Water Heater Install & Replacement

A water heater swap is one of the highest-margin, highest-liability jobs a residential plumber runs. The work looks simple - two water connections, a gas or electric feed, a vent if combustion is involved - but the number of ways to fail an inspection, burn down a house, scald a child, or leave the customer with a $300 gas bill is remarkable. This guide walks through the major water heater types, the code requirements every apprentice needs to internalize, the step-by-step installation sequence, and the punch list a typical inspector will run against your work.

Tank vs Tankless - Pick the Right Unit for the House

The first question on any replacement job is whether the customer stays with a storage tank or upgrades to tankless. Both are valid and both are wrong in the wrong house.

Storage tank water heater:

• 30, 40, 50, 75, or 80 gallon capacity typical residential
• First-hour rating drives selection (gallons of hot delivered in first 60 minutes)
• Recovery rate (gallons per hour) depends on BTU input and element wattage
• Lower up-front cost, 10-12 year typical service life, simpler install
• Standing losses all day even when nobody uses hot water

Tankless (on-demand) water heater:

• Rated in gallons per minute (GPM) at a given temperature rise
• Needs larger gas line (typically 3/4 inch minimum for a 180-200 kBTU unit, sometimes 1 inch)
• Needs a dedicated 240V 40-50A circuit for electric models
• Requires annual descaling in hard-water regions
• Longer service life (20+ years with maintenance), but higher up-front cost
• Delivers infinite hot water but limited GPM - a 199 kBTU gas unit doing 70 degree rise runs roughly 5 GPM

When a customer has a 3-bathroom house with two teenagers and a jetted tub, a single mid-size tankless will disappoint them. When a customer lives alone and showers once a day, a tankless pays for itself in standby losses. Size the unit for the house, not the customer's enthusiasm.

Fuel Type - Gas vs Electric

Electric tank - Two 240V heating elements (upper and lower), one upper thermostat with an ECO cutoff, one lower thermostat. Simpler, no venting, no combustion air. Recovery is slow (typically 18-22 GPH for a 4500W element). Cheaper install, higher operating cost in most markets.

Natural gas or propane tank - Burner under the tank, flue tube up the center, thermostat gas valve with thermocouple or flame sensor. Faster recovery, lower operating cost, but requires venting and combustion air.

Electric tankless - Wall-mounted, needs 2-3 dedicated 240V circuits totaling 80-150 amps. Only practical where gas is not available and the panel has the capacity.

Gas tankless - Wall-mounted, fan-assisted combustion, sealed burner, heat exchanger (stainless or copper). Nearly all modern units are condensing and vent through PVC or polypropylene.

Confirm the existing gas meter and gas line can actually feed the new unit BEFORE you promise the customer a tankless. A 150,000 BTU tankless dropped onto a 1/2 inch gas line sagging through a crawlspace will not fire correctly even if you hook it up - the pressure drop starves the burner.

Venting Categories - Know Before You Cut a Hole

Gas-fired water heaters are classified by the condition of their flue gas:

| Category | Flue Pressure | Flue Temp      | Common Use                       |
|----------|---------------|----------------|----------------------------------|
| I        | Negative      | Above dew pt   | Atmospheric B-vent tank units    |
| II       | Negative      | Below dew pt   | Rare in residential              |
| III      | Positive      | Above dew pt   | Power-vent non-condensing        |
| IV       | Positive      | Below dew pt   | Condensing tankless, PVC vent    |

In plain English, four venting types show up in residential work:

• Atmospheric B-vent (Cat I) - Double-wall galvanized B-vent going up through the roof by natural draft. Draft hood at the top of the heater. Slope the horizontal run 1/4 inch per foot upward toward the vertical rise. Simple, old-school, ventless-combustion-air friendly but cannot share a vent with a furnace that has been converted to 90+ efficiency.
• Power vent (Cat III) - Blower on top of the heater pushes flue gas through galvanized single-wall or AL29-4C stainless. Sidewall termination is allowed. Still non-condensing, so it cannot vent in PVC.
• Direct vent / sealed combustion - Two-pipe concentric or side-by-side: one pipe brings combustion air from outside, one exhausts flue gas. Isolates the appliance from interior air. Required in tight-sealed newer homes.
• Condensing PVC / polypropylene (Cat IV) - Modern condensing tankless. Flue gas cool enough to be vented in Schedule 40 PVC or manufacturer-approved polypropylene. Runs downhill to the unit (slope toward the heater) because condensate forms in the pipe. A condensate drain must be plumbed to an approved receptor.

Never mix vent materials. Never reuse an old B-vent for a power-vent appliance. Never vent a condensing appliance into a clay-lined masonry chimney - the acidic condensate will destroy it in a season.

Combustion Air Requirements

A gas water heater in a confined space needs combustion air. Two rules you will use constantly:

• Two 100-square-inch openings (one within 12 inches of ceiling, one within 12 inches of floor) when drawing air from inside the building, sized at 1 square inch per 1,000 BTU input as a starting point.
• Sealed-combustion (direct vent) or power vent eliminates the requirement entirely because combustion air comes from outside.

If you install an 80-gallon 75 kBTU atmospheric tank in a small utility closet without louvered doors, the unit will either short-cycle, backdraft, or set off the carbon monoxide alarm. That is an inspector failure every time.

T&P Valve and Discharge Piping

Every water heater has a temperature-and-pressure relief valve (T&P). It must be factory-rated for the tank's pressure (typically 150 psi) and temperature (210 degrees F). The discharge pipe rules are tested on every inspection:

• Same diameter as the T&P outlet - typically 3/4 inch - and no smaller.
• Rigid material approved for the temperature (copper, CPVC, galvanized). No PVC - it cannot handle 210 degrees.
• Terminates within 6 inches of the floor, over a floor drain, or outdoors. Never into a sealed pan or capped.
• No threaded end, no valve, no tee. The discharge must be a straight run that cannot be blocked.
• Downward slope the entire length, no sagging traps.

A T&P that cannot relieve is a tank bomb. Apprentices who "cap the leaky T&P until we come back" have killed homeowners. Never, ever do this.

Expansion Tank Sizing

In a closed system (check valve, PRV, or backflow device on the service line) thermal expansion has nowhere to go, and pressure will climb until the T&P weeps. Code in most jurisdictions requires a thermal expansion tank on a closed system.

Quick sizing rule for residential:

| Water Heater | Incoming Pressure | Expansion Tank |
|--------------|-------------------|----------------|
| 40 gallon    | Up to 60 psi      | 2.0 gallon     |
| 50 gallon    | Up to 60 psi      | 2.0 gallon     |
| 75-80 gallon | Up to 60 psi      | 4.4 gallon     |
| 50 gallon    | 80 psi            | 4.4 gallon     |

Pre-charge the expansion tank air side to match static line pressure BEFORE you plumb it in. Installing a 40 psi tank on an 80 psi line guarantees a waterlogged tank within months.

Dielectric Protection

Where galvanized pipe meets copper, or copper meets the iron tank nipples, dissimilar-metal corrosion can open a pinhole in a year. Options:

• Dielectric unions - Old-school, but many fail internally within a few years.
• Brass couplings or brass-bodied heat trap nipples - Preferred. Brass is an acceptable neutral metal between copper and galvanized steel.
• Heat trap nipples - Most modern heaters ship with factory heat trap nipples that double as dielectric barriers. Use them unless the manufacturer says otherwise.

Gas Line Sizing for Tankless (Quick Reference)

A 180-200 kBTU condensing tankless is a serious gas appliance. The gas line must deliver full input pressure under firing load:

• 1/2 inch black iron - Not acceptable for most tankless units at any meaningful length.
• 3/4 inch black iron or CSST - Minimum for most tankless, short runs only (typically under 20-25 feet).
• 1 inch - Required on longer runs, or when two appliances share the branch.

Always run a full sizing calculation using the Longest Length Method per IFGC (International Fuel Gas Code) or the local amended code. Do not eyeball it.

Every gas appliance needs:

• A shutoff valve within 6 feet of the appliance, in the same room.
• A sediment trap ("drip leg") downstream of the shutoff, upstream of the appliance regulator. Typically a 3-inch nipple capped at the bottom of a tee, arranged so debris and condensate fall into it instead of the control valve.
• A union downstream of the shutoff so the appliance can be removed without cutting pipe.

Water Side Hookups

• Full-port ball valve on the cold inlet - Never a gate valve. Gates stick and shear.
• A union or service valve on the hot outlet - Makes future replacement painless.
• Heat trap nipples - Reduce standby loss from convective loops.
• Hammer arrestors on quick-closing appliance lines - Dishwashers and washing machines especially. Not strictly water heater scope, but note the house condition.

Thermostat Setting - 120 F vs 140 F

Code default and manufacturer shipping default is 120 degrees F. This is the right setting for most homes:

• Prevents scald injuries (third-degree burn in 5 seconds at 140 F, 30 seconds at 130 F, 5 minutes at 120 F)
• Reduces standby losses
• Compatible with dishwashers that have internal boosters

Raise to 140 F only where there is a Legionella risk - typically an immunocompromised resident, a recirculation loop that dead-legs, or a large building with long storage times. If you raise the setpoint, install an anti-scald mixing valve at the heater outlet to temper the distribution to 120 F while storing at 140 F. That is the code-compliant way.

Anode Rod - The Sacrificial Hero

Every tank heater has a magnesium or aluminum anode rod threaded into the top (sometimes combined with the hot outlet nipple on newer tanks). The anode corrodes instead of the tank lining. When it is gone, the tank starts rusting.

• Inspect every 3-5 years, replace when more than 6 inches of the core wire is exposed.
• Magnesium - Best sacrificial performance, but produces hydrogen sulfide (rotten egg smell) in some water.
• Aluminum - Tolerates sulfur water better, but some research links aluminum anodes to dietary concerns. Many shops use aluminum-zinc alloy for smelly water.
• Powered / impressed-current anode - Aftermarket retrofit for problem installs, wires to a low-voltage transformer. Permanent replacement.

Selling an anode inspection is one of the highest-value upsells a service plumber can offer - a $40 part extends a $1,500 tank by 5 years.

Tankless Descale Procedure

Every 12 months in hard water, every 18-24 months in softened water, run a descale flush:

1. Shut off the gas and the cold-water inlet valve.
2. Close the hot-water outlet valve.
3. Connect a submersible pump and a 5-gallon bucket to the service valves (most modern tankless units ship with isolation service valves - install them if the previous plumber did not).
4. Fill the bucket with white vinegar or a manufacturer-approved descaling solution (CLR diluted per label for heavy scale).
5. Circulate through the heat exchanger for 45-60 minutes.
6. Drain, flush with clean water for 5 minutes.
7. Close service valves, open cold inlet and hot outlet, bleed air, re-light.

A unit that has never been descaled in 8 years of hard water may require multiple cycles or an exchanger swap. Always quote descale on new tankless installs as part of the annual maintenance plan.

Electric Element Replacement - Power OFF, Meter It

When an electric tank drops cold:

1. Turn off the breaker. 240V across a chest will kill you instantly.
2. Meter the element terminals to verify zero volts. Trust your meter, not the switch label.
3. Drain the tank below the element being replaced.
4. Remove the access panel, insulation, and thermostat cover.
5. Unscrew the old element (1-1/2 inch socket for screw-in, 6-point wrench for flange-mount).
6. Clean the gasket surface, install new element with new gasket, torque per spec.
7. Refill the tank, verify zero air at the hot tap before re-energizing. Firing a dry element burns it out in seconds.

Earthquake Strapping

In seismic zones (California, Pacific Northwest, parts of the Mountain West), two straps are required - one in the upper third of the tank, one in the lower third. Use pre-approved metal strap kits and lag into studs, not drywall.

Day 1 Checklist

• Permit pulled or confirmed unneeded for jurisdiction
• Gas input verified (meter capacity, line size, pressure drop under load)
• Expansion tank, T&P discharge pipe, isolation valves, dielectric protection on truck
• Vent type confirmed and materials on truck (B-vent, PVC, polypropylene, or concentric kit)
• Condensate drain plan for condensing units (neutralizer if required)
• Earthquake straps if seismic zone
• Combustion-air calculation done for confined-space installs
• Setpoint plan (120 F default or 140 F with mixing valve)

Expert Tips

• "Measure the door before you buy the heater." A 75-gallon tank does not fit through every interior doorway.
• "Pipe the T&P to daylight on every install." The time it costs is the cheapest insurance premium you will ever pay.
• "Photograph the gas pressure reading at the manifold under full fire." Proof for the customer and proof for the warranty.
• "Sell the expansion tank on every PRV install." The two go together. A closed system without one is a callback waiting to happen.
• "Quote annual descale on every tankless." Customers who skip maintenance blame the plumber when the exchanger fouls.
• "Torque the anode rod to manufacturer spec." Over-tight cracks the tank liner at the threads. A common cause of early tank failure.