Every home needs hot water. Most of us never think about the water heater until it fails, usually on a cold morning with a shower full of soap. When replacement day comes, you face one main choice: another tank, or a switch to tankless.
Both make hot water. They do it in very different ways, and the differences matter for your budget, your utility bills, and how much hot water your house can deliver at once. This guide lays out the honest tradeoffs so you can decide with confidence.
How Each One Works
How a Tank Water Heater Works
A tank water heater is the big insulated cylinder most homes have. Cold water flows in from the top. A gas burner underneath, or electric heating elements inside, warms the whole tank to your set temperature, usually around 120 degrees.
The water sits in the tank, staying hot, waiting for someone to open a tap. When hot water leaves, cold water enters to replace it, and the burner or elements fire up again. The tank keeps cycling like this all day and night, even when nobody is home.
How a Tankless Water Heater Works
A tankless unit is a compact box that hangs on a wall. It stores no water at all. Open a hot water tap and cold water flows through a heat exchanger while a powerful burner or element fires up. The water leaves the unit hot, seconds later. Close the tap and the unit shuts off completely. No standby heating, no stored hot water waiting around.
Installed Cost Ranges
This is where the tank usually wins. A conventional tank swap is one of the cheapest major appliance installs in a house.
| Type | Rough installed cost | Typical scenario |
|---|---|---|
| Gas or electric tank, like-for-like swap | $800 to $1,800 | Same fuel, same spot, minimal changes |
| Whole-home gas tankless | $2,000 to $4,500 | New venting, possible gas line upgrade |
| Whole-home electric tankless | $1,500 to $3,000 | Panel and wiring upgrades often needed |
These are broad ranges, and labor is the wild card. A tank replacement in the same spot with the same fuel type is often a half-day job. Switching from tank to tankless is a bigger project. Gas units may need new venting and a larger gas line. Electric units often need a panel upgrade, because whole-home electric tankless models draw a great deal of power.
Permits and local code requirements vary by town. Get itemized quotes from at least two or three plumbers before deciding, and make sure each quote lists the unit model plus all the venting, gas, electrical, and permit work.
Lifespan
Tanks wear out from the inside. Sediment builds up on the bottom, the steel slowly corrodes, and the anode rod that protects the tank gets used up. A conventional tank water heater typically lasts 8 to 12 years. Some fail earlier in areas with hard water. A few make it to 15 with good maintenance, but plan on 8 to 12.
Tankless units last longer because there is no big tank of water sitting and corroding. A well-maintained tankless heater commonly lasts 15 to 20 years. The heat exchanger is the heart of the unit, and many manufacturers carry long warranties on that part specifically.
Over a 20-year span, you might buy two tanks or one tankless unit. That narrows the lifetime cost gap more than most people expect.
Energy Use and Operating Cost
A tank heats water around the clock. Even with good insulation, heat escapes through the tank walls and the flue. That is called standby loss, and you pay for it every hour of every day, whether anyone showers or not.
A tankless unit only burns fuel when water is flowing. Government efficiency studies commonly find tankless units use roughly a quarter to a third less energy than conventional tanks for a typical household. The exact savings depend on how much hot water you use. Homes that use less hot water tend to save a bigger percentage.
In dollars, the savings are real but modest for most families. You will not cut your whole utility bill in half. You might shave a noticeable amount off the water heating portion. Whether that pays back the higher install cost depends on your fuel prices and how long you stay in the home. A common payback estimate is 5 to 10 years.
One caveat: in cold climates, incoming winter water is very cold, so the unit works harder and delivers a lower flow rate. Sizing matters more up north, which brings us to the next section.
Sizing: Gallons vs. GPM
Tanks are sized by gallons, which is simple. A 40-gallon tank is the most common size in average homes. A 50-gallon tank suits larger families or homes with big soaking tubs. Smaller 30-gallon tanks work for apartments or one-bath homes. If your current tank kept up with demand, replacing it with the same size is usually safe.
Tankless units are sized by flow rate, measured in gallons per minute (GPM). This is the part people get wrong. The unit must handle every hot water tap running at the same time. A shower uses about 2 to 2.5 GPM. A dishwasher adds about 1.5. If two showers and a dishwasher run together, you need roughly 6 to 7 GPM of capacity at your temperature rise.
Temperature rise is the other half of sizing. It is the difference between your incoming cold water temperature and your target hot water temperature. In warm climates, incoming water might be 65 degrees, so the rise to 120 is only 55 degrees. In cold climates, incoming water can be 40 degrees, so the rise is 80. Bigger rises mean lower flow rates from the same unit. Always size for your winter groundwater temperature, not the yearly average.
When in doubt, have a plumber do the math. An undersized tankless unit is the most common reason people regret the switch.
Gas vs. Electric
Gas tankless units dominate the whole-home market for a reason. Gas delivers a lot of heat fast, so gas units can serve several fixtures at once. They cost more to install but usually cost less per unit of hot water in most areas.
Electric tankless units are simpler to install. No venting, no gas line. But whole-home electric models need serious electrical capacity, often 120 amps or more across multiple breakers. Many older homes need a panel upgrade first, which wipes out the install savings. Electric tankless works best as a point-of-use heater for a single bathroom or sink.
For tank models, the same logic applies in miniature. Gas tanks recover faster after heavy use. Electric tanks are cheaper to install and need no venting, but they heat water more slowly.
If your home has no gas service, running a gas line just for a tankless heater rarely pays off. Stick with a tank, or look into a heat pump water heater. That is a different tank-style option that uses far less electricity than a standard electric tank.
Venting and Install Complexity
A like-for-like tank swap is straightforward. The vent, gas line, and water connections are already there. The plumber drains the old tank, hauls it out, and sets the new one in place.
Switching from tank to tankless is more involved:
- Gas tankless units need dedicated venting. Modern condensing units can often vent through PVC pipe out a side wall, which is easier to route than a metal flue. The termination location still has to meet code clearances.
- The gas line often needs upsizing. A tankless burner can draw two to three times the gas of a tank burner at full fire. The existing line may be too small.
- Condensing tankless units produce acidic condensate that needs a drain line. Non-condensing units are simpler but less efficient.
- Electric tankless units need heavy wiring and open breaker space. The panel may need an upgrade.
- Some units need a minimum water flow to fire the burner. Very low-flow faucets can cause short cycling in edge cases.
This is why tankless quotes vary so much. The unit itself might cost $1,000 to $2,000. The venting, gas, electrical, and condensate work makes up the rest. Always compare full installed quotes, not unit prices.
Maintenance
Tanks need little attention but benefit from two simple habits. Drain a few gallons from the bottom valve once a year to flush sediment. And have the anode rod checked every few years. The anode rod is a sacrificial metal rod inside the tank that corrodes so the tank itself does not. Replacing a spent rod is cheap and can add years to a tank’s life.
Tankless units need annual descaling, especially if you have hard water. Mineral buildup coats the heat exchanger, cutting efficiency and eventually damaging the unit. The process is called flushing. A plumber circulates a descaling solution through the unit with a small pump. Many installers add isolation valves at install time to make this easy. Skip the flushing for years in a hard-water area and you can kill a tankless unit early.
Both types need the temperature and pressure relief valve checked occasionally. And if either unit is past about 10 years old, start planning for replacement on your schedule rather than waiting for a leak to decide for you.
Most of this falls under basic DIY plumbing upkeep, and the right tools make it a quick weekend job.
Brands Worth Knowing
A few names dominate the water heater aisle:
- Rheem makes both tank and tankless units and is one of the most widely installed brands in the country. Parts and service are easy to find.
- Rinnai is best known for tankless. The company helped popularize tankless water heating in the U.S. and offers a wide range of gas condensing models.
- A. O. Smith is a major tank manufacturer with deep roots in the plumbing trade. Plumbers know the brand well, and parts are widely available.
Brand matters less than proper sizing and installation. A correctly sized unit from any major brand, installed by a competent plumber, will serve you well. An undersized unit from the best brand will not.
The Verdict: Which Should You Pick?
Pick a conventional tank if:
- Your budget is tight and you need hot water this week. A tank swap is fast and affordable.
- Your household uses a lot of hot water at once and you do not want to think about flow rates.
- You have an older home where venting, gas, or electrical upgrades would be expensive.
- You plan to sell the house within a few years. You are unlikely to recoup a tankless premium on resale.
Pick tankless if:
- You are staying long term and want the lower operating cost and longer lifespan.
- Space is tight. A wall-hung unit frees up real floor space in a utility room or garage.
- You have a large household that keeps running out of hot water with a tank. Tankless never runs out, as long as it is sized right.
- You are already doing a bigger home renovation or new build, when venting and gas line work is cheapest to add.
Either way, do not buy based on the unit price alone. Get installed quotes that include venting, gas or electrical work, permits, and the first maintenance visit. The cheapest quote on the unit alone is rarely the cheapest finished job.
Frequently Asked Questions
Can I replace my tank with a tankless unit myself?
This is a pro job. Gas line sizing, venting, condensate drainage, and combustion air all have code requirements. A bad install can be dangerous with gas units and can void the warranty. Hire a licensed plumber.
Does a tankless water heater work during a power outage?
No, not normally. Even gas tankless units need electricity for the controls, ignition, and freeze protection. A conventional gas tank with a standing pilot can still deliver hot water during an outage. Some homeowners add a small battery backup for their tankless unit.
Will a tankless unit really lower my bills?
Usually yes, but by less than the marketing suggests. Expect the water heating portion of your bill to drop noticeably, not your total utility bill. The payback period on the extra install cost is often 5 to 10 years.
Is hard water a problem for tankless heaters?
Yes, more so than for tanks. Hard water leaves scale on the heat exchanger. In hard-water areas, plan on annual flushing and consider a water softener or scale inhibitor. Check your water hardness before buying.
How long does installation take?
A straightforward tank swap takes a few hours. A tank-to-tankless conversion usually takes a full day, sometimes two, depending on the venting and gas or electrical work involved.
