Quick Answer: A tankless water heater installation runs 2–4 hours for a tankless-to-tankless swap, 6–10 hours when converting from a storage tank, and 2–3 days if it also needs a gas line upgrade, new venting, or an electrical panel upgrade, per installation-time data compiled by residential plumbing contractors. Almost every jurisdiction requires a permit — and per the International Code Council’s model codes, a 199,000 BTU/h gas unit in a single-family home falls under the residential (IRC) code path, while the same unit in a mixed-use commercial building follows the separate IPC/IFGC commercial path with its own inspection requirements. A plug-in electric point-of-use unit is a realistic DIY project; a whole-home gas conversion generally isn’t, both because of code and because most manufacturers require a licensed contractor’s gas and electrical hookup to preserve the warranty.
Every buying guide on this site talks about which heater to buy. This one covers what happens after the box shows up — the real timeline, the permit and inspection steps people skip at their own risk, and an honest read on which parts of the job are DIY-reasonable versus which parts you’re better off not touching yourself.
The real installation timeline
| Scenario | Typical time | Why |
|---|---|---|
| Tankless-to-tankless swap | 2–4 hours | Venting, gas line, and mounting point already exist and fit |
| Tank-to-tankless conversion | 6–10 hours | New venting run, often a gas line upsize, new wall mounting |
| Complex conversion (gas line upgrade + electrical + venting reroute) | 2–3 days | Multiple trades, multiple inspections |
| Electric point-of-use swap | Under 1 hour | Plugs into an existing outlet, no venting |
The number that matters isn’t “how long does the heater take to bolt to the wall” — it’s how much of your existing infrastructure has to change to support it. A straight tankless-to-tankless replacement is a half-day job. A first-time conversion from a tank is a full workday, and if the gas line, electrical service, or venting path needs real work, it stretches into a multi-day project with its own permit and inspection for each trade involved.
What actually happens during the install
- Load calculation and sizing. Before anything gets mounted, the flow rate and BTU requirement get calculated against your fixture count and groundwater temperature — see our tankless water heater sizing guide for the GPM × 500 × temperature-rise formula. Getting this wrong means a heater that runs out of hot water at two showers running at once.
- Shutoff and removal of the old unit. Water, gas (or electrical), and any existing venting get disconnected and the old tank or unit comes out.
- Mounting. Tankless units mount to a wall bracket, indoors or outdoors depending on the model and climate — a different footprint than a floor-standing tank, so the mounting location often shifts.
- Gas or electrical hookup. Gas units may need the line upsized — see the gas-line-sizing FAQ below — while electric units need a load calc against panel capacity, sometimes triggering a service upgrade. Both are the parts of the job most manufacturers want a licensed contractor handling.
- Venting (gas units only). Condensing and non-condensing units use entirely different vent materials and length limits — our tankless water heater venting guide covers the PVC-vs-stainless-steel rule and the equivalent-length math inspectors check.
- Isolation valves and commissioning. Valves get installed on the inlet and outlet, the system gets purged of air, and the unit gets fired up and checked for leaks and correct temperature rise before the installer leaves.
Permits and inspections
| Permit type | Triggered by | What it covers |
|---|---|---|
| Mechanical | Any water heater installation | Baseline permit almost every jurisdiction requires |
| Plumbing | New water line routing, isolation valves | Rough-in and final plumbing inspection |
| Gas | New or upsized gas line | Gas-specific inspection, separate from plumbing |
| Electrical | New circuit, panel upgrade, or electric unit install | NEC-governed inspection, separate from mechanical |
Per the International Code Council’s model codes, the residential-vs-commercial code path is drawn largely by BTU input and occupancy: a 199,000 BTU/h gas tankless unit in a single-family home follows the IRC (International Residential Code), while the same unit in a mixed-use commercial structure follows the separate IPC/IFGC commercial path — different inspectors, different checklist. Most tankless jobs stack more than one permit type at once (mechanical plus gas, or mechanical plus electrical), and each trade typically gets its own inspection rather than one combined sign-off. Skipping the permit to save time is the most common way an otherwise-correct DIY install turns into a problem — either at resale, when an inspector flags unpermitted gas or electrical work, or with your manufacturer, since several tankless warranties are contingent on a code-compliant, permitted installation.
DIY vs. hire a pro
- Electric point-of-use under a sink: realistic DIY. It’s a plug-in unit with simple water-line connections — see our best point-of-use water heater guide for models built around this exact swap.
- Whole-home electric tankless: DIY-possible on the plumbing side, but the electrical load calc and any panel upgrade belong to a licensed electrician — get that check done before you buy the unit, not after.
- Tankless-to-tankless gas swap: borderline. If the gas line, venting, and mounting point already fit code and don’t need to change, an experienced DIYer can do the mechanical work, but most manufacturers still specify a licensed contractor for the gas connection to preserve the warranty.
- Tank-to-tankless gas conversion: hire a pro. New venting, a probable gas line upsize, and a full permit-and-inspection cycle make this the scenario where DIY mistakes are most expensive to unwind.
Rheem RTG20124 Isolation Valve Kit
- The manufacturer-matched isolation valve set for Rheem tankless units — install it at the same time as the heater rather than retrofitting it after the fact.
- Full-port valves let a descaling pump circulate vinegar through the heat exchanger without disconnecting the water lines, turning an annual maintenance job into something you can do yourself in under an hour.
- Skipping isolation valves at install time is the single most common regret homeowners report once their first descaling is due — see our tankless water heater descaling guide for why hard water makes this a when-not-if maintenance item.
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2. EASTMAN Full-Port Isolation Valve Kit — Best Universal/Budget Pick
EASTMAN Full-Port Tankless Water Heater Isolation Valve Kit
- Not brand-locked to a single manufacturer, unlike the OEM Rheem kit above — a straightforward pick if you're installing a Rinnai, Navien, Noritz, or Takagi unit and don't need a brand-matched kit.
- Same full-port design that lets a maintenance pump push descaling solution through the unit without breaking the plumbing connections.
- Confirm your unit's inlet/outlet size before ordering — most residential tankless units use 3/4-inch connections, but check your specific model's installation manual.
3. BrassCraft ProCoat Stainless Steel Gas Connector — Best for Gas Hookup
BrassCraft ProCoat Stainless Steel Gas Connector
- The flexible connector between a rigid gas line and the heater itself — sized to match your unit's gas inlet, not the branch line diameter, so confirm both before ordering.
- A flexible connector isn't a substitute for upsizing an undersized branch line — if your existing 1/2-inch line can't flow enough gas for a 160,000+ BTU unit, the fix is a larger line, not a different connector, and that's plumber-or-gas-fitter work in nearly every jurisdiction.
- Buy this alongside your unit rather than assuming the old tank's connector will fit — tankless units commonly sit in a different location with a different connection point than the tank they're replacing.
4. SharkBite Tankless Water Heater Installation Kit — Best for Water Line Hookup
SharkBite Tankless Water Heater Installation Kit
- Push-to-connect fittings let a DIYer make the water-line connections to a new wall-mounted unit without a torch or solder — useful on the plumbing side even on jobs where the gas or electrical hookup goes to a licensed pro.
- Compatible across PEX, copper, and CPVC, which matters when a tankless conversion moves the connection point and you're tying into whatever pipe material is already in the wall.
- Pairs with the isolation valve kits above — install both in the same work session rather than as separate trips.
How to choose
- Swapping tankless-for-tankless with existing venting and gas line intact: budget 2–4 hours and the shortest permit cycle — an experienced DIYer can often handle the mechanical work.
- Converting from a tank: budget 6–10 hours, a full permit-and-inspection cycle, and a likely gas line or electrical upgrade — see our tankless water heater cost guide for what those upgrades typically add to the bill.
- Going electric point-of-use: the most DIY-friendly path by far — see our best point-of-use water heater picks.
- Unsure if your gas line or panel can handle the new unit: get the load calculation done before you buy, not after — it’s the single most common source of mid-install surprises.
- Any gas or electrical hookup: check your specific unit’s warranty terms first. Many manufacturers require a licensed contractor for these connections specifically, regardless of how comfortable you are with the plumbing side.
The bottom line
Tankless water heater installation isn’t one job — it’s several, and which ones apply to you depends entirely on what you’re replacing. A tankless-to-tankless swap runs 2–4 hours; a tank conversion runs 6–10 hours; a job that also needs a gas line upgrade, new venting, or electrical service can run 2–3 days. Almost every jurisdiction requires a permit, and per the International Code Council’s model codes, even the BTU rating of your unit can determine which code path — residential IRC or commercial IPC/IFGC — governs the inspection. Electric point-of-use units are a realistic DIY project; whole-home gas conversions generally aren’t, both for code reasons and because most manufacturers tie their warranty to a licensed, permitted installation. Install isolation valves at the same time as the heater regardless of who does the work — it’s the cheapest insurance against an expensive first descaling.