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Tankless water heater installation requirements come from the manual and local codes. Use this checklist so there are no surprises on install day.
| Requirement | Gas / propane | Electric |
|---|---|---|
| Correct fuel (NG or LP) | Yes, must match the unit | n/a |
| Gas line sized for BTU input | Yes | n/a |
| Manual gas shutoff and sediment trap | Yes | n/a |
| Venting | Indoor models (outdoor models vent at the unit) | No |
| Combustion air | Indoor non-direct-vent models | No |
| Condensate drain and neutralizer | Condensing models | No |
| 120V power | Yes, for controls and fan | n/a |
| Dedicated 240V circuits | n/a | Yes, sized to amperage |
| Isolation (service) valves | Strongly recommended | Strongly recommended |
| Pressure relief valve | As required by manual and code | As required by manual and code |
| Expansion tank | If closed system and code requires | If closed system and code requires |
| Clearances | Per manual (sides, top, front, vent terminal) | Per manual |
| Freeze protection | Indoor in heated space, or outdoor model powered | Indoor in heated space |
| Permit and inspection | Usually | Usually |
A tankless unit burns far more gas per hour than a tank, though for much less time. The gas line must deliver the unit’s maximum BTU input plus every other appliance that may run at the same time, at the pressure the manual specifies. Installers use the fuel gas code tables, which depend on pipe material, diameter and length. Flexible CSST lines are common and must be bonded per code.
A properly sized tankless water heater gas line is the most common upgrade in conversions.
Gas tankless units need a 120V supply for the control board, fan and freeze protection. Electric tankless water heaters need dedicated 240V circuits. Whole-house models commonly draw 100–150 amps, split across several double-pole breakers. Wire gauge must match breaker size and run length. A load calculation confirms your panel can handle it.
Tankless water heater isolation valves (service valves) are installed on the hot and cold lines at the unit. Each has a main shutoff and a small hose port. They let you shut off the unit and connect a flush kit without draining the house. Many brands sell valve kits that include a pressure relief valve.
These are also called tankless water heater service valves, and most valve kits include both tankless water heater valves and a relief valve.
A tankless water heater pressure relief valve protects against excessive pressure. Many manuals require a pressure-only relief valve rated for the unit’s maximum pressure and BTU input, installed on the hot outlet and piped to a safe drain location. Follow the manual and local code exactly.
A tankless water heater expansion tank absorbs thermal expansion in closed plumbing systems. Whether you need one depends on whether your system is closed and on local code. Your installer or local building department can confirm.
A wall mounted tankless water heater needs a solid mounting surface. Some locations require a tankless water heater drain pan under the unit.
A standard tankless water heater installation diagram shows, from left to right under the unit: the gas supply with shutoff and sediment trap, the cold water inlet with isolation valve and inlet filter, the hot water outlet with isolation valve and pressure relief valve, and the condensate line to a neutralizer and drain. Vent pipe exits the top. Electric tankless water heater installation diagrams show the cold inlet, hot outlet and several 240V circuits entering the bottom terminal block. Always use the diagram in your manual, since layouts differ by model.
See our dedicated venting guide and condensing vs non-condensing guide.
U.S. jurisdictions adopt model codes such as the International Plumbing Code (IPC), International Fuel Gas Code (IFGC), Uniform Plumbing Code (UPC), National Fuel Gas Code (NFPA 54) and National Electrical Code (NFPA 70), often with local amendments. Your installer and local building department determine which version applies.
Isolation valve kits, flush kits and other parts installers use with tankless units.
Sometimes. A thermal expansion tank is needed when your home has a closed water system, for example a check valve or pressure-reducing valve at the meter, and local code requires it. Tankless units heat less water at once than tanks, so some jurisdictions don’t require one. Ask your installer and local code office.
Most manufacturers require a pressure relief valve, and some units include one. It must be rated for the unit and piped to an approved location. Follow the manual and local code.
It depends on the unit’s BTU input, the length of the pipe run and the other gas appliances. Many 199,000 BTU units need a 3/4-inch line or larger from the meter. Installers size it from fuel gas code tables.
A drain pan is required in some situations, such as installations above finished living space, depending on the manual and local code. Many wall-mounted units don’t need one, but check locally.
Whole-house units often need two to four double-pole breakers of 40–60 amps each. Point-of-use units may need a single 20–40 amp breaker. The manual specifies breaker and wire size.
This guide is general information for U.S. homeowners, not professional advice. Gas, electrical and venting work must follow the manufacturer’s instructions and local codes, and in most areas requires a licensed plumber, electrician or gas fitter and a permit. Specifications and prices change; always confirm details on the product page and in the installation manual.
