As homeowners and builders look for more efficient ways to supply hot water, electric tankless water heaters have become a popular alternative to traditional storage tank systems. Unlike conventional water heaters that maintain a reservoir of heated water, tankless units heat water on demand, delivering hot water only when needed. This article explores the features, benefits, and important installation considerations for electric tankless water heaters, drawing on the example of the Stiebel Eltron Tempra 29 system.
For context on how modern homes manage their mechanical systems, see our article on residential ventilation principles and how ERV systems support indoor air quality, which covers another key aspect of high-performance home design.
How Electric Tankless Water Heaters Work
Electric tankless water heaters operate on a straightforward but effective principle. When a hot water tap is opened, cold water travels through a heat exchanger within the unit. High-power electric heating elements quickly raise the water temperature to the desired set point. The system delivers a continuous supply of hot water as long as the flow rate stays within the unit’s capacity.
Heating Element Technology
Most residential electric tankless water heaters use one of two types of heating elements:
- Exposed heating wire elements where the water flows directly over electric resistance coils. These heat up quickly and transfer energy directly to the water with minimal thermal lag.
- Incoloy sheathed elements where the heating wire is encased in a metal sheath. These are more durable and resistant to corrosion but have a slightly slower thermal response.
The Tempra 29, for example, uses advanced bare-wire heating elements that provide rapid heat transfer and high energy density in a compact housing. This design allows the unit to deliver up to 29 kW of heating power while maintaining a footprint small enough to mount on a wall.
Flow Rate and Temperature Rise
The performance of any tankless water heater depends on two variables: the incoming water temperature and the desired output temperature. The difference between these is called the temperature rise. A higher temperature rise means less usable flow rate because the heater must work harder to raise the water temperature.
As an example, a 29 kW unit like the Tempra 29 can deliver approximately:
| Incoming Water Temperature | Desired Output Temperature | Temperature Rise | Maximum Flow Rate (GPM) |
|---|---|---|---|
| 50 degrees F (10 degrees C) | 105 degrees F (41 degrees C) | 55 degrees F | 4.4 GPM |
| 60 degrees F (16 degrees C) | 110 degrees F (43 degrees C) | 50 degrees F | 4.9 GPM |
| 70 degrees F (21 degrees C) | 120 degrees F (49 degrees C) | 50 degrees F | 4.9 GPM |
| 40 degrees F (4 degrees C) | 120 degrees F (49 degrees C) | 80 degrees F | 3.0 GPM |
Colder incoming water — common in northern climates during winter — reduces the unit’s effective flow rate. Builders and homeowners must consider this when sizing a unit for their specific region.
Key Benefits of Going Tankless
Electric tankless water heaters offer several advantages over traditional tank-style heaters that make them attractive for new construction and retrofit projects alike.
Energy Efficiency and Standby Loss Elimination
One of the primary benefits of tankless technology is the elimination of standby heat loss. A conventional storage tank water heater continuously heats 40 to 60 gallons of water, even when no one is using hot water. This standby loss can account for 10 to 20 percent of the unit’s total energy consumption. Electric tankless heaters, by comparison, only draw power when a hot water tap is open, which can reduce overall energy use by 25 to 35 percent in typical household applications.
For homeowners interested in other energy-saving technologies, our article on thermal breaks and heat pump water heaters explores another high-efficiency water heating approach that pairs well with tankless systems in hybrid configurations.
Space Savings and Longevity
A typical tank-style water heater occupies roughly 6 to 8 square feet of floor space in a basement, garage, or mechanical closet. Electric tankless units mount directly on a wall and have a much smaller footprint. The Tempra 29, for instance, measures approximately 17 inches tall by 11 inches wide and 4 inches deep. This compact size frees up valuable floor space and allows installation in locations where a tank unit would not fit, such as under a sink or in a small utility closet.
Tankless water heaters also tend to have a longer service life than tank-style units. While conventional water heaters typically last 8 to 12 years, electric tankless units can last 20 years or more with proper maintenance. The absence of a standing water reservoir reduces corrosion and sediment buildup, two of the primary failure modes in tank-style heaters.
Endless Hot Water Supply
For households with high hot water demand, the continuous supply offered by tankless systems is a major advantage. As long as the flow rate does not exceed the unit’s capacity, the system can provide hot water indefinitely. This makes tankless water heaters particularly well suited for:
- Homes with large bathtubs or spa-style showers
- Households with multiple family members who shower at the same time
- Kitchens with commercial-grade dishwashers that require sustained hot water
- Addition of a second bathroom or a mother-in-law suite
Installation Requirements and Considerations
Installing an electric tankless water heater involves more than simply mounting the unit and connecting the water lines. Several critical factors must be addressed to ensure safe and reliable operation.
Electrical Service Requirements
Electric tankless water heaters draw significant power. A 29 kW unit requires a 240-volt, 125-amp dedicated electrical circuit. This typically means:
- Upgrading the main electrical panel to accommodate the additional load. Many older homes with 100-amp service panels will need a panel upgrade to 200 amps or higher.
- Running appropriately sized conductors from the panel to the unit. For a 125-amp circuit, this usually requires 1 AWG or 2 AWG copper wire, depending on the distance.
- Installing a dedicated double-pole breaker rated for the full load of the unit.
- Ensuring local code compliance for wire routing, conduit, and disconnect requirements.
The electrical demands of tankless water heaters are significantly higher than those of standard tank units. Builders should factor this into the project budget, as panel upgrades and heavy-gauge wiring can add considerable cost.
Water Supply and Connection Details
On the plumbing side, installation requires attention to several details:
- Water quality: Hard water can accelerate scale buildup inside the heat exchanger. In areas with hard water, a whole-house water softener or a descaling system is strongly recommended.
- Pressure relief valve: The unit must be equipped with a temperature and pressure relief valve, typically rated at 150 PSI and 210 degrees F.
- Isolation valves: Full-port ball valves on both the hot and cold sides allow the unit to be isolated for maintenance without draining the entire house system.
- Sediment filter: A Y-strainer or sediment filter on the inlet side protects the heat exchanger from debris that may be present in the water supply.
- Recirculation loop: Some installations benefit from a dedicated recirculation line that keeps hot water circulating near the fixtures. This reduces the wait time for hot water at distant taps.
For builders working on plumbing systems, our guide to PEX expansion tools and the M12 ProPEX expander covers essential techniques for making reliable plumbing connections in tight spaces.
Venting and Location
One significant advantage of electric tankless water heaters over gas-fired units is that they require no venting. Gas tankless heaters need stainless steel exhaust flues that must terminate outside the building envelope. Electric units produce no combustion byproducts, so they can be installed in bedrooms, bathrooms, closets, and other interior spaces where gas units would not be permitted by code.
This flexibility allows for point-of-use installations near the fixtures that consume the most hot water. Placing a smaller tankless unit near a master bathroom or kitchen reduces the length of pipe run and the time required for hot water to reach the tap.
Comparing Electric Tankless to Other Water Heating Options
Electric tankless water heaters are not the only option on the market. Understanding how they compare to alternative systems helps builders and homeowners make informed decisions.
Electric Tankless vs. Gas Tankless
Gas tankless water heaters use natural gas or propane burners to heat water. They typically have higher flow rates than electric units, making them suitable for larger households. However, gas units require:
- Combustion air intake vents
- Exhaust flues that must pass through the roof or exterior wall
- Regular maintenance of burners and heat exchangers
- Gas line sizing that accommodates peak demand
Electric units, by comparison, are simpler mechanically, require less maintenance, and have no combustion-related safety concerns. The trade-off is that electric units often need a substantial electrical service upgrade, which can offset the lower purchase price.
Electric Tankless vs. Heat Pump Water Heaters
Heat pump water heaters (also called hybrid water heaters) use electricity to move heat from the surrounding air into the water, making them two to three times more energy efficient than electric tankless units. However, they have a larger physical footprint, require installation in a space with sufficient air volume (typically at least 1,000 cubic feet), and produce cool exhaust air that must be managed.
For a detailed look at how heat pump systems compare, see our article on thermal breaks for heat pump water heaters and DIY dense pack insulation techniques, which covers the installation and performance characteristics of these systems in cold climates.
Cost Comparison Overview
The total cost of a water heating system includes purchase price, installation, and ongoing operating expenses. The table below summarizes typical ranges:
| Water Heater Type | Unit Cost (Equipment Only) | Installation Cost | Annual Energy Cost (Est.) | Service Life |
|---|---|---|---|---|
| Electric Tankless (29 kW) | $650 – $1,100 | $1,500 – $3,000 | $450 – $600 | 20+ years |
| Gas Tankless | $900 – $2,000 | $1,500 – $3,500 | $350 – $500 | 15 – 20 years |
| Electric Storage Tank (50 gal) | $400 – $900 | $500 – $1,000 | $600 – $800 | 8 – 12 years |
| Heat Pump Water Heater (50 gal) | $1,200 – $2,500 | $800 – $1,800 | $200 – $350 | 10 – 15 years |
While electric tankless units have a higher upfront installation cost than traditional tank heaters, the longer service life and lower energy consumption often result in a lower total cost of ownership over a 20-year period.
Sizing Guidelines
Selecting the right size electric tankless water heater requires calculating the peak hot water demand for the home. Here is a simplified sizing process:
- List all fixtures that may be used simultaneously. Common flow rates: shower (1.5 – 2.5 GPM), bathroom faucet (0.5 – 1.0 GPM), kitchen faucet (1.0 – 1.5 GPM), dishwasher (1.0 – 1.5 GPM).
- Determine the total flow rate by adding the GPM of all fixtures that could run at the same time. For most homes, 3 to 5 GPM is a reasonable peak estimate.
- Measure the incoming water temperature during the coldest month of the year. This is the worst-case scenario for temperature rise.
- Match the unit’s output to the required temperature rise at the peak flow rate. Use the manufacturer’s performance charts to select a model that meets the demand.
- Consider a multiple-unit configuration for larger homes. Two smaller units installed in parallel can provide higher total flow capacity while allowing one unit to serve as backup.
For advice on fitting water heaters and other plumbing fixtures into tight spaces, our guide on the FlashBob technique for precision plumbing vent pipe layout offers practical tips for accurate pipe routing during new construction and remodels.
Maintenance and Long-Term Care
Electric tankless water heaters require less maintenance than gas-fired units, but they are not zero-maintenance appliances.
Descaling Schedule
Mineral scale accumulates inside the heat exchanger over time, reducing heat transfer efficiency and restricting flow. The descaling frequency depends on water hardness:
- Soft water (0 – 3 grains per gallon): Descale every 24 to 36 months
- Moderately hard water (3 – 7 grains per gallon): Descale every 12 to 18 months
- Hard water (7+ grains per gallon): Descale every 6 to 12 months
Descaling is performed by circulating a food-grade descaling solution (typically white vinegar or a commercial descaler) through the heat exchanger using a bucket and a small pump. Most manufacturers provide a descaling kit and detailed instructions.
Filter Maintenance
The inlet water filter should be cleaned or replaced every three to six months. A clogged filter restricts flow and can cause the unit to short-cycle or fail to fire. This simple step takes only a few minutes and prevents many common service calls.
Performance Monitoring
Modern electric tankless water heaters include digital displays that show the current output temperature, flow rate, and power consumption. Homeowners should periodically verify that the actual output temperature matches the set point. A noticeable drop in output temperature at a given flow rate is often the first sign that descaling is needed. Some units also include diagnostic codes that indicate specific issues, such as a failed heating element or a flow sensor malfunction.
Conclusion
Electric tankless water heaters offer a compelling combination of energy efficiency, space savings, and long service life that makes them an excellent choice for many residential applications. The Stiebel Eltron Tempra 29 demonstrates how modern tankless technology delivers continuous hot water on demand in a compact wall-mounted package. However, successful installation requires careful planning of the electrical service, water quality management, and proper sizing to match the household’s peak demand. For homes with appropriate electrical capacity and a commitment to regular maintenance, electric tankless water heaters provide reliable, efficient hot water for decades.