A one-bathroom home is the easiest tankless sizing problem there is, and also the easiest to get wrong. A heater rated for a four-bathroom house will cost you more and run you out of power headroom, while one rated for a single lavatory will leave you shivering halfway through a shower. We spent six weeks reading spec sheets, warranty terms and owner feedback across ten of the most-reviewed tankless water heaters on the market to find the ones that actually fit one full bathroom.
The short version: you are looking for 3 to 5 gallons per minute of flow at a realistic temperature rise, a 240V circuit your panel can spare, and a body small enough to mount outside the shower wall. Our complete tankless water heater roundup covers the whole category if you are still deciding between fuels or want a larger home covered.
Everything below is updated for 2026, and every number in the spec lines comes straight from the manufacturer data rather than a marketing roundup somewhere else. Where reviewers disagree with the spec sheet, we say so.
Table of Contents
What One Bathroom Actually Needs From a Tankless Heater
A one-bathroom home typically needs 3 to 5 GPM of hot water from a low-flow showerhead and a bathroom sink. That single number is the whole decision. It assumes a showerhead of 2.0 GPM or less and a lavatory faucet of roughly 1.0 GPM, with both fixtures running at once while the water reaches 120 degrees Fahrenheit.
The catch is temperature rise. Manufacturers rate flow at a given temperature rise, so a unit quoted at 4.4 GPM might only deliver 2.9 GPM when your incoming water is 50 degrees Fahrenheit instead of the 70 used for the rating. In a cold climate that derating is what turns a generous-looking spec into a shower that runs cold at minute eight.
Everything else is downstream of flow. A 40 or 50 gallon tank stores a fixed volume, which is why it runs out; a tankless heater has no volume limit but a hard flow ceiling. Pick the ceiling that matches your fixtures and the shortage disappears.
Top 3 Picks for One Bathroom
These three cover the realistic range of one-bathroom installs: an electric whole-home unit, an electric point-of-use unit, and a gas unit for homes that already have a gas line.
Rheem RTEX-18
- 18 kW at 240V
- 4.4 GPM flow rate
- Digital LED temperature control
- Self-modulating power
Rheem RTEX-13
- 13 kW at 240V
- Up to 3.17 GPM
- 1 degree temperature increments
- 99.8 percent efficient
All 10 Best Tankless Water Heaters for One Bathroom (October 2026)
Flow rate, fuel and the electrical or venting requirement are the three fields worth reading side by side. We have listed every model we reviewed, with the features that actually decide a one-bathroom fit.
| Product | Specifications | Action |
|---|---|---|
| Rheem RTEX-18 18kW Electric Tankless |
|
Check Latest Price |
| Rheem RTEX-13 13kW Electric Tankless |
|
Check Latest Price |
 ThermoMate ET110HCUS 11kW Point of Use |
|
Check Latest Price |
 Rheem RTEX-11 11kW Point of Use |
|
Check Latest Price |
 Stiebel Eltron Tempra 29 Plus 28.8kW |
|
Check Latest Price |
| Ranein RH10K 4.3 GPM Natural Gas |
|
Check Latest Price |
 Ranein RE13K 13kW Slim Electric |
|
Check Latest Price |
 WINTEMP WN18 18kW WiFi Electric |
|
Check Latest Price |
 ThermoMate CZ422NG 5 GPM Natural Gas |
|
Check Latest Price |
 FOGATTI InstaGas Comfort 5.1 GPM |
|
Check Latest Price |
1. Rheem RTEX-18 18kW Electric Tankless – the Most Proven Fit for One Full Bathroom
Rheem 18kW 240V Tankless Electric Water Heater, Gray
18 kW at 240V
4.4 GPM flow
14.5 x 18.25 x 18.25 inch
140 F max temp
Pros
- Deep review history at 4.2 stars across 2185 ratings
- 4.4 GPM covers a shower plus a running sink
- External digital thermostat with 1 degree LED readout
- Field-serviceable copper immersion elements
- Compact wall-mount body with 3/4 inch NPT bottom hookups
Cons
- Field-serviceable elements may need attention sooner than sealed-element designs
- 240V hardwired install means no hot water during an outage
The RTEX-18 is the model we kept coming back to while sizing one-bathroom installs, and the reason is unglamorous: it has the deepest owner feedback base of anything in this group. More than 2,000 ratings averaging 4.2 stars is a much better reliability signal than a perfect score built on 150 reviews, and roughly 71 percent of those buyers gave it five stars.
At 4.4 GPM with 18 kW of output, it is the one unit here that covers the full one-bathroom case without asking you to compromise. Shower at 2.0 GPM, sink at 1.0 GPM, and you still have headroom for someone filling a bucket in the adjacent room.
The controls are where this unit separates itself from cheaper imports. The external digital thermostatic control shows live temperature on an LED display with one-degree accuracy, and self-modulating power control ramps the elements to match demand instead of cycling full-blast. Owners report the delivered temperature holding steady rather than hunting.
Two practical notes. The copper immersion elements are field serviceable, which means a plumber can replace them instead of the whole heater, but it also means mineral buildup shows up as maintenance sooner than it would in a sealed chamber. And the 240V hardwired connection means an outage takes your hot water with it, something a gas unit with a battery-free pilot does not.
What to check before you order the RTEX-18
Confirm your panel has a spare 40 amp double-pole slot. An 18 kW draw at 240V is the biggest single load most households already have, so it competes directly with a range or a clothes dryer on the same panel.
Also decide whether you are replacing an existing tank in place or relocating. The body is 14.5 inches wide by 18.25 inches deep and weighs 14.8 pounds, so a wall-hung location near the bathroom is straightforward if a stud is available for the mount.
Where the RTEX-18 falls short
If your incoming water is very cold year round, the 4.4 GPM headline number is optimistic. That rating assumes a modest temperature rise, and a 50 degree inlet cuts delivered flow substantially. In a northern climate, size up or plan for a lower-flow showerhead.
It also does nothing for a gas-only household with a small panel. If your electrical service is already at capacity, an 18 kW load is a non-starter and you should read the gas section before the rest of this list.
2. Rheem RTEX-13 13kW Electric Tankless – Best Value for a Single Bathroom
Rheem 240V Heating Chamber RTEX-13 Residential Tankless Water Heater, GRAY
13 kW at 240V
Up to 3.17 GPM
54 amp draw
8.25 x 12.62 inch
Pros
- 4.4 star rating across 1408 reviews
- Up to 3.17 GPM sized for one bathroom
- 99.8 percent efficiency with self-modulating control
- Compact body with simple side compression hookups
Cons
- Copper immersion elements in hard water need periodic descaling
- Requires a dedicated 54 amp 240V circuit
The RTEX-13 is the pick we recommend to most one-bathroom households, and the gap between it and the models above it is a half-step rather than a category. It holds a 4.4 star average across 1,408 ratings, with 74 percent five-star marks, which is the highest rating-to-review depth combination in this group.
Up to 3.17 GPM is a genuinely correct size for one full bathroom. It handles a 2.0 GPM shower with the sink running, and it will not carry a second shower, which is exactly the constraint most single-bathroom homes live with.
Temperature control runs in 1 degree Fahrenheit increments through the external digital thermostat, and the self-modulating power control adjusts output to demand. Buyers consistently call out how little they have to think about it after the initial set-and-forget adjustment.
The installation profile is friendly for a small space. Side 1/2 inch compression connections with 1/2 NPT adapters included mean it can sit close to the shower without fighting for elbow room, and the whole body is 8.25 inches wide by 12.62 inches tall.
Why this is the safe one-bathroom buy
Its 54 amp draw at 240V is a meaningful electrical commitment, so check that before anything else. If your panel has the capacity, the efficiency figure of at least 99.8 percent means no standby losses and no standby fuel bill while the family is out.
If you have hard water, factor in descaling. Copper immersion elements are serviceable, which keeps repair cost down, but they are also the surface where scale accumulates fastest in mineral-heavy areas.
When to size up instead
If your bathroom has a soaking tub, a dual-head shower or a hand shower plus a body sprayer, treat those as simultaneous fixtures and the 3.17 GPM rating will not hold. Either drop to a 2.0 GPM showerhead or move up a size.
It is also not the right choice if a gas line is already available and you want higher flow without a bigger circuit. On-demand hot water at 4 GPM or above with a gas burner is a different category of purchase.
3. ThermoMate ET110HCUS 11kW Point-of-Use Electric – Compact and Cheap to Run
ThermoMate Tankless Water Heater Electric, 11kW 240 Volt On Demand Instant Hot Water Heater for Point of Use, Self-Modulating LED Display Knob Control, ETL Certified, 2.15GPM at 35°F Rise
11 kW at 240V
2.15 GPM at 35 degree rise
9.72 x 13.78 inch
11 pounds
Pros
- Very compact wall-mount body
- Cast aluminum chamber separates element from water
- 99 percent efficiency with self-modulating regulation
- Knob control plus LED display and ETL certification
Cons
- Output falls well short of advertised flow at higher demand
- Some buyers report temperature overshoot and cycling
- Needs a 50 amp double-pole breaker and 6 AWG wiring
The ET110HCUS is the smallest sensible electric option here at 11 pounds and under 14 inches tall, and its cast aluminum heating chamber is a design choice worth understanding. The element sits apart from the water path, which limits the scale and corrosion exposure that plagues copper immersion designs in hard water.
Its honest limit is flow. The 2.15 GPM rating is measured at a 35 degree temperature rise, and a substantial share of owners report that output drops well below that once you push past roughly 1.25 GPM, leaving only warm water. That makes it a sink-and-shower answer for a low-flow bathroom, not a household heater.
Maximum outlet temperature is 131 degrees Fahrenheit, which is adequate for a shower but tighter than units that reach 140. If you have young children or anyone with limited mobility, factor that ceiling into the decision before anything else.
A few buyers describe temperature overshoot and cycling rather than smooth modulation, and one reported the behavior persisting through a brand replacement. The majority, though, report meaningful reductions in their water heating bills after moving off a tank, which is exactly the on-demand advantage showing up in a real number.
When this undersink unit makes sense
Install it close to the bathroom, not at the property line. Point-of-use placement exists to keep the run of hot pipe short, and every foot of extra pipe is hot water sitting in the wall cooling down before it reaches the tap.
Verify the electrical run before purchase. The unit draws a maximum of 46 amps and needs a double-pole 50 amp breaker with 6 AWG wire, which is a real load on a 100 amp panel.
Where it will frustrate you
Any household expecting a shower and a kitchen sink to run together will notice the ceiling quickly. The unit is also ventless, which is a genuine advantage over gas in a tight closet, but that also means there is no stored hot water to fall back on during a power cut.
Choose this if your bathroom has a genuinely low-flow showerhead. Choose something larger if the existing head is a 2.5 GPM standard-flow model you are not planning to change.
4. Rheem RTEX-11 11kW Multi-Fixture Point of Use – Cheapest Path Into Tankless
Rheem 11kW Multi Fixture Tankless Electric Water Heater – 240V
11 kW at 240V
2.68 GPM
8.25 x 12.63 inch
80 to 140 degree control
Pros
- Compact enough for a closet or under-sink install
- 99.8 percent efficiency with no standby losses
- No venting required versus a gas unit
- Digital control in 1 degree increments
- Multiple owners report three-plus years of solid service
Cons
- Some buyers report weak output or only lukewarm water at the top setting
- Housing is largely plastic and awkward to plumb
- Threaded adapters can be hard to seal against drips
- Power modulation can cause LED flicker and AM radio static
The RTEX-11 is the entry point most people meet on the way to a larger unit, and at 2.68 GPM it is sized for multiple fixtures in a small home. It requires no venting at all, which removes the single largest install complication on a gas tankless purchase.
At 4.0 stars from 519 ratings it sits at the bottom of this group’s ratings, and the complaints are consistent enough to plan around. A steady minority report weak output or lukewarm water at the highest setting, which is a flow ceiling talking rather than a defect.
The housing is largely plastic, with a screw-secured cover reached from the back, and that detail matters more than it sounds. It complicates both the wiring run and the mounting clearance, so allow more wall space than the 8.25 by 12.63 inch footprint suggests.
Two smaller annoyances get repeated reviews. The threaded adapters supplied with the outlets can be hard to seal against drips, and power modulation can cause LED bulb flicker and AM radio static on the same circuit, which is a good reason to keep it on its own dedicated branch.
What to confirm before installation
This model needs 240V single-phase power on one 50 amp double-pole breaker with 8 AWG copper conductors and ground, and Rheem recommends a minimum 100 amp household service panel. Check that panel rating first; it is the most common reason an otherwise good install gets delayed.
Plan on a 5-year leak and 1-year parts warranty. That is shorter coverage than the German-made unit in this list, so treat it as a fixture with a defined service life rather than a long-term asset.
Where it falls short
Two. The lukewarm-at-highest-setting reports are frequent enough that you should test with your actual showerhead in hand before committing. And the 2.68 GPM figure assumes a modest temperature rise, so cold incoming water shrinks it further.
If your bathroom has any fixture above roughly 2.0 GPM, the two 13 kW and 18 kW models are the safer buy. If your shower is genuinely low-flow, this is a perfectly serviceable choice.
5. Stiebel Eltron Tempra 29 Plus 28.8kW – the Most Stable Temperature Control
Stiebel Eltron Tempra 29 Plus Electric Tankless Water Heater, 28.8kW, 240V
28.8 kW at 240V
3 GPM
16.63 x 14.5 inch
Made in Germany
Pros
- Rock-steady temperature over four to five years of ownership
- Two temperature memory presets
- German build with copper heat exchanger and shielded element
- Completely silent with no venting
- Owners report simultaneous showering with no comfort drop
Cons
- Needs 200 amp service and three dedicated 40 amp circuits
- Very sensitive to low flow
- Mounting holes at 15 inches on center miss standard stud spacing
- Third-generation cover change can leave existing wiring too short
The Tempra 29 Plus earns its place for one specific reason: owners describe its temperature as the steadiest of any unit in this group. Multiple buyers report four to five years of trouble-free service, and the 3 GPM rating is delivered with very little fluctuation even when flow changes mid-shower.
It is also genuinely premium hardware. Made in Germany with a copper heat exchanger and a shielded element, and it runs completely silently with no venting requirement, which is unusual for a unit this powerful.
Two temperature memory presets handle the set-and-forget part well. You set the shower setting and the lavatory setting once, and the unit recalls both without anyone touching the controls at 6 in the morning.
The electrical requirement is the headline drawback. It needs hard-wired 240 or 208 volt service at up to 28,800 watts, three separate dedicated 40 amp breakers with 8 gauge copper wiring, and a minimum 200 amp total residence service. If your panel is not already 200 amp, this is a panel upgrade conversation before it is a water heater conversation.
Whether your panel can take it
Run this check first, before you fall in love with the temperature control. Three 40 amp breakers is more panel capacity than most one-bathroom homes have going spare, and a service upgrade is a separate project with its own cost and timeline.
The 3 GPM rating is also measured at a 28.8 kW input, which means it holds flow at a much lower temperature rise than a small electric unit. In a cold climate that advantage is worth real money in comfort.
Known install quirks to plan around
It is very sensitive to low flow. Some fixtures will not trigger it at all, and one reviewer had to keep a second faucet open to hold temperature steady. That is a real design tradeoff, and it is why low-flow showerheads matter more here than elsewhere.
Mounting holes at 15 inches on center do not align with standard US stud spacing, and the third-generation hinged cover and relocated terminal block can leave existing wiring too short to reuse. Both add install time.
6. Ranein RH10K 4.3 GPM Natural Gas Indoor – Best If You Already Have a Gas Line
Ranein Natural Gas Tankless Water Heater, 4.3 GPM, 100,000 BTU, Indoor
100000 BTU
4.3 GPM
14.17 x 22.05 inch
95 to 155 degree range
Pros
- 4.3 GPM and 100000 BTU cover several fixtures running at once
- Four one-touch preset modes
- LED display with live temperature and fault codes
- Vent pipe included in the box
- Heats quickly even on small flow rates
Cons
- Natural gas and LP versions are not interchangeable
- Resets to 129 degrees after a power interruption
- Half inch non-tapered threading complicates service valve kits
- Some buyers report control board failures within roughly 8 months
Gas changes the sizing conversation entirely. Where an electric unit is limited by amperage, a 100,000 BTU burner is limited by flue capacity, so if you have a gas line you get flow without touching your electrical panel at all. At 4.3 GPM this comfortably handles a single bathroom with multiple fixtures running.
It is also the only unit here that ships with its vent pipe in the box. That sounds minor until you price a trip to a plumbing supply house because the box did not include one, which is a common complaint about competitors in this category.
The control experience is good. Four one-touch preset modes cover season, kitchen, bathtub and shower, and the LED display shows live temperature, operating status and fault codes while remembering your set points. The adjustable range runs 95 to 155 degrees Fahrenheit with a high-temperature lock.
Reliability is the open question. At 4.1 stars from 734 ratings, with 68 percent five-star, it trails the Rheem units on both measures, and a recurring critical theme is control board failure within roughly eight months alongside unresponsive warranty support. The stated manufacturer warranty is 2 years.
Decisions that cannot be undone
Natural gas and LP versions are not interchangeable, and the gas type cannot be changed after purchase. Confirm which gas serves the house before ordering, because getting it wrong means a replacement unit rather than a conversion.
Plan the venting location now. This is an indoor unit rated for elevations up to 2,000 feet, and it needs a licensed professional installation with a properly sized flue run. Anti-freeze protection is rated to 14 degrees, which covers most climate zones but not all.
Where owners report friction
It resets to a 129 degree default after any power interruption with no remote to restore your settings, which is an annoyance during any outage longer than a few minutes. The 1/2 inch non-tapered connections also complicate standard service valve kits.
If you already run gas for cooking or a furnace, this is the most natural fit in the roundup. If you do not, a gas install adds a line run and a permit on top of everything else.
7. Ranein RE13K 13kW Slim Electric – the Thinnest Unit That Still Covers a Shower
Ranein 13kW Tankless Electric Water Heater, Max 3.1 GPM, 240V, LED Display
13 kW at 240V
3.1 GPM
10.7 x 7.9 x 2.8 inch
86 to 131 degree range
Pros
- 3.1 GPM covers a sink and shower at once
- Very thin 2.8 inch profile for under-sink and closet installs
- 99 percent efficiency with up to 60 percent heating cost savings
- Heating elements separated from the water path to reduce scale
- Wiring accessible without removing the housing
Cons
- Requires a 60 amp breaker
- Maximum outlet temperature limited to 131 degrees
The RE13K solves a physical problem the other units do not: wall depth. At 10.7 inches wide by 7.9 inches tall and only 2.8 inches deep, it fits between studs or under a counter where a conventional body will not go, and it still delivers up to 3.1 GPM.
That 3.1 GPM figure is what makes it interesting for one bathroom. It is sized for one to two usage points, meaning a sink and a shower, which is the exact demand pattern of a full bathroom rather than a single lavatory.
Its separated-element design reduces scale buildup, which matters in hard water, and the manufacturer claims 99 percent thermal efficiency with heating cost savings of up to 60 percent. Wiring is accessible without removing the housing, a small thing that saves real time on the install.
Two constraints deserve a close look. It requires a single 60 amp breaker at 240V, which is a heavier commitment than the 50 amp circuits used by several smaller units here, and the maximum outlet temperature is 131 degrees Fahrenheit rather than 140.
Where a slim body changes the decision
If your existing tank sits in a closet too shallow for a conventional heater, this is the unit that gets installed instead of a project that gets abandoned. The 2.8 inch profile also suits a kitchen sink cabinet where point-of-use placement keeps the hot run short.
Confirm the 60 amp requirement against your panel. A 60 amp double-pole breaker is larger than most people expect, and on a 100 amp service it often means reassigning existing loads.
Limits worth weighing
The 131 degree ceiling is the one to think about if anyone in the household is young or elderly. It is hot enough for a normal shower, but there is less headroom for a preheated tank to top up.
At 4.1 stars from 152 ratings the feedback base is still thin, so treat the spec sheet as more informative here than the review count. It is a capable small unit, not a proven one, at least not yet.
8. WINTEMP WN18 18kW WiFi Electric – App Control With Real Flow
WINTEMP 18kW Electric Tankless Water Heater 240V ETL Certified WiFi
18 kW at 240V
4.3 GPM
11.1 x 19.2 x 4 inch
Remote WiFi control
Pros
- Remote WiFi temperature control from a phone
- 4.3 GPM covers a shower plus other household demand
- Self-modulating power adjusts to real-time flow for up to 99 percent savings
- Compact 4 inch deep body takes about 90 percent less space than a tank
- ETL certified with leakage overheat and dry-fire protection
Cons
- Needs 150 amp service and two 40 amp double-pole breakers
- ABS plastic housing rather than metal
- 2-year limited manufacturer warranty
The WN18 is the only unit here with remote WiFi temperature control, and in a one-bathroom home that has a specific use: you start the shower temperature from another room rather than standing at the wall unit with a towel on the floor.
Underneath the app, the numbers are serious. At 18 kW it delivers up to 4.3 GPM, matching the gas unit in this roundup while staying ventless, and it pairs that with self-modulating power control that adjusts to real-time flow and set temperature.
It scores 4.4 stars from 202 ratings with 75 percent five-star marks, the highest five-star share among the electric units we looked at, and reviewers single out both the remote and the flow capacity as the standouts.
Watch the electrical side closely. The unit draws a maximum of 75 amps and needs two 40 amp double-pole breakers with 2x 8/2 AWG wiring on a recommended 150 amp household service. That is a genuine panel-capacity conversation for most single-bathroom homes.
Is WiFi worth it for one bathroom
If anyone in the house routinely runs long showers, remote pre-heating removes the wait and means nobody has to stand under cold water while the unit catches up. It is a small daily convenience that adds up.
The 4 inch deep body is compact, and the manufacturer claims it occupies about 90 percent less space than a tank heater. The trade-off is the ABS plastic housing rather than metal, which is lighter to mount but less thermally massive.
When it is the wrong choice
On a 100 amp panel, two 40 amp double-pole breakers simply are not available, and a service upgrade changes the economics of the whole purchase. Measure your panel first.
The 2-year limited warranty is also the shortest of the group. If you want long heat exchanger coverage as a reliability proxy, the 5-year heat exchanger warranty on the last unit in this roundup is a stronger position.
9. ThermoMate CZ422NG 5 GPM Natural Gas – for Two Showers, Not One
ThermoMate CZ422NG, Natural Gas Tankless Water Heater Indoor, Up to 5 GPM
113000 BTU
5 GPM
13.77 x 21.65 x 7 inch
0.81 UEF
Pros
- 5 GPM and 113000 BTU run two showers at a stable 105 degrees
- Flow sensor and control chip adjust heating 20000 times per second
- 98 percent thermal efficiency with a 0.81 UEF
- Compact body takes under a third of a tank heater space
- DC fan resists backdraft and flameout in high wind
Cons
- Annual operating costs are roughly 30 percent lower than a tank rather than near zero
- Gas installation must be handled by a licensed professional
The CZ422NG is the most capable heater in this roundup, and for a strict one-bathroom home that capability is more than you need. At 113,000 BTU and up to 5 GPM it sustains 105 degrees Fahrenheit through two simultaneous showers, which is a two-bathroom problem by definition.
That said, the modulation engineering is the most advanced here. A flow sensor and control chip adjust heating 20,000 times per second, and the claimed result is plus or minus 1 degree consistency even as flow varies, which is what keeps temperature steady during a long shower.
Its 0.81 UEF and 98 percent thermal efficiency are the highest efficiency figures in the roundup, and freeze protection activates at 41 degrees, which is more conservative than the 14 degree rating on the other gas unit. The body is 21.65 by 13.77 by 7 inches, under a third of a tank heater footprint.
Be realistic about the running cost framing, though. Owners describe annual operating costs as roughly 30 percent lower than a traditional heater rather than near-zero, and gas installation has to be handled by a licensed professional, which is a real line item in the total.
Why oversizing a gas unit is low risk
Unlike an electric heater, an oversized gas unit does not demand a bigger electrical panel. It runs on a 120V plug, so the capacity question moves entirely to the gas line and the flue, both of which a licensed installer can confirm.
The DC high-speed fan is rated for 360Pa static pressure and resists backdraft and flameout even in hurricane-force wind, which is a meaningful advantage in coastal areas where draft-induced flameout is a common complaint.
When it is more unit than you need
If your home has one bathroom and one showerhead, you are buying flow capacity you will never draw. That is not a dealbreaker, but it is money aimed at a future you may never have.
Consider it instead if you are adding a bathroom, converting a guest room, or want the option of a tub plus shower running together later. Under those plans, 5 GPM pays for itself in comfort.
10. FOGATTI InstaGas Comfort 5.1 GPM – the Longest Heat Exchanger Warranty Here
FOGATTI Tankless Water Heater Natural Gas Indoor On Demand CSA Certified
120000 BTU
5.1 GPM
15.5 x 23.9 x 7.37 inch
2 to 5 year coverage
Pros
- 5.1 GPM and 120000 BTU serve two to four outlet points
- Water servo and sensors hold output within plus or minus 1 degree
- Quiet operation
- Operates down to 15 PSI water pressure
- 2-year unit coverage with a 5-year heat exchanger warranty
Cons
- Minimum flow sensor can let output spike near 170 degrees at very low flow
- Only the heater and manual are in the box
- Must be pressed back on manually after a power loss
- Some buyers in hot climates reported recurring error codes
The InstaGas Comfort holds the longest heat exchanger warranty in this roundup at 5 years, with 2-year full unit coverage, and warranty length on the heat exchanger is the reliability benchmark plumbers and builders most often cite. If you are buying on a reliability signal rather than a feature list, this is the one that scores best.
Its distinguishing practical feature is the low-pressure tolerance. It works from 15 to 150 PSI, which matters in older homes or on municipal systems where a restrictive regulator or aging supply line leaves less pressure than a conventional unit would tolerate.
At 120,000 BTU and 5.1 GPM it serves two to four outlet points and handles two showers at once, and the water servo technology with temperature sensors holds output within plus or minus 1 degree even as flow changes. The four-stage combustion system is designed to keep that stable across seasons.
It is also the quietest unit we recommend, which matters if the heater is mounted near the bathroom rather than in a basement.
What the low pressure tolerance buys you
If you have ever had a shower weaken when another fixture ran, that is a pressure problem, not a heater problem. A unit that keeps modulating down at 15 PSI keeps delivering usable hot water where a unit with a higher minimum flow threshold would simply stall.
Its 15.5 by 23.9 by 7.37 inch body is about a third the space of a tank heater, and it runs on standard 120V AC with an integrated active anti-freeze device.
Real drawbacks to weigh
The minimum flow sensor can allow output to spike to roughly 170 degrees at very low flow, which is above scalding range. That is a household with children or elderly residents you should think carefully about.
Only the heater and manual ship in the box, with no vent pipe, valves or fittings, and the unit must be pressed back on manually after any power loss. Some buyers in hot climates reported recurring error codes requiring repeated replacement, which is the main reason it sits at 4.0 stars from 167 ratings.
Electric vs Gas for a One-Bathroom Home
Electric wins when you have panel capacity and want a ventless, wall-mountable install with no gas work. Gas wins when your panel is full, when you want flow above about 4 GPM, or when you already have a gas line in the room where the heater needs to go.
The deciding factor for most one-bathroom homes is amperage, not preference. An 18 kW electric unit pulls 75 amps, the Stiebel Tempra 29 Plus needs 200 amp service and three dedicated 40 amp circuits, and the 11 kW and 13 kW point-of-use models land between 46 and 60 amps. A gas unit uses a 120V plug and asks nothing of your panel.
Against that, gas brings venting. A condensing unit needs a flue or a PVC vent run, and in a tight indoor closet that is often where an install starts to cost more than the heater. Venting distance and code requirements vary locally, so this is a conversation with a plumber before you pick a model.
Cold climate performance splits the two as well. Gas capacity holds flow at a lower temperature rise, so it degrades less when incoming water is 45 degrees instead of 65. Electric units sized to the same GPM number lose more in the same conditions.
Our broader electric tankless water heater guide and gas tankless water heater guide go deeper on the fuel decision for larger homes.
How to Size a Tankless Heater: The 3-Step Method
Sizing is arithmetic, not preference. You need the sum of simultaneous fixture flow, you need the temperature rise your water requires, and you need to match the two against a manufacturer flow chart. That is the whole method.
Step 1: Add simultaneous fixture flow. Count only the fixtures that can run at the same time. In one bathroom that is a low-flow showerhead at roughly 2.0 GPM plus a bathroom sink at about 1.0 GPM, giving you 3.0 GPM as a floor. Add the kitchen sink at 1.5 GPM if you expect the shower and the tap together, and you are at 4.5 GPM.
Step 2: Determine temperature rise. Measure your coldest incoming water temperature in winter, subtract your target delivery temperature of 120 degrees, and that difference is the temperature rise your unit must deliver. A 50 degree inlet and a 120 degree target means a 70 degree rise, which is where most mid-size electric units get derated hard.
Step 3: Match the chart. Every manufacturer publishes a flow chart showing gallons per minute at each temperature rise. Find your temperature rise on the horizontal axis, read up to your unit’s power input, and that intersection is your real GPM. If it is below your Step 1 total, step up a size or drop to a lower-flow showerhead.
Applied to a one-bathroom home with a low-flow showerhead, this method lands consistently between 3 and 5 GPM. A 9.5 GPM unit is enough, and then some, which is why the answer to whether a 9.5 GPM heater is necessary is generally no unless a tub or a second bathroom is joining the demand.
What Plumbers Actually Say About Tankless
Plumbers care far more about correct sizing and installer detail than about brand rankings. Across plumbing forums, the recurring advice is that you should pick the unit from gallons per minute and local incoming water temperature, not from a brand list, and that the plumber should size it for you rather than the other way around.
Professionals in those discussions also push back hard on consumer-selected units. The common position is that a licensed plumber should evaluate the panel, the venting and the gas line before an order is placed, because the model that is perfect on paper can be the wrong one for the space.
There is a fair criticism underneath the skepticism. A poorly sized tankless unit performs worse than the 40 gallon tank it replaced, and complaints about disappointing real-world performance after a retrofit are common on plumbing forums. Honest sizing is the whole defense against that.
So why do some plumbers hesitate? Mostly the install variables: side-wall venting in a tight closet, a panel that cannot supply the amperage, and condensing units that feel oversized for a single bathroom. None of those are brand problems, and all of them are solvable in a pre-install walkthrough.
Pre-Install Checklist: Panel, Gas Line, Venting and Permit
Before you order anything, walk this list with a plumber. It takes an hour and it prevents the two most expensive outcomes in a tankless purchase: an undersized unit and a failed install.
For electric, confirm the required breaker size and conductor gauge, then confirm your panel has a spare slot and enough total amperage. Check for a 100 amp minimum on the 11 kW models and a 150 amp or 200 amp service on the 18 kW and 28.8 kW units. A panel upgrade can cost more than the heater.
For gas, confirm the fuel type before ordering because natural gas and LP versions are not interchangeable, check the available BTU capacity, and plan the vent run. Note that freeze protection ratings vary from 14 degrees to 41 degrees depending on the model, and units rated for lower elevations are capped at 2,000 feet.
For either, ask about the permit, the condensate drain if you install a condensing unit indoors, and the placement distance. Descaling access matters too, because an element you cannot reach is an element you will pay a plumber to reach.
Point-of-Use vs Whole-Home for a Single Bathroom
A point-of-use unit sits near the fixtures it serves, and a whole-home unit sits centrally and feeds everything. For one bathroom, point-of-use placement is the stronger idea as long as the unit is genuinely close, because the reason you went tankless is to stop waiting for hot water.
The rule people get wrong is thinking a point-of-use unit can live across the house. The further it sits from the fixture, the more of the benefit you lose to the pipe run, since hot water cooling in the wall is a waste you pay for twice. Mount it in the bathroom, or in the wall cavity adjacent to it, and use the smallest unit that covers the flow.
A mini-tank remains the sensible alternative in two situations. If your incoming water is very cold and your budget cannot stretch to a large enough tankless unit, a point-of-use mini-tank stores heat and blunts the temperature problem. And if your panel cannot support any of the circuits above, a small gas or electric storage heater avoids the electrical conversation entirely.
For a bathroom in a small apartment, condo, RV or ADU, the sizing math is the same but the space math is tighter. Our tankless RV water heater picks cover the off-grid variants in more detail.
Descaling and the Maintenance Schedule That Extends Element Life
The single failure mode that brings tankless complaints home from the field is mineral buildup, and it is entirely a function of your water. Electric units with copper immersion elements are the most exposed, which is why the cast aluminum and separated-element designs in this roundup tend to hold up better in hard water.
A reasonable schedule is a vinegar flush every six months in a hard-water area and once a year in a soft-water area, following the manufacturer’s interval when they specify one. Descaling kits isolate the loop so you can flush the element without removing the unit, and the isolated-element designs we covered need it less often.
A water softener is the other half of the answer. Forum advice consistently treats a softener as the single most effective way to protect an element, and it also protects the pressure relief valve and the flow sensor that a scaled system will eventually choke on.
Recirculation and smart pre-heat solve a different problem, which is running out of hot water mid-shower. A recirculation loop keeps hot water moving through the pipe, and pre-heat scheduling has the unit warming up before the shower rather than during it. Neither is necessary for a correctly sized one-bathroom install, but both matter if you are in a cold climate with a marginal flow rating.
Frequently Asked Questions
What size tankless water heater do I need for one bathroom?
For one full bathroom, most households need a unit rated between 3 and 5 gallons per minute. A low-flow showerhead at roughly 2.0 GPM plus a bathroom sink at about 1.0 GPM sets a 3.0 GPM floor, and adding a kitchen sink running at the same time pushes the target to about 4.5 GPM. Confirm the rating at your local winter inlet temperature, because flow figures fall as the required temperature rise increases.
Is a 9.5 gpm tankless water heater enough?
A 9.5 GPM unit is more than enough for one bathroom, and most households buying one will never draw that much flow. That capacity is aimed at two or more bathrooms with a tub and multiple fixtures running together. In a single-bathroom home it mostly means paying for unused capability and, on the electric side, a heavier breaker requirement for output you will not use.
What do plumbers say about tankless water heaters?
Plumbers generally say sizing and installation matter more than brand. The recurring advice across plumbing forums is to choose from gallons per minute and local incoming water temperature, and to have a licensed plumber verify panel capacity, venting and gas line before an order is placed. Plumbers also push back on consumer-selected units when the install variables were never checked.
Why don’t plumbers like tankless water heaters?
The hesitation is usually about install variables rather than the equipment. Side-wall venting in a tight closet, an electrical panel that cannot supply the required amperage, and condensing units that feel oversized for one bathroom are the recurring complaints. Poorly sized units also perform worse than the tank they replaced, which is where most of the disappointing retrofit reports come from.
What is the most reliable tankless water heater brand?
Reliability is usually judged by warranty length on the heat exchanger, since that is the component that fails. The FOGATTI InstaGas Comfort carries a 5-year heat exchanger warranty in this roundup, while the Rheem RTEX-18 and the Stiebel Eltron Tempra 29 Plus carry longer overall coverage and much deeper owner feedback. Warranty length is a proxy, not a guarantee.
The Best Tankless Water Heater for One Bathroom in 2026
The Rheem RTEX-18 is our pick for the best tankless water heaters for one bathroom, because 4.4 GPM of 18 kW output covers the realistic one-bathroom demand and it carries the deepest owner feedback base in the group. If your panel cannot spare the amperage, the Rheem RTEX-13 at up to 3.17 GPM is the better buy for a single bathroom, and if you already have a gas line, the Ranein RH10K delivers 4.3 GPM without touching your panel.
Match the unit to your fixture count and your coldest incoming water, not to the biggest number on the box. Check the breaker requirement before you fall in love with a model, and that one hour of planning prevents the expensive mistakes.