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The most popular way to get endless hot water for a whole house. Natural gas tankless water heaters deliver high flow even when incoming water is cold.
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A natural gas tankless water heater uses a powerful burner, typically 120,000 to 199,000 BTU, to heat water instantly as it flows through a heat exchanger. That power means more flow at a bigger temperature rise than an electric unit, which is why gas is the usual choice for whole homes in colder climates. Natural gas is also generally cheaper per unit of heat than electricity in most U.S. markets.
Replacing a natural gas hot water heater tank? A natural gas tankless hot water heater fits the same fuel supply once the gas line is sized correctly. Whether you call it a gas water heater tankless, a gas hot water heater tankless or one of the many natural gas tankless hot water heaters, the buying criteria are BTU input, flow at your rise and venting.
| Condensing | Non-condensing | |
|---|---|---|
| Efficiency (UEF) | About 0.90–0.97 | About 0.80–0.83 |
| Venting | PVC or polypropylene (cooler exhaust) | Stainless steel or special category III vent |
| Condensate drain | Required, usually with neutralizer | Not required |
| Unit price | Higher | Lower |
| Best for | Indoor installs, long-term savings | Outdoor installs, lower upfront cost |
Read the full comparison in condensing vs non-condensing tankless water heaters.
Indoor models need a direct vent through a wall or roof and combustion air. Outdoor models mount on an exterior wall with no vent pipe, which simplifies installation in mild climates and frees up indoor space. Outdoor units have freeze protection that requires power. Browse outdoor tankless water heaters.
An outdoor tankless gas water heater is a common choice in the South and West, where freezing is rare.
| Home | Simultaneous use | Typical choice |
|---|---|---|
| 1 bathroom | 2.5–3.5 GPM | 120,000–150,000 BTU |
| 2 bathrooms | 4–6 GPM | 150,000–180,000 BTU |
| 3+ bathrooms or cold climate | 6–8+ GPM | 180,000–199,000 BTU, or two units linked |
Budget for installation in our tankless water heater installation cost guide.
Our natural gas range includes Rinnai, Navien, Marey and Eccotemp, with each listing showing BTU input, rated flow, UEF, venting type and condition.
Whole-house natural gas units are typically rated 6.5–11 GPM, but that maximum is measured at a small temperature rise. At a 70°F rise, a 199,000 BTU condensing unit delivers about 5–5.5 GPM; at a 45°F rise in a warm climate, about 8 GPM.
Often, yes. A 199,000 BTU unit needs far more gas than a 40,000 BTU tank heater, and many homes need a larger line from the meter, typically 3/4 inch or larger depending on length and other appliances. Your installer sizes it using code tables.
Only if the model is sold for propane or the manufacturer provides an approved conversion kit installed by a qualified technician. Buy the version that matches your fuel. See propane tankless water heaters.
No. Gas tankless units need electricity for the controls, igniter and fan. Some homeowners use a small backup power supply or generator.
