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A conventional gas storage water heater burns gas beneath an insulated tank, transfers that heat into the stored water, and sends hot water from the top of the tank to fixtures. A thermostat and gas control switch the burner on and off to maintain the selected temperature, while a vent carries combustion products outdoors. Gas tankless heaters work differently: they heat water as it flows through a heat exchanger.
How a gas storage water heater heats and delivers water
- Cold water enters near the bottom. The inlet and dip tube direct incoming water toward the lower part of the tank, helping keep cooler replacement water below the hotter water already stored.
- The thermostat calls for heat. When tank water cools below the control’s target, the thermostat and gas control start the heating cycle. Ignition designs vary by model; not every heater has a continuously burning pilot.
- The burner transfers heat to the tank. Gas burns in the combustion chamber beneath the tank. Hot combustion gases pass through a flue, whose heat-transfer surfaces warm the surrounding water. The water is heated through the tank base and flue surfaces, not by direct contact with exhaust.
- Hot water leaves from the top. Heated water rises, so the hottest water collects near the top of the tank and flows through the hot-water outlet when a fixture is opened. Incoming cold water replaces the water drawn off.
- The burner cycles off and on. Once the water reaches the control’s target, the gas control shuts off the burner. It starts the burner again when the stored water cools enough to need reheating.
- Exhaust is routed outdoors. The heater’s approved venting system carries combustion products out of the building. The appliance design determines how it gets combustion air and how exhaust is vented.
What the main parts do
| Part | Job |
|---|---|
| Tank and insulation | Store hot water and slow heat loss. |
| Cold-water inlet and dip tube | Bring replacement water into the lower part of the tank. |
| Hot-water outlet | Draw hot water from the upper part of the tank. |
| Thermostat and gas control | Sense or respond to tank-water temperature and regulate burner operation. |
| Burner and combustion chamber | Burn gas and produce heat beneath the tank. |
| Flue and vent | Transfer heat from combustion gases into the water and route exhaust outside. |
| Anode rod | Help protect the tank from corrosion. It is a maintenance and replacement component; compatibility depends on the heater. |
| Temperature/pressure relief valve | Provide a safety function to help keep operation within design limits. Do not block or remove its discharge piping. |
| Drain valve | Provide a way to drain the tank during service. |
The U.S. Department of Energy describes storage water heaters in terms of the tank, inlet and outlet, heater or heat exchanger, and safety features and controls. See DOE’s overview of gas storage water heaters and its discussion of water-heater construction and energy models.
How a gas tankless heater differs
A storage heater keeps a reservoir of water hot and reheats it periodically. A gas tankless heater has no large storage tank: a flow sensor detects water moving through the unit, activates a burner, and heats the water in a heat exchanger. Heating stops when the flow stops. The U.S. Department of Energy identifies a high-powered burner, fan, and heat exchanger as basic components of a gas tankless heater; see its gas tankless heater overview.
Tankless sizing depends on the greatest flow likely to be used at once and the temperature rise needed for the home’s location. A tankless unit avoids the standby heat losses associated with maintaining a tank of hot water, but that fact alone does not determine which type is better for a particular household. The U.S. Department of Energy’s consumer water heater guidance provides broader selection context.
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#1 Best Overall
Sale
Rheem RTG Series 7.0 GPM Natural Gas Tankless Water Heater Indoor | 160,000 BTUs High Efficiency On Demand, Continuous Hot Water | Non-Condensing
- High Performance Natural Gas Tankless Water Heater delivers endless instant hot water on demand for whole house residential use | 7.0 GPM at 35°F temperature rise provides 160,000 BTU/hr output for 2-3 simultaneous showers | Never run out of hot water again
- Energy Efficient Non-Condensing Gas Tankless Water Heater with 0.82 Uniform Energy Factor (UEF) saves up to 34% on energy costs vs. 50-gallon electric tank heaters | Indoor installation with direct vent system | Ultra-low gas bills with on-demand heating technology
- Ideal Whole House Tankless Gas Water Heater for large families and multi-bathroom homes with 2-3 water points running simultaneously | Perfect replacement for aging tank water heaters | Space-saving wall-mount design frees up valuable floor space
- CSA Certified Natural Gas Tankless Water Heater with advanced safety features | Digital LED temperature control panel with precision ±1°F adjustments for consistent hot water temperature | Built-in freeze protection, flame failure protection, and multi-point safety system
- Leak Protection – LeakGuard Leak Detection & Prevention protects your home from water damage that detects any internal leak, both large or small
Why combustion air and venting matter
Gas combustion needs an appropriate air supply and a working route for exhaust. Designs differ: a direct-vent sealed-combustion appliance draws combustion air from outdoors and sends exhaust outdoors, while an atmospheric-vent appliance relies on surrounding air. Venting and combustion-air arrangements must match the appliance and installation; the U.S. Department of Energy explains direct-vent equipment and defines backdrafting and combustion-appliance-zone testing. Its guidance says combustion-safety testing of atmospheric-vent appliances should be done by a certified rater.
Incorrect installation or a blocked or unsuitable vent can interfere with safe exhaust removal. Installation, venting, clearances, and service should follow the heater’s instructions and applicable local requirements. This general explanation cannot establish whether a particular appliance or vent is operating safely.
Quick Recap
SaleBestseller No. 1
Best Value
ThermoMate CZ422NG, Natural Gas Tankless Water Heater Indoor, Up to 5 GPM
- [Max 5 GPM & 113,000 BTU] – The ThermoMate instant water heater delivers hot water on-demand for two showers at the same time, maintaining a stable 105°F (based on 65°F inlet temperature). No waiting, no cold-water interruptions—perfect for busy homes. Its smart flow sensor and control chip adjust heating 20,000 times per second, ensuring ±1°F temperature consistency.
- [Safety & Reliable] - Certified by CSA, this residential gas water heater features boil-dry, freeze, flame-out, overheat, over temperature, timeout, pipe blockage, high-temperature lock, and excessive air pressure protections for safe and dependable operation. The freeze protection automatically activates at 41°F (5°C).
- [Energy Efficient & Save Money] – Built with an oxygen-free copper heat exchanger for efficient heat transfer and corrosion resistance, this natural gas water heater delivers up to 98% thermal efficiency with a UEF of 0.81. Its energy-efficient design reduces energy waste and can lower annual operating costs by approximately 30% compared with traditional water heaters.
- [Easy to Install & Save Space] – Designed for whole house use, this indoor water heater measures only 21"H × 13"W × 7"D and occupies less than one-third of the space of traditional water heater models. Standard 3/4" NPT water inlet/outlet connections, a 1/2" NPT gas inlet, a 3" OD vent connection, and UL-approved 120V plug make installation simple in most homes.
- [Smart Diagnostic Display] - Easily adjust water temperature (90–140°F) and keep track of flow, fan, and flame status on the 3.5-inch LED display. Advanced error codes allow fast troubleshooting, while the UL-certified 120V/5-foot cord keeps the system powered and monitored at all times.
Rank #4
Natural Gas Tankless Water Heater, 4.74 GPM 18L Indoor,Constant propane water heater,Instant Hot Natural Gas Water Heater with Fahrenheit Digital Display,Multi-Protection
- 【Tankless Water Heater】- Natural Gas Tankless Water Heater with Steady Temperature Control & Multiple Safety Protections (Flame Failure Device, Overheating Protection). Only 3.6PSI water pressure required, outputs 4.74 gallons of hot water per minute, maximum temperature raise up to 75℃. Applicable exclusively to Natural Gas (LNG).
- 【Efficient & Energy Saving】- The 4.74 GPM Instant Natural Gas Water Heater features a maximum power output of 81,888 BTU/hour, providing on-demand hot water for 2-3 points of simultaneous water supply to meet bathroom and kitchen hot water demand. This Natural Gas Water Heater’s heat exchanger and main burner are designed with advanced combustion technology, which effectively decreases gas consumption and reduces burning noise.
- 【Quality Material】- Our Water Heater is Crafted from stainless steel, and this Natural Gas Water Heater exhibits high temperature resistance and corrosion resistance. It also features a built-in oxygen-free copper water tank, which helps reduce heat loss and ensures a prolonged service life. It is equipped with an LED smart digital display, allowing you to effortlessly adjust the temperature between 38℃-75℃ according to your preferences.
- 【Easy Assemble and Use】- This water heater is applied with standard 1/2" NPT water fittings and gas inlet, and comes with a 2.4" horizontal vent pipe for quick installation. Note: For normal operation, this product must be installed with proper ventilation and equipped with a smoke exhaust pipe.
- 【Widely Application】- Compared to tank water heaters, it saves over 50% of available space, and is suitable for both bathroom and kitchen hot water demand. It is great for indoor installation in homes, small cabins and remote apartments, and there is no need to install an expansion tank. Note: In cold weather, if the water heater is not used for a long time, it is necessary to open the pressure relief valve to drain the residual water inside the water heater.
Rank #3
Novieex Natural Gas Tankless Water Heater Up to 4.3 GPM 100,000 BTU, Indoor
- 【Note 】 This is 4.3GPM natural gas model. Gas rated pressure: 8.04" w.c. (2.0 kPa), Gas Inlet & Cold Water Inlet & Hot Water Outlet：NPT 1/2", Power Supply: 110V-120V AC/60Hz, Water Pressure Range: 1.5-116 psi (0.01-0.8MPa)
- 【Reliable Hot Water on Demand】 With 100,000 BTU power, this on demand water heater delivers up to 4.3 GPM instant hot water. You can comfortably run a shower, kitchen faucet, and bathroom sink simultaneously—all without any noticeable temperature drop.
- 【Precise Temperature Control】 Enjoy consistent water temperature with a wide adjustable range from 95°F to 149°F, ±1℉ temperature control . The built-in digital thermostat of the tankless water heater ensures your water stays at the desired setting, delivering comfort and safety for every shower and household task.
- 【High Efficiency, Lower Bills】 Say goodbye to the energy and electricity waste of traditional heaters that reheat water nonstop. By leveraging a selectional burner system, our natural gas hot tankless water heater precisely adjusts the flame for the water temperature. You will see a noticeable drop in your energy bills, making it an efficient and cost-effective choice.
- 【Compact & Space-Saving】Compared bulky traditional tanks. Our on demand water heater is wall-mounted, saving over 50% of space and blends seamlessly into utility rooms, basements, or closets. Its sleek appearance and LED touchscreen interface allow for easy and intuitive operation.
Rank #2
Outdoor Propane Tankless Water Heater - 6L 1.58 GPM Portable Gas Water Heater with Handle & Overheating Protection, Instant Hot Water for RV Cabin Camping Boat Barn
- 【Ultra Slim Design】 6L portable water heater product size 11.8 (L) x 5.3(W) x 17.3 (H) inches, only weight 12lb. The compact design is suitable for carrying on the go. This outdoor water heater comes with foldable handle, easy to hang and store. Package includes 5 feet premium on/off shower head, 5 feet gas regulator, 1 set US-standard garden hose adapter, and 1 installation hardware pack. It only takes you a few minutes to install it.
- 【Endless Instant Hot Water】 Maximum power output 40,000BTU/Hr. Get instant endless hot water easily wherever you are. No need to preheat before using tankless water heater. As soon as you turn on the tap, the water flows with your desired temperature. Also, 1.58 gallons of hot water can be produced per minute at the flow rate of 1.58 GPM. Low water pressure start up, just need 2.5 PSI of water pressure to start.
- 【Water Pressure Requirement】 This unit requires a minimum 2.5 PSI starting pressure to operate properly. If your water pressure falls below this threshold, the water heater may fail to start or deliver fluctuating temperatures (hot & cold) during use. Please check your home water pressure BEFORE purchasing — for low-pressure homes, we recommend installing a booster pump. Ensure compatibility to avoid installation issues and enjoy a stable hot water experience.
- 【6-in-1 Protection 】 This propane water heater with CSA approved regulator, safe as per Canadian and US Standard. Features over-heating protection, low water flow protection, dry combustion protection, anti-freezing protection, high water pressure protection and flame failure device. Superior manufacturing quality ensures reliable and long-lasting performance.
- 【Idea for Off-Grid】The camping water heater runs on a standard 20 lb. liquid propane tank and is equipped with an electronic ignition powered by two D-cell batteries. Can be utilized while camping, in RV, hunting, poolside, washing pets or horses, cleaning a vehicle, or even in a tiny house, greenhouse, or cabin. No more chilly outdoor shower anymore.
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 at the time of purchase will apply.
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