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You stop paying to keep a tank of water warm all day when you switch to a tankless water heater. It burns gas or draws electricity only while a hot-water tap is open. Every pick below is a tankless, high-efficiency model rather than a tank that idles. Two numbers decide your bill: the flow rate in GPM (gallons per minute — how much hot water the unit can keep up with) and the efficiency rating (how much of the fuel becomes heat instead of waste). Buy too small and the temperature sags when two fixtures run at once. Buy the wrong fuel type and the unit will not connect to your gas line or panel.
I’m Mohammad Maruf — the founder and writer behind Gardening Beyond. This guide is built by comparing the manufacturers’ published specifications and the patterns across verified customer reviews, so you get each pick’s real strengths and trade-offs instead of marketing spin.
You are about to spend real money on a plumber and a wall-mounted appliance you will live with for years. So the guide below sorts the confusing numbers into plain language, shows which units suit which households, and flags the install traps before you buy. By the end you will know exactly which energy efficient water heater fits your home.
Our Picks at a Glance
How To Choose The Best Energy Efficient Water Heater
A tankless heater saves money by heating water on demand instead of keeping a tank warm all day. That is the efficiency you are paying for. The catch is that you have to match three things to your house or the savings never show up: the fuel you already have, the flow your household actually uses, and the space and venting your install allows. Get those right and a tankless unit runs for years with one annual flush. Get them wrong and you are re-plumbing a wall and re-venting the exhaust.
Match the fuel to your home first
Natural gas, propane, and electric units are not interchangeable. A natural gas heater needs a gas line, while a propane model needs a propane tank — and the two gas types often cannot be converted after the fact. If you are not sure what feeds your home, check the gas meter or panel label before you buy, because returning a gas appliance is painful. Electric models skip venting entirely, but they draw serious power and usually need a large electrical service panel.
Size the flow rate to your bathrooms
The flow rate in GPM is the number that decides whether your shower goes cold when the dishwasher kicks on. Two showers plus a sink generally need more flow than a single-bathroom home. If you size too small, the unit caps how much hot water it can make at once and the temperature sags. Size slightly larger than you think you need, especially in cold climates where 40°F incoming water takes more energy to heat.
Plan for venting and condensate
Gas tankless units need a vent, and some need a drain for condensate — the mildly acidic water a condensing unit produces as it squeezes extra heat out of the exhaust. Condensing models are the most efficient, but they add that drain line to your install. Non-condensing models skip the condensate but run a little less efficient. Factor the vent and drain into your plan before you commit to a model.
Quick Comparison
| Model | Best For | Flow Rate | Fuel Type | Efficiency | Amazon |
|---|---|---|---|---|---|
| Rinnai RU199iN★ Best Overall | Whole-home, 7 appliances | 11 gpm | Natural Gas | ENERGY STAR, UEF 0.105 | Amazon |
| APUS Propane TanklessBest Value | 2 showers plus a sink | 4.42 gpm | Propane | UEF 0.81 | Amazon |
| ThermoMate CZ422NGBest for Busy Homes | Two showers at once | 5 gpm | Natural Gas | UEF 0.81 | Amazon |
| Rheem RTEX-24 | No-vent electric installs | 5.9 gpm | Electric | 99.8% energy efficient | Amazon |
| CAMPLUX Natural Gas | 3-bathroom homes | 6.86 gpm | Natural Gas | CSA Certified | Amazon |
| FOGATTI Gas Propane | 3-5 outlet points | 7.5 gpm | Propane | CSA Certified | Amazon |
| Ranein Natural Gas | 4-6 outlet points | 8.5 gpm | Natural Gas | Energy saving | Amazon |
| Rheem RTG Series | 2-3 showers at once | 7 gpm | Natural Gas | UEF 0.82 | Amazon |
| Rinnai RUCS65IN | Up to 5 appliances | 6.5 gpm | Natural Gas | Highly efficient | Amazon |
| MIZUDO Natural Gas | 2-3 bathrooms, WiFi control | 10.3 gpm | Natural Gas | On-demand efficiency | Amazon |
| ThermoMate RTC68iN | Three showers at once | 9.38 gpm | Natural Gas | 0.91 UEF | Amazon |
In‑Depth Reviews
1. Rinnai RU199iN Condensing Tankless Hot Water Heater, 11 GPM, Natural Gas, Indoor
Our pick — over 4★ from 350+ verified ratings; the strongest balance of quality and price.
The whole-home powerhouse that feeds seven appliances from a single wall unit.
This is the pick that keeps up when the shower, the dishwasher, and the laundry all run at once. Its flow rate of 11 GPM (gallons per minute — how much hot water it can heat on the fly) is the highest of any unit here, which is why the maker says it can serve up to 7 appliances at once. It runs on natural gas and mounts indoors, so it needs a gas line and a vent rather than a propane tank or a big electrical circuit.
What wins people over is the performance and reliability — the two things reviewers bring up most, by a wide margin. Owners describe stable, endless hot water with no pressure drop when the bath, shower, and dishwasher all run together. It earned ENERGY STAR qualification with a Uniform Energy Factor of 0.105, which is the efficiency score that reflects how much gas becomes hot water. It also carries Circ-Logic for pairing with an external pump to cut the wait for hot water. On the durability front, it comes with a 15-year residential heat-exchanger warranty, so you are covered on the costly part.
Buyers report real-world savings that make the math plain: one noted the unit plus a labor-only install came in far under a full plumber quote, and others mention a lower natural gas bill and full rebate and tax-credit eligibility. Now the honest part — this is a heavy, professional-grade unit at 74.1 pounds, and it wants a qualified technician for install, not a weekend DIY. Cold-start can take 15 to 30 seconds longer in winter on the upper floors, and the heat output is the one area reviewers rate mixed, with some noticing small hot-cold blips.
Why it earns the top spot
- 11 GPM flow feeds up to 7 appliances at once — the highest flow here
- ENERGY STAR qualified with UEF 0.105, the documented efficiency score
- 15-year heat-exchanger warranty on the most expensive part
- Circ-Logic recirculation support trims the wait for hot water
What to weigh before buying
- 74.1 pounds and best installed by a qualified technician
- Cold-start can run 15-30 seconds longer on upper floors in winter
- Heat output is the mixed-feedback area, with occasional hot-cold blips
Buy it if: you want the highest documented flow here and endless hot water for a full house.
skip it if: you are set on a hands-off plug-and-play install rather than a licensed gas hookup.
2. APUS Propane Tankless Water Heater, 95,000 BTU, 4.42 GPM, Indoor
A propane unit light enough to hang solo that still runs two showers and a sink.
At 19.2 pounds, this is the featherweight of the group — a full 74.1 pounds lighter than the Rinnai RU199iN up top. That gap matters when you are lifting a heater onto a wall bracket in a tight closet. It runs on propane and delivers 95,000 BTU (British thermal units — a measure of heating power) with a 4.42 GPM flow rate, enough per the maker for two showers plus one sink at the same time. The 4.42 GPM also trails the Rinnai’s 11 GPM, so this is a two-bathroom unit, not a whole-estate one.
Efficiency is the headline for a value pick: it reaches UEF 0.81, the same efficiency score as the ThermoMate CZ422NG further down, and the maker states it uses about 30% less energy than a conventional tank model. It holds water within ±1°F of your setting, adjustable from 95°F to 140°F, so the shower does not lurch hot and cold. It also runs quiet at 36dB — roughly a library whisper — and fires up on just 0.66 GPM of low flow, which means a single faucet can trigger it. A built-in freeze protection kicks in as temperatures fall toward 41°F.
Owners are happiest with reliability and performance, the two most-mentioned areas, and the support team gets steady praise for troubleshooting and sending replacements fast. One concrete scene: owners mention hot water arriving in less than 20 seconds, and one coffee-trailer owner found it easily met the trailer’s water needs. The catch is the heat output, which reviewers rate mixed — some say it heats fast, a few find it only reaches lukewarm — and it is propane-only, so it will not run off a natural gas line.
The efficiency read: UEF 0.81 matches the ThermoMate CZ422NG exactly, so neither wins on the score — you pick between propane here and natural gas there.
Reach for this if: your home runs on propane and you want a light, quiet two-bathroom unit with strong support behind it.
Look elsewhere if: you need whole-home flow or your house is plumbed for natural gas.
3. ThermoMate CZ422NG, Natural Gas Tankless Water Heater Indoor, Up to 5 GPM
A natural gas unit built to shrug off hurricane-force wind and hold its temperature.
This one is for the homeowner in an exposed or storm-prone area who wants hot water that does not flicker when the weather turns. Its DC high-speed fan pushes 360Pa of static pressure at 3000 RPM, which the maker says lets it stand up to hurricane winds of Category 3 and above. The flow rate is 5 GPM with 113,000 BTU of heating power, sized for two showers running at the same time, and the control chip re-adjusts the burn 20,000 times a second to hold temperature within ±1°F.
It is built to save on running costs, too: an oxygen-free copper heat exchanger (a copper part that moves heat without rusting) delivers up to 98% thermal efficiency with a UEF of 0.81 (Uniform Energy Factor — a score for how well the unit turns fuel into heat), and the maker states it can cut annual operating costs by roughly 30% versus a traditional heater. The included 3.5-inch LED display lets you set 90°F to 140°F, and the freeze protection auto-starts at 41°F. Installation is made simpler by standard 3/4″ NPT water connections (a common threaded pipe size) and a UL-approved 120V plug (a standard safety-tested plug).
Performance and installation are what buyers like most. Owners describe it heating well water from 55°F up to 130°F and running dishes, laundry, and a shower all at once without trouble. The honest trade-off is the temperature control, which reviewers rate mixed — a few report it not holding steady, with one saying the water swings between barely warm and nearly too hot. It also needs a Class 3 vent, and the proper adapter is not sold with it, so plan that piece before install day.
The wind-safety angle: that 360Pa fan rating (a measure of how hard the fan can push against wind) is the only hurricane-wind spec on this list, which separates it for coastal and open-exposure homes.
The install watch-out: no matching vent or adapter ships in the box, so budget for that part or you will be improvising one.
Best for: a two-bathroom home in a wet or windy region that wants steady temperature and lower bills.
Think twice if: you want a self-contained vent kit in the box or you live in deep-freeze country.
4. Rheem RTEX-24 24kW 240V Electric Tankless Water Heater, Gray
The no-gas, no-vent pick that frees up a whole corner of your utility room.
If running a gas line or a vent through your wall is a deal-breaker, this electric unit sidesteps both. It needs no venting at all, which is why it suits a summer cottage, a garage, or any spot where a flue is impossible. It delivers a 5.9 GPM flow rate from 24KW of heating and wears a 99.8% energy-efficient rating, so nearly all the electricity it draws turns into heat rather than waste. It also takes up 90% less floor space than a traditional tank, per the maker.
The electrical requirements are the big thing to check first, because this is where most surprises live. It needs 240V single-phase power, three 40-amp double-pole breakers, three sets of #8 AWG copper conductors with ground, and a recommended minimum 200-amp household service panel. If your panel is smaller than that, plan for an upgrade. You also get digital thermostat control in 1-degree steps from 80°F to 140°F.
Performance is the runaway favorite among owners. It handles two showers plus laundry, and one long-term owner found the annual descale-and-clean easy while gaining a large chunk of utility-room space back. Set realistic expectations on temperature: reviewers rate heat output mixed, and the far shower can run 2-4°F cooler. It is also a fit for warm groundwater, not cold climates — in cooler regions the incoming water is too cold for it to reach your target.
The wiring reality: three 40-amp breakers and a 200-amp panel is the make-or-break spec. Confirm it with an electrician before you order, or the unit sits in the box.
Get this if: you have no gas line, want zero venting, and are installing in a warm region with a strong electrical panel.
Pass if: your home has a small service panel or your winter groundwater runs cold.
5. CAMPLUX Tankless Water Heater Natural Gas Indoor, Up to 7.6 GPM 199,000 BTU
A near-silent natural gas unit that swaps a leaking tank for a wall-mounted box.
Replacing a worn-out 50-gallon tank heater with this means gaining back your garage or basement corner. It runs on natural gas at up to 199,000 BTU with a 7.6 GPM flow rate, and the maker says that serves 6 points at once. It also operates at 45dB — quieter than a refrigerator hum — thanks to a bottom-fan design with dual undercarriage blowers that keeps exhaust from backflowing. For cold snaps, it carries -20°F freeze protection with an automatic drain assist.
It is built to be easy to connect: 3/4″ NPT connections (a common threaded pipe size), a 120V UL plug (a standard safety-tested plug), and a 2.95″ universal vent pipe streamline the install, and the maker states the unit stays within a 130°F to 164°F output window and fires up about 80% faster than standard models. The oxygen-free copper heat exchanger (a copper part that moves heat without rusting) handles the heat transfer. There is one maintenance rule the maker stresses: descale it annually (flush out mineral buildup), and winterize it below 20°F. Altitude compensation runs up to 2,000 feet.
Owners bring up quality and functionality most, and the tech support gets repeated mentions for solving problems fast. One owner had a dead-on-arrival unit that shipped back as a working replacement after photos and videos with support; another swapped a leaking 50-gallon tank and found it fit a 3-bath home with a touchscreen on the white glass front. The one nitpick to know: the mounting brackets are too narrow to reach two studs, so many mount it on 2x4s instead.
The quiet-and-tidy case: that sealed, bottom-fan design is what keeps it at 45dB and keeps bugs out — a real plus in warm, humid regions.
Right for you if: you want a whisper-quiet gas unit with strong freeze protection and easy 3/4″ connections.
Not for you if: you dislike adding a descale to your annual to-do list or need to span two studs with the included bracket.
6. FOGATTI Tankless Water Heater Gas Propane Indoor On Demand CSA Certified
A propane unit that runs on weak water pressure and serves 3-5 outlets at once.
For a multitasking household on well water or an older low-pressure system, this is the fit. It works with water pressure as low as 15 PSI and as high as 150 PSI, so a weak supply will not stall it. It runs on propane at 170,000 BTU with a 7.5 GPM flow rate and, per the maker, handles 3 to 5 outlet points. It also fits in about a third of the space of a traditional tank heater.
Temperature control is the engineering story: water servo technology and sensors adjust flow and gas together to hold within ±1°F, backed by a four-stage combustion system for steady comfort summer and winter. It is CSA certified and built with a copper heat exchanger and a 304 stainless steel burner. Coverage runs 2 years on the whole unit and 5 years on the heat exchanger, and it powers off a standard 120V AC connection.
Installation is the most-praised area — owners say it goes in cleanly, and one added a whole-house filter while doing it. But be an informed buyer here: functionality and value draw mixed feedback, with some units arriving non-operational and a couple of owners reporting early failures they could not get past. That said, several of those same stories end in replacement units or parts from support that got them running. It is propane-only and needs a filter or softener, and the install is more involved than swapping a tank.
Pressure flexibility: the 15-150 PSI window is the widest tolerance here, which is the whole reason to consider it on an older plumbing system.
The reliability caveat: most owners are happy, but a small number hit early failures — keep your receipt and lean on the 2-year coverage.
Strong choice if: you are on propane with low water pressure and want adjustable ±1°F control for a 3-5 outlet home.
Consider alternatives if: you want the simplest possible reliability record or run on natural gas.
7. Ranein Natural Gas Tankless Water Heater, 8.5 GPM, 190,000 BTU, Indoor
A high-flow natural gas unit aimed squarely at a big family with many faucets.
If your house has four to six hot-water points fighting for supply, this 8.5 GPM unit with 190,000 BTU of heating power is sized for you — the maker rates it for 4-6 outlet points. It is one of the higher-flow picks here, well above the 5 GPM of the ThermoMate CZ422NG, so it takes more simultaneous demand in stride. Its exhaust fans sit underneath the unit, which the maker says prevents smoke backflow and provides steady combustion.
The blower auto-adjusts its speed to match outside wind pressure and water-temperature changes, keeping the burn clean and the noise low. It uses standard 3/4″ NPT water inlets for a quick hookup and mounts on a wall in a compact footprint. On the service side, it carries a 2-year warranty, and the maker notes it is not recommended for areas above 2,000 feet or for long stretches of winter non-use without draining the relief valve.
Owners find it reliable and easy to install, and it saves on the electric bill while heating fast — several report it responding quickly to small flow changes. One family with three adults and five kids ran it hard for about a year and came through winter with no breaks, and a converted-from-electric owner saw the bill drop by a meaningful monthly amount. The one consistent knock is support: if a part fails out of warranty, customers note it is hard to reach anyone to get a replacement.
The flow match: 8.5 GPM is enough for multiple simultaneous draws — the right size for a 4-6 outlet home that used to run a tank.
Suits you if: you have a large family, several hot-water points, and want high flow for the money.
pass on it if: you want an easy path to out-of-warranty parts, or live above 2,000 feet.
8. Rheem RTG Series 7.0 GPM Natural Gas Tankless Water Heater Indoor, 160,000 BTUs
A mid-flow natural gas unit that watches its own pipes for leaks.
Rheem built this one for households that worry about water damage as much as efficiency. It carries LeakGuard leak detection and prevention, which the maker says detects any internal leak, large or small, and protects your home. That is a feature unique to this pick. It runs at 7 GPM with 160,000 BTU/hr, giving 7.0 GPM at a 35°F temperature rise for 2-3 simultaneous showers, per the maker.
Efficiency lands it here: a UEF of 0.82 is the highest efficiency score in this article, just above the 0.81 shared by the APUS and the ThermoMate CZ422NG. The maker states it can save up to 34% on energy costs versus a 50-gallon electric tank heater — that comparison is the maker’s own figure. It is non-condensing, so it skips the acidic condensate drain a condensing unit needs. You also get a digital LED panel with ±1°F adjustments plus freeze and flame-failure protection.
Owners call it effective and well made; one ran it for about a year and a half in a hard-water area, flushing it twice a year with distilled white vinegar and finding it far more economical than the old electric unit, able to run shower, laundry, and dishwasher at once. It backs that up with a 15-year heat-exchanger warranty. The buyer’s homework here is flow: if you have multiple bathrooms, size up so you are not caught short, and plan for new venting since this is a dedicated indoor direct-vent install.
The efficiency edge: UEF 0.82 edges out the 0.81 units here, and the LeakGuard system adds internal-leak detection no other pick lists.
Best for: a family of four or a multi-bath home that wants top-efficiency gas heat plus built-in leak detection.
Look elsewhere if: you need more than 2-3 showers at once or want a unit that ships with its own venting.
9. Rinnai RUCS65IN Tankless Hot Water Heater, Natural Gas
A natural gas Rinnai for tight spaces that need flexible venting options.
This Rinnai is the one to consider when where you mount things decides the install. It offers multiple venting options, including running PVC/CPVC pipe with an adaptor, so you can route the exhaust where the room allows. It runs on natural gas at 130,000 BTU with a 6.5 GPM flow rate that the maker says serves up to 5 appliances at once. It is a smaller, more compact Rinnai than the 11 GPM RU199iN up top, aimed at a mid-size home rather than an estate.
The efficiency and space angle is the same family idea: it heats only when needed and mounts compactly to free floor space. Build quality is the standout — this is a refined design, and parts are individually replaceable, which the maker side touts for long life. It comes with a 12-year heat-exchanger, 5-year parts, and 1-year labor residential warranty. Rinnai also strongly recommends a qualified technician, since venting, gas, and electricity are all easy to get wrong.
Quality and installation are what buyers rate highest. One long-time buyer notes PVC exhaust that stays cool to the touch and precise 120°F output that auto-adjusts to flow, and a repeat owner installs them across multiple properties and finds them reliable once complete. The honest catch shows up with low-flow fixtures: one owner found that nudging the thermostat above the default cut the kitchen tap’s hot water off entirely. Plan the valve kit and brass hardware into your budget, because they are not included.
The space-and-venting play: PVC/CPVC venting plus a compact body is why it fits rooms where a rigid metal flue would never work.
The low-flow note: if your kitchen tap runs weak, check the minimum activation flow before you commit.
Pick this if: you need flexible vent routing for up to 5 appliances and value a refined, repairable build.
Step back if: your home has very low-flow faucets or you want every fitting included in the box.
10. MIZUDO Natural Gas Tankless Water Heater, 199,000 BTU, Indoor
High-flow gas heat you can check and adjust from your phone.
If you like seeing and steering your home systems from an app, this is the pick. Beyond its LED display, you manage it remotely — checking operating status, fine-tuning temperature, and getting instant alerts — and it works with Amazon Alexa for hands-free control. Under the hood it delivers 199,000 BTU of heating input and up to 10.3 GPM, sized per the maker for 2-3 bathrooms plus kitchen, sinks, laundry, and dishwasher. That flow is second only to the 11 GPM Rinnai up top.
Temperature precision is a strong suit: advanced gas modulation holds water within ±1°F, with a default 107°F everyday setting and an adjustable range up to 149°F, which the maker points to for deep soaking tubs and heavy laundry. It fires on a low 0.53 GPM of flow, so a single faucet activates it, and it heats only when needed to cut standby loss. The sealed concentric direct-vent system pulls outdoor air for combustion and uses a 3″ × 5″ vent kit, keeping backdraft out in windy weather. It is CSA certified and takes a standard 120V supply with 3/4″ NPT connections.
Owners rate quality and installation highest, and they like that it heats in seconds and fits a tiny home or a 400-square-foot cabin with room to spare. One owner ran it heavily for a month — two ten-minute showers a day at max temp — and found it very efficient. Set expectations on output, though: heat output draws mixed feedback, with some reporting it runs 60-90 seconds of hot water before turning cold, and it is not recommended above 3,000 feet.
The connected-home angle: app control plus Alexa is the one setup here you can run from your phone, which is the whole draw of this model.
Right for you if: you want the highest non-commercial flow with app and Alexa control for a small-to-mid home.
Reconsider if: you live above 3,000 feet or you have seen enough reports of short hot-water windows to want a simpler unit.
11. ThermoMate RTC68iN, 0.91 UEF Condensing Tankless Water Heater
The highest efficiency score in this article, built to recycle its own waste heat.
This is the most efficient unit on the list, and it earns that with a second heat exchanger. Its 0.91 UEF is the top efficiency score here, well above the 0.81 shared by the APUS and the ThermoMate CZ422NG. That rating comes from a dual heat-exchange system — a stainless steel condensing exchanger plus an oxygen-free copper one — that captures and reuses waste heat traditional models simply lose. The maker states it cuts annual hot water costs by about 17% versus traditional models.
Flow and capacity suit a busy household: it can supply three showers at once, each delivering 105°F at 2 GPM (gallons per minute — how much hot water it can heat on the fly), based on a 62°F inlet, and in warm climates with an inlet above 77°F it can hit up to 9.38 GPM. A smart water mixing valve regulates flow to hold a stable output with no sudden swings, and the sealed combustion design keeps it quiet. It is CSA certified (a safety standard) with 8 built-in protections, including boil-dry, freeze, flame-out, and overheat, and frost protection kicks in at 41°F.
Installation is DIY-friendly for the basic mount, and it comes with the wall basket, anchors, and vent-pipe sealant you need. The one thing to plan for is the trade-off of a condensing unit: it produces mildly acidic condensate that must drain away, and the maker recommends adding an external neutralizer kit, which is not included. Routing that condensate to PVC or PEX rather than copper protects your pipes. This is also the newest, least-reviewed model here, so treat the efficiency as a spec on paper.
The efficiency headline: 0.91 UEF is the highest score in this article, and the dual heat exchanger is the mechanism behind it.
The condensing trade-off: mildly acidic condensate must be drained, and the neutralizer kit is a separate purchase.
Go for this if: you chase the best efficiency score, want three simultaneous showers, and can route a condensate drain.
Hold off if: you cannot drain condensate near the unit or you prefer a model with a longer track record.
Understanding the Specs
Flow Rate (GPM)
GPM stands for gallons per minute — it is how much hot water the heater can produce while the tap is running. This number, not the tank size, decides whether two showers can run at once. A higher GPM feeds more faucets without the temperature sagging. Match your GPM to the number of hot-water points you run together, and size up in cold climates where incoming water is colder and takes more energy to heat.
Efficiency (UEF)
UEF, or Uniform Energy Factor, is the efficiency score that shows how much of the fuel becomes hot water instead of waste. Higher is better. On this list the scores you will see stretch from 0.81 up to 0.91. Condensing models reclaim waste heat with a second heat exchanger and tend to score higher, but they add a condensate drain to the install. A high UEF is what turns a tankless unit’s on-demand heating into real savings on your monthly bill.
FAQ
How many GPM do I need for a family of four?
Can a natural gas tankless water heater run on propane?
Do tankless water heaters need a vent?
What is a good UEF rating for a tankless water heater?
Will a tankless water heater save me money on my gas bill?
How much electrical service does an electric tankless water heater need?
Do tankless water heaters freeze in winter?
How often should I flush a tankless water heater?
Can I install a tankless water heater myself?
What is the difference between condensing and non-condensing tankless?
Does a tankless water heater work with low water pressure?
Can I switch from a tank water heater to a tankless one?
Final Thoughts: The Verdict
For most households, the energy efficient water heater winner is the Rinnai RU199iN, because its 11 GPM flow is the highest here and it carries ENERGY STAR qualification with a 15-year heat-exchanger warranty. It is the safe pick if your home runs on natural gas and you want endless hot water for the whole house. If efficiency is your reason for buying, the ThermoMate RTC68iN tops this article at 0.91 UEF with its dual heat exchanger — just be ready to route a condensate drain. And if you want app control on a high-flow gas unit, the MIZUDO at 10.3 GPM with WiFi and Alexa is the one to reach for.
How We Picked
We do not accept paid placement. Every pick is matched to a real buyer and a real use-case; we do not hands-on test units.
Sources & Methodology
Specifications: manufacturer listings and product documentation. Review insights: verified customer reviews, as of September 2026. Pricing: not shown on this page (it changes often); check the current price via the retailer link.
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