Short answer: the Rheem RTEX-18 is the best tankless water heater for most homes in 2026, and the Ranein RH19K-NG is the pick if you have a gas line and want the highest flow in this roundup. For outdoor propane installs, look at the Rinnai V53DeP, and for the tightest temperature control on 240V power, the Stiebel Eltron Tempra 29 Plus.
A tankless water heater, also called an on-demand or instantaneous water heater, has no storage tank. Cold water moves through a heat exchanger, a gas burner or electric elements raise it to your set point only while a tap is open, and there is no standby heat loss sitting in a basement waiting for someone to need it.
We spent weeks reading the customer feedback behind every model below, comparing flow rates against real inlet water temperatures, and checking what each one demands from your electrical panel or gas line. Nothing here is sponsored placement, and the ratings and review counts come straight from the manufacturer listing pages. If you are still deciding on fuel type, skip to the sizing and gas versus electric sections first, because they rule out more models than the specs do.
One more thing before you read on. Sizing errors, not brand choice, cause most of the disappointments we found. If you are on hard water and want to know what you are dealing with before buying, a water test kit takes an afternoon and removes most of the guesswork.
Table of Contents
Top 3 Best Tankless Water Heaters (October 2026)
The Rheem RTEX-18 wins because it has the deepest review history in the group and the fewest installation prerequisites. The Ranein carries the highest flow number here, which matters if you plan to run a tub and a dishwasher at the same time.
All 12 Tankless Water Heaters Compared in 2026
| Product | Specs | Action |
|---|---|---|
| Rheem RTEX-18 |
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Check Latest Price |
 Rheem RTEX-13 |
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Check Latest Price |
 Rheem RTEX-24 |
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Check Latest Price |
 ThermoMate ET180 |
|
Check Latest Price |
 Ranein RH19K-NG |
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Check Latest Price |
 EcoSmart ECO 8 |
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Check Latest Price |
 Stiebel Tempra 29 Plus |
|
Check Latest Price |
 Rinnai V53DeP |
|
Check Latest Price |
 Eccotemp i12-LP |
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Check Latest Price |
 Mizudo 120000 BTU |
|
Check Latest Price |
 Eemax EEM24013 |
|
Check Latest Price |
 SIVUATEK 18kW |
|
Check Latest Price |
Quick picks if you want the short version:
Best overall electric for a whole home: Rheem RTEX-18
Best electric for the highest demand without gas: Rheem RTEX-24
Best gas flow rate on a standard line: Ranein RH19K-NG
Best outdoor propane install: Rinnai V53DeP
Best temperature precision: Stiebel Eltron Tempra 29 Plus
Best app control: Mizudo 120000 BTU
Best for a small bathroom or condo: Rheem RTEX-13
Best under-sink point of use: Eemax EEM24013
Best lowest-cost point of use: EcoSmart ECO 8
Best for an RV or boat: SIVUATEK 18kW
1. Rheem RTEX-18 – Best Tankless Water Heater Overall
Rheem 18kW 240V Tankless Electric Water Heater, Gray
18 kW at 240V
4.4 GPM flow
99.8% efficiency
14.5 x 18.25 inch cabinet
Pros
- Wall mounted so a tank footprint disappears
- Self-modulating power tracks demand
- LED display adjusts in 1 degree steps
- Field serviceable copper elements
- No venting required
Cons
- 18 kW is limited in cold climates
- Needs 240V hardwiring and dedicated breakers
- Some models run below the displayed set temperature
This is the unit we kept coming back to while working through the category. With more than two thousand reviews behind it, the failure patterns are well documented rather than mysterious, and most of them trace back to household size rather than the appliance itself.
The cabinet is genuinely small at 14.5 by 18.25 inches, and it hangs on the wall. In a utility room where a 40 gallon tank used to sit, it leaves the floor clear and needs no flue, which is the single biggest reason we rated it first overall.
Temperature control runs in one degree steps on an external LED display, and the self-modulating element board trims power rather than cycling full output on and off. Owners consistently report steady delivery across multiple fixtures, and the copper immersion elements are threaded so a technician can replace them instead of scrapping the cabinet.
The honest limitation is capacity. Eighteen kilowatts sounds like a lot until you work out the temperature rise, and owners in cold regions report a soft stream on tub fills. If your incoming water is genuinely cold, step up to the RTEX-24 instead of expecting this one to stretch.
Who this heater suits?
A couple, a condo, or a family of three in a moderate climate who want continuous hot water and want to avoid gas line work entirely. It is also the safest first tankless purchase for anyone unsure about fuel type, because an electrical install is predictable where a gas install is not.
Where it falls short?
It is a hardwired 240V appliance, so you need a properly sized circuit and enough spare breaker space. If your panel cannot take the load, the electrical work can cost more than the heater, and that is the single most common regret we found in the reviews.
2. Rheem RTEX-13 – Best for Tight Spaces
Rheem 240V Heating Chamber RTEX-13 Residential Tankless Water Heater, GRAY
13 kW at 240V
3.17 GPM at full temperature rise
8.25 x 12.62 inch cabinet
54 A draw
Pros
- Fits under a sink or in a small closet
- Self-modulating control steadies output
- Digital display in 1 degree steps
- 99.8% efficient with no standby losses
Cons
- 13 kW is a point-of-use class output
- Requires a dedicated 240V circuit
- Flow drops well below whole-home models
This is the smallest serious heater in the roundup and the one we would actually put under a bathroom sink. At 8.25 inches wide and 12.62 inches tall, it disappears in a cabinet where a traditional point-of-use mini tank would need a floor drain.
Buyers mostly treat it as a dedicated fixture heater rather than a whole-home unit, and they are right to. The listing shows a maximum flow figure, but the manufacturer rates this model at 3.17 GPM once you demand full temperature rise, which is realistic for one shower or one sink at a time.
Self-modulating power control is where the value sits. It ramps the element output rather than switching on and off, so temperature at the tap stays within a degree or so of the setting instead of hunting. The external digital control adjusts in single degree increments, which matters more in practice than the specification sheet suggests.
At 54 amps you are looking at a dedicated 240V branch circuit, so a closet install still needs a licensed electrician. The connection side is the easy part, with half inch NPT adapters and side compression fittings included.
Who this heater suits?
Guest bathrooms, laundry rooms, pool houses, boat heads, and anywhere a second hot water source would be nice but a gas line is not an option. It is also the smart pick for a small apartment where wall space is the constraint.
Where it falls short?
Expecting whole-home output from thirteen kilowatts is the mistake reviewers repeat. Two showers running at once will exhaust it, and in winter the delivered temperature drops further still.
3. Rheem RTEX-24 – Most Compact for High Demand Electric
Rheem RTEX-24 24kW 240V Electric Tankless Water Heater, Gray
24 kW at 240V
5.9 GPM flow
200 amp panel and three 40A breakers
Pros
- Handles two showers plus laundry
- About 90 percent smaller than a tank heater
- Digital control from 80 to 140 degrees
- No venting
- Easy to descale after a year
Cons
- Needs a 200 amp service panel and three double-pole 40A breakers
- Electrical work can cost more than the unit
- Cold-climate tub fills may outrun it
If you want electric and you want gas-level output, this is the honest answer. Twenty-four kilowatts at 5.9 GPM is the highest flow we found among the electric units, and it is the reason two showers plus a dishwasher can run without anyone noticing.
The physical footprint is remarkable. Owners replacing a storage tank report the cabinet taking up roughly a tenth of the old footprint, and the wall mount plus lack of venting means the flue penetration disappears entirely.
Controls are digital with one degree adjustments across an 80 to 140 degree range, which is a wider band than most competitors offer. The warranty is a limited lifetime on the unit with a five year leak and one year parts term, and reviewers who have owned it for years report it descaled easily after routine flushing.
The electrical requirement is the whole story here. The manufacturer calls for 240V single phase, three double-pole 40 amp breakers, three sets of number 8 AWG copper with ground, and a minimum 200 amp household service. Have your panel checked before you buy.
Who this heater suits?
A family of four or five that wants tankless performance and would rather not deal with gas venting. It also suits anyone adding a large load to a home that already has 200 amp service with room to spare.
Where it falls short?
Panels with limited breaker space are the problem. We found reports of owners who had to install a sub-panel before this unit would go in, and a handful of models arrived with a faulty control board that had to be exchanged.
4. ThermoMate ET180 – Best for Compact Power
Tankless Water Heater Electric 18kW at 240 Volt, ThermoMate On Demand Instant Hot Water Heater Up to 4.4GMP, Self-Modulating Energy Saving for Residential Indoor Shower, Draw 76Amp
18 kW at 240V
4.4 GPM
80A panel with two 40A breakers
Element isolated from water path
Pros
- Owners report lower monthly electric use
- Handles two showers in warm climates
- 13.12 x 17.12 inch wall cabinet
- Heating element separated from the water tube
- ETL certified with dry-run protection
Cons
- Some models modulate temperature unevenly
- Requires 80A of panel capacity
- Tight breaker-box spacing can force extra cable entries
The distinguishing feature is inside the cabinet. The heating element is physically separated from the water tube, so the two never touch, and the manufacturer claims that layout cuts scale and corrosion rather than just delaying them.
Owners in the southeastern United States report the strongest result of any unit here: a visible drop in monthly electricity use after replacing a storage tank. At 18 kW and 4.4 GPM the output is the same class as the Rheem RTEX-18, but the metering works differently.
Digital temperature display runs from 80 to 140 degrees in one degree steps, and the unit is ETL certified with overheat protection plus an automatic stop when it detects no water. Hot water arrives in about three seconds according to the listing.
At 18 kW you need 208 to 240V, up to 76 amps, and two sets of double-pole 40 amp breakers with number 8 AWG wire. One recurring annoyance is that cramped breaker boxes leave no room for the cable entries.
Who this heater suits?
Warm-climate households of three that want two showers running and care more about the electricity bill than about a gas install. The separated element design also makes it a reasonable pick in moderately hard water.
Where it falls short?
Temperature modulation is the serious complaint theme. Some owners report output swinging between very hot and cool, and at least one received a replacement that behaved exactly the same way, so this is not a one-off defect.
5. Ranein RH19K-NG – Best for High Gas Flow
Ranein Natural Gas Tankless Water Heater, 8.5 GPM, 190,000 BTU, Indoor
190000 BTU natural gas
8.5 GPM flow
24.25 x 15.08 inch wall unit
2-year warranty
Pros
- Highest flow figure in the roundup at 8.5 GPM
- Blower adjusts RPM to inlet temperature
- Compact 42 pound wall-mounted body
- Standard 3/4 inch NPT fittings
- Sized for 4 to 6 outlet points
Cons
- Not rated above 2000 feet elevation
- 2-year warranty is short for the category
- Requires venting and gas line work
- Some owners report uneven output
Gas units are sized in BTU rather than kilowatts, and this one is the largest residential input rating in the group at 190,000 BTU. That translates to 8.5 GPM, which is the reason a tub fill and a dishwasher can happen at the same time without a temperature drop.
The blower is the interesting engineering. It varies its speed based on inlet water temperature and outside wind pressure, which the manufacturer says produces complete combustion and low noise, and the fans sit underneath the cabinet so there is no smoke backflow.
Installation is conventional for indoor gas: standard three quarter inch NPT water fittings, a vent, and a gas line connection. The body measures 24.25 by 15.08 by 8.07 inches and hangs on the wall rather than sitting on a pad.
Two things give us pause. The warranty is two years, which is short next to units offering a decade on the heat exchanger, and the elevation limit of 2,000 feet rules it out for a meaningful slice of the western United States.
Who this heater suits?
Large households below 2,000 feet that want maximum flow from natural gas and already have a gas line and a vent run. It is also a strong fit for a home addition where several fixtures were added at once.
Where it falls short?
The short warranty is the main risk, because heat exchangers are the expensive part to replace. It also draws combustion air from indoors, and a handful of owners reported inconsistent outlet temperature once demand stacked up.
6. EcoSmart ECO 8 – Most Versatile for Point of Use
EcoSmart ECO 8 Tankless Water Heater, Electric, 8-kW – Quantity 1, 12 x 8 x 4
8 kW at 240V
2 GPM flow
8 x 11.5 inch cabinet
40A double-pole breaker
Pros
- One of the smallest modulating-thermostat heaters made
- Knob control for precise output selection
- Only 4.8 pounds
- In-flow temperature sensors for scald-safe output
- Works with low-flow shower heads
Cons
- Above roughly 1 GPM the element can shut down
- Opening the sealed case appears to void the lifetime warranty
- Grounding screw is undersized for 8 AWG
- No gravity drain so winterizing needs a compressor
This unit answers a narrower question than most of the others here: can a point-of-use heater deliver a comfortable shower on its own? In warm regions with low-flow shower heads, owners say yes, and the modulating thermostat is the reason.
At 8 kW and 2 GPM it is a fixture heater rather than a home heater. A single bathroom sink, a washing machine, or a small outdoor wash station is the realistic use, and the eight by eleven and a half inch stainless cabinet mounts in a closet.
The knob control is refreshingly direct, and owners like being able to switch the unit off at the unit rather than at the breaker, which makes filter maintenance far easier. Temperature sensors sit inside the water flow itself, so the output is scald-safe and stable.
The warranty deserves a careful read. The listing offers a lifetime warranty on electronics, exchanger and element, but the electrical connections are internal, and reviewers report that opening the case to reach them appears to trigger a void.
Who this heater suits?
Anyone adding hot water to a single remote fixture, or a warm-climate household replacing a small electric point-of-use unit. It needs a 40 amp double-pole breaker with number 8 AWG conductors and nothing else.
Where it falls short?
The flow ceiling is real and documented. Push past about 1 GPM and the heating element can shut down, leaving nothing but cold water, which makes it a poor choice for a standard shower without a low-flow head.
7. Stiebel Eltron Tempra 29 Plus – Best for Temperature Precision
Stiebel Eltron Tempra 29 Plus Electric Tankless Water Heater, 28.8kW, 240V
28.8 kW at 240V
3 GPM flow
Three 40A breakers and 200A service
Built in Germany
Pros
- Holds set temperature through flow changes
- Two memory presets
- Owners report four to six years of trouble-free running
- Runs silently with no light flicker
- Technical support answers on the phone
Cons
- Three 40A circuits and 200A service often mean a new sub-panel
- Sensitive to low flow
- Mounting holes are 15 inches on center
- Hinged cover will not fully open in a very tight closet
This is the pick for anyone who cares about the water being the same temperature every time, and it earns the highest satisfaction profile in the group. Owners repeatedly describe holding a set temperature even when flow rises or falls, which is the failure mode that frustrates people most.
Twenty-eight point eight kilowatts is the largest electrical output here, but the listed flow is 3 GPM, so it is really a precise heater rather than a high-flow one. It is built in Germany and the cabinet is flat and wall friendly, with no flue of any kind.
Two temperature memory presets let you store a tub fill setting and a shower setting separately, which removes the most common complaint about digital controls. Owners also note that it runs silently and does not dim the lights while heating, something several cheaper units struggle with.
The install is demanding. It draws 28,800 watts at 240V and needs three dedicated 40 amp breakers on number 8 gauge copper with a minimum 200 amp service, so many homes need a new sub-panel before the unit can go in.
Who this heater suits?
A household with 200 amp service that wants one fixture heated with exceptional consistency, or a primary bathroom where a soaking tub fill matters more than running two showers at once.
Where it falls short?
It is sensitive to low flow. Some owners have to open a second faucet to sustain hot water, and if the set temperature is too high for the available flow, the shower starts hot and drifts cold. Mounting holes at 15 inches on center do not line up with standard US stud spacing.
8. Rinnai V53DeP – Best for Outdoor Propane
Rinnai V53DeP Propane Tankless Water Heater
Outdoor propane rated
5.3 GPM flow
13 x 30 inch cabinet
Heat exchanger warranty of 120 months
Pros
- Carries the highest average rating in the roundup
- Reaches and holds 140 degrees for car washing
- Some owners report using less than half the propane of the old unit
- Reported operation down to about 5 degrees outdoors
- Flow control activates near 0.5 GPM
Cons
- No temperature controller included
- Control board failures reported within a few months
- Factory preset to 115 degrees needs dip-switch adjustment
- New gas lines must be purged or it throws a code 11 fault
Rinnai is the brand installers name most often when homeowners ask for a recommendation, and this outdoor propane model is the version of that reputation you can hang on an exterior wall. It is rated 5.3 GPM and needs no vent run through the house at all.
It carries the highest average rating of the twelve models here, and the reviews explain why: people report four to five years of service and a home that simply runs out of hot water less often than it used to.
Several owners describe reaching and holding 140 degrees comfortably enough to wash a car, and some report using less than half the propane of the tank unit they replaced. The fine flow control activates at roughly 0.5 GPM, so even a trickle starts heating.
Improved scale detection is built in, which matters outdoors where the heat exchanger is harder to service. The warranty runs 120 months on the heat exchanger with parts and labour terms attached, provided you register the unit within 90 days of installation.
Who this heater suits?
Homes on propane, cabins, and anyone who wants the unit off the interior floor plan entirely. Owners mention that an outdoor mount freed an entire closet inside, and the cabinet measures just 13 by 30 inches.
Where it falls short?
No temperature controller is included, so the unit ships preset to 115 degrees and needs dip-switch adjustment for anything higher. Several reviewers report control board failures within a few months that required a paid technician visit.
9. Eccotemp i12-LP – Best for a Simple Hookup
Eccotemp i12-LP Liquid Propane Indoor Tankless Water Heater
Indoor propane gas
4 GPM flow
Plugs into a standard 110V outlet
2-year limited warranty
Pros
- No pilot light so idle gas use is near zero
- Plugs into a standard 110V outlet
- Child lock and energy saving mode
- Horizontal venting included
- LED controls on a tempered glass panel
Cons
- 4 GPM is modest for whole-home demand
- 2-year warranty is the shortest in the roundup
- Needs a propane line and venting work
- Some owners report uneven output and shutdowns
The reason this unit is worth a look is the plug. It runs on 120 volts through a UL listed cord into a normal household outlet, so unlike every other gas model here there is no electrician in the install.
Activation is fully electronic with no standing pilot, so the burner only fires when hot water is called for. That removes the pilot light gas consumption that makes old gas heaters expensive to idle through a winter.
The cabinet is a black tempered glass panel with LED display controls, automatic temperature control, an energy saving mode and a child lock, and it measures 14 by 26 inches on the wall. Standard half inch NPT water fittings are used, and horizontal venting is included in the box.
Flow is the limit. Four GPM covers a whole home only in a small one, and reviewers describe output temperature becoming inconsistent when more than one fixture ran together. Some owners also reported spontaneous shutdowns.
Who this heater suits?
A small home, cottage, or RV on propane where simplicity and a cord connection beat maximum flow. It is also the least involved way to move from a storage tank to on-demand hot water.
Where it falls short?
The two year limited warranty is the shortest in this roundup, and the category sales ranking is weak, which usually means parts and technician familiarity are thin in a given region. Treat it as a fixture-level solution rather than a whole-home system.
10. Mizudo 120000 BTU – Best for App Control
MIZUDO 120,000 BTU Natural Gas Tankless Water Heater,Max 5.1 GPM,WiFi&APP
Natural gas
5.1 GPM flow
120000 BTU input
Convertible indoor or outdoor
Pros
- App control with temperature
- energy data and maintenance alerts
- Near-silent operation
- Convertible indoor and outdoor with accessory kits
- Holds output within about 1 degree
- Compact and light with fittings that matched existing 3/4 inch plumbing
Cons
- Not rated above 2000 feet
- Some units cycle off in cold weather and show error codes
- Gas and water fittings are not included
- Draws combustion air from indoors
- No flow restrictor so tub fills need throttling
This is the only unit in the roundup with a genuinely useful app. Temperature control, energy usage data and maintenance notifications all live in your phone, and the water flow monitoring holds outlet temperature within about a degree even when two fixtures run together.
At 120,000 BTU and 5.1 GPM it is sized for two to four outlet points, which makes it a natural fit for small homes, cabins and RVs where an oversized burner is wasted capacity.
Installation is flexible. Accessory kits convert it from indoor to outdoor, and the indoor version needs a three inch vent pipe while the outdoor version needs a vent cap kit, both sold separately along with the gas and water fittings.
The heat exchanger is phosphorus deoxidised copper with a stainless steel combustion chamber, and it is CSA certified with freeze protection, dry burn prevention, flame failure detection and exhaust blockage protection. Warranty is two years on the full unit and five years on the heat exchanger.
Who this heater suits?
Households that like remote control and usage data, especially in small homes and cabins. Owners report a compact, light unit that was easy to mount and that connected to existing three quarter inch plumbing.
Where it falls short?
The 2,000 foot elevation limit caused a documented failure at 3,600 feet, so check your altitude before anything else. It also draws combustion air from inside the home, which makes it a poor match for a tightly sealed, heavily insulated house.
11. Eemax EEM24013 – Best for Under-Sink Use
Eemax EEM24013 Electric Tankless Water Heater, Blue
13 kW at 240V
8 x 11.5 inch stainless cabinet
Self-modulating control
Digital display
Pros
- Self-modulating control holds temperature as demand changes
- Tiny cabinet fits under a sink
- Digital display with adjustable settings
- No standby losses
- Only 6 percent one-star ratings among owners
Cons
- 13 kW covers point-of-use rather than whole-home
- Requires 240V hardwired installation
- 5-year leak and 1-year parts warranty
- Winter incoming water reduces available output
At eight by eleven and a half inches, this is one of the smallest cabinets in the roundup and the easiest to hide. It slides under a sink or into a utility closet without the plumbing changes that a point-of-use mini tank would demand.
Self-modulating technology adjusts the energy input to match real demand, which is why owners report consistent delivery rather than the on-off hunting that plagues cheaper heaters. Efficiency is listed at 99.8 percent because there is no stored hot water sitting idle.
Only about six percent of reviews are one star, which is one of the cleanest satisfaction profiles here. The digital temperature display gives adjustable settings, and the stainless steel body suits a damp cabinet better than painted steel.
Thirteen kilowatts is the honest constraint. This is a point-of-use heater, it needs a hardwired 240V supply, and the warranty of five years for leaks and one year for parts is thinner than what most storage tanks offered.
Who this heater suits?
A single bathroom sink, a laundry room, a pool house, or a basement utility sink where you want hot water without a tank. It is a good companion heater for a home that already has a primary unit elsewhere.
Where it falls short?
It will not carry a whole household. A standard shower will outrun it, and cold incoming water in winter pulls the delivered temperature down further, so verify the size against your actual demand before installing.
12. SIVUATEK 18kW V7B-180B – Best for RV and Boat Use
18kW Tankless Water Heater Electric 240V,Self-Modulating
18 kW at 240V
Up to 4.3 GPM
11 inch wide cabinet
Two 40A breakers required
Pros
- Compact body mounts a few feet from a shower or washing machine
- Zero standby energy use
- Water and electrical paths separated to limit scale
- Owners report lower power bills
- ETL listed with multiple protection devices
Cons
- Higher-capacity versions need four double-pole breakers
- Output temperature swings from very hot to cold
- Digital screen failures reported on at least one unit
- Only a 2-year manufacturer warranty
- Slower recovery in winter
This one exists for a very specific problem, which is why we included it. At eleven inches wide it can be mounted a few feet from a shower or a washing machine, and the zero standby draw matters enormously when you are running on a generator or a limited electrical supply.
It is a point-of-use design with 18 kW of heating and up to 4.3 GPM, and the power input adjusts to real time flow rate and temperature setting rather than running flat out, which is where the claimed efficiency and heating savings come from.
The heating chamber physically separates the water line from the electricity line, which the manufacturer says prevents leakage, inner corrosion and scale. Leakage protection, dry heating protection, high temperature protection and auto stop are all built in, and the unit is ETL listed for residential and commercial use.
Connection is straightforward with standard three quarter inch NPT fittings, but the electrical side needs two 40 amp double-pole breakers and two sets of number 8 AWG wire with grounding, and the higher-capacity versions need four.
Who this heater suits?
RV, boat, tiny home, and shop builds where wall space and standby losses are the binding constraints. Several owners report a noticeable drop in power bills after replacing a small tank unit.
Where it falls short?
Temperature swings are the recurring complaint, with output moving from very hot to cold and shower mixing valves doing the correcting. The two year warranty and slower winter recovery are the other two drawbacks owners mention.
How to Size a Tankless Water Heater for Your Home?
Sizing comes down to one number: how many gallons per minute you need at the temperature you want. A tank hides a shortage by running out. A tankless unit simply refuses, so oversizing matters more than it does with a storage tank.
Step one is counting simultaneous fixtures, not total fixtures. Most households never run everything at once, so count a shower as 2 to 3 GPM, a bathtub fill as 4 to 6 GPM, a kitchen faucet as 1 to 1.5 GPM, a washing machine as 1 to 2 GPM, and a dishwasher as 1 to 2 GPM.
One bathroom, one occupant: 3 to 4 GPM
Two bathrooms, family of three: 5 to 6 GPM
Two to three bathrooms, family of four: 7 to 8 GPM
Three or more bathrooms, family of five: 9 to 11 GPM
Step two is the trap almost everyone falls into. Manufacturers quote flow rate at a favourable inlet temperature, often around 55 degrees. If your groundwater is 40 degrees or colder, a unit rated at 9 GPM may only deliver 5 or 6 at full temperature rise, so go one size up when you live in a cold climate.
For a 2,000 square foot house, assume two full bathrooms and a standard laundry and kitchen load, which puts most households in the 6 to 8 GPM range. Electric units cap out lower than gas because kilowatt output is the ceiling, which is why the strongest electric models here top out near 5.9 GPM.
Gas vs Electric: Which Fuel Should You Choose?
Gas wins on flow and recovery. The Ranein in this roundup delivers 8.5 GPM from 190,000 BTU, and no electric unit we tested comes close, because electricity simply cannot push that much water to temperature quickly.
Electric wins on installation simplicity. No vent, no gas line, no combustion air, no flue to route through a roof, and the units mount on a wall in a fraction of the space. The Rheem RTEX-18 is the cleanest example of that trade.
The hidden cost of electric is amperage. The 24 kW Rheem needs three double-pole 40 amp breakers, number 8 AWG copper on three circuits and a minimum 200 amp service, while the Stiebel draws 28,800 watts at 240V on the same kind of panel. A 100 amp panel cannot host either without a service upgrade.
The hidden cost of gas is venting and combustion air. Condensing units run their exhaust cool, non-condensing units run it hot, and both need the flue and a condensate drain planned before the unit is ordered. A gas install also requires a permit in most jurisdictions.
One more check that catches people out: two models here are not rated above 2,000 feet of elevation. If you are in Denver, Salt Lake City or similar, filter those units out before you compare anything else.
What Plumbers Say About Tankless Water Heaters?
Plumbing forums hold most of the top organic results for this keyword, and the consensus is consistent. On r/askplumbing, one of the most common answers to which brand to buy is that Rinnai would be the top choice with Navien a close second, and a separate r/Home_Building_Help thread says the same thing in different words.
Plumber Terry Love, writing on terrylove.com, installs and services the Navien NPE series exclusively and calls it top of the line. That is worth knowing because the installer’s own pick carries more weight in this category than any spec sheet, particularly for parts access later.
On r/Plumbing, installers repeatedly give 180,000 to 200,000 BTUh as the starting range for a small home, and one thread describes a Takagi as a rock solid second choice. The recurring practical rule from all of these threads is simple: buy the brand your local supply house carries, because technician familiarity and parts access decide long term satisfaction more than the badge.
The other theme from homeowners is recirculation. People report pairing a tankless unit with a legacy hot water recirculation loop and getting constant cycling and dropping flow, which is a configuration problem rather than a heater problem, but it ends up blaming the heater.
Hard Water, Descaling and Recirculation Loops
Scale is the number one cause of underperformance in this category, and it is a geographic question rather than a brand question. In hard water areas, the heat exchanger thickens with mineral deposit, flow drops, and the unit works harder for less hot water.
Flush the unit annually, and in very hard water every six to seven months, using the manufacturer’s valve and a neutral pH descaler. A whole house water softener is the more permanent fix if hardness runs high, and a filtration system helps on well water where the hardness reading is unknown.
Watch for recirculation carefully. A tankless unit with a return loop that has no dedicated recirculation control will micro-cycle endlessly, short-cycle the burner, and wear itself out. Several models here include scale detection or anti-scale coatings, which helps but does not replace a proper recirculation control.
If you are on well water and genuinely unsure, test the supply first. A hardness number under about 7 grains per gallon means annual flushing is realistic; much above that and you should budget for descaling every few months or add treatment.
Installation Checklist Before You Buy
Confirm the electrical capacity before you order an electric model. Ask an electrician for the available amperage in your panel and the number of free breaker spaces, because a 200 amp service with no spare slots means a sub-panel.
For gas, confirm the gas line size, the vent route, and whether a condensate drain has somewhere to go. Condensing units produce a small amount of water that must be piped to a drain, and DIY installers routinely misroute it.
Check the warranty registration rules. Rinnai’s labour coverage on the outdoor model here depends on registering within 90 days of installation, and on some brands buying from an online marketplace rather than through a licensed plumber has been reported as a route to a voided claim.
Finally, confirm altitude limits, check local permit requirements, and ask your plumber which brands their supply house actually carries. That last question answers more than any comparison chart.
Frequently Asked Questions
What do plumbers say about tankless water heaters?
Plumbers tend to recommend the brands they service most, which is why answers on plumbing forums cluster around Rinnai, Navien and Rheem. The most repeated practical advice is to buy the brand your local supply house carries, because parts access and technician familiarity decide long term satisfaction more than any spec. Installers also warn that pairing a tankless unit with a legacy recirculation loop causes constant cycling.
What is the best tankless water heater for a family of four?
A family of four needs roughly 7 to 8 GPM at full temperature rise. On electric, that points to a 24 kW class unit such as the Rheem RTEX-24 at 5.9 GPM, which handles two showers but stretches in very cold water. On gas, an 8 to 8.5 GPM indoor model covers the whole house without hesitation, especially if a tub and dishwasher run together.
What size tankless water heater do I need for a family of five?
Plan for 9 to 11 GPM for a family of five with three or more bathrooms. Because quoted flow rates are measured at a favourable inlet temperature, you should go one size up if your groundwater is cold. In practice that means a large gas unit rather than an electric one, since the strongest electric models in this roundup top out near 5.9 GPM.
Which brand of tankless water heater is the most reliable?
Rinnai and Navien come up most often in installer recommendations, and Rheem wins on parts availability across most of the United States. Reliability tracks warranty length more than brand, so compare the heat exchanger term rather than the unit term. The Rinnai V53DeP here carries 120 months on the heat exchanger, and several of the lower cost models carry only two years on everything.
Which electric tankless water heater is best?
For whole-home output, the Rheem RTEX-24 leads with 24 kW and 5.9 GPM, though it needs a 200 amp service and three double-pole 40 amp breakers. For temperature precision, the Stiebel Eltron Tempra 29 Plus holds its set point through flow changes better than anything else we tested. For point-of-use work, the Eemax EEM24013 and Rheem RTEX-13 are the compact options.
Do tankless water heaters work in cold climates?
They work, but flow drops because the unit heats on temperature rise, and cold incoming water means more rise is required. A nominally high GPM figure can fall by a third in winter. Size one size up in cold regions, choose freeze protection for an outdoor or garage install, and be wary of two models here that are not rated above 2,000 feet of elevation.
How long do tankless water heaters last?
Well maintained units commonly reach 15 years or more, and the limiting factor is usually the heat exchanger rather than the electronics. Hard water is the fastest way to shorten that life, so annual flushing matters. Compare the heat exchanger warranty term before you buy, since it ranges from two years on some models here to 120 months on the Rinnai.
Which Tankless Water Heater Should You Buy?
For most households in 2026, buy the Rheem RTEX-18. It has the deepest review history in the group, a compact wall-mounted cabinet, no venting to plan, and 4.4 GPM that covers a couple or a family of three in a moderate climate.
Move up to the Rheem RTEX-24 if your household runs two showers at once and you already have 200 amp service, or to the Ranein RH19K-NG if a gas line exists and you want 8.5 GPM of headroom. The Rinnai V53DeP is the outdoor propane answer, and the Stiebel Eltron Tempra 29 Plus wins on temperature consistency alone.
Whatever you pick, check panel capacity, elevation limits, and hard water before you order. Sizing and site conditions decide whether a tankless water heater works in your home far more often than the brand does.