Quick Answer: A tankless retrofit — replacing an existing storage tank rather than swapping tankless-for-tankless — changes three things a straight swap doesn’t: the vent has to be a dedicated new run rather than tied into the old metal flue (condensing exhaust runs too cool to draft safely up it and can rust it from the inside, per professional home-inspector consensus), the gas line may or may not need upsizing depending on whose claim you trust, and the unit typically moves from the floor to a wall, which has to land on real framing. Together, industry cost data pegs the premium for these changes at roughly $500 to $2,000 on top of a like-for-like install.
Our installation guide already covers the general timeline, permits, and DIY-vs-pro call for any tankless job. This one zooms in on the part that’s specific to retrofitting: what has to change about your home’s existing infrastructure when the unit you’re removing is a tank, not another tankless heater.
The venting trap: you can’t just tie into what’s already there
An old atmospheric-vent storage tank runs a vertical metal flue up through the roof, relying on hot exhaust rising on its own. A condensing tankless unit’s exhaust is cool enough — low enough temperature that it needs a powered fan to push it out — that splicing new PVC into that same old metal flue doesn’t work the way it looks like it should. Inspectors discussing this exact scenario on professional forums generally reject the tie-in approach for two reasons: the cooler exhaust doesn’t draft reliably upward on its own and can spill back into the house, and the acidic condensate in that exhaust will rust an existing metal flue that was never designed to carry wet exhaust.
The fix is a dedicated vent run in a material actually approved for condensing exhaust — Schedule 40 or 80 PVC, CPVC, ABS, polypropylene, or UL 1738 Category III stainless steel, per manufacturer installation manuals (foam-core PVC is never permitted). Most retrofits route that new vent horizontally through a side wall instead of reusing the old roof penetration, which introduces its own rule: a concentric or two-pipe sidewall termination has to clear manufacturer- and code-specified distances from windows, doors, and other building openings — distances that simply didn’t apply to the old vertical flue. See our venting guide for the PVC-vs-stainless material rule by unit type, and our direct-vent-vs-power-vent guide for how that sidewall clearance math actually works.
The gas line question: two answers that don’t agree
| Source | Claim | What it means for you |
|---|---|---|
| Manufacturer (e.g. Rinnai's own "myths" page) | A zero-governor gas valve means many installs don't need a bigger gas line | Best-case scenario, not a guarantee for your specific run length and BTU rating |
| Independent plumbing guidance | Gas supply is one of the first things to check when converting from a tank | A 160,000–199,000 BTU unit draws far more gas at once than the 40,000 BTU tank it replaces |
Both can be true at once: some retrofits genuinely don’t need a bigger line, and plenty do. The honest answer is that you won’t know which camp you’re in until someone runs the math against your unit’s BTU rating, your existing pipe diameter, and your run length — not from a manufacturer’s general marketing claim. Our gas line size guide covers that sizing math directly, including the CSST-vs-black-iron capacity-table difference that trips up a lot of retrofit quotes.
Universal Tankless Water Heater Wall Mounting Bracket Kit
- Built for exactly the situation a retrofit creates: a unit that used to sit on the floor as a tank now has to hang securely on a wall, often in a new location your old tank never occupied.
- An adjustable plate accommodates the mounting-hole spacing of most major brands, but confirm against your specific unit's installation manual before drilling — hole patterns aren't universal across manufacturers.
- Pair with a stud finder before you drill (see below) — the bracket is only as strong as what it's screwed into.
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Floor tank to wall-hung unit: the weight problem
A retrofit almost always means a location change, and a wall-mounted unit is heavier than it looks. A whole-home gas unit like the Rinnai RU199iN weighs about 74 pounds and the Navien NPE-240A2 about 77 pounds, per each manufacturer’s own spec sheet — enough that the mounting bracket needs to land on real framing, not drywall alone. Electric units run lighter, generally 15–30 pounds depending on kW rating, which gives more flexibility on wall placement, but code-compliant mounting hardware still matters on every unit: a heater that’s even slightly out of level can affect condensate drainage on condensing models. See our dimensions guide for the full weight-and-clearance specs by model.
Stud Finder Wall Scanner with Metal and AC Wire Detection
- Confirms actual stud locations before a contractor commits to a mounting spot — important on a retrofit, since the new wall-mounted location almost never lines up with where the old tank's floor footprint was.
- The AC-wire detection mode matters specifically on an electrical retrofit: you're drilling into a wall that may already carry a circuit feeding a nearby outlet or panel.
- A $20–$40 tool that prevents a $70-plus-unit bracket from being anchored into drywall alone — cheap insurance relative to what it protects.
What the retrofit premium actually costs
A true tank-to-tankless conversion costs more than a tankless-to-tankless swap, and industry cost guides are consistent on why: any location change, fuel-type switch, or infrastructure upgrade — new venting, a gas line upsize, a panel upgrade for an electric unit — typically adds roughly $500 to $2,000 on top of a like-for-like replacement. That’s a range, not a guaranteed number, because it depends entirely on which of the three changes above actually apply to your specific install. Our full cost guide breaks down the complete unit-plus-install price; this premium is what pushes a given retrofit toward the higher end of that range rather than the lower end.
How to plan a retrofit
- Don’t assume the old flue works for the new unit. Budget for a dedicated vent run in an approved material, not a tie-in — see our venting guide.
- Get the gas line checked, not just told. A manufacturer’s “you probably don’t need a bigger line” claim and an independent plumber’s “check the gas supply first” instinct can both be right in different houses — only a load calculation against your actual BTU rating and run length settles it.
- Plan the new mounting location before the installer arrives. Confirm it lands on studs capable of carrying 75-plus pounds for a gas unit, and that it leaves the manufacturer’s required service clearance — our dimensions guide has the specifics.
- Price the premium in, not as a surprise. Treat $500–$2,000 as part of the retrofit’s real cost from the start, not an add-on your contractor springs on you mid-job.
- Going electric instead of gas? The retrofit’s infrastructure question shifts from gas line and venting to panel capacity — see our electrical requirements guide for breaker count and amperage by kW rating.
The bottom line
A tankless retrofit isn’t just a bigger version of a tankless-to-tankless swap — it changes what your venting, gas line, and mounting location actually need to be, and industry cost data puts the premium for those changes at roughly $500 to $2,000. The vent almost always has to be a dedicated new run rather than a tie-in to the old metal flue; the gas line may or may not need upsizing depending on whose claim you check first; and the unit typically moves from the floor to a wall that has to carry 75-plus pounds on real framing. Confirm all three before you get a quote, not after. Once the retrofit itself is sorted, our best tankless water heater guide has the current flagship picks across gas and electric.