Endless hot water and a reclaimed corner of the utility room are the twin rewards of going tankless, but the process is far more involved than swapping a tank. A proper tankless hot water heater installation touches your plumbing, gas supply, venting, and electrical, which is why it demands careful planning and, for gas units, professional handling. Understanding the full scope helps you make smart choices and avoid the mistakes that leave homeowners with lukewarm showers.
This guide walks through sizing, the differences between gas and electric setups, the installation sequence, and the permits and pro tips that separate a smooth job from a callback. Even if you hire out the work, knowing the steps helps you supervise and budget intelligently.
How Tankless Heaters Work
Unlike a storage tank that keeps 40 to 50 gallons hot around the clock, a tankless unit heats water only on demand. When you open a hot tap, cold water flows through the unit, a gas burner or electric elements fire, and water is heated instantly as it passes through a heat exchanger. When you close the tap, the unit shuts off.
That on-demand design is what eliminates standby heat loss and delivers a continuous supply. The catch is that flow rate, measured in gallons per minute, is finite, so sizing the unit to your household’s peak demand is the single most important decision.
Sizing the Unit Correctly
An undersized tankless heater is the top complaint, so calculate your peak demand before buying. Add up the flow rates of the fixtures you might run at once, then account for how much the unit must raise the incoming water temperature, called temperature rise.
- A shower uses about 2 to 2.5 gallons per minute; a kitchen faucet about 1.5.
- Groundwater enters at roughly 40°F in the North and 70°F in the South, so cold climates need more capacity for the same output.
- A whole-home gas unit typically delivers 7 to 11 gallons per minute; electric whole-home units manage 3 to 8 depending on the model and climate.
- For a family running two showers at once in a cold climate, plan on a high-output gas unit around 9 to 11 GPM.
Gas vs. Electric Installation
The fuel type dictates most of the installation work. Gas units heat more water and suit whole-home use in cold regions, but they require venting and often a larger gas line. Electric units install more simply with no venting but may need an electrical service upgrade.
Gas Setup
Gas condensing units like the Navien NPE-2 or Rinnai RSC series need a 3/4-inch gas line in many cases, plus dedicated venting. Condensing models vent through inexpensive PVC and produce acidic condensate that needs a drain, while non-condensing units require stainless steel Category III venting.
Electric Setup
Electric whole-home units such as the Rheem RTEX or Stiebel Eltron Tempra draw heavy amperage, often needing two or three double-pole breakers and sometimes a panel upgrade. There is no venting, which simplifies placement, so these are popular for additions and warm climates.
Tools and Materials
A gas install calls for pipe wrenches, a tubing cutter, gas-rated pipe and fittings, venting components, mounting brackets, isolation valves, Teflon tape or pipe dope, a level, and a drill. Add a voltage tester and wire for the control power. Always confirm you have the manufacturer’s approved venting kit, since mismatched venting is both a code violation and a safety hazard.
The Installation Sequence
Here is the general order a professional follows for a gas unit. Even as a supervising homeowner, knowing the flow helps you spot shortcuts:
- Shut off utilities: Turn off water, gas, and electrical, then drain and remove the old tank if replacing.
- Mount the unit: Fasten the heater to a solid wall at the manufacturer’s clearances, usually indoors near an exterior wall for short vent runs.
- Run venting: Install the intake and exhaust venting through the wall or roof, sloped for condensate drainage on condensing models.
- Connect gas: Run or upgrade the gas line to the required diameter and install a shutoff valve and sediment trap.
- Plumb the water lines: Connect cold in and hot out, adding isolation valves for future descaling and a pressure relief valve.
- Wire the electrical: Provide the required circuit for controls on gas units or the heavy-gauge circuits for electric units.
- Add the condensate drain: Route condensate to a drain, often through a neutralizer.
- Test and commission: Check for gas and water leaks, purge air, set the temperature, and verify flow.
Permits, Codes, and Safety
Tankless installations almost always require permits and inspection because they involve gas, venting, and electrical. Skipping permits can void insurance, fail a future home sale, and, more importantly, risk carbon monoxide or gas leaks. Set the output temperature no higher than 120°F to prevent scalding while limiting mineral scale.
Gas work should be done by a licensed plumber, and any panel or heavy-circuit electrical by a licensed electrician. A DIY point-of-use electric unit under a sink is reasonable for a skilled homeowner; a whole-home gas conversion is not.
Protecting Your Investment
Hard water is the enemy of tankless heaters because scale coats the heat exchanger and cuts efficiency. In hard-water areas, install a water softener or an inline scale filter, and flush the unit with white vinegar annually using the isolation valves. This simple maintenance keeps the unit running efficiently for its full 20-plus-year lifespan.
- Descale annually, or every six months on very hard water.
- Clean the inlet water filter screen a couple of times a year.
- Check the venting and condensate drain for blockages each year.
- Keep the manufacturer’s clearances clear of stored items.
Where to Locate the Unit
Placement affects both performance and installation cost. The ideal spot is an interior wall near an exterior wall, which keeps vent runs short and centralizes hot-water delivery to your main fixtures. Locating the unit far from the bathrooms and kitchen it serves increases the wait for hot water and may push you toward a recirculation system.
Gas units must maintain manufacturer clearances from combustibles and need access to combustion air and venting, so a garage, basement, or utility closet with proper ventilation works well. Electric units are more flexible since they need no venting, letting you tuck a whole-home model into a closet or even a large cabinet, provided the electrical service reaches it.
Indoor vs. Outdoor Models
In mild climates, outdoor-rated tankless units mount on an exterior wall and vent directly to the air, eliminating indoor venting entirely and freeing up interior space. They include built-in freeze protection down to certain temperatures, but in regions with hard freezes an indoor unit is the safer, more reliable choice.
If you choose an outdoor model in a cold climate, confirm its freeze-protection rating and consider a recirculation loop or pipe insulation to guard the water lines. For most northern homes, an indoor installation with proper venting remains the standard.
Common Installation Mistakes to Avoid
A handful of errors account for most tankless disappointments. Undersizing the unit tops the list, leaving homeowners with lukewarm showers when two fixtures run at once. Improper venting is both dangerous and a code violation, and using an undersized gas line starves a gas unit so it cannot reach its rated output.
- Skipping isolation valves, which makes annual descaling far harder.
- Ignoring water hardness and letting scale ruin the heat exchanger.
- Setting the temperature too high, wasting energy and risking scald.
- Forgetting the condensate drain and neutralizer on condensing gas models.
- Pulling no permit, which can void insurance and complicate a home sale.
A well-planned tankless hot water heater installation rewards you with decades of endless hot water, lower energy bills, and valuable reclaimed space. Size the unit to your peak demand, respect the venting and gas requirements, pull the proper permits, and stay on top of descaling, and your tankless system will outlast two or three conventional tanks with room to spare.
