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Quick answer from a licensed plumber: Pick by flow at your winter water temperature and by what your gas line and vent allow. The Rheem RTGH-84DVLN-3 is the pick for most retrofits: condensing, with plastic vent. The RTG-95DVLN-3 is the bigger non-condensing unit and needs stainless vent. The RTG-84XLN-3 mounts outside and needs no vent. Natural gas and propane models are not interchangeable, and the gas line, vent and permit are licensed work.
Our pick for most homes on natural gas: the Rheem RTGH-84DVLN-3 condensing tankless. Check the Rheem RTGH-84DVLN-3 on Amazon
A gas tankless heater makes hot water as fast as its burner can heat it, so two numbers decide whether it works for your house: how many gallons per minute it can heat at your winter water temperature, and whether your gas line and vent can support it. The heater is the easy part. This guide picks three Rheem models and then covers the work around them, which is licensed work: gas piping, venting and the condensate drain.
Why only Rheem? We only link a unit if it is in stock and sold or shipped by Amazon or the maker. On September 28, 2026, the Rheem units below were sold and shipped by Amazon. The Rinnai, Noritz and Takagi units we checked were sold and shipped by third-party sellers, and our Navien searches returned only parts, no water heaters. If you are choosing between gas and electric first, start with our best tankless water heater guide and gas vs electric water heater.
Gas Tankless Water Heaters Compared
| Model | Install | Max input (BTU/h) | GPM at 35°F rise | GPM at 67°F rise | Vent |
|---|---|---|---|---|---|
| Rheem RTGH-84DVLN-3 | Indoor, condensing | 157,000 | 8.4 | 4.6 | 2-in. or 3-in. PVC-family |
| Rheem RTG-95DVLN-3 | Indoor, non-condensing | 199,900 | 9.5 | 5.0 | 3-in./5-in. Category III stainless |
| Rheem RTG-84XLN-3 | Outdoor, non-condensing | 180,000 | 8.4 | 5.1 | None (outdoor) |
| Rheem RTG-70XLN-3 | Outdoor, non-condensing | 160,000 | 7.0 | 4.5 | None (outdoor) |
Figures come from the Rheem RTG and RTGH specification sheets. Each linked heater was listed on Amazon, sold and shipped by Amazon, when we checked on September 28, 2026. These are natural gas models. Each has a propane version with a different model number, which is not interchangeable with the natural gas unit.
High-Efficiency Gas Tankless: Condensing or Non-Condensing
Shopping for a high-efficiency tankless water heater? Compare the Uniform Energy Factor (UEF) printed on each spec sheet. Rheem lists 0.93 UEF for the condensing RTGH-84DVLN-3 (0.90 to 0.93 across the RTGH series) and 0.82 for every non-condensing RTG model, indoor and outdoor. Rheem sells both lines as high efficiency: the RTGH sheet says “Super High Efficiency Condensing” and the RTG sheet says “High Efficiency Non-Condensing.”
Per Rheem, the condensing RTGH has two heat exchangers, a copper primary and a stainless steel secondary, and a 1/2-in. condensate connection, so it needs a condensate drain (covered below). It vents in Schedule 40 PVC, CPVC or polypropylene, while the indoor non-condensing RTG-95DVLN-3 needs 3-in./5-in. Category III stainless vent or a Rheem-approved vent system. Of the picks on this page, the RTGH-84DVLN-3 is the only condensing one.
Rheem RTGH-84DVLN-3 Condensing Indoor Gas Tankless
Natural gas · indoor direct vent · condensing · 157,000 BTU/h maximum input · 8.4 GPM at a 35°F rise, 4.6 GPM at a 67°F rise · 2-inch or 3-inch PVC-family vent · 120 V · 15-yr heat exchanger / 5-yr parts / 1-yr labor
The RTGH-84DVLN-3 is Rheem’s condensing model in the middle of the RTGH range. Rheem’s specification sheet lists 157,000 BTU/h maximum input and 11,000 BTU/h minimum, 8.4 GPM at a 35°F rise, 6.7 at 45°F and 4.6 at 67°F, and sizes it for three bathrooms. Rheem lists plastic vent for its condensing design: 2-inch vent up to 60 feet or 3-inch up to 150 feet of straight pipe. Plastic pipe is generally easier to run in a retrofit than stainless.
The price of that is a condensate drain. Rheem’s manual says the condensate is acidic, recommends an external neutralizer, and says to run the drain in PVC, CPVC or flexible tube and never to connect it directly to the sewer. If the drain blocks, the unit shows an error code. The heat exchanger is copper on the primary and stainless steel on the secondary, and Rheem lists a 0.4 GPM minimum activation flow. It is a 120 V unit and Rheem says not to put it on a GFCI or AFCI circuit.
- Plastic vent pipe, up to 60 feet of 2-inch or 150 feet of 3-inch straight run (Rheem)
- Rheem lists a 0.4 GPM activation flow and a 0.26 GPM minimum continuous flow
- Copper primary and stainless secondary heat exchanger, 15-year heat exchanger warranty (Rheem)
- Sold and shipped by Amazon
- Needs a condensate drain, and Rheem recommends a neutralizer
- Only 4.6 GPM at a 67°F rise, so it is a three-bathroom unit, not a four
- Indoor only unless you add Rheem’s RTGH-X outdoor conversion kit, sold separately
- Parts and labor coverage (5 years and 1 year) is much shorter than the heat exchanger term
Verdict: The pick for most retrofits where you can run PVC vent and a drain, and the one to choose if you want the condensing design.
Rheem RTG-95DVLN-3 Non-Condensing Indoor Gas Tankless
Natural gas · indoor direct vent · non-condensing · 199,900 BTU/h maximum input · 9.5 GPM at a 35°F rise, 5.0 GPM at a 67°F rise · 3-inch/5-inch Category III stainless concentric vent · 120 V · UEF 0.82 · 15-yr heat exchanger / 5-yr parts / 1-yr labor
The RTG-95DVLN-3 is the biggest of Rheem’s non-condensing RTG units: 199,900 BTU/h maximum input, 9.5 GPM at a 35°F rise and 7.4 GPM at 45°F. Rheem sizes it for three and a half bathrooms. In Rheem’s ground-water zone table it delivers 4.9 GPM with 52°F incoming water and 4.0 GPM at 37°F, the highest of the three RTG sizes.
The trade-off is the vent. This is a non-condensing unit, so Rheem requires 3-inch/5-inch UL-approved Category III stainless steel vent, or a Rheem-approved concentric kit from Metal Fab, ProTech or UBBINK, and says no other vent material is allowed and that it must not be vented through a chimney. Rheem lists up to 43 feet of straight pipe, less with elbows. The manual also says to connect the condensate trap on the vent to a proper drain. One more point from the table: at a 67°F rise Rheem lists 5.0 GPM for this model and 5.1 GPM for the smaller RTG-84, so the extra size pays off in mild weather more than in cold.
- Highest flow of the RTG line: 9.5 GPM at a 35°F rise (Rheem)
- Rheem lists 4.9 GPM in a 52°F ground-water zone and 4.0 GPM at 37°F
- Concentric vent kits from Metal Fab, ProTech and UBBINK are approved
- Sold and shipped by Amazon
- Category III stainless vent is a stricter, more specialized system than PVC
- At a 67°F rise it lists 5.0 GPM, no better than the RTG-84 (5.1 GPM)
- Highest gas input listed here: 199,900 BTU/h, which puts the most load on the gas line
- Amazon showed low stock when we checked, so confirm availability
Verdict: The pick for a larger house in a mild climate where a condensing unit is not an option and stainless vent is already in place or planned.
Rheem RTG-84XLN-3 Outdoor Gas Tankless
Natural gas · outdoor only · non-condensing · 180,000 BTU/h maximum input · 8.4 GPM at a 35°F rise, 5.1 GPM at a 67°F rise · no vent pipe · 120 V, hard-wired or plugged · 15-yr heat exchanger / 5-yr parts / 1-yr labor
The RTG-84XLN-3 is Rheem’s outdoor version: 180,000 BTU/h maximum input, 8.4 GPM at a 35°F rise, 6.7 at 45°F and 5.1 at 67°F, sized by Rheem for three bathrooms. Mounted on an exterior wall, it needs no vent pipe at all, which skips the biggest cost and design question of an indoor gas swap. Rheem’s manual calls it “for outdoor installation only” and says never to install it indoors or in a confined area.
Outdoor means winter matters. Rheem says these units have a built-in freeze protection heater good to roughly -4°F outdoors, but that it only protects the inside of the heater and stops working if the power is cut. The pipes, valves and drain outside need their own protection, and the manual suggests insulation or a small trickle of hot water. The manual says outdoor models are hard-wired, or you add a plug. For a smaller house, the RTG-70XLN-3 is the 160,000 BTU/h, 7.0 GPM version, and a propane RTG-70XLP-3 is listed separately.
- No vent pipe or condensate line to run (Rheem)
- Rheem lists 5.1 GPM at a 67°F rise and 4.4 GPM with 52°F incoming water
- Also offered as the 160,000 BTU/h, 7.0 GPM RTG-70XLN-3 for a smaller house (Rheem)
- Sold and shipped by Amazon
- Outdoor only: Rheem says never to install it indoors or in a confined area
- Freeze protection only works while it has power and only covers the unit, not your pipes
- Exterior gas, water and electrical connections all have to be weatherproofed and protected
- Non-condensing: Rheem lists UEF 0.82, against 0.90 to 0.93 for the condensing RTGH series
Verdict: The pick where an indoor vent is impossible or too complicated, and the winters are mild enough for the unit and its pipes to be protected.
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How to Size a Gas Tankless Heater
Temperature rise is the set temperature minus the temperature of the water coming in. A 120°F setting with 52°F incoming water is a 68°F rise (our arithmetic). A tankless heater has a fixed amount of heat to give, so the colder the incoming water, the fewer gallons per minute it can bring up to temperature. That is why the box number, measured at a 35°F rise, is the best case. Rheem publishes flow at 35°F, 45°F and 67°F rise, and a second table by U.S. ground-water zone. Use the one closest to your coldest month.
| Rheem model | GPM with 37°F incoming water | GPM with 52°F incoming water | GPM with 67°F incoming water |
|---|---|---|---|
| Rheem RTG-95 | 4.0 | 4.9 | 6.3 |
| Rheem RTG-84 and RTGH-84 | 3.6 | 4.4 | 5.7 |
| Rheem RTG-70 | 3.2 | 3.9 | 5.0 |
The zone table is Rheem’s, calculated for a 120°F set temperature and a 105°F mixed shower. To compare with what you run at once, add the fixtures. Using Rheem’s average flow rates of 2 GPM per shower and 2 GPM for a dishwasher, two showers and a dishwasher add up to 6.0 GPM at the fixtures (our arithmetic). Showers are mixed with cold water at the valve, so the hot side is somewhat lower, which is why Rheem’s zone numbers run higher than its plain 67°F rise column. If your total is above the number for your climate, run fewer things at once or move up a size. Our tankless water heater size calculator walks through the same steps.
A quick check on any model (our arithmetic): heat needed in BTU per hour is about GPM times rise times 500. Five gallons per minute at a 67°F rise is 167,500 BTU/h of heat delivered, which is why a 199,900 BTU/h burner tops out near 5 GPM in cold water.
Gas Line, Vent and Condensate: Licensed Work
Gas piping and venting can hurt people if they are wrong. Rheem’s installation guide says it recommends the heater be installed and serviced by a licensed plumber or professional service technician, and its manual warns that a badly vented unit can cause death from carbon monoxide. Use this section to know what to ask for, not as install instructions. A licensed gas fitter sizes the pipe.
BTU input and gas line size
A tankless heater draws far more gas at once than a tank heater, so the meter, regulator and pipe all need to carry it. Rheem’s manual gives the method: add the BTU/h of every gas appliance on the line and divide by the heating value of the gas, which it puts at 1,024 BTU per cubic foot for natural gas and 2,546 for propane. If the total is more than the meter or regulator rating, the gas company has to upgrade it. Here is that arithmetic for the models above (our arithmetic, using Rheem’s figures), for the heater alone:
| Model | Max input (BTU/h) | Natural gas (cubic feet per hour) | Propane (cubic feet per hour) |
|---|---|---|---|
| RTGH-84DVLN-3 | 157,000 | 153 | 62 |
| RTG-84XLN-3 | 180,000 | 176 | 71 |
| RTG-95DVLN-3 | 199,900 | 195 | 79 |
| RTG-70XLN-3 | 160,000 | 156 | 63 |
Add your furnace, range and dryer to that. Rheem’s spec sheets say the RTG and RTGH units work on a half-inch gas line up to 24 feet, subject to local code, but the manual is stricter. It says half-inch line is allowed only if the system pressure is at least 8.0 inches of water column and the pipe capacity table is met, that the supply line should be a minimum of 3/4-inch black steel or other approved piping, and that a flexible connector must be at least 3/4-inch inside diameter with a BTU rating at or above the heater’s. It also calls for a manual shutoff valve at the heater and a sediment trap at the lowest point of the line. Rheem lists an allowable inlet pressure of 4.0 to 10.5 inches of water column for natural gas and 8.0 to 13.0 for propane, and says a regulator is required if the supply is outside that range.
Venting: concentric PVC or stainless
The vent type follows the design. Condensing units (RTGH) cool the exhaust, so Rheem lists plastic vent. The spec sheet names Schedule 40 PVC, CPVC and polypropylene; the installation guide lists PVC, CPVC and ABS; the manual lists a longer set including Schedule 80 PVC, ABS and stainless, and says foam-core pipe is never allowed. Because the documents differ, follow the manual and your installer’s local code. Non-condensing units (RTG) run hotter and need 3-inch/5-inch UL-approved Category III stainless steel vent, or Rheem-approved Metal Fab, ProTech or UBBINK concentric vent, and cannot use a chimney. Rheem says to inspect the vent terminal and air intake at start-up and at least once a year afterward. Outdoor units need no vent pipe.
Condensate drain
The RTGH is a condensing unit, and Rheem’s manual says never to run it without a drain. The condensate is acidic, an external neutralizer is recommended, the drain line should be PVC, CPVC or flexible tube sloped downward with an open end, and it cannot connect directly to the sewer. In cold climates the drain can freeze, and a blocked drain will trip an error code. Even the non-condensing RTG manual says not to run the unit without the condensate trap on the vent connected to a proper drain.
Natural gas or propane
Buy the one that matches your fuel. Rheem’s manuals say never to convert a unit from natural gas to LP or the reverse, and that a natural gas water heater will not function safely on LP. Propane models use a P in the model number. Propane is also heavier than air, which Rheem notes is why gas detectors are recommended.
Permits
Rheem’s manuals require installation to local codes and the National Fuel Gas Code but do not discuss permits. In our experience most places require a permit and an inspection for a gas water heater replacement (a plumber’s note, not from a maker document), and the local building department decides. Call yours before you buy, and ask the contractor who pulls the permit to confirm what your area requires.
Before You Buy
- Find your winter water temperature and your peak demand. Add up what runs at once and compare it with the maker’s flow for your zone, not the 35°F box number.
- Check your fuel. Natural gas or propane, and buy the matching model. They are not interchangeable.
- Have the gas line checked. A licensed gas fitter should read the meter and regulator rating, measure the supply pressure and size the pipe for the total load.
- Pick the vent path first. If a vent to the outside is easy and short, indoor condensing is simplest. If not, an outdoor model avoids the vent. See our tankless water heater installation cost guide for what changes the scope.
- Look at your water. Rheem says its warranty does not cover damage from water outside its limits, which include pH 6.5 to 8.5 and up to 200 mg/L total hardness, and it recommends water treatment and regular flushing. See water heater flush kits and how to descale a tankless water heater.
- Plan the electrical and water side. Rheem lists a 120 V supply, says not to use a GFCI or AFCI circuit, and says 65 psi of water pressure is needed for maximum flow. It also calls for service valves and a 150 psi relief valve.
Care and Warranty
Rheem recommends flushing the heat exchanger every year to year and a half. Set the optional maintenance notice and the display shows code 88 at 500, 750 or 1,000 hours of burner time. The manual also lists a monthly water filter check, an annual look at the condensate neutralizer and drain, and a yearly check of the pressure relief valve. Rheem prints a 15-year heat exchanger, 5-year parts and 1-year labor warranty and adds that all warranties are void after 12,000 hours of operation, and that it requires registration, so read the warranty before you count on it. If your heater errors out, our Rheem tankless error codes guide covers the codes.
FAQ
Can I switch a gas tankless water heater from natural gas to propane?
Not with these Rheem units. Rheem’s manuals say never to convert a water heater from natural gas to LP or the reverse, and that it must use the fuel on its data plate. Rheem sells natural gas and propane models under different model numbers (swap the N for a P), so buy the one that matches your fuel.
Can a gas tankless water heater use my existing half-inch gas line?
Sometimes, and that is a licensed gas fitter’s call. Rheem’s spec sheets list half-inch gas line compatibility up to 24 feet, subject to local code. Its manual is stricter: half-inch line is allowed only if the system pressure is at least 8.0 inches of water column and the pipe capacity table is met, and it says the supply line should be at least 3/4-inch black steel or other approved piping.
Do I need a permit to install a gas tankless water heater?
In most places, in our experience. Rheem’s manuals say the unit must be installed to local codes and the National Fuel Gas Code, but they do not discuss permits. Permit and inspection rules come from your local building department, so call yours before you buy. In our experience the licensed contractor usually pulls the permit.
How much flow do I need from a gas tankless heater?
Add up what runs at once, then check the maker’s flow at your incoming water temperature. Rheem’s average flow rates are 2 GPM for a shower, 2 for a dishwasher, 1.5 for a kitchen faucet and 3 for a washing machine. Rheem’s zone table lists 3.9 to 4.9 GPM for these models with 52°F incoming water and 3.2 to 4.0 GPM at 37°F.
How often should a gas tankless water heater be flushed?
Rheem recommends flushing the heat exchanger every year to year and a half. The units count burner hours and can show error code 88 as a reminder at 500, 750 or 1,000 hours if you set it. Rheem does not print the steps in its manual, so see our guide on how to descale a tankless water heater and Rheem’s own flush kit instructions.
Why Trust Plumbing Picks
Plumbing Picks is written by a licensed Georgia journeyman plumber. Every size, rating, dimension and spec on this page comes from the manufacturer pages and documents listed below, which we opened and read, or from the Amazon listing we link; nothing here comes from hands-on testing. Each linked product was shipped or sold by Amazon, new and in stock, when we checked on September 28, 2026. Where a link goes to Amazon we may earn a commission at no extra cost to you. Nobody pays to be included here. More on who writes this and how we pick.
Sources
- Rheem: RTGH series condensing tankless gas water heaters specification sheet (form TK-RTGH Series Rev. 2)
- Rheem: RTGH series tankless gas water heater Use & Care manual
- Rheem: RTGH series Tankless Easy Installation Guide
- Rheem: RTG/RTG-R series non-condensing tankless gas water heaters specification sheet (form RH-RTG-RTGR-SS Rev.1)
- Rheem: RTG series tankless gas water heater Use & Care manual
- Rheem: RTG series Tankless Easy Installation Guide
- Amazon listings for the five linked Rheem models (linked above)
Bottom Line
Size the heater from the maker’s flow at your coldest incoming water, not the 35°F number on the box. Choose the RTGH-84DVLN-3 if you can run plastic vent and a condensate drain, the RTG-95DVLN-3 if you need the extra capacity and stainless vent is available, and the RTG-84XLN-3 if the heater should go outside. Match natural gas or propane exactly, and have a licensed gas fitter size the gas line, install the vent and pull the permit.