Guide to Installing a Tankless Water Heater

Planning, Codes, and Sizing for a Tankless Upgrade

Before any tools come out of the toolbox, planning sets the tone for a smooth tankless upgrade. Start with local building and mechanical codes, which govern everything from venting materials to gas line sizing. Permits are often required, and inspections verify safety-critical items such as combustion air and pressure relief routing. A conversation with your local authority helps clarify expectations, avoids rework, and keeps you aligned with safety standards.

Correct sizing is the backbone of performance. Unlike storage tanks that buffer demand, tankless units must meet peak flow in real time. Sizing typically revolves around gallons per minute (GPM) at a specific temperature rise (ΔT). For example, raising incoming water from 45°F to 120°F (a 75°F rise) for a 2.0 GPM shower plus a 0.5 GPM sink means 2.5 GPM at a 75°F rise. A unit’s rated GPM drops as ΔT increases, which is why cold-climate households often select higher-capacity models. Typical household fixtures use: – Showers: 1.8–2.5 GPM – Bathroom faucet: 0.5–1.2 GPM – Kitchen faucet: 1.5–2.2 GPM – Dishwasher: 1.0–2.0 GPM – Washing machine: 1.5–3.0 GPM.

Consider lifestyle and simultaneity. Two back-to-back showers, a laundry cycle, and a kitchen sink draw can push any unit. Map your household’s peak pattern over a week to predict real demand. If you prefer reliable multi-tap performance, plan for a capacity buffer rather than chasing edge-case maximums. This approach reduces temperature dips and extends comfort.

Elevation and water chemistry also matter. Higher altitude reduces gas combustion efficiency, which can derate capacity. Hard water—in many regions above 7 grains per gallon—invites scale that impairs heat transfer. Planning for a scale inhibitor or softening strategy at the outset protects performance. Finally, think through placement: a central location shortens hot-water runs, while proximity to an exterior wall can simplify venting. In short, accurate sizing, code awareness, and thoughtful layout are the trio that make installation smoother and daily use more satisfying.

Site Prep and Utilities: Gas, Electric, Venting, and Combustion Air

Preparing the site transforms a wish list into a workable plan. Gas-fired tankless units demand significant input, often in the 150,000–199,000 BTU/h range, which can exceed the capacity of legacy 1/2-inch gas lines designed for storage tanks. Long runs, elbows, and tees further reduce deliverable volume. Many installations require upsizing to 3/4-inch or 1-inch lines to meet manufacturer-specified pressure at full fire. Include a sediment trap (drip leg) and accessible shutoff valve to protect the appliance and simplify service.

Electric tankless units can be highly efficient at point-of-use but may require substantial amperage for whole-home service. Common configurations demand multiple 40–60A double-pole breakers at 240V, sometimes totaling 100–150A or more. Many homes need a service upgrade to comfortably support an electric whole-home unit. That upgrade changes both cost and schedule, so verify panel capacity early.

Venting options are dictated by the technology. Non-condensing gas units typically use Category III stainless steel venting that resists corrosive flue gases. Condensing units, which reclaim more heat, produce cooler exhaust and condensate; they often use PVC or polypropylene venting with precise pitch for drainage. Keep vent runs short and within specified equivalent lengths; count elbows carefully. Maintain clearances to windows, doors, and property lines to avoid re-circulation and code violations. For indoor models, dedicated combustion air can be ducted from outdoors, improving safety and minimizing negative pressure in tight homes.

Think through condensate management for condensing units. The acidic condensate should be neutralized before discharge to protect plumbing and comply with many codes. A neutralizer cartridge and an air gap to the drain are common. Plan for periodic replacement of neutralizer media.

Finally, staging the work area matters: – Clear a safe working zone with good lighting and a grounded extension for tools – Mount a fire extinguisher within reach when working with gas – Shut off and label existing utilities – Protect floors from solder flux, debris, and condensate. These seemingly small steps add up to a professional finish, reduce rework, and make passing inspection much easier.

Installation Walkthrough: Mounting, Venting, Commissioning, and Safety Checks

With planning complete, the installation proceeds in a deliberate sequence. Start by mounting the unit on a solid wall using rated anchors or studs, keeping clearances per the manual. A cardboard template, if provided, or a measured layout helps align hangers, water lines, and vent penetrations. Leave space below for service valves and filters, and ensure you can remove the front cover for future maintenance without obstruction.

Next, route water lines. Many installers prefer a compact manifold with full-port isolation valves on hot and cold, plus drain ports for descaling. Incorporate: – Cold inlet sediment filtration to catch debris – A check valve where required – A thermal expansion solution if your system has check valves or a PRV – A pressure reducing valve when static pressure exceeds 80 psi. Pressure relief routing should discharge to an approved drain with an air gap, complying with local rules.

Gas piping follows, with attention to correct sizing, leak testing, and support intervals. Use approved thread sealant for gas, pressure test the new line, and soap-test all joints. Include a sediment trap. For electric models, pull appropriately sized conductors, verify grounding, and label the panel. Breakers must match the unit’s specification, and conductors should be sized for run length to minimize voltage drop.

Venting and combustion air are next. Keep penetrations clean, pitched correctly (condensing: pitch back to the unit to return condensate; non-condensing: follow the specified slope), and sealed with approved fire-blocking and exterior flashing. Maintain required terminal clearances. If ducting combustion air, mirror the vent’s routing to minimize pressure imbalances.

Commissioning is where many installs succeed or stumble. Flush the lines to clear flux and debris before connecting to the unit. Fill the heat exchanger, purge air, and check for leaks under static pressure before firing. Start-up typically includes setting outlet temperature (120°F is a widely recommended balance of comfort and safety), verifying combustion parameters, and confirming stable operation across several flow rates. Log: – Inlet and outlet water temperature – Gas manifold pressure (or electrical current draw) at low and high fire – Flue temperature and draft – Vent and combustion air integrity. Conclude with a safety review: proper relief discharge, secure wiring, confirmed leak-free joints, and documentation of settings for the homeowner.

Understanding Efficiency: UEF, Real-World Performance, and Cost of Ownership

Efficiency is a headline feature of tankless technology, but the details matter. The Uniform Energy Factor (UEF) captures how effectively a unit converts energy into hot water over standardized tests. Gas non-condensing units often post UEF values in the 0.80–0.85 range, while condensing gas models commonly reach 0.90–0.96. Electric tankless heaters can approach near-1.00 conversion at the point of use, though the whole-home suitability depends on service capacity. The key practical advantage lies in reduced standby losses: no big tank of water idling all day.

Real-world savings vary with usage patterns. Households that use less than 41 gallons of hot water per day often see notable reductions in energy use for water heating because the system fires only when you open a tap. Larger families can still benefit, especially with condensing models and efficient fixtures, but total savings hinge on inlet temperature, draw length, and simultaneously used fixtures. Recirculation, if used, must be thoughtfully controlled; continuous recirculation erodes savings, while on-demand or timer-based systems preserve most efficiency gains.

From a cost-of-ownership perspective, weigh the higher upfront price and potential utility upgrades against lower fuel consumption and longer service life. Gas piping, venting, or electrical upgrades can meaningfully affect payback timelines. A simple back-of-the-envelope example: – Extra install costs: $800–$2,500 depending on venting and gas/electric work – Annual energy savings: commonly cited in the 8–34% range for water heating, depending on usage and climate – Service life: many units are designed for a long lifespan with proper maintenance. If your household’s pattern favors short, infrequent draws, payback shortens; heavy simultaneous use with recirculation may lengthen it.

Comfort is part of efficiency too. Right-sized units maintain steady outlet temperatures when flow is above the minimum activation rate, often around 0.4–0.6 GPM for modern models. Smart fixture choice—aerated faucets, low-flow showerheads—keeps flow in the unit’s sweet spot. Meanwhile, setting 120°F minimizes scald risk and reduces mineral precipitation inside the heat exchanger, supporting sustained efficiency. The bottom line: efficiency is both a rating on a label and a result of how you design, install, and use the system day to day.

Plumbing Integration and Maintenance: Piping Layouts, Recirculation, and Water Quality

A thoughtful plumbing layout makes a tankless system feel effortless in daily life. Central placement shortens hot-water runs and reduces wait times. Where long runs are unavoidable, insulated piping and strategic branch routing help. Many installers incorporate a service valve kit beneath the unit to streamline flushing and future repairs. A small point-of-use heater in a distant bath or kitchen is sometimes used to trim delays without resorting to continuous recirculation.

Recirculation deserves careful design. It solves the “cold sandwich” and long waits, but it can raise energy use if it runs constantly. Better options include: – Demand-activated recirc with a wall switch or motion sensor – Timer-controlled cycles during busy hours – Temperature-based recirc with tight differential control – Thermostatic balancing valves on returns for even distribution. A dedicated return line gives the cleanest results, though retrofit crossover valves can work when a return is impractical. Insulate both supply and return lines to preserve heat and reduce short-cycling.

Water quality is the quiet variable that decides long-term performance. Hard water accelerates scale on the heat exchanger, raising outlet temperature fluctuations and fuel consumption. A scale inhibitor cartridge or a properly set softener upstream can dramatically slow deposition. Plan on periodic descaling—many manufacturers suggest annual flushing in hard-water regions—using a pump, hoses, and a mild acid like food-grade citric acid. The process is simple with isolation valves: – Close service valves, attach hoses – Circulate solution for 30–45 minutes – Rinse thoroughly and return to service. A sediment filter on the cold inlet traps debris that would otherwise lodge in tight passages.

Don’t overlook safety and code compliance within the plumbing system. If your home has a pressure reducing valve or check valves that create a closed system, provide an expansion solution per local rules. Verify that the pressure relief discharge terminates to an approved drain with the correct air gap. Label shutoffs, leave a simple diagram of the system in the mechanical area, and document filter and neutralizer change intervals. With these practices, your tankless system delivers smooth, consistent hot water while staying efficient and easy to service—the kind of quiet reliability that makes a home feel well tuned.