Code and quoting
Tankless water heater installation: everything to quote beyond the unit
Going from a tank to a tankless heater is not a swap. The gas line, the vent, the condensate drain, the outlet, the service valves, the water and the permit all change. Here is what each one involves, with a checklist so none of it is missing from the quote.
· 9 min read
On this page
The customer sees one box on the wall replacing one tank. You see a gas line that may be too small, a vent that can't be reused, a condensate drain, an outlet, service valves and a permit that may need a gas piping plan. Quote it like a tank swap and you lose money on each.
This guide covers what a gas tankless install involves beyond hanging the unit, and ends with a checklist to run before you send the price.
The examples below come from one manufacturer's manual, Rinnai's installation and operation manual for its RU-series condensing heaters. Other brands and models have different numbers, so use the manual for the unit you're installing. Your local code and inspector have the final word.
What changes from a tank swap
| Item | Typical tank swap | Tank to tankless |
|---|---|---|
| Gas supply | Reuse the line | Check sizing for a much higher input; often a larger or new line |
| Venting | Reuse the vent | New listed vent system for the unit; the old vent usually can't be used |
| Condensate | None | Drain line, often a neutralizer, sometimes a pump (condensing units) |
| Electrical | None for an atmospheric tank | A 120-volt outlet or a disconnect |
| Service valves | Shutoff on the cold side | Isolation valves with service ports for flushing |
| Permit | Usually a plumbing permit | Often plans or gas sizing calculations as well |
Gas line sizing
A whole-house tankless heater burns a lot of gas at full fire. The Rinnai RU199i, for example, is rated from 15,000 to 199,000 Btu/h. Read the rating plate on the old tank before you quote: the difference between the two inputs is what the gas piping now has to carry, on top of everything else in the house.
Rinnai's manual says the installation must follow local codes or, where there are none, the National Fuel Gas Code (ANSI Z223.1/NFPA 54), and it tells you to size the pipe from the tables in the applicable code. The load to size for is the heater plus every other gas appliance at the location, all at full input. Its worked example:
Cubic feet per hour = (heater input + other appliances) ÷ heating value of the gas
With a 199,000 Btu/h heater, 65,000 Btu/h of other appliances and gas at 1,000 Btu per cubic foot, that's 264 cubic feet per hour. In the manual's NFPA 54 table for Schedule 40 metallic pipe at a 0.5 in. water column pressure drop, 3/4 in. pipe carries 360 cubic feet per hour at 10 feet but only 247 at 20 feet. So the same heater needs 3/4 in. pipe on a 10-foot run and 1 in. pipe on a 20-foot run. That is why you measure the run before you price it.
Two more numbers from the same manual matter on site:
- Supply pressure. 3.5 to 10.5 in. water column for natural gas, and 8.0 to 13.5 in. for propane, measured with all gas appliances running.
- Shutoff and testing. A manual gas valve in the supply to the heater, and a leak and pressure test before it runs.
If the meter or regulator may not keep up with the new total load, ask the gas utility before you commit to a price.
Venting
Tankless heaters don't vent like an atmospheric tank. Foster City, California's tankless checklist puts it plainly: most use positive-pressure venting, installed per the manufacturer's instructions with a listed system for Category III or IV appliances, sealed airtight to the termination, and Type B vent is not permitted for positive-pressure venting. In practice, the old B-vent comes out or gets capped, and you're pricing a new vent run and new penetrations.
For the condensing RU series, Rinnai's manual sets these rules, among others:
- Materials. Listed Category IV vent or, when local code accepts it, Schedule 40 PVC (for room-air installs), or approved direct-vent products. No cellular core PVC/CPVC, no Radel or galvanized, and no connection to an existing vent or chimney.
- Length. Up to 65 equivalent feet with 2 in. vent and 150 feet with 3 in. (concentric 2 x 4 in. and 3 x 5 in.), counting each 90° elbow as 6 feet and each 45° as 3 feet.
- Slope and support. Horizontal exhaust sloped 1/4 in. per foot back toward the heater; supports every 4 feet horizontally and 6 feet vertically, or per the vent maker.
- Termination. At least 12 in. above grade or the expected snow level. For US direct-vent installs, 12 in. from an opening window or door, and 3 feet above a mechanical air intake within 10 feet horizontally.
Condensate drain and neutralizer
A condensing unit makes acidic water, and it has to go somewhere. Rinnai's manual calls for corrosion-resistant drain material such as PVC, at least 1/2 in. along the whole length, sloped to the drain, with the end open to the air and never submerged. It must not share a line with the pressure relief discharge or connect to an air conditioner's evaporator drain. The unit has its own trap, so you don't add one.
If there's no floor drain low enough, the manual says to install a condensate pump, and to wire its safety switch so the heater shuts down if the pump fails. Rinnai also sells a neutralizer kit that raises the condensate's pH to help protect the drain and the public sewer; whether one is required is a local code question. Foster City's checklist, for example, allows condensate to discharge onto soil but not over concrete or walkways.
Electrical outlet
Gas tankless heaters still need power for the controls and fan. The Rinnai units need 120 volts from a properly grounded circuit. Indoor models come with a 5-foot cord for a standard three-prong outlet, with no extension cords or adapters; outdoor models need a disconnect switch suitable for outdoor use.
Local rules add to that. Foster City requires a GFCI-protected receptacle in a garage, a GFCI weather-resistant outlet with a bubble cover (or a hard-wired disconnect in sight) outdoors, a receptacle and switched light for attic or basement installs, and an electrical permit for new electrical work. If there's no outlet within cord length, price the electrician.
Whole-house electric tankless is a different job: HomeAdvisor's guide notes it needs at least a 240-volt, 80-amp hard-wired circuit, and a home with at least 150-amp service.
Isolation valves and flushing
Scale is what kills a tankless heat exchanger. Rinnai's manual says damage from lime build-up isn't covered by the warranty and strongly recommends isolation valves so the heat exchanger can be flushed. The RU units ship with an isolation valve kit. Its flushing procedure circulates four gallons of undiluted white vinegar through the heater with a pump for at least an hour, through the service ports on those valves.
The same manual requires a pressure relief valve rated to 150 psi and at least the heater's maximum Btu/h, with nothing between the valve and the heater, and a 3/4 in. discharge line ending 6 in. above a drain where it can be seen. Price the valve, the discharge line and the first flush. A yearly flush is also a natural maintenance agreement.
Water quality and softening
Rinnai lists water quality limits for its heaters, including total hardness up to 200 mg/L, chlorides up to 250 mg/L, pH between 6.5 and 8.5 and total dissolved solids up to 500 mg/L. In hard-water areas, its manual says the water must be treated and may need more frequent flushing, and damage from water quality isn't covered by the warranty.
Check the hardness before you quote. If it's over the limit, offer treatment as an option, and write down that you recommended it.
Thermal expansion
Rinnai's manual says that when the heater is installed on a closed system, for example one with a backflow preventer on the cold supply, a means to control thermal expansion must be provided, and to contact the water supplier or local plumbing inspector on how. It also notes that a relief valve that discharges from time to time may be a sign of thermal expansion in a closed system. Check your brand's manual and ask your inspector. Our guide to the water heater expansion tank requirement covers how to tell if a system is closed.
Permits
Expect more paperwork than a tank swap. Two examples:
- Foster City, California applies its tankless checklist to all installations, replacements included. It asks for a floor plan, whether the job is like-for-like or new, and, for new installations, gas line sizing calculations with lengths and an isometric riser diagram. The manufacturer's instructions must be on site for inspections, and new or altered gas piping must be pressure tested.
- The City of Las Vegas requires a site or floor plan showing the gas line from start to finish, the gas line's total developed length, total demand, pipe size and material, how the heater will be vented, and how it gets combustion air, regardless of the size of the work.
Put the plan-drawing time, the permit fee and the inspection visit on the quote.
What homeowners are told it costs
Consumer cost guides publish national averages. They tell you what the homeowner has read, not what to charge.
| Source (homeowner averages) | Figure | Range |
|---|---|---|
| HomeAdvisor, updated June 17, 2026 | Tankless installation, average $2,641 | $1,406 – $3,902 |
| Fixr, updated January 31, 2025 | Gas tankless, total installed | $1,400 – $3,500 |
| HomeAdvisor, updated June 20, 2026 | Gas line installation, average $598 | $272 – $936 |
Your own number should come from your hourly cost, your materials markup and the real time the job takes. See how to price plumbing jobs.
Plumbo, now on the App Store and Google Play, turns a voice note into a quote from your own price book, then checks it for forgotten items like an expansion tank, shutoffs or the permit before it goes out. Try the 60-second demo →
Tankless quote checklist
- Unit: model, Btu/h input and flow rate for the house's peak demand.
- Gas line: measured length, total connected load, pipe size from the code table, new runs or upsizing.
- Gas pressure: supply pressure check, meter or regulator question for the utility, pressure test.
- Venting: material, diameter, equivalent length, terminations, wall or roof penetrations, capping the old vent.
- Condensate: drain line, neutralizer, pump and safety switch wiring if there's no low drain.
- Electrical: outlet within cord length, GFCI or disconnect, electrician and permit if needed.
- Isolation valves with service ports, and the first flush.
- Pressure relief valve and its discharge line.
- Thermal expansion control, if the system is closed.
- Water treatment, as a line or an option, based on a hardness test.
- Drain pan where a leak would cause damage.
- Mounting, haul-away of the old tank, and wall patching.
- Permit: plan and gas calculation time, fee, inspection visit.
Also run the water heater replacement quote checklist: the items that go missing on tank quotes go missing here too.
Sources
- Rinnai, RU199iN product page, and the N Series Residential Condensing Installation and Operation Manual linked from it (100000467, rev. 07: specifications, gas sizing tables and example, venting, condensate, power, isolation valves, relief valve, flushing, water quality, thermal expansion), checked October 8, 2026.
- City of Foster City Building Division, Tankless Water Heater Check List (PDF: permit documents, venting, condensate, electrical, gas piping), checked October 8, 2026.
- City of Las Vegas Building & Safety, Tankless Water Heater Guideline (PDF: plan and gas line requirements), checked October 8, 2026.
- HomeAdvisor, How Much Does a Tankless Water Heater Cost? (updated June 17, 2026: average and normal range, electric circuit requirement), checked October 8, 2026.
- HomeAdvisor, How Much Does Gas Line Installation Cost? (updated June 20, 2026: average and normal range), checked October 8, 2026.
- Fixr, How much does it cost to install a tankless water heater? (updated January 31, 2025: gas installation range), checked October 8, 2026.