---|---|---|---|---|
| 1 | Bosch Electric Mini-Tank Water Heater Tronic 3000 US ES4 4-Gallon | 4 gallons | Best overall | 9.4 | Check price |
| 2 | Stiebel Eltron Tempra 12 Plus Tankless Electric Water Heater | 240V, up to 11.5 kW | Best for high-demand homes with multiple fixtures | 9.2 | Check price |
| 3 | EcoSmart ECO 11 Electric Tankless Water Heater | 240V, 13 kW | Best value for a single busy sink | 9.0 | Check price |
| 4 | Rheem RTEX-13 Tankless Electric Water Heater | 240V, 13 kW | Best budget for multiple-fixture point-of-use | 8.8 | Check price |
| 5 | Titan N-120 Tankless Water Heater | 240V, 12 kW | Best for tight under-sink cabinets | 8.6 | Check price |
| 6 | EcoSmart ECO 8 Electric Tankless Water Heater | 240V, 8 kW | Best for a single bathroom vanity sink | 8.3 | Check price |
| 7 | Rheem RTEX-06 Tankless Electric Water Heater | 240V, 6.5 kW | Best for a small kitchen prep sink | 8.0 | Check price |
Frequently asked questions
Do under-sink water heaters need a dedicated circuit?
Most tankless models in this comparison need a dedicated 240V circuit sized to the unit’s kilowatt rating. Mini-tank units like Bosch’s ES4 run on a standard 120V outlet instead, avoiding new wiring.
What is the difference between a mini-tank and a tankless under-sink heater?
A mini-tank stores a small volume of hot water, typically 2.5 to 6 gallons, and reheats it as it’s used. A tankless unit has no storage and heats water only while it flows through.
Can an under-sink water heater supply an entire shower?
Units like Bosch’s ES4 mini-tank are sized for a sink, not a shower’s flow rate and volume. Larger tankless units, like the Stiebel Eltron Tempra 12 Plus, supply more flow but are still usually installed for sinks in a point-of-use setup.
How many gallons per minute does a kitchen sink need?
A kitchen faucet typically needs around 1 to 1.5 GPM of hot water for washing dishes or filling pots. Units like the EcoSmart ECO 11, rated near 1.5 GPM, match that demand without oversizing the installation.
Is a 120V under-sink water heater as effective as a 240V model?
A 120V mini-tank like Bosch’s ES4 heats a stored volume slowly over time rather than instantly. A 240V tankless unit delivers hot water on demand at a higher flow rate but needs more electrical capacity to do it.
Under-sink water heater vs tank water heater: which uses less standby energy?
A full-size tank heater keeps 40 to 80 gallons hot around the clock, even when no one is home. A small under-sink mini-tank or tankless unit only heats a few gallons or heats on demand, cutting standby losses for that one fixture.
Can these units be installed without a plumber or electrician?
A 120V mini-tank like Bosch’s ES4 can sometimes be installed by an experienced DIYer familiar with basic plumbing. Any 240V tankless unit in this comparison should get installed by a licensed electrician and plumber due to circuit and code requirements.
Do under-sink water heaters work well in cold climates?
Tankless units lose flow rate as incoming water gets colder, since more temperature rise takes more energy per gallon. Rheem’s RTEX-13 and Stiebel Eltron’s Tempra 12 Plus both list reduced GPM figures at higher temperature rise, which matters more in cold climates.
Which under-sink water heater is best for a rental or apartment?
A 120V plug-in mini-tank like Bosch’s ES4 suits a rental well, since it needs no new wiring and can move with a tenant. A 240V tankless unit generally requires landlord approval and a licensed electrician, making it a poor fit for a short-term rental.
Final word
A 120V mini-tank like Bosch’s ES4 avoids electrical upgrades but only serves one sink at a time. Tankless units on a 240V circuit, such as the Stiebel Eltron Tempra 12 Plus or Rheem RTEX-13, support more fixtures but need professional wiring. Match the flow rate and voltage to the actual number of fixtures involved before buying.
