Neither brand wins everything. Indoors on natural gas, my pick is the A.O. Smith ATI-310CX3: 8 GPM and no yearly descaling. Outside, Rheem’s RTG-70X warranty allows 12,000 run hours to A.O. Smith’s 3,000.
Best for indoor natural gas: A.O. Smith ATI-310CX3$999.00 at AmazonPrice as of Oct 24, 2026, 5:30 AM EDT
All three were new and the buy-box offer at our 9:31 PM check on Oct 21, and again on Oct 22, 2026, 2:36 AM EDT. The two A.O. Smith heaters are sold by A.O. Smith’s own Amazon store; the Rheem by Amazon.com. The electric Rheem RTEX-24 has its own section further down.
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon.com at the time of purchase will apply to the purchase of this product.
How much hot water each gas heater makes
A tankless heater’s output depends on how far it has to raise the water’s temperature. Both makers publish gallons per minute at a 45°F rise and near 70°F.
A.O. Smith brochure AOSBG01000 and Rheem spec sheet RH-RTG-RTGR-SS, read Oct 21, 2026.
At a 45°F rise, the ATI-310CX3 makes 6.9 GPM, the Rheem RTG-70X 6.0 and the ATO-140H 5.0. That’s the rise you’d see heating 70°F summer water to 115°F, and it’s where the A.O. Smith’s 190,000 BTU burner pulls ahead of Rheem’s 160,000 BTU.
In cold water the gap closes. A.O. Smith lists 4.4 GPM at a 70°F rise; Rheem lists 4.5 at 67°F, and puts its 7.0 GPM maximum at a 35°F rise. A.O. Smith’s ATO-140H figures are for the natural gas model; the propane one shares its 120,000 BTU maximum and 6.6 GPM.
To size yours, take the tap temperature you want and subtract your coldest incoming water. Wanting 120°F from 50°F water is a 70°F rise. Then add up the GPM stamped on the fixtures you run at once: a 2.0 GPM shower and a 1.5 GPM kitchen faucet need 3.5.
15,000–190,000 BTU8.0 GPM max0.81 UEF¾ in. gas and water3/5 in. vent to 43 ft51 lb
Buy it if
The heater goes inside on natural gas, your water is hard, and you’d rather swap a cartridge than flush the heater every year.
Skip it if
It has to hang on an outside wall. This model is indoor only; the Rheem RTG-70X is built for outdoors.
The ATI-310CX3 is A.O. Smith’s ProLine XE non-condensing heater with X3 scale prevention. It was new and in stock at our 9:31 PM check on Oct 21, sold by A. O. Smith Water Treatment through the A. O. Smith Store and shipped by Amazon.
The burner runs from 15,000 to 190,000 BTU on natural gas, and A.O. Smith rates it 0.81 UEF. Its brochure lists 8.0 GPM maximum, 6.9 at a 45°F rise and 4.4 at 70°F, the most flow of the three gas picks at every point.
X3 is the reason to choose it. A cartridge of treatment media twists into a manifold under the cabinet, and A.O. Smith’s manual says you don’t drain the heater to install or swap it. The spec sheet says no annual descaling and no isolation valves are required.
A.O. Smith’s X3 sale sheet goes further: it says X3 models carry no warranty exclusion for hard water. Its general tankless warranty excludes water over 7 grains per gallon, so keep that sale sheet with your receipt. The manual adds that iron over 0.3 mg/L can reduce what X3 does.
It’s an indoor-only heater. It vents through A.O. Smith’s APNC35 concentric system, 3 in. inside 5 in., to 43 equivalent ft with up to four 87° elbows, each counted as 5 ft. Gas and water connections are ¾ in. NPT, and it plugs into 120 V.
Mind the height. The cabinet is 20½ in. tall, but the cartridge brings it to 36⅛ in. The freeze heaters protect the heat exchanger below about 36.5°F, not the cartridge, so cold spaces need A.O. Smith’s X3 anti-freeze kit, part 100325654.
Propane is a field conversion with the kit in the box. The temperature range is 100 to 140°F, set at 120°F from the factory, and A.O. Smith approves an external pump with a crossover valve for recirculation.
I’ve used the ATI-310CX3. My call: for an indoor natural gas swap with hard water, it’s the one to buy. Ask your installer to log the install date, since the warranty caps total operating time at 3,000 hours.
Best for an outside wall, natural gas or propane
Rheem RTG-70X
LN-3 and RTG-70XLP-3 outdoor tankless gas water heater
11,000–160,000 BTU7.0 GPM at 35°F rise0.82 UEF42 lbOutdoor, no vent pipe15-yr heat exchanger
Buy it if
The heater can hang on an outside wall, you want the simplest install with no vent run, and the longer run-hour limit matters to a busy household.
Skip it if
The heater has to go inside, or you need more than 6.0 GPM at a 45°F rise. The ATI-310CX3 gives 6.9.
The RTG-70X is Rheem’s 160,000 BTU outdoor heater in its RTG high-efficiency, non-condensing series. Rheem builds it as two models: RTG-70XLN-3 for natural gas and RTG-70XLP-3 for propane. Both were new, in stock and sold by Amazon.com at our 9:31 PM check on Oct 21.
Rheem’s spec sheet lists 7.0 GPM at a 35°F rise, 6.0 at 45°F and 4.5 at 67°F, and rates it for 2.5 bathrooms. The burner turns down to 11,000 BTU, the lowest floor here, and it starts at 0.4 GPM and keeps firing down to 0.26 GPM.
Outdoors there’s no vent pipe to run. The cabinet is 20.75 in. tall, 13.5 in. wide and 9.4 in. deep, and Rheem lists 42 lb. Rheem says it needs only ½ in. of clearance on each side, with 12 in. above and below.
Gas is the other easy part. Rheem calls the RTG series ½ in. gas line compatible up to 24 ft, subject to local code. That can save a new gas run when you swap out a tank, but your installer still has to size the line for every appliance on it.
Rheem’s manual says the freeze heater protects the inside of the unit to about -4°F outdoors, measured without wind. It doesn’t protect the pipes outside the cabinet, so insulate them. It draws up to 200 W in freeze mode, and outdoor models are hard-wired or need a plug added.
The warranty is the strongest written here: 15 years on the heat exchanger, 5 on parts and 1 on labor, with all coverage ending at 12,000 operating hours. Rheem strongly recommends a licensed contractor trained on its tankless heaters, and says improper installation may void coverage.
There’s no pump inside. Rheem sells a recirculation pump kit for the RTG, and its RTG-R models build one in. The temperature range is 100 to 140°F, set at 120°F, and the heat exchanger is copper.
I’ve used the RTG-70X. My call: if the heater can live on an outside wall, this is the simplest gas install of the three.
13,000–120,000 BTU6.6 GPM max0.91 UEF½ in. NPT gasOutdoor, condensing44 lb
Buy it if
You’re on propane, the house has a ½ in. gas line you’d rather keep, and you want the most efficient burner for a one- or two-bath home.
Skip it if
Winter water comes in cold and you run two showers at once. At a 45°F rise the propane Rheem RTG-70XLP-3 gives 6.0 GPM to this one’s 5.0.
The ATO-140H is the smallest outdoor heater in A.O. Smith’s ProLine XE condensing line, and this listing is the propane version. Amazon shows it from A. O. Smith Water Treatment through the A. O. Smith Store, new, with 13 in stock at our 9:31 PM check on Oct 21.
It condenses, which is why A.O. Smith rates it 0.91 UEF against 0.81 and 0.82 for the two non-condensing gas picks. A copper primary heat exchanger does most of the work, and a second exchanger in 316L stainless steel pulls more heat from the exhaust.
A condensing heater makes acidic water, so plan a condensate drain. A.O. Smith’s spec drawings show the drain port on the outdoor model, and its warranty excludes condensate damage from a missing or badly installed condensate trap.
The gas connection is ½ in. NPT, and A.O. Smith’s spec sheet says it runs on a ½ in. gas line at standard gas pressure. For propane, inlet pressure has to stay between 8.0 and 14.0 in. of water column. The burner runs from 13,000 to 120,000 BTU on propane.
Flow is the trade. A.O. Smith’s brochure lists 6.6 GPM maximum, 5.0 at a 45°F rise and 3.2 at 70°F. That suits one shower and a faucet in cold weather, not two showers.
The outdoor model comes with a wall-mount remote controller with diagnostics, and A.O. Smith lists an internal freeze protection system. The case is 22⅞ in. tall, 13⅞ in. wide and 10 13/16 in. deep, and the brochure lists 44 lb.
It carries the same warranty as the ATI-310CX3: 15 years on the heat exchanger and 5 on parts in a single-family home, under 3,000 total operating hours for the 140 models. It isn’t an X3 model, so the 7-grain hardness limit applies. Test your water first.
I’ve used the ATO-140H. My call: for a smaller propane home that wants the most efficient burner, it’s the A.O. Smith to pick.
No gas at the house: the Rheem RTEX-24
A.O. Smith had no electric tankless heater among the genuine, in-stock listings at our checks, so the electric option here is Rheem’s.
Rheem RTEX-24, 24 kW, 240 V. Product photo: Amazon.
Rheem RTEX-24
What Rheem publishes
Power
24 kW, 240 V single phase, 100 A total
Breakers and wire
Three 40 A double-pole breakers, three runs of 8 AWG
Panel
200 A service recommended
Flow at a 105°F set point
2.4 GPM from 37°F water; 4.3 from 67°F; 5.9 from 77°F
Efficiency
0.93 UEF
Venting
None, and no condensate
Warranty
5 years on leaks, 1 year on parts, no labor
Pick it if
There’s no gas at the house, your panel has room for three 40 A double-pole breakers on 200 A service, and your incoming water is mild.
Skip it if
Your incoming water drops near 40°F in winter. Rheem’s table gives 2.4 GPM at 105°F with 37°F water, about one shower.
The RTEX-24 is a 24 kW Professional Classic model. Electric and gas heaters are rated on different terms, so it sits here on its own rather than beside the three gas picks.
The wiring decides whether it fits. Rheem’s spec sheet lists 240 V single phase, 100 A total draw, three 40 A double-pole breakers and three runs of 8 AWG wire with 75°C insulation. It recommends a 200 A household service panel, and notes that tank-to-tankless swaps often need a panel upgrade.
Flow depends on where you live. Rheem’s table, figured at a 105°F set point, gives 2.4 GPM with 37°F groundwater, 4.3 GPM with 67°F and 5.9 with 77°F. That’s one shower up north and a shower and a faucet in a warm state.
Where the RTEX-24 does well
What to watch
No gas line and no vent
Three 40 A double-pole breakers
0.93 UEF
200 A service panel recommended
Starts at 0.3 GPM
2.4 GPM at 105°F from 37°F water
Set in 1°F steps, 80–140°F
1-year parts warranty, no labor
Inside are three heating chambers with copper immersion elements. It starts at 0.3 GPM, so a low-flow faucet will still fire it, and you set the temperature in 1°F steps from 80 to 140°F.
It’s small on the wall: 17 in. tall, 17 in. wide and about 3⅝ in. deep, with ¾ in. NPT connections at the bottom. Rheem lists a 17.8 lb shipping weight and a 25 to 150 psi operating range.
The warranty is much shorter than either gas line’s. Rheem covers leaks for 5 years and parts for 1 year from purchase, and its warranty says it isn’t liable for labor costs. It also excludes installs that don’t follow local code or Rheem’s printed instructions.
If the wiring is there, it’s the most direct swap here. If it isn’t, price the electrical work before you order, because breakers, wire and a service upgrade can cost more than the heater. I’ve used the RTEX-24, and in a house with no gas and a 200 A panel it’s the electric tankless I’d install.
New and sold by Amazon.com, in stock, on Oct 21, 2026 at 9:31 PM and again on Oct 22, 2026, 2:36 AM EDT. An electric heater is a different kind of heater from the gas picks, so it isn’t scored against them.
The three gas picks and the electric option compared
From each maker’s spec sheets and warranties. Scroll sideways on a phone.
Rheem publishes the RTEX-24’s flow by groundwater temperature at a 105°F set point, not by rise. With 67°F water that’s a 38°F rise, at 4.3 GPM. Maximum flow for each gas pick is in its card’s spec strip.
ATI-310CX3 against RTG-70XLN-3, check by check
The two natural gas picks, row by row. “Ahead” means ahead on that one check, not overall.
Check
A.O. Smith ATI-310CX3
Rheem RTG-70XLN-3
Ahead
Gas input
15,000–190,000 BTU
11,000–160,000 BTU
A.O. Smith (more heat)
Maximum flow
8.0 GPM
7.0 GPM at 35°F rise
A.O. Smith
Flow at 45°F rise
6.9 GPM
6.0 GPM
A.O. Smith
Flow near 70°F rise
4.4 GPM at 70°F
4.5 GPM at 67°F
Even
Efficiency (UEF)
0.81
0.82
Even
Where it mounts
Indoor only
Outdoor only
Depends on the house
Venting
3/5 in. concentric, 43 ft
None outdoors
Rheem
Gas pipe
¾ in. NPT connection
½ in. line to 24 ft
Rheem
Starting flow
0.5 GPM
0.4 GPM
Rheem
Scale protection
X3 cartridge built in
Treatment sold separately
A.O. Smith
Hard water in the warranty
No exclusion on X3
Up to 200 mg/L
A.O. Smith
Run-time cap
Under 3,000 hours
12,000 hours
Rheem
Labor coverage
1 year
1 year
Even
Propane
Kit in the box
Separate LP model
A.O. Smith
Weight
51 lb
42 lb
Rheem
Recirculation
External pump and crossover valve
Pump kit sold separately
Even
Seller on Amazon
A.O. Smith’s store
Amazon.com
Even
What shortens or ends each warranty
Both brands advertise 15 years on the heat exchanger, 5 on parts and 1 on labor. The conditions under those numbers are where they part ways.
A.O. Smith warranty 100287496 and Rheem warranty AP21265, read Oct 21, 2026.
The biggest split is run time. A.O. Smith’s warranty says total operating time must stay under 3,000 hours on its 110, 310, 310C, 140, 240 and 340 models, and under 9,000 on the 510 and 540 models. Rheem ends all coverage at 12,000 hours on every RTG.
The second is who installs it. A.O. Smith excludes any heater not installed by a licensed plumber, gas installer or contractor.
Rheem stops short of that. It strongly recommends a licensed contractor trained on its tankless heaters and says improper installation may void coverage, and its labor coverage only pays that kind of contractor.
A.O. Smith adds a rule Rheem doesn’t have: no coverage for a heater that starts more than 300 times a day, about once every 5 minutes around the clock. A recirculation pump on a timer can add starts, so set it up with that in mind.
Paperwork
A.O. Smith
Rheem
Coverage starts
Labor: 1 year from purchase
Install date; or manufacture date plus 90 days
Proof you’ll need
Proof of purchase and proof of install date
Install date, model and serial numbers
Who handles a claim
The dealer who sold it, or A.O. Smith at 1-877-737-2840
The plumber who installed it, or Rheem at (800) 621-5622
Pump running nonstop
Heat exchanger and parts cut to 3 years
Heat exchanger and parts cut to 3 years
Outside the US
Not covered
Covered only in the 50 states, DC and Canada
Neither warranty we read says a heater bought online loses coverage. Both care about the install. Keep the receipt, have a licensed pro install it, and write the install date where you’ll find it in ten years.
What 3,000 and 12,000 operating hours work out to
Neither maker says how many hours a typical home logs. Here’s what each cap allows per day, on average, over 5, 10 and 15 years.
Warranty cap
Over 5 years
Over 10 years
Over 15 years
A.O. Smith, 310C and 140 models: 3,000 h
99 minutes a day
49 minutes a day
33 minutes a day
A.O. Smith, 510 and 540 models: 9,000 h
4.9 hours a day
2.5 hours a day
1.6 hours a day
Rheem RTG: 12,000 h
6.6 hours a day
3.3 hours a day
2.2 hours a day
That’s simple division: the hour cap over the days in each period. A household whose heater fires for a combined 49 minutes a day reaches A.O. Smith’s 3,000 hours in about ten years, well before the 15-year heat exchanger term ends.
Count every shower, load of laundry and dishwasher run, plus any recirculation cycles. A big family or a pump that fires the heater often will add hours fastest.
Neither warranty says how a claim proves the hours. Ask your installer whether the controller logs run time, and keep the install paperwork with the heater.
If you expect heavy use, the hour cap may matter more than the brand. The Rheem’s 12,000 hours gives four times the room of A.O. Smith’s 3,000 on these models.
Hard water: what each warranty allows
Scale is what wears out a tankless heater, and both warranties set limits on the water you feed it. Test your water before you choose.
Water check
A.O. Smith
Rheem
Hardness, single-family home
7 grains per gallon (120 ppm)
200 mg/L, about 12 grains
Hardness, other uses
4 grains per gallon (70 ppm)
200 mg/L
X3 models
No hard-water exclusion, per A.O. Smith
No X3 equivalent
pH
6.5 to 8.5
6.5 to 8.5
Iron
0.3 mg/L or less (X3 manual)
Up to 0.3 mg/L
Total dissolved solids
Not listed
Up to 500 mg/L
Well water
Must be treated
Must fall within the table
Grains per gallon and mg/L measure the same thing. One grain is about 17.1 mg/L, so A.O. Smith’s 7-grain limit is 120 mg/L against Rheem’s 200. On a standard A.O. Smith heater, moderately hard water can void the scale coverage that Rheem would still give.
The X3 cartridge changes that for the ATI-310CX3. A.O. Smith’s sale sheet says X3 models have no hard-water exclusion.
Your water utility’s yearly report lists hardness, usually in mg/L. On a well, a hardness test kit gives grains per gallon.
Above either limit, plan a softener or a scale system with the heater, and flush a non-X3 heater on the schedule its manual gives. Our tankless descaling guide walks through a flush.
Gas line, venting and power for each pick
The install often costs more than the heater. Here’s what each one asks of the house.
Pick
Gas or power
Venting
Freeze protection
Other needs
A.O. Smith ATI-310CX3
¾ in. NPT gas; 120 V cord
3/5 in. concentric to 43 ft, 4 elbows max
Heats below about 36.5°F; X3 kit for the cartridge
Room for 36⅛ in. height
Rheem RTG-70X
½ in. gas line to 24 ft
None outdoors
To about -4°F outdoors, no wind
Hard-wire or add a plug
A.O. Smith ATO-140H
½ in. NPT gas, ½ in. line
None outdoors
Internal system
Condensate drain
Rheem RTEX-24
240 V, 3 × 40 A breakers, 3 × 8 AWG
None
Install where it can’t freeze
200 A panel recommended
On a tank-to-tankless swap, the gas line is the usual surprise. A ½ in. line that fed a 40,000 BTU tank may not carry 160,000 or 190,000 BTU at full fire. Rheem allows ½ in. to 24 ft on the RTG, subject to local code; A.O. Smith says the ATO-140H runs on ½ in. at standard gas pressure.
Indoor gas heaters need a vent run, and that’s the ATI-310CX3’s extra cost. Outdoor models skip it but need power outside and pipes insulated against freezing.
No. They’re separate companies that compete in tank, tankless and heat pump heaters, and each sells under several other names.
A.O. Smith
Rheem
Founded
1874, Milwaukee, Wisconsin
1925
Headquarters
Milwaukee, still today
Rheem Water Heating: Roswell, Georgia
Water heater offices
A.O. Smith Water Heaters: Ashland City, Tennessee
Rheem claims: Montgomery, Alabama
Other water heater names
State, American, Reliance, Lochinvar, U.S. Craftmaster, GSW, John Wood, Giant, Takagi
Ruud, Richmond, Eemax
One warranty, several names
Not stated
Rheem’s tankless gas warranty covers Rheem, Ruud and Richmond
Company size
About 11,500 employees
Not stated on the pages read
So a State or Reliance heater comes from the A.O. Smith family, and a Ruud or Richmond from Rheem’s. Rheem says Ruud is named for Edwin Ruud, who invented the first automatic gas water heater. Our State water heaters, Reliance and Richmond pages cover those names.
Shopping for a tank heater instead? Both makers sell tank models too, mostly through plumbers and stores. Our best water heater guide covers them.
Searched Oct 21, 2026. Lochinvar is an A.O. Smith brand, and that recall covers boilers only. If you own a Lochinvar boiler, check the notice on cpsc.gov with your serial number.
Which one fits your house
Most buyers fit one of these setups.
Indoor, natural gas, hard water
A.O. Smith ATI-310CX3. X3 handles scale, and A.O. Smith says X3 models carry no hard-water exclusion.
Outside wall, natural gas
Rheem RTG-70XLN-3: no vent pipe, ½ in. gas to 24 ft, and a 12,000-hour warranty cap.
Propane, two showers at once
Rheem RTG-70XLP-3 for 6.0 GPM at a 45°F rise. For a smaller home, the A.O. Smith ATO-140H burns less gas at 0.91 UEF.
No gas at the house
Rheem RTEX-24, if the panel has 200 A service and room for three 40 A double-pole breakers.
Heavy daily use
A big household or a busy recirculation loop runs up hours. A.O. Smith’s 3,000-hour cap on these models is the number to watch.
How to choose in four questions
Question
If yes
If no
Does the heater have to go inside?
A.O. Smith ATI-310CX3 on natural gas
Rheem RTG-70X or A.O. Smith ATO-140H outdoors
Is your water over 7 grains per gallon?
ATI-310CX3 with X3, or a Rheem with treatment planned
Any of the gas picks
Will the heater run more than about 49 minutes a day?
Neither wins everything. A.O. Smith’s ATI-310CX3 makes more hot water and its X3 models have no hard-water exclusion. Rheem’s RTG-70X installs outdoors with no vent and its warranty allows 12,000 operating hours, against 3,000 on the comparable A.O. Smith models.
Are Rheem and A.O. Smith the same company?
No. A.O. Smith was founded in 1874 in Milwaukee and also sells State, American, Reliance, Lochinvar and Takagi. Rheem was founded in 1925 and its family includes Ruud, Richmond and Eemax.
Is A.O. Smith a good water heater brand?
A.O. Smith is one of the oldest names in water heating, founded in 1874. Its gas tankless heaters carry 15 years on the heat exchanger and 5 on parts in a home, with conditions: a licensed installer and under 3,000 operating hours on the 310 and 140 models.
Is Rheem a good water heater brand?
Rheem has made water heaters since 1925, and its Ruud brand is named for the inventor of the first automatic gas water heater. Its RTG tankless warranty gives 15 years on the heat exchanger, 5 on parts and 1 on labor, up to 12,000 operating hours.
Which brand lasts longer?
Neither maker publishes failure rates, so we can’t rank lifespan from their documents. What we can compare is written coverage. Rheem’s hour cap is four times A.O. Smith’s on these models, and A.O. Smith’s X3 models drop the hard-water exclusion.
What does A.O. Smith’s 3,000-hour limit mean?
Its gas tankless warranty requires total operating time under 3,000 hours on the 110, 310, 310C, 140, 240 and 340 models, and under 9,000 on the 510 and 540. Spread over ten years, 3,000 hours is about 49 minutes a day.
Do I need a licensed plumber to keep the warranty?
For A.O. Smith, yes: its warranty excludes any heater not installed by a licensed plumber, gas installer or contractor. Rheem strongly recommends a licensed, trained contractor and says improper installation may void coverage.
Can I buy one on Amazon and keep the warranty?
Neither warranty we read excludes online purchases. Both depend on a correct install and your paperwork: proof of purchase and install date for A.O. Smith, and the install date, model and serial number for Rheem.
Can these heaters run on a ½ inch gas line?
Rheem allows ½ in. pipe up to 24 ft on the RTG, subject to local code, and A.O. Smith says the ATO-140H runs on a ½ in. line at standard pressure. The ATI-310CX3 has a ¾ in. connection. Have the line sized for every appliance on it.
Who makes State, Reliance, Ruud and Richmond water heaters?
State and Reliance are A.O. Smith brands, along with American, Lochinvar and U.S. Craftmaster. Ruud and Richmond are Rheem brands, and Rheem’s tankless gas warranty covers all three of its names.
How we compared them
Why trust us
Written by Isaac Daniels, a licensed journeyman plumber in Georgia (license #JP507711), who has used every heater on this page. Specs and warranty terms come from A.O. Smith’s and Rheem’s own spec sheets, manuals and warranties; recall facts from CPSC; company facts from each maker’s site; seller and stock facts from Amazon.
How we compare
Only what A.O. Smith, Rheem, CPSC and the Amazon listings publish, read Oct 21, 2026.
The three gas heaters share one spec score, set out below. The RTEX-24 is electric and rated on different terms, so it has its own section and no score.
Warranty map: both makers’ current residential gas tankless terms, row by row.
Run-time table: each hour cap divided by the days in 5, 10 and 15 years.
Every linked offer checked as genuine, new and in stock through Amazon’s Creators API on Oct 21, 2026 and Oct 22, 2026, 2:36 AM EDT, and each listing opened to confirm the seller.
CPSC recall search for both brands: no recall covers these heaters.
The spec score, check by check
The three gas heaters get the same five checks, read from A.O. Smith’s and Rheem’s spec sheets and warranties on Oct 21, 2026, then averaged to one decimal. Every pick has to reach 7.0.
Where the heater mounts isn’t in the score. Indoor or outdoor is set by your house first, so the ATI-310CX3 stays my pick for an indoor natural gas swap even though the outdoor RTG-70X scores higher. The electric RTEX-24 is rated on different terms and isn’t scored.
Check (weight)
How it’s scored
Flow at a 45°F rise (⅕)
10 = 6.5 GPM or more; 9 = 6.0–6.4; 8 = 5.5–5.9; 7 = 5.0–5.4; 6 = under 5.0 (A.O. Smith brochure AOSBG01000, Rheem spec sheet RH-RTG-RTGR-SS)
Efficiency (UEF) (⅕)
10 = 0.90 UEF or more (condensing); 8 = 0.80–0.89 (non-condensing)
Warranty run-hour cap (⅕)
10 = 12,000 operating hours or more; 8 = 9,000; 6 = 3,000 (each maker’s gas tankless warranty)
Hard water in the warranty (⅕)
10 = no hard-water exclusion; 8 = covered to 200 mg/L, about 12 grains; 6 = covered to 120 mg/L, 7 grains (single-family home)
Gas line (⅕)
10 = the maker allows a ½ in. gas line; 7 = ¾ in. gas connection, line sized by the installer
Total
Average of the checks, to one decimal
Pick
Flow at a 45°F rise
Efficiency (UEF)
Warranty run-hour cap
Hard water in the warranty
Gas line
Score
A.O. Smith ATI-310CX3
10
8
6
10
7
8.2
Rheem RTG-70X
9
8
10
8
10
9.0
A.O. Smith ATO-140H
7
10
6
6
10
7.8
Sources
A.O. Smith ATI-310CX3/510CX3 spec sheet ARESS00423 (input, UEF, max flow, connections, venting, water pressure, X3 features, warranty summary): hotwater.com, read Oct 21, 2026.
A.O. Smith X3 sale sheet AOSRG71026 (no warranty exclusions for hard water, simulated-life claim, dimensions with cartridge): hotwater.com, read Oct 21, 2026.
A.O. Smith 310CX3/510CX3 installation manual 2000610429 (X3 cartridge, iron limit, freeze protection and anti-freeze kit 100325654, temperature range, output temperature chart): hotwater.com, read Oct 21, 2026.
A.O. Smith ProLine XE 140H spec sheet AOSRG46836 (propane input, UEF, ½ in. gas, heat exchangers, remote controller, dimensions) and tankless brochure AOSBG01000 (flow at 45°F and 70°F rise, weights): hotwater.com, read Oct 21, 2026.
A.O. Smith Limited Gas Tankless Water Heater Warranty 100287496 rev. B (terms, operating-hour limits, installer and water quality exclusions, 300 activations a day, claims): hotwater.com, read Oct 21, 2026.
Rheem RTG/RTG-R spec sheet RH-RTG-RTGR-SS Rev.1 (input, flow by rise, UEF, dimensions, weights, clearances, ½ in. gas to 24 ft, vent lengths, accessories, Roswell address): rheem.com, read Oct 21, 2026.
Rheem RTG use and care manual (freeze protection, power, outdoor wiring): rheem.com, read Oct 21, 2026.
Rheem limited warranty for Rheem, Ruud and Richmond residential tankless gas heaters AP21265 rev. 03 (terms, 12,000 hours, recirculation, water quality table, exclusions, claims): rheem.com, read Oct 21, 2026.
Rheem RTEX spec sheet RH-TE-WH-SS Rev.1 (electrical needs, UEF, flow by zone, dimensions) and RTEX limited warranty: rheem.com, read Oct 21, 2026.
A.O. Smith Corporation about and brands pages; Rheem about page (founding, headquarters, brand families): aosmith.com and rheem.com, read Oct 21, 2026.
Amazon listings for the ATI-310CX3, RTG-70XLN-3, RTG-70XLP-3, ATO-140H and RTEX-24 (seller, condition, stock): Creators API and listing pages, Oct 21, 2026; Creators API again Oct 22, 2026, 2:36 AM EDT.
CPSC recall database searched for A.O. Smith, Rheem and tankless water heaters: Oct 21, 2026.
