A tankless water heater heats water on demand instead of storing it, giving you continuous hot water and better efficiency in a compact, wall-mounted unit.
If you’re tired of running out of hot water or want to reclaim floor space, a tankless water heater is a strong choice. But it’s not a simple swap-in replacement for your old tank. Understanding how these units work, what installation requires, and how to size one correctly is the difference between years of reliable hot water and a costly mistake.
How a Tankless Water Heater Works
A tankless water heater, also called a demand-type or instantaneous water heater, heats water only when you turn on a hot-water tap. Cold water travels through the unit, where a powerful gas burner or electric element warms it instantly. Because there is no storage tank, you don’t pay to keep 40 or 50 gallons hot around the clock. The U.S. Department of Energy (DOE) notes this design avoids standby losses—heat that radiates away from a traditional tank—making tankless units generally more energy efficient.
The trade-off is a limit on flow. The DOE says typical units deliver about 2 to 5 gallons per minute, and gas-fired models usually achieve higher flow rates than electric ones. That’s plenty for one or two fixtures at a time, but running the shower while the washing machine fills can strain a single small unit.
What to Know Before You Buy One
Proper sizing is essential. The DOE states the heater must be selected based on your peak hot-water demand and the temperature rise needed between incoming water and desired output. A unit sized for a warm climate may struggle with cold winter groundwater.
Installation is rarely a simple swap. Homes with high simultaneous demand may need multiple units or a much larger model. Gas models may require bigger gas lines, upgraded venting, and combustion-air considerations, while electric units may need dedicated circuits or a service-panel upgrade. Check all of these before you purchase.
Match the unit to its location. Some products are indoor-only, some outdoor-only, and the DOE notes that venting must meet both manufacturer instructions and local code. A model in the wrong spot can be a safety hazard or simply refuse to work.
Does a Tankless Water Heater Last Longer?
Yes, with proper maintenance. The DOE reports tankless heaters commonly last around 20 years, compared with 10–15 years for a typical storage tank. That longevity, combined with lower standby losses, is why many homeowners find the higher upfront cost worthwhile.
The catch: maintenance is not optional. Hard water builds up mineral scale inside the heat exchanger over time, reducing efficiency and eventually shortening the unit’s life. Annual descaling—flushing the system with a vinegar solution—is the standard preventive step. Skipping it turns a 20-year appliance into a 10-year one.
Common Installation Mistakes to Avoid
The most frequent include undersizing for peak demand, ignoring incoming-water temperature, failing to verify gas supply or electrical service before purchase, and installing a model somewhere that conflicts with its indoor/outdoor rating.
Proper installation involves clean, sequential steps.
Manufacturer manuals reinforce the stakes. Rheem’s condensing unit requires a minimum water pressure of 14 psi (97 kPa). Rinnai’s manual tells you to keep the area around the heater clear of combustible materials. Following the manual step by step—or hiring a licensed pro—is genuinely worth it.
If you’re exploring a compact, plug-in option for a small space, our roundup of the best 110v tankless water heaters covers tested models that run on a standard outlet.
| Spec or Requirement | Typical Value | Why It Matters |
|---|---|---|
| Typical flow rate | 2–5 gallons per minute | Limits how many fixtures can run at once |
| Expected lifespan | Around 20 years | Outlasts storage tanks; needs annual descaling |
| Minimum water pressure (Rheem) | 14 psi (97 kPa) | Low pressure prevents proper operation |
| Gas vs. electric | Gas: higher flow; Electric: may need panel upgrade | Determines installation cost and feasibility |
| Venting | Model-specific; indoor vs. outdoor rating | Wrong venting risks carbon-monoxide hazards |
Several manufacturers offer condensing models that recover extra heat from exhaust gases, pushing efficiency higher. The DOE notes these units generally beat storage water heaters on energy use, but exact savings depend on your household’s water usage pattern.
The bottom line: a tankless heater is a long-lived, efficient appliance when it is sized for your actual peak demand, installed per the manufacturer’s instructions, and flushed annually. Homes that treat it like a drop-in replacement often end up chasing insufficient flow or paying for an emergency service call—all preventable with planning before you buy.
References & Sources
- U.S. Department of Energy. “Measure Guideline: Tankless Water Heaters.” Explains how tankless units work, sizing, lifespan, and installation considerations.