How to Install a Tankless Water Heater

Why this matters

Tankless water heater installs are a premium-margin plumbing service: typical project, 4-8 hours labor. Customer gets endless hot water, smaller footprint, longer equipment life (20+ years vs 10-12 for tank). Done wrong (undersized gas line, wrong venting, no condensate drain), tankless underperforms + frustrates customers. This is the working procedure.

When tankless wins

Strong candidates:

• Family with high hot-water demand (4+ bath, large laundry, frequent guests)
• Existing tank failing replacement (cost differential smaller)
• Small mechanical room (tankless is wall-mounted, frees floor space)
• High utility costs (energy savings 20-40% over tank gas)
• Owner staying long-term (20+ year ROI)
• Owner replacing functional tank (often not worth swapping early)

Weak candidates:

• Light hot water demand (single occupancy, small home)
• Cold incoming water (Northeast winter; needs larger unit)
• Old gas service that can't be upsized
• Hard water (scale builds on heat exchanger; descale required annually)

Sizing

Tankless rated by flow rate at temperature rise:

• Cold-climate (50°F incoming): aim for 6-7 gpm continuous
• Mid-climate (60°F incoming): 7-8 gpm
• Warm climate (70°F incoming): 8-9 gpm

Translation to fixtures:

• Shower: 1.5-2.5 gpm
• Dishwasher: 1.5-2 gpm
• Washing machine: 1.5-2 gpm
• Kitchen sink: 1.5-2 gpm

If customer wants 2 showers simultaneously + dishwasher: 5-6 gpm. Pick a tankless rated for that flow at customer's incoming temp.

Most popular sizes:

• 6-7 gpm: unit (Rinnai RU130, Navien NPE-240, A.O. Smith ATI-340)
• 7-9 gpm: unit (Rinnai RU180, Navien NPE-A2 240)
• 9-11 gpm: unit (Rinnai RU199, premium series)

Gas line sizing

Tankless units consume more BTU than tank water heaters:

• Tank: 30,000-40,000 BTU
• Tankless: 150,000-200,000 BTU (5x higher peak demand)

Most homes need gas line upgrade for tankless:

• 1/2" black iron OK up to ~30-50 ft for 150,000 BTU
• 3/4" needed for longer runs OR larger tankless OR additional appliances on the same line
• Verify per NFPA 54 Table 6.2

If existing line undersized: re-pipe OR install dedicated line for tankless. add to project.

Venting

Two types of tankless:

Non-condensing:

• Combustion gases ~300°F
• Stainless steel Category III vent (positive pressure)
• More expensive vent + harder to install
• Less common now

Condensing (modern standard):

• Combustion gases ~120°F
• PVC vent (Schedule 40)
• Cheaper + easier vent
• Slightly higher efficiency
• Produces condensate (drain needed)

For most installs: condensing. PVC venting through wall or roof, 2-3" diameter typical, max length per manufacturer.

Electrical

• 120V dedicated 15A circuit for controls + igniter
• Some premium units 240V (verify)
• Outdoor disconnect required (NEC + manufacturer)

Materials + tools

• Tankless heater unit
• Mounting bracket + lag screws (or plywood backer)
• Black iron pipe + fittings for gas (or CSST flex if certified)
• Pipe dope (gas-rated)
• 3/4" copper or PEX for water supply + outlet
• PVC for vent (Sch 40, primer + cement)
• Condensate drain line (PVC or 1/2" PE)
• 240V or 120V cable + disconnect
• Wall plate / penetration sleeve
• Standard plumbing + HVAC tools
• Pressure test gauge for gas
• Combustion analyzer for commissioning

Procedure

Step 1: Site survey + plan

1. Locate mounting position (interior wall, mechanical room, garage)
2. Verify clearances: 12-24" front, 6-12" sides, 12+" top
3. Plan gas line route
4. Plan vent route (through-wall or through-roof)
5. Verify electrical access
6. Verify condensate drain path
7. Discuss with customer; photograph
8. Pull permit

Step 2: Remove old water heater (if applicable)

1. Shut gas + water at supplies
2. Drain old tank to laundry tub or floor drain
3. Disconnect water + gas
4. Cap old gas line (or repurpose)
5. Disconnect electric (if electric tank) or vent
6. Remove old tank

Step 3: Mount tankless

1. Mount bracket level on wall (stud or plywood backer)
2. Verify clearances + access
3. Hang unit on bracket
4. Some units have specific weight rating; verify

Step 4: Plumbing connections

1. Cold supply: 3/4" copper or PEX with shutoff
2. Hot output: 3/4" copper or PEX
3. Service valves (isolation valves at unit are highly recommended; manufacturer often supplies)
4. Recirculation loop if applicable
5. Pressure relief valve + discharge pipe (typically to floor)
6. Mixing valve at outlet if scalding protection required

Step 5: Gas connection

1. Run gas line if needed (size per NFPA 54)
2. Shutoff valve within 6 ft of unit
3. Sediment trap (drip leg)
4. Union for service
5. Flexible appliance connector to unit (or hard-pipe)
6. Pressure-test new gas piping per code
7. Soap-test connections

Step 6: Vent + intake

For condensing (typical):

1. PVC vent from unit, slight upward slope
2. Through wall or roof per manufacturer
3. Intake air pipe also (concentric vent: pipe-in-pipe; or separate intake)
4. Termination clearances (10+ ft from windows, doors, intakes)
5. Bird screen at exit

Step 7: Electrical

1. 120V or 240V cable from disconnect to unit
2. Verify breaker matches
3. Ground per NEC

Step 8: Condensate drain

1. Connect drain from unit to floor drain, condensate pump, or laundry tub
2. Acid-neutralizer cartridge inline (condensate pH 2.5-3.5; neutralizer raises to safe levels)
3. Verify slope + no traps

Step 9: Commission

1. Open water supply; bleed air from system
2. Open gas supply
3. Power up
4. Verify ignition
5. Test hot water at fixture; verify flow + temperature
6. Combustion analysis: CO2 7-9%, CO < 100 ppm
7. Verify recirculation (if applicable)
8. Test temperature setpoint

Step 10: Customer briefing + paperwork

1. Operating temperature setting
2. Annual descaling recommendation
3. Filter cleaning (water inlet screen)
4. Error code basics + Wi-Fi setup
5. Manuall service contract offer
6. Warranty registration
7. Photos + permits documentation

Acceptance criteria

• Hot water at all fixtures (flow + temperature)
• Gas pressure test passed
• Combustion within spec
• No leaks (gas, water, condensate)
• Customer signed + briefed
• Permit closed

Common pitfalls

• Undersized gas line: appliance starves at peak demand
• Wrong vent material: B-vent on condensing unit fails (corrosion)
• No condensate drain: water on the floor + acid damage
• No service valves: future servicing requires draining whole loop
• No customer education: customer doesn't descale; warranty void
• Skipping permit: AHJ + insurance issue
• Recirculation oversized: water heater short-cycles + fails early
• Hard water without softener: scale buildup; failure in 3-7 years

IRA + utility incentives

References

• NFPA 54 / ANSI Z223.1 National Fuel Gas Code
• ASME water heater + storage tank standards
• ENERGY STAR Water Heater Product List
• Manufacturer install manuals (Rinnai, Navien, A.O. Smith, Bosch, Bradford White)
• Manuall internal: Annual Tankless Water Heater Service, Heat Pump Water Heater Reference