Traditional water heaters keep dozens of gallons hot around the clock, whether or not anyone is home. A tankless water heater takes a different approach: it heats water only when you open a tap. That simple change affects energy use, space, installation and the way hot water behaves in your house. This guide explains how tankless units work, the main types, the benefits and drawbacks, and the kinds of households that tend to get the most from them.
This article is general, independent information. It does not describe any particular manufacturer’s products, prices or warranties. Always follow the installation manual and local codes, and use a licensed professional for gas, venting and electrical work.
How a Tankless Water Heater Works
Also called an on-demand or instantaneous water heater, a tankless unit has no storage tank. The process looks like this:
- You open a hot tap or start an appliance.
- A flow sensor detects water moving through the unit.
- The control board fires the burner (gas models) or energizes the heating elements (electric models).
- Cold water passes through a heat exchanger and leaves at the set temperature.
- When you close the tap, the flow stops and the heating switches off.
Because water is heated as it passes through, you are never “running out” of a stored supply. The limit is instead how much water the unit can heat at once, which depends on its capacity and on how cold the incoming water is.
Gas, Propane and Electric Models
Natural gas and propane
Gas models use a burner and typically offer higher flow rates than electric ones. They need a gas supply line of adequate size, a vent to carry exhaust outdoors and, usually, an electrical outlet for the controls. Propane versions need a suitable conversion or model and a reliable tank supply. For installation details, see our guide to gas vs electric tankless installation requirements.
Electric
Electric units use heating elements and need no venting, which makes siting easier. However, they draw a lot of current, often requiring dedicated high-amperage circuits and possibly a panel upgrade. Whole-home electric models in cold climates can be demanding on the electrical service.
Condensing versus non-condensing gas units
Condensing models capture extra heat from the exhaust gases, which can improve efficiency but produces acidic condensate that needs a drain and a neutralizer in many installations. They often use plastic venting. Non-condensing units use metal venting and run hotter exhaust. Which is suitable depends on the model, local codes and your home.
Point-of-use units
Small electric units installed near a single sink or shower. They can fix a far-away tap that takes a long time to get hot, but they are not a whole-house solution.
Key Concepts: Flow Rate and Temperature Rise
Two numbers determine whether a tankless heater will keep up:
- Flow rate (gallons per minute, GPM): how much hot water you want at the same time.
- Temperature rise: the difference between incoming cold water temperature and your desired hot water temperature.
Colder incoming water means a bigger rise, which reduces the flow a given unit can deliver. In winter, a unit that works well in a mild climate may struggle in a cold one. Sizing is covered in detail in our guide to sizing a tankless water heater.
Benefits
- Continuous hot water. You can run a long shower without waiting for a tank to recharge, provided demand stays within the unit’s capacity.
- Space saving. Most are wall-mounted and take far less floor space than a tank.
- Potential efficiency gains. Because there is no standing tank to keep hot, standby heat loss is mostly eliminated. How much you save depends on your usage, local energy prices and the model.
- Long expected service life. Many tankless units are rated for a longer life than tank heaters when well maintained, though actual lifespan varies with water quality and care. See our FAQ on lifespan.
- No tank to leak. A failing tank can flood a basement; tankless units have far less stored water.
Drawbacks
- Higher upfront cost. The unit and installation typically cost more than a like-for-like tank replacement, especially if gas lines, venting or electrical work need upgrading.
- Installation complexity. Venting, gas sizing and electrical capacity can all require changes.
- Flow limits. Running multiple fixtures at once can exceed capacity, resulting in lower temperature or reduced flow.
- Cold-water sandwich. A short burst of cold water can appear between uses, and small flows may not trigger the unit. Our troubleshooting guide on lukewarm water explains this.
- Maintenance. Hard water can cause scale buildup, so periodic flushing is important; see how to flush a tankless heater.
- Power dependence. Most gas units need electricity to run the controls and fan, so they will not work during a power outage unless you have backup power.
Who Is Most Likely to Benefit?
A tankless heater tends to suit:
- Households that run out of hot water with a tank because of large families or long showers
- Homes with limited space, such as apartments, closets or utility rooms
- People planning a renovation who can adapt gas lines, venting and wiring
- Homeowners who prefer low standby losses and plan to stay for many years
- Vacation or part-time homes, where heating a tank around the clock is wasteful
It may be a poorer fit for:
- Homes with tight budgets and a functioning tank
- Houses where upgrading gas supply or venting would be very costly
- Very cold-climate homes with high demand, unless sized correctly
- People who use very low flows that may not trigger the unit
How Does It Compare to a Tank?
A detailed comparison is in our article on tank vs tankless water heaters, but in short: tanks cost less up front and are simple, while tankless units offer endless hot water within capacity and a smaller footprint, at a higher installation cost. Our broader overview of home water heating options covers heat pumps, solar and others.
What to Check Before Switching
- Hot water demand. List fixtures you use at the same time.
- Incoming water temperature. Check seasonal temperatures in your area.
- Fuel and infrastructure. Gas line size, vent routes, electrical panel capacity.
- Local codes and permits. Many jurisdictions require permits and inspections.
- Water quality. Hard water means a maintenance plan.
- Budget. Include installation, venting, gas or electrical upgrades, and any rebates or incentives you can verify.
Our checklist, Questions to Ask Before Buying a Tankless Unit, goes deeper.
Getting It Installed Safely
Gas combustion, venting and high-amperage electrical work carry real safety risks, including carbon monoxide and fire. Use a licensed, insured plumber or HVAC contractor, ensure permits and inspections are handled and install carbon monoxide detectors according to local requirements.
Sizing a Tankless Water Heater: Flow Rate and Temperature Rise
Tank vs Tankless Water Heater: Cost and Efficiency Compared
Frequently Asked Questions
Does a tankless water heater really provide unlimited hot water?
It provides continuous hot water as long as demand stays within the unit’s capacity. If too many fixtures run at once, temperature or flow can drop.
Is a tankless water heater worth the extra cost?
It depends on your usage, local energy prices, installation costs and how long you plan to stay. Get quotes and compare total cost of ownership.
Do tankless heaters work in a power outage?
Most do not, as they need electricity for controls and fans. Ask about backup power options.
Can I install one myself?
Gas, venting and high-amperage electrical work should be done by qualified professionals and may require permits.
Conclusion
A tankless water heater trades a storage tank for on-demand heating, offering space savings, continuous supply within capacity and lower standby losses, at the cost of higher upfront and installation expenses. Size it properly, check your gas or electrical infrastructure and maintain it, and it can serve a home well. If you are unsure, ask a licensed professional to assess your house.
