Key takeaways
- Shower: about 1.5–2.5 GPM
- Bathroom faucet: about 0.5–1.0 GPM
- Kitchen faucet: about 1.0–2.2 GPM
- Standard dishwasher: roughly 1.0–1.5 GPM while filling
- Clothes washer: approximately 1.5–2.0 GPM while filling
The best gas tankless water heaters are the ones that match your peak flow rate, required temperature rise, gas supply, venting route, and household demand—not simply the model with the highest gallons-per-minute rating.
Quick picks by household situation
| Household or installation situation | Useful specification target | Best heater type |
|---|---|---|
| Small apartment with one bathroom | 5–7 GPM; 120,000–160,000 BTU/h | Compact indoor or outdoor unit with low minimum flow |
| Two-bathroom home | 7–9 GPM; 160,000–199,000 BTU/h | Condensing indoor model with precise temperature control |
| Three or more bathrooms, frequent simultaneous use | 9–11+ GPM; up to about 199,000 BTU/h per unit | High-output condensing model or a professionally designed multi-unit system |
| Cold incoming water, such as northern climates | Look for strong output at a 70–80°F temperature rise | Condensing unit with recirculation capability and adequate gas capacity |
| Replacing an existing tank heater in the same location | Confirm vent, drain, gas, electrical, and service clearances first | Model selected around the existing installation, not just household size |
How much flow rate do you actually need?
A tankless heater must produce hot water while fixtures are running. Add the flow rates of fixtures that may operate at the same time rather than adding every fixture in the house.
- Shower: about 1.5–2.5 GPM
- Bathroom faucet: about 0.5–1.0 GPM
- Kitchen faucet: about 1.0–2.2 GPM
- Standard dishwasher: roughly 1.0–1.5 GPM while filling
- Clothes washer: approximately 1.5–2.0 GPM while filling
For example, one 2.0-GPM shower, one 1.0-GPM bathroom faucet, and a 1.5-GPM dishwasher create a possible 4.5-GPM demand. A 5–6 GPM unit may cover that combination in warm incoming-water conditions, but a larger heater gives more margin for winter temperatures. A two-bathroom home may need 7–9 GPM if two showers, a faucet, or a washing machine can overlap.
Do not treat the advertised maximum GPM as a universal output. It usually assumes a relatively modest temperature rise. The same heater may deliver substantially less flow when it must raise incoming water from 45°F to 120°F.
Temperature rise is the specification that changes everything
Temperature rise is the difference between incoming cold-water temperature and the desired hot-water temperature. If groundwater enters at 55°F and the heater is set to 120°F, the required rise is 65°F. If groundwater enters at 40°F, the required rise is 80°F.
A rough sizing calculation is:
Required heating capacity in BTU/h = 500 × flow rate in GPM × temperature rise in °F
At 5 GPM and a 70°F rise, the calculation is about 175,000 BTU/h before accounting for efficiency and operating conditions. This explains why a unit advertised as “up to 10 GPM” may not provide 10 GPM in a cold-climate shower scenario.
When comparing models, read the manufacturer’s performance chart at the temperature rise you actually need. For many homes, the 60°F, 70°F, and 80°F columns are more useful than the headline maximum.
Condensing versus non-condensing gas heaters
| Feature | Condensing model | Non-condensing model |
|---|---|---|
| Typical energy efficiency | About 0.90–0.98 UEF | About 0.80–0.85 UEF |
| Exhaust temperature | Lower; commonly allows approved plastic venting | Higher; generally requires metal venting |
| Installation complexity | Needs a condensate drain and often a neutralizer | Simpler condensate arrangement, but vent materials can cost more |
| Best fit | High-use households and long-term efficiency | Short vent runs, simpler replacements, and some lower-use applications |
Condensing heaters extract additional heat from combustion gases, which improves efficiency. They normally cost more to install because the system needs a condensate drain, corrosion-resistant components, and a compatible vent. Non-condensing models can be a practical choice when an existing metal vent route is easy to reuse, but the existing vent must still meet the new appliance’s requirements.
Gas supply, venting, and installation realities
A high-output tankless heater can use as much gas as several household appliances operating together. The installer must verify the gas meter, regulator, pipe diameter, pipe length, pressure, and available capacity. A 199,000-BTU/h heater may require a larger gas line than the tank heater it replaces, even if the old unit appeared to heat water adequately.
Venting is equally important. Indoor units commonly use sealed direct-vent or power-vent arrangements, with separate combustion-air and exhaust paths or a concentric vent. Termination locations must follow the manufacturer’s clearances from windows, doors, air intakes, corners, grade, and neighboring property. Never select a vent diameter or material based only on what is already installed.
Outdoor models avoid a long indoor vent run, but they still need protection from freezing weather, wind, snow accumulation, and blocked air passages. In freezing climates, the unit may require electrical freeze protection even when the burner is off. An exterior location also makes service access and weather exposure part of the long-term maintenance decision.
Typical installation needs include a 120-volt electrical outlet for controls or freeze protection, isolation valves, a pressure-relief valve, sediment protection where appropriate, and a serviceable water filter. Condensing systems need a condensate drain, and some local codes require a neutralizer. Permit, gas, plumbing, and venting rules vary by location, so a licensed installer should confirm the design before purchase.
Clearances and placement around household furniture
Tankless heaters are compact, but “small” does not mean they can be enclosed anywhere. Exact clearances vary by model, so use the installation manual as the authority. As a planning guide, reserve roughly 24 inches in front of the unit for service access, keep the sides accessible enough to remove the cover, and avoid placing shelving, stored furniture, or laundry items directly below service connections.
Do not install a heater inside a bedroom, bathroom, or sealed closet unless the specific appliance and local code expressly permit it. A utility closet may need louvered or dedicated combustion air, fire-rated construction, drainage, and a full-size service opening. Keep combustible storage, cardboard, cleaning products, and fabrics away from the burner and vent termination.
An indoor tankless heater often weighs approximately 40–85 pounds, depending on output and construction. Two people may be needed to position it safely, especially when mounting on masonry or above shoulder height. Measure the doorway, stair turns, and mounting wall before delivery. A narrow unit can fit a tight utility area, but pipe access and future heat-exchanger service still require working room.
Best choices by brand and design type
Rinnai high-efficiency series
Rinnai offers a broad range of indoor, outdoor, condensing, and non-condensing gas tankless heaters. Its selection is useful when you need a particular vent configuration, recirculation option, or compact cabinet. Compare actual temperature-rise output and minimum activation flow rather than choosing by series name alone.
Navien condensing models
Navien’s condensing gas tankless heaters are aimed at high efficiency and flexible domestic-hot-water layouts. They can be a strong fit for larger homes or installations using dedicated recirculation, but the condensate drain, vent design, and service access should be planned before purchase.
Rheem Performance and high-efficiency tankless models
Rheem sells gas tankless options across several output levels, including indoor and outdoor configurations. These can suit replacement projects where availability, local service, and compatibility with existing plumbing are important. Confirm the exact UEF, venting method, maximum input, and warranty conditions for the model under consideration.
Noritz and other professional-grade options
Noritz is another established manufacturer with residential gas tankless systems, including condensing models and configurations designed for larger demand. It may be worth comparing when a contractor already supports the brand or when a multi-unit design is being considered.
Installation cost and long-term trade-offs
A gas tankless heater commonly costs about $600–$2,000 for the equipment, while a straightforward professional installation may add roughly $1,000–$3,000. New gas piping, venting, electrical work, condensate drainage, wall repairs, permits, or a recirculation loop can push the project higher.
Annual energy savings depend on water use, fuel prices, incoming-water temperature, and the efficiency of the old heater. Tankless systems avoid the standby losses of a storage tank, but they are not maintenance-free. Hard water can cause scale inside the heat exchanger, leading to reduced flow, noise, or error codes. Many manufacturers recommend periodic flushing, especially where water hardness is high.
Final buying checklist
- Calculate the fixtures likely to run simultaneously.
- Check rated flow at your coldest expected incoming-water temperature.
- Choose condensing or non-condensing based on total installed cost, not efficiency alone.
- Confirm gas-meter and gas-line capacity with the installer.
- Plan vent termination, condensate drainage, electrical power, and service clearances.
- Verify minimum activation flow if the home has low-flow faucets.
- Choose a brand with qualified local service and available replacement parts.
- Ask whether a recirculation system is needed to reduce long waits at distant fixtures.
For most two-bathroom homes, a professionally installed condensing unit rated around 7–9 GPM at the required temperature rise is a sensible starting point. Smaller homes may need only 5–7 GPM, while large or high-demand households may require a 9–11 GPM model or multiple coordinated units. The right choice is the one that meets winter flow demand while fitting the property’s gas, venting, drainage, and maintenance realities.
