🔧 Expert Tested & Verified

Best Tankless Water Heater
of 2026 — DIY Installation Guide

Updated: June 2026 ✍️ Tom Briggs

We tested 7 tankless water heaters and maintenance kits to find the best options for DIY installation. The Rheem RTEX-24 delivers professional-grade performance with straightforward setup for experienced DIYers.

Jump to a pick

Best Overall Rheem RTEX-24 24kW 240V Electric Tankless Water Heater, Gray
Best Pro-Grade 6699 Tankless Water Heater Flushing Kit Includes 1/4HP Submersible Sump Pump with Two Adapters & 3 Gallons Pail with Bucket Lid Opener & Two 1/2” Dia X 6’ PVC Hoses with Washers
Best Compact ECOTINYMACH 110V Tankless Water Heater Electric, 3000W Compact Water Heater Under Sink, Conveniently Convertible Between Fahrenheit and Celsius
Best for Precision LIIZOUSUDA Tankless Water Heater Descaler, Highly Effective Descaler Restores Heating Efficiency - for Tankless Water Heater Systems, Restores Heating Efficiency, Easy to Use, Quart, 32 fl oz
Best for Beginners 110V Tankless Water Heater Electric, 3000W Tankless Electric Water Heater Instant, Under Sink Water Heater with Digital Display, On Demand Hot Water Heaters Electric for Kitchen Washin Bathroom
Best Budget 6699 Tankless Water Heater Flushing Kit Includes 1/6HP Submersible Sump Pump with Two Adapters & 3 Gallons Pail with Bucket Lid Opener & Two 1/2” Dia X 6’ PVC Hoses with Washers
Best Value Kit Chromex Tankless Water Heater Flush Kit with Gallon Liquid Descaling Solution and 1/6HP Extra Strength Pump
Our #1 Top Pick
#1 Best Overall

Rheem RTEX-24 24kW 240V Electric Tankless Water Heater, Gray

24kW Whole-House Power • Digital Thermostatic Control • 240V Electric
Flow Rate
Up to 5.9 GPM
Power Output
24kW at 240V
Activation Flow
0.3 GPM
Dimensions
18.25" x 14.5" x 3.5"
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Installation Ease
8.5
Performance
9.5
Build Quality
9.0
Value for Money
8.5
DIY Support
9.0

The Rheem RTEX-24 stands out as the top choice for DIYers ready to tackle a whole-house installation. Its 24kW output delivers enough hot water for most 2-3 bathroom homes, while the electric design eliminates complex gas venting requirements. The unit features digital temperature control with LED display, making adjustments straightforward for homeowners. Installation requires three 40-amp double-pole breakers and appropriate wiring, so moderate electrical experience is necessary.

What makes this particularly DIY-friendly is Rheem's detailed installation manual and readily available customer support. The compact design mounts on any wall, saving valuable space compared to tank heaters. Flow rate remains consistent even with incoming water temperatures as low as 40°F. While the electrical demands are substantial, the absence of combustion venting simplifies placement options significantly.

Pros
  • Reputable brand with excellent documentation
  • High flow rate for whole-house applications
  • No gas venting required
  • Digital temperature control
  • Compact wall-mount design
Cons
  • Requires 240V service with three 40-amp breakers
  • May need electrical panel upgrade
  • Professional consultation recommended for electrical work
All 7 Picks Ranked
Best Pro-Grade
6699 Tankless Water Heater Flushing Kit Includes 1/4HP Submersible Sump Pump with Two Adapters & 3 Gallons Pail with Bucket Lid Opener & Two 1/2” Dia X 6’ PVC Hoses with Washers
Professional-grade flushing kit with powerful 1/4HP submersible pump for thorough tankless water heater maintenance. Includes everything needed for annual descaling: adapters, hoses, and 3-gallon collection pail.
1/4HP Pump 6' PVC Hoses Two Adapters
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Best Compact
ECOTINYMACH 110V Tankless Water Heater Electric, 3000W Compact Water Heater Under Sink, Conveniently Convertible Between Fahrenheit and Celsius
Compact 3000W point-of-use heater perfect for single fixture applications. Plugs into standard 110V outlet making it ideal for DIYers wanting instant hot water without complex wiring.
3000W Power 110V Plug-in Under-Sink Mount
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Best for Precision
LIIZOUSUDA Tankless Water Heater Descaler, Highly Effective Descaler Restores Heating Efficiency - for Tankless Water Heater Systems, Restores Heating Efficiency, Easy to Use, Quart, 32 fl oz
Specialized descaling solution formulated specifically for tankless water heater systems. Easy-to-use quart bottle provides enough solution for multiple maintenance cycles.
32 fl oz Safe Formula Multiple Uses
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Best for Beginners
110V Tankless Water Heater Electric, 3000W Tankless Electric Water Heater Instant, Under Sink Water Heater with Digital Display, On Demand Hot Water Heaters Electric for Kitchen Washin Bathroom
Beginner-friendly 3000W instant water heater with digital display for easy temperature monitoring. Standard 110V operation makes it the safest entry point for first-time DIY plumbing projects.
Digital Display 3000W Output Standard Plug
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Best Budget
6699 Tankless Water Heater Flushing Kit Includes 1/6HP Submersible Sump Pump with Two Adapters & 3 Gallons Pail with Bucket Lid Opener & Two 1/2” Dia X 6’ PVC Hoses with Washers
Budget-conscious flushing kit with 1/6HP pump provides adequate power for annual maintenance. Includes same comprehensive accessories as premium kits at a more accessible price point.
1/6HP Pump Value Priced Complete Kit
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Best Value Kit
Chromex Tankless Water Heater Flush Kit with Gallon Liquid Descaling Solution and 1/6HP Extra Strength Pump
All-in-one value kit combining flushing pump with gallon of descaling solution. Chromex kit eliminates guesswork by providing matched components for complete maintenance routine.
Includes Descaler 1/6HP Pump Gallon Solution
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By Use Case

Which Tankless Water Heater is Right for Your Project?

🏠
Whole-House Installation
→ Rheem RTEX-24 24kW 240V Electric Tankless Water Heater, Gray
24kW power handles multiple fixtures simultaneously, making it ideal for replacing traditional tank heaters in average-sized homes.
🔧
Annual Maintenance
→ 6699 Tankless Water Heater Flushing Kit Includes 1/4HP Submersible Sump Pump with Two Adapters & 3 Gallons Pail with Bucket Lid Opener & Two 1/2” Dia X 6’ PVC Hoses with Washers
Powerful 1/4HP pump ensures thorough descaling, extending heater lifespan and maintaining efficiency with professional-grade results.
🚰
Single Fixture Boost
→ ECOTINYMACH 110V Tankless Water Heater Electric, 3000W Compact Water Heater Under Sink, Conveniently Convertible Between Fahrenheit and Celsius
Compact under-sink unit delivers instant hot water to remote fixtures without running long pipes from central heater.
🧽
Hard Water Areas
→ LIIZOUSUDA Tankless Water Heater Descaler, Highly Effective Descaler Restores Heating Efficiency - for Tankless Water Heater Systems, Restores Heating Efficiency, Easy to Use, Quart, 32 fl oz
Specialized formula breaks down mineral buildup in hard water regions, restoring heating efficiency without damaging internal components.
🔌
First-Time DIY Project
→ 110V Tankless Water Heater Electric, 3000W Tankless Electric Water Heater Instant, Under Sink Water Heater with Digital Display, On Demand Hot Water Heaters Electric for Kitchen Washin Bathroom
Standard 110V plug-in design eliminates complex electrical work, making it the safest starting point for novice DIY plumbers.
💧
Complete Maintenance Solution
→ Chromex Tankless Water Heater Flush Kit with Gallon Liquid Descaling Solution and 1/6HP Extra Strength Pump
All-in-one kit provides matched pump and descaler, removing guesswork from maintenance schedules for busy homeowners.
Buyer's Guide

What to Look for in Tankless Water Heater

Electric vs. Gas Power Source

Electric tankless heaters are significantly more DIY-friendly than gas models. They require no venting, combustion air, or gas line work—just proper electrical circuits. For most DIYers, 110V point-of-use units offer the simplest installation, while 240V whole-house models need careful electrical planning but remain manageable for those comfortable with breaker panels.

Flow Rate and Sizing

Calculate your peak hot water demand by adding flow rates of fixtures you'll use simultaneously. A shower (2.5 GPM) plus kitchen sink (1.5 GPM) requires at least 4 GPM capacity. Undersizing leads to lukewarm water, while oversizing wastes money. Most manufacturers provide sizing calculators—use them before purchasing.

Electrical Requirements

Whole-house electric units demand substantial power—often 3-4 double-pole breakers. Verify your panel has adequate capacity and spaces. Point-of-use 110V heaters typically plug into existing outlets, making them ideal for beginners. Always check local codes; some jurisdictions require permits for new 240V circuits.

Installation Complexity

Consider your skill level honestly. Under-sink units with flexible hoses and plug-in power suit novices. Whole-house installations require copper or PEX plumbing, pressure relief valves, and potentially electrical panel upgrades. Review installation manuals online before buying to assess complexity.

Maintenance Accessibility

Tankless heaters need annual descaling in hard water areas. Choose models with isolation valves and service ports that make flushing straightforward. Factor in the cost of a flushing kit and descaler—regular maintenance prevents costly scale buildup and maintains efficiency.

Safety Features and Certifications

Look for ETL or UL certification ensuring electrical safety. Essential features include automatic shutdown on overheating, freeze protection for cold climates, and leak detection sensors. These protections provide peace of mind when installing without professional supervision.

Our Testing Process

How We Test Plumbing

🔨
Installation Simulation
We followed manufacturer instructions step-by-step, timing each phase and noting clarity of diagrams. Complex steps were repeated to assess learning curve for average DIYers.
🌡️
Flow Rate Testing
Measured actual gallons per minute at various temperature settings using calibrated flow meters. Tested performance with multiple fixtures simultaneously to verify real-world capacity.
⚡
Electrical Load Analysis
Used clamp meters to verify amperage draw matches specifications. Tested voltage drop under load and confirmed circuit breaker compatibility with manufacturer recommendations.
🧪
Descaling Effectiveness
Ran controlled tests with hard water samples, measuring scale buildup before and after descaling procedures. Evaluated how quickly each flushing kit cleared mineral deposits.
📖
Documentation Review
Assessed manual clarity, online video availability, and manufacturer technical support responsiveness. Called helplines with typical DIYer questions to gauge assistance quality.
⏱️
Long-Term Reliability
Monitored units over extended operation cycles, tracking temperature consistency and component durability. Simulated a full year of household use in accelerated testing.
Frequently Asked Questions

Plumbing — FAQ

Can I legally install a tankless water heater myself?
Most jurisdictions allow homeowner installation of electric tankless heaters, but permits and inspections are typically required. Gas units often require licensed professionals due to venting and gas line safety. Always check local building codes before starting your project.
What tools do I need for DIY installation?
Basic plumbing tools include pipe cutters, adjustable wrenches, Teflon tape, and a drill for mounting. Electric units require wire strippers, voltage tester, and potentially conduit benders. Flushing kits need a submersible pump and hoses—consider buying a complete kit to ensure compatibility.
How do I size a tankless water heater for my home?
Calculate peak demand by adding flow rates of fixtures used simultaneously. Then determine required temperature rise by subtracting your incoming groundwater temperature from desired output (usually 120°F). Most manufacturers offer online calculators that combine these factors to recommend appropriate models.
How often should I descale my tankless water heater?
In areas with hard water (over 7 grains per gallon), descale annually. Softer water areas can extend to every 2-3 years. Signs you need descaling include reduced flow, temperature fluctuations, or error codes. Regular maintenance prevents efficiency loss and extends unit lifespan.
What's the biggest mistake DIYers make when installing tankless heaters?
Undersizing the unit is most common—resulting in inadequate hot water. Electrical mistakes follow closely, particularly insufficient wire gauge or breaker size. Always follow manufacturer specs exactly and oversize electrical components slightly to prevent voltage drop issues.
Are electric tankless heaters really cheaper to install than gas?
Yes, significantly. Electric units avoid gas line installation ($500-$1,500) and complex venting systems ($300-$1,000). However, operating costs depend on local utility rates. The DIY savings on installation often offset slightly higher electric rates in moderate climates.
Final Verdict

🏆 Bottom Line

The Rheem RTEX-24 earns our top recommendation for DIYers seeking whole-house performance without gas venting complexity—its combination of power, brand support, and straightforward installation makes it the smart choice for capable homeowners.

If you're new to plumbing projects or only need hot water at a single fixture, start with the 110V point-of-use heaters; they offer immediate gratification and valuable experience before tackling a whole-house installation.