Tankless Water Heaters 101:
Is One Right for Your Home?
Published Updated
5 min read
It can be, if your household runs out of hot water with a tank or you want a heater that lasts longer and takes up less room. ENERGY STAR puts a tankless unit's life expectancy at 20 years, much longer than a tank, and gas models are small enough to hang on a wall. Whether it fits your house comes down to two things: how much hot water you use at the same time, and whether your gas line, venting and electrical service can support the unit. A plumber should look at both in person before you decide.
Key takeaways
- A tankless unit warms water only while a hot tap is open, so no energy is lost keeping a tank hot all day.
- ENERGY STAR gives tankless water heaters a 20-year life expectancy.
- Size is measured in gallons per minute: the more fixtures you run at once, the bigger the unit you need.
- Coming off a tank means checking the gas line, venting and power first, and that install can run 6 to 10 hours.
- Even soft water leaves minerals behind, so plan on descaling the unit every 12 to 18 months.
How does a tankless water heater work?
It heats water as it passes through, instead of keeping a tank of it hot. When you open a hot tap, cold water flows into the unit, a flow sensor lights the burner, and the water leaves at the temperature you set. Exhaust goes out through a sealed vent. With no stored water to keep warm, there is no standby heat loss.
Gas or electric, we repair and replace tankless units of both kinds. More on that work: Tankless water heaters
What's the lifespan of a tankless water heater?
About 20 years, by ENERGY STAR's figure, which is much longer than a standard tank. If your tank is more than 10 years old, ENERGY STAR's advice is to start planning its replacement now, before a failure forces a rushed choice.
When a tankless unit acts up, the repair-or-replace call turns on how old it is, its condition, and whether we can still get parts for it. Either way, we track down what failed before we suggest anything.
Will a tankless water heater use less energy?
It can. A tank keeps its water hot around the clock, and the heat that escapes while no tap is open is wasted. A tankless unit only heats water when a hot tap is open, which is how ENERGY STAR says it can reach greater efficiency than a standard tank. How much that shows on a bill depends on how much hot water your household uses, so we don't put a number on it.
What size tankless water heater do I need?
One that can heat enough water for everything you run at the same time. Tankless units are rated in gallons per minute (GPM), not in tank gallons. ENERGY STAR lists these averages: a shower or tub uses about 2.5 GPM, a clothes washer 3.3, a kitchen or bath sink 2.2 and a dishwasher 1.3. Add up what tends to run together in your house, and that is the flow the unit has to keep up with. We size every new unit for the home it goes in.
What changes when you switch from a tank to tankless?
That depends on four checks an installer makes in your home. They confirm the right size, check whether your gas line can handle the unit, see whether there is power near the heater for models with forced venting, and work out how the exhaust will be vented. High-efficiency condensing models vent through a vertical PVC pipe and need a condensate drain.
When we replace a water heater with a tankless unit, we pull the permit and haul the old one away.
Does a tankless water heater need maintenance?
Yes: a flush and descale every 12 to 18 months, which is the schedule we keep on the units we service. Soft water doesn't make that step optional, since minerals and sediment still collect in any water heater over time. It matters most on a private well, where hard water is more likely.
Which tankless brands do you work on?
On gas units, Rinnai's locator lists us as a Rinnai PRO. On electric units, we install Stiebel Eltron. We also repair Navien, Noritz and Rheem tankless units, though we don't install those three.
Quick answers about tankless water heaters
Will a tankless water heater ever run out of hot water?
Not the way a tank does. ENERGY STAR describes the supply as an endless flow, but every unit has a gallons-per-minute rating. Run more fixtures at once than that rating covers, and the unit can't keep up.
Do tankless systems work with well water?
It depends on the water. A home on a private well is more likely to have hard water, and that is where the 12 to 18 month flush and descale matters most.
Can I swap my tank water heater for a tankless one?
Yes, and ENERGY STAR says gas tankless units are becoming a popular replacement for gas tanks. Before anyone gives you a number, the gas line, venting and power near the heater should be checked in person.
Do tankless water heaters need electricity?
Electric models run on it, and gas models with forced venting need power near the heater too.
How long does installation take?
A typical install runs 4 to 6 hours. Coming off a tank takes longer, about 6 to 10 hours.
Want this handled?
Weighing tankless, or nursing a unit that keeps throwing errors? We find the cause first and give you the straight answer on repair or replacement.