Replacing a Tank With Tankless

1. The Short Answer: It Is a Conversion, Not a Swap

When a tank water heater fails and you replace it with another tank, the installer disconnects the old one, sets the new one in the same spot, and reconnects the same gas line, the same flue, the same water lines and the same relief drain. Nothing upstream changes, which is why a like-for-like replacement is usually a two-to-four-hour job and why the price is predictable.

Replacing a tank with a tankless unit breaks almost every one of those assumptions. A whole-house gas tankless heater burns fuel at four to five times the rate of the tank it replaces, so the gas pipe is often too small. It vents differently, so the old flue usually cannot be reused. It needs electricity even when it burns gas. And because it hangs on a wall rather than sitting on the floor, the water connections move too.

None of this means converting is a bad idea. A well-installed tankless heater can run for 20 years, frees up floor space, and never runs out mid-shower as long as it is sized for your flow. If you are still deciding whether tankless suits your house at all, our plain explanation of how a tankless water heater works and the broader guide to the types of water heaters are the better starting points. This page assumes you are leaning yes and want to know what the job involves.

The money follows the work. Our tankless water heater cost breakdown puts a gas tankless replacing a tank at roughly $3,000 to $6,500 installed, against roughly $1,500 to $3,000 to replace a gas tank with another tank. The gap is almost entirely the gas line, the venting and the labor to rebuild them. If your house happens to have a large gas line and an easy outside wall nearby, you land at the bottom of that range. If it does not, you land at the top.

2. The Gas Line: Why the Old Pipe Usually Is Not Enough

A typical 40- or 50-gallon gas tank is rated somewhere around 30,000 to 50,000 BTU per hour. A whole-house tankless heater is usually rated between 150,000 and 199,000 BTU per hour, because it heats water as it flows instead of slowly warming a stored tankful. That burst of demand is the single biggest reason conversions cost more than people expect.

Most tanks are fed by a half-inch gas line, which is plenty for 40,000 BTU. Whether a half-inch line can carry nearly 200,000 BTU depends on its length and on everything else drawing from the same pipe, and in most houses the answer is no once you get past a very short run. The installer sizes the pipe from the code tables using the total connected load and the longest run from the meter, and the common result is a new three-quarter-inch line, sometimes larger, run back toward the meter or to a trunk line that can carry the load.

The meter itself is the next check. Many residential gas meters are rated around 250 cubic feet per hour, which is roughly 250,000 BTU per hour of natural gas. Add a 199,000 BTU tankless heater to a furnace, a range, a dryer and perhaps a fireplace, and the total connected load can exceed what the meter is rated to deliver. The fix is a call to the gas utility for a larger meter, which is usually inexpensive or free but can add days or weeks to the schedule.

Tankless heaters are also fussier about pressure than tanks are. The installation manual specifies a minimum inlet gas pressure, and if the pressure sags when the furnace and the heater fire together, the heater can throw an error code or deliver lukewarm water. A good installer checks the static and running pressure at the heater with everything on, not just whether the burner lights.

3. Venting and Condensate: The Old Flue Gets Retired

Most older gas tanks are atmospheric: hot exhaust rises naturally through a draft hood into a metal B-vent flue or a masonry chimney. A gas tankless heater is a sealed, fan-driven appliance that pushes its exhaust under positive pressure. Manufacturers do not allow that exhaust into a B-vent, so the existing flue is typically abandoned and a new vent is run, usually straight out the nearest exterior wall.

What the new vent is made of depends on the heater. Non-condensing units run hot exhaust and usually need Category III stainless steel venting or a manufacturer-approved equivalent, which is expensive per foot. Condensing units extract so much heat that the exhaust leaves cool enough for PVC, CPVC or polypropylene pipe, which is cheap and easy to run. That difference alone can make a more expensive condensing heater the cheaper project overall, and it is covered in detail in our condensing vs non-condensing comparison.

Condensing heaters create a new chore: condensate. They produce a steady trickle of mildly acidic water that has to go to a floor drain, a standpipe or a condensate pump. Many manufacturers recommend, and some local codes require, a neutralizer cartridge filled with limestone or similar media to raise the pH before the water reaches cast-iron drains. Budget for the drain route and a neutralizer when you plan a condensing unit, and replace the media every year or two as the manual directs.

There is one more venting trap that catches people. In many houses the old water heater shared a flue with the furnace. Remove the water heater, and the furnace can be left alone in a flue sized for two appliances, which can draft poorly and condense inside a masonry chimney. Installers call this an orphaned furnace. The fix is usually a properly sized chimney liner or a new vent for the furnace, and it should be checked before the job is priced rather than discovered on the day.

Where the vent comes out matters too. The manual and local code set minimum clearances from windows, doors, air intakes, gas meters, decks and the ground, and those rules can push the heater to a different wall than you expected. If the nearest clear spot is a long run away, the manual also caps the maximum vent length and number of elbows, which may favor a different location entirely.

4. Power, Water Quality and Valves

Most atmospheric gas tanks need no electricity at all. A gas tankless heater needs a 120-volt outlet near the unit for its fan, controls and ignition. That is usually a short, cheap circuit, but it has two consequences: an electrician may need to be part of the job, and the heater stops working during a power outage unless you plug it into a backup source. If you lose power often, that is worth knowing before you remove a tank that would have kept working.

Electric whole-house tankless heaters are a different scale of problem. An electric tank typically runs on a single 30-amp, 240-volt circuit. A whole-house electric tankless unit is commonly 18 to 36 kW and needs two to four large double-pole breakers, with total draw that can exceed 100 amps on the biggest models. Many houses with 100-amp or even 150-amp service simply cannot add that without a service upgrade. Our tankless sizing guide shows how to work out the kW you actually need, and in cold-groundwater regions the answer is often more than the panel can give.

Water quality matters more on a tankless heater than on a tank. The heat exchanger has narrow passages, and hard water deposits scale in them faster than it settles to the bottom of a tank. Above roughly 7 grains per gallon, many manufacturers recommend treatment or more frequent descaling, and some tie warranty coverage to water quality. If your water is hard, factor in a water softener or at least a scale-reduction cartridge, and commit to annual flushing.

That flushing is only practical if the installer fits isolation valves, the pair of service valves with hose ports that let you close off the heater and circulate descaling solution through it. They add little to the parts bill and turn a plumber visit into a 90-minute DIY job. Insist on them in the quote, along with a pressure relief valve if the model does not include one, and a check of whether your local code requires a thermal expansion tank on a closed system.

Finally, think about recirculation. A tankless heater does not deliver hot water any faster to a distant bathroom than a tank does, and some people notice the wait more because of the short burst of cold water between back-to-back uses. Many units support a recirculation pump or a crossover valve under the farthest sink. Deciding this during installation is far easier than adding it later.

5. Where the New Heater Goes

The appeal of tankless is partly physical: a unit roughly the size of a carry-on suitcase hangs on the wall and frees the floor space a 50-gallon tank used to take. In practice most conversions mount the new heater on the wall right above or beside the old tank location, because the water lines are already there and the vent route is known. Moving it farther than that adds plumbing, and every extra foot of hot-water pipe adds to the wait at the tap.

In warm climates, an outdoor-rated model mounted on an exterior wall avoids indoor venting entirely, since the unit exhausts directly from its own cabinet. That can make the cheapest conversion of all. Outdoor units have freeze protection heaters that need power, and the water lines feeding them need insulation, so they suit places where hard freezes are rare and power outages short.

Clearances still apply indoors. The manual lists minimum distances to the ceiling, the floor and the sides for service access and combustion air, and the wall has to carry the weight of a filled unit. A garage or basement wall framed in studs is usually fine; a crumbling block wall may need a mounting board. In a closet, check that the door does not block service access or the drain route for the pressure relief valve.

If the household has one bathroom far from the others, or a kitchen at the opposite end of a long house, it is worth asking whether one central heater is the right design at all. A smaller main heater plus a point of use water heater at the remote fixture can beat one large unit on both the gas line and the wait for hot water.

6. Permits, Inspection, Timeline and DIY vs Pro

Almost every jurisdiction in the US requires a permit to replace a water heater, and a conversion involves exactly the work inspectors care most about: new gas piping, a new vent through the building envelope and new electrical. Expect the inspector to check the gas line sizing and a pressure test on new piping, the vent material, length and termination clearances, the relief valve discharge, and the bonding and electrical connections. A licensed installer pulls the permit as part of the job; ask to see it.

A straightforward conversion with a short gas run and an easy outside wall is typically done in one long day. Add a long gas line, a roof vent, an orphaned furnace flue or a condensate pump, and it can stretch to two. If the gas utility needs to upgrade the meter, or an electric unit needs a service upgrade, the schedule can stretch to weeks. That is the strongest argument for planning a conversion before the old tank fails rather than the morning it floods the basement.

If your tank has already failed and you need hot water tonight, the practical move is often a like-for-like tank replacement now and a planned conversion later, or not at all. Our guide to how long water heaters last explains how to read the serial number for the age of your tank, which tells you how much runway you have to plan.

Can you do it yourself? Mounting the unit and re-piping the water lines are within reach of a capable DIYer. Gas piping, venting and the electrical circuit are a different matter: many jurisdictions require a licensed professional for gas work, and most manufacturers condition the warranty on a licensed installation. Because the heat exchanger warranty is often the most valuable part of a tankless purchase, a DIY install that voids it is rarely a saving.

When you collect quotes, make them itemize the gas line, vent, condensate, electrical, valves and permit separately. A single bottom-line price hides whether one installer skipped the gas line upgrade that another included. Ask which brands they service locally too; a heater with no nearby parts or trained techs is a liability, which is part of why water heater brand support matters as much as the spec sheet.

7. When You Should Not Convert

Tankless is not the right answer for every house, and the conversion cost makes the wrong choice expensive. The clearest case against it is a small household with modest hot water use on a cheap gas rate. The fuel savings of a tankless over a decent tank are often under $100 a year, so a conversion that costs $2,000 to $4,000 more than a new tank may take decades to pay back, if it ever does.

The second case is an all-electric home with limited panel capacity. A whole-house electric tankless heater can need a service upgrade that costs as much as the heater itself, and in a cold climate it may still struggle to keep up with two showers in January. For most of those homes a heat pump water heater is the better buy: it uses a fraction of the electricity of a standard electric tank, runs on an ordinary circuit, and qualifies for larger rebates in many areas.

The third case is a house where the gas line, meter and venting would all need work at once. When every one of those boxes is ticked, the conversion can climb toward the top of the cost range, and a high-efficiency power-vent or condensing tank delivers most of the benefit for less. Our gas water heater picks cover those options.

Where conversion clearly makes sense is a larger household that runs out of hot water with the current tank, a home with an existing three-quarter-inch or larger gas line and an outside wall close to the heater, and owners planning to stay long enough to collect on a 20-year lifespan. If that is you, size it carefully, specify condensing to keep venting cheap, and choose from our tested tankless water heater picks.