Switching to a tankless water heater is one of the smartest home upgrades you can make. No more running out of hot water mid-shower. No more heating 50 gallons of water around the clock just in case someone needs it. Tankless units heat water on demand, last twice as long as tank heaters, and cut energy bills noticeably over time.
Knowing how to install tankless water heater correctly matters more than most people realize. A poor install leads to leaks, venting failures, and gas pressure issues that show up fast. This guide covers every step from choosing the right unit to testing the finished install.
What Is a Tankless Water Heater?
A tankless water heater also called an on-demand water heater or instantaneous water heater heats water directly as it flows through the unit. There’s no storage tank involved.
When you turn on a hot water tap, cold water enters the unit. A gas burner or electric element fires and heats the water as it passes through a heat exchanger. Hot water arrives at the tap within seconds and keeps flowing as long as the tap is open.
The result is unlimited hot water with no standby heat loss and the energy wasted keeping a tank warm 24 hours a day.
Benefits of Installing a Tankless Water Heater
Before getting into the water heater tankless installation process, it helps to understand what you’re getting.
Unlimited hot water. A properly sized tankless unit never runs out. It heats on demand three showers at once is possible with the right unit size and gas supply.
Longer lifespan. Tank heaters last 8 to 12 years. Tankless units last 15 to 25 years with proper maintenance. That’s one fewer replacement over a 25-year homeownership period.
Lower energy costs. The U.S. Department of Energy estimates that tankless water heaters are 24% to 34% more energy efficient than tank units for homes using under 41 gallons of hot water per day. Savings are higher in households with low to moderate usage.
Space savings. A tankless unit mounts on the wall. It’s roughly the size of a small suitcase. A 50-gallon tank heater takes up a significant floor footprint. The difference is meaningful in small utility rooms or tight spaces.
Federal tax credit eligibility. High-efficiency tankless units qualify for the federal Energy Efficient Home Improvement Credit (25C) up to $600 in tax credits in 2026. Confirm the specific unit’s UEF (Uniform Energy Factor) rating meets the qualifying threshold before purchasing.
Types of Tankless Water Heaters
Choosing the right type is the first step in any water heater installation tankless project.
Gas Tankless Water Heater
Gas units are the most common choice for whole-home installations. They deliver higher flow rates typically 6 to 12 GPM and work well even when incoming water is cold.
Natural gas and propane models are available. Natural gas is more common in urban and suburban areas. Propane units work in areas without natural gas access.
Condensing gas tankless units are the high-efficiency option. They capture heat from exhaust gases that non-condensing units vent outside. Condensing units have UEF ratings above 0.90 and use PVC vent pipes instead of metal flue pipes.
Non-condensing gas tankless units are less expensive upfront but slightly less efficient. They require metal venting (stainless steel or Category III aluminum) due to higher exhaust temperatures.
Electric Tankless Water Heater
Electric units are simpler to install with no gas line, no venting required. But they have two limitations:
First, flow rate. Electric whole-home units max out at 4 to 6 GPM in most configurations lower than gas equivalents. In cold climates where incoming water is below 55°F, output is further reduced.
Second, electrical demand. A whole-home electric tankless unit requires 150 to 200 amps dedicated to the unit alone. Most homes have 200-amp total service. Upgrading the electrical panel to support a whole-home electric tankless unit is expensive and not always practical.
Electric point-of-use units small heaters serving one fixture are a different story. They’re affordable, easy to install, and work well for a bathroom or kitchen sink far from the main water heater.
Pre-Installation Considerations
Several things must be confirmed before the install begins. Skipping this step leads to surprises mid-job.
Gas Line Capacity
Most tankless gas units require a 3/4-inch or 1-inch gas line with adequate pressure at the unit — typically 4 to 14 inches water column for natural gas. Many homes have 1/2-inch lines running to the existing water heater location. That’s undersized for most tankless units.
Have a licensed plumber check gas line size and pressure before ordering the unit. An undersized gas line causes performance problems and error codes that are difficult to diagnose after the fact.
Electrical Requirements
Every tankless water heater, even gas models, needs electrical power for the control board, ignition, and fan. Gas units typically need a standard 120-volt outlet nearby. Confirm one is accessible at the planned mounting location.
Venting for Gas Units
Gas tankless units need dedicated venting separate from any other appliance. Sharing a flue with a furnace or conventional water heater is not permitted.
Direct vent (concentric vent): Uses two pipes one for combustion air intake from outside, one for exhaust. This is the most common setup and the safest, since all combustion air comes from outside.
Power vent: Uses indoor air for combustion and a powered fan for exhaust. Requires adequate indoor air volume for safe combustion.
Confirm vent run length doesn’t exceed the manufacturer’s maximum. Longer vent runs reduce draft efficiency and can trigger error codes. Most manufacturers allow 35 to 100 feet of total vent run depending on the unit.
Water Quality
Hard water builds scale on the heat exchanger. If your water hardness exceeds 11 grains per gallon (GPG), a water softener or dedicated scale filter is worth installing at the same time. Scale buildup on a heat exchanger causes the same damage as sediment in a tank, reduced efficiency and early failure.
Permits
Pull a permit before starting. Most jurisdictions require one for tankless water heater tankless installation both gas and electric. The permit triggers an inspection that confirms the installation meets code. Skipping the permit creates problems at home sale and can void the manufacturer’s warranty.
Step-by-Step Installation Guide: How to Install Tankless Water Heater
This guide covers a gas condensing tankless unit, the most common whole-home installation type. Electric installations follow similar steps but omit gas lines and venting work.
Step 1: Choose the Mounting Location
Mount the unit on an exterior wall when possible. This minimizes vent run length, which keeps the install simpler and within manufacturer limits.
The unit needs:
- Clearance on all sides per the manufacturer spec (typically 12 inches minimum on sides and top)
- Access to the gas line
- Access to water supply lines
- A 120-volt outlet within reach
- A condensate drain nearby (condensing units only)
Step 2: Mount the Bracket and Hang the Unit
Most tankless units come with a mounting bracket. Mark the bracket position on the wall, confirm its level, and anchor it into wall studs or use appropriate wall anchors for the unit’s weight.
Hang the unit on the bracket. Confirm it sits level a unit mounted at an angle affects condensate drainage on condensing models.
Step 3: Run the Vent Pipes
For a condensing unit, PVC vent pipes (typically 3-inch or 4-inch diameter) run from the unit through the wall to the exterior. The exhaust and intake pipes exit through concentric holes or separate penetrations within manufacturer-specified spacing.
Seal all wall penetrations with fire-rated caulk or expanding foam rated for this use. Cap the exterior ends with proper vent termination fittings these prevent wind, rain, and pests from entering the vent.
Keep elbows to a minimum. Each 90-degree elbow reduces effective vent run length by 5 to 10 feet depending on the manufacturer spec.
Step 4: Connect the Water Lines
The cold water inlet connects to the unit’s cold port (typically marked with blue or “C”). The hot water outlet connects to the hot port (typically marked with red or “H”).
Install isolation valves on both the cold inlet and hot outlet before connecting to the unit. These allow you to service the unit without shutting off the whole house water supply.
Install a pressure relief valve on the cold water inlet if not already present in the supply line.
For hard water areas: Install a scale filter on the cold water inlet line at this stage. It fits in line and traps minerals before they reach the heat exchanger.
Use Teflon tape on all threaded connections. Flexible braided stainless connectors make the water connections easier to manage than rigid copper in tight spaces.
Step 5: Connect the Gas Line
This step requires a licensed plumber or gas fitter in most states. Do not attempt this without the right credentials and tools.
The gas supply line connects to the gas inlet port on the unit. Use a new flexible gas connector rated for the BTU output of the unit. Never reuse an old gas connector.
Apply gas-rated thread sealant (not Teflon tape) to the threaded connections. Tighten all connections securely.
After connections are made, test for gas leaks before proceeding. Apply a gas leak detection solution to every joint and watch for bubbling. A digital gas detector is even more reliable. Any leak must be corrected before going further.
Step 6: Connect the Condensate Drain (Condensing Units Only)
Condensing tankless units produce acidic condensate water with a pH of 3 to 5 as a byproduct of their high-efficiency combustion. This drain must route to an appropriate drain point.
Most manufacturers require a condensate neutralizer kit if draining to a sanitary sewer the acidic condensate can damage cast iron drain pipes over time. The neutralizer contains limestone chips that raise the pH before the condensate enters the drain.
Step 7: Connect the Electrical
Plug the unit’s power cord into the dedicated 120-volt outlet. If the outlet isn’t present, have a licensed electrician install one before proceeding. Do not use an extension cord.
Step 8: Fill the System and Check for Leaks
Open the cold water inlet valve slowly. Let the unit fill. Open a hot water faucet in the house to bleed air from the system. Once water flows steadily from the faucet, the unit is full.
Inspect all water connections carefully. Even a very small drip needs to be corrected now — not after the unit has been running for a month.
Step 9: Power On and Test
Turn the gas supply valve on. Restore electrical power to the unit. Following the manufacturer’s startup sequence most units require a specific power-on process before ignition.
Open a hot water faucet. The unit should ignite and begin heating water within a few seconds. Hot water should arrive at the tap within 15 to 30 seconds depending on pipe run length.
Check for error codes on the unit’s display. Most tankless units have self-diagnostic systems that flag any issue of gas pressure, venting, water flow, or ignition problems.
Let the unit run for 10 to 15 minutes. Check all connections against water, gas, and venting before declaring the install complete.
You can read about: How Much to Install Tankless Water Heater
Installation Cost
Install tankless water heater costs in 2026 break down like this:
Unit cost: $500 to $2,000 depending on type and output capacity. Condensing high-efficiency gas units sit at the higher end.
Labor cost: $300 to $1,000 for a standard installation. Complex jobs with gas line upgrades and full vent runs cost more.
Additional materials: Gas line fittings, flexible connectors, venting materials, isolation valves, condensate neutralizer add $100 to $400 depending on what’s already in place.
Total typical range: $1,200 to $3,500 for a professional whole-home gas tankless installation.
Gas line upgrade (if needed): add $300 to $800. Electrical panel work (if needed for electric unit): add $500 to $1,500.
Common Mistakes During Tankless Water Heater Installation
These are the errors that cause the most callbacks and failures:
Undersizing the unit. The most common mistake. Always size for peak demand at the lowest expected incoming water temperature.
Inadequate gas line. A 1/2-inch gas line that worked fine for a tank heater often can’t supply the higher BTU demand of a tankless unit. Always confirm line capacity before ordering the unit.
Exceeding vent run limits. Too many elbows or a run that’s too long reduces draft efficiency. Error codes and inconsistent ignition follow.
No isolation valves. Installing without isolation valves means the whole house water supply must be shut off for any future servicing. This is a small cost to add upfront and a significant inconvenience to skip.
Skipping the condensate neutralizer. Acidic condensate in a cast iron drain line corrodes it from the inside. Over several years, this causes drain failures that are expensive and messy to repair.
No scale protection in hard water. A heat exchanger coated in calcium scale within two years of install is a direct result of skipping water quality assessment. A $50 scale filter avoids a $400 heat exchanger descaling job.
Maintenance Tips for Tankless Water Heaters
Installing tankless water heater is only the beginning. Maintenance determines how long it performs.
Annual descaling. Flush the heat exchanger with a food-grade descaling solution (white vinegar or commercial descaler) once a year. Hard water areas should do this every 6 months. Most manufacturers void the warranty if descaling is not performed.
Inlet filter cleaning. Most tankless units have a small screen filter at the cold water inlet. Check and clean it every 6 to 12 months. A clogged screen reduces flow and triggers low-flow error codes.
Venting inspection. Check the vent termination on the exterior wall annually. Bird nests, wasp nests, and debris block exhaust vents and cause ignition failures. Clear any obstruction immediately.
Error code log. Most modern tankless units record error codes. Review them annually with a plumber. A pattern of recurring codes often signals a developing problem before it becomes a failure.
Professional service every 2 to 3 years. A licensed technician can check gas pressure, combustion efficiency, heat exchanger condition, and all safety interlocks. This is especially important after year 10.
Conclusion
How to install tankless water heater the right way comes down to three things: choosing the correct unit size, ensuring adequate gas supply and proper venting, and getting every connection right before the unit fires for the first time. Done correctly, a tankless unit delivers unlimited hot water, lower energy bills, and 20 or more years of reliable service.
This is not a project to rush or cut corners on. Gas connections and venting work require a licensed professional in most states and for good reason. The consequences of a gas leak or a blocked flue are serious.
For homeowners in the Eagle Rock area ready to make the switch, Tankless Water Heater Installation in Eagle Rock is a service that handles every step correctly. Derks Plumbing provides licensed installation, permit pulling, gas line assessment, and full testing so your new unit starts right and runs for years without issues.
FAQs
Q: How to install tankless water heater if I’m replacing a tank unit?
The process requires removing the old tank, capping or rerouting old vent connections, running new vent pipes through an exterior wall, confirming gas line size, and making new water connections. Most conversions take 4 to 8 hours with a licensed plumber.
Q: Can I install a tankless water heater myself?
Electric point-of-use units are DIY-friendly for homeowners with basic electrical and plumbing skills. Whole-home gas units are not gas line work and venting requires licensed contractors in most states.
Q: What size tankless water heater do I need for a 3-bedroom home?
Most 3-bedroom homes need a unit rated for 7 to 9 GPM at the expected temperature rise for your climate. In warmer climates with high incoming water temperatures, a 7 GPM unit often suffices. In colder climates, size up.
Q: How long does tankless water heater installation take?
A standard whole-home gas tankless installation takes 4 to 6 hours. Conversions from tank units with new gas line runs and full vent installation can take 6 to 10 hours or span two days.
Q: Does water heater tankless installation require a permit?
In most U.S. jurisdictions, yes. Gas appliance installations and new electrical circuits both require permits. The permit triggers an inspection that confirms the install meets code which protects you, your warranty, and your homeowner’s insurance coverage.
