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newSchoolWorks.com › VoTechWorks › Building & Construction › Plumbing & Pipefitting › Plumbing Fixtures: Selection, Rough-In, and Trim-Out › Lesson 9
BLD-PLUM06-05 · Lesson 9 · 15 clock hours · lab

Water Heater Installation: Tank and Tankless

Objective. Students will connect a tank-type gas or electric water heater and a tankless unit to supply and drain lines, install a T&P relief valve with a compliant discharge pipe, and set the thermostat to the recommended temperature.

Water Heater Installation: Tank and Tankless

Water heaters are among the most common fixtures a plumber installs and services. Whether the unit stores hot water in a tank or heats it on demand, the fundamental goals are the same: safe connections, proper safety-device installation, and correct temperature settings.

Tank-Type Water Heaters

A tank water heater stores a reservoir of heated water — typically 30 to 80 gallons — and cycles on to maintain a set temperature. Units may be fueled by natural gas, propane, or electricity.

Cold and hot connections. Cold water enters through the dip tube at the cold-water inlet (usually marked with a blue or "C" label at the top of the unit). Heated water exits from the hot-water outlet at the top. Both connections are typically ¾-inch threaded. Always install a union on each connection so the heater can be removed without cutting pipe. Where copper pipe meets the steel tank nipples, install a dielectric union on each port. Dielectric unions contain a plastic or nylon insulating sleeve that prevents galvanic corrosion — the electrochemical reaction that destroys pipe and fittings when dissimilar metals are in direct contact with water.

Isolation valves. Install a full-port ball valve or gate valve on the cold-water supply line so the heater can be isolated without shutting off the building supply.

Drain valve. A hose-bib-style drain valve is factory-installed near the base of the tank. Connect it to a floor drain with appropriate pipe so the tank can be drained for maintenance or replacement without flooding the space.

Temperature and Pressure Relief (T&P) Valve

The T&P relief valve is the most critical safety device on any tank water heater. It opens automatically if tank pressure or temperature exceeds safe limits, preventing a catastrophic tank failure. Every tank-type water heater must have a listed T&P valve installed in the designated opening — never a plug or any other fitting.

Discharge pipe requirements (per model plumbing codes including the IPC):

After installing the T&P valve and discharge pipe, have your instructor verify both before pressurizing the unit.

Thermostat Setting

The U.S. Department of Health and Human Services and the plumbing industry recommend setting tank water heaters to 120 °F for most residential applications. This temperature is hot enough to meet household needs and inhibit bacterial growth, while reducing the risk of scalding. Some codes or health guidelines require higher settings in certain commercial or healthcare settings — always check the applicable code. On gas heaters, the thermostat dial is typically on the gas valve; on electric heaters, one or two thermostats are located behind panels on the tank body and are adjusted with a flathead screwdriver.

Tankless (On-Demand) Water Heaters

A tankless unit heats water only when flow is detected; there is no storage tank. Key components include:

Tankless units require larger gas lines or dedicated high-amperage electrical circuits than tank heaters of equivalent capacity. In this lab, you will identify components and document connections on the display unit only — no live gas or electrical energization takes place in the shop.

Safety Requirements for This Lab

Documentation

Record all connections, valve positions, T&P valve model and rating, discharge pipe size and termination point, and thermostat setting in your lab log. Note differences between the tank and tankless unit in a side-by-side comparison table. This log is part of your lab grade.

Key terms8

T&P Relief ValveTemperature and pressure relief valve — a safety device installed on tank water heaters that opens automatically if tank temperature or pressure exceeds safe design limits, preventing tank failure.
Dielectric UnionA pipe union containing an insulating sleeve that electrically isolates two sections of dissimilar metal piping, preventing galvanic corrosion at the joint.
Dip TubeA plastic tube inside a tank water heater that directs incoming cold water to the bottom of the tank so it does not mix with and cool the hot water at the top.
Discharge PipeThe pipe connected to the outlet of a T&P relief valve that safely directs discharged water to a floor drain or other approved safe location; must be full-size and unobstructed.
Galvanic CorrosionElectrochemical deterioration that occurs when two dissimilar metals are in electrical contact in the presence of water, causing the less noble metal to corrode accelerated.
Tankless Water HeaterAn on-demand water heater that activates only when flow is detected and heats water as it passes through a heat exchanger, storing no water between uses.
Isolation ValveA full-port ball valve or gate valve installed on the cold-water supply line to a water heater so the unit can be shut off and serviced without interrupting the entire building supply.
Inlet Filter ScreenA mesh screen at the cold-water inlet of a tankless water heater that traps sediment before it can damage the unit's heat exchanger; requires periodic cleaning.

Worked examples4

1. The T&P relief valve on a tank water heater has a ¾-inch outlet. The plumber has ½-inch copper tube available. What size discharge pipe must be installed, and how far from the finished floor must it terminate?
  1. Step 1 — Identify the T&P outlet size: the valve outlet is ¾ inch.
  2. Step 2 — Apply the code rule: the discharge pipe must be the same diameter as the valve outlet and must never be reduced. A ½-inch pipe is smaller and is not permitted.
  3. Step 3 — Select the correct material: use ¾-inch copper tube (or another listed material rated for the expected discharge temperature).
  4. Step 4 — Determine termination height: the discharge pipe must terminate within 6 inches of the floor or over an approved drain receptor.
Answer: Install ¾-inch copper discharge pipe; terminate the open, unthreaded end within 6 inches of the floor or directly over an approved drain. The ½-inch tube is not acceptable.
2. A plumber is connecting a copper hot-water outlet line to the steel nipple on top of a gas water heater. What fitting is required, and why?
  1. Step 1 — Identify the metals: copper pipe and a steel (iron) tank nipple are dissimilar metals.
  2. Step 2 — Recognize the hazard: direct contact between dissimilar metals in the presence of water creates a galvanic cell, causing accelerated corrosion of the less noble metal.
  3. Step 3 — Specify the solution: install a dielectric union between the steel nipple and the copper pipe. The union's insulating sleeve breaks the electrical path and stops galvanic corrosion.
  4. Step 4 — Apply to both ports: a dielectric union is required at both the cold-water inlet and the hot-water outlet wherever the pipe material differs from the steel tank nipple.
Answer: A dielectric union must be installed at the connection. It electrically isolates the copper from the steel, preventing galvanic corrosion at both the inlet and outlet connections.
3. A tank water heater thermostat is currently set to 140 °F. The standard recommended setting for a typical residential installation is 120 °F. By how many degrees Fahrenheit should the thermostat be reduced, and what is the main reason for the lower setting?
  1. Step 1 — Identify current setting: 140 °F.
  2. Step 2 — Identify target setting: 120 °F.
  3. Step 3 — Calculate the difference: 140 °F − 120 °F = 20 °F reduction.
  4. Step 4 — State the reason: 120 °F is the commonly recommended temperature to reduce scalding risk while still meeting hot-water demand and inhibiting bacterial growth.
Answer: Reduce the thermostat by 20 °F, from 140 °F to 120 °F, to reach the standard recommended residential setting and reduce the risk of scalding.
4. A discharge pipe from a T&P valve currently terminates 10 inches above the floor with a threaded cap on the end. List two code violations present in this installation.
  1. Step 1 — Check termination height: the pipe ends 10 inches above the floor. The code requires termination within 6 inches of the floor. 10 inches exceeds the 6-inch maximum — Violation 1.
  2. Step 2 — Check the pipe end: a threaded cap is installed. The discharge pipe end must remain open (not capped, plugged, or valved) so pressure can escape freely — Violation 2.
  3. Step 3 — Confirm the correction: re-route or extend the pipe so it terminates within 6 inches of the floor or over an approved drain, and remove the cap, leaving the end open and unthreaded.
Answer: Violation 1: the pipe terminates 10 inches above the floor — it must end within 6 inches. Violation 2: the pipe end is capped — it must be open and unthreaded. Both must be corrected before the heater is used.
In class: Instructor demo on a tank water heater: connect cold-water inlet and hot-water outlet with unions, install a T&P relief valve, run a full-size discharge pipe to within 6 inches of the floor or to a safe drain location, connect the drain valve to a floor drain, and set the thermostat. · Students connect their assigned tank water heater, install the T&P valve and discharge piping, and connect a dielectric union on each port where dissimilar metals meet; instructor verifies T&P discharge pipe size and termination before students pressurize. · Instructor demonstrates a tankless water heater connection (water and gas or electric supply side only — no live gas or electrical energization in the shop); students identify inlet filter, cold-water isolation valve, and hot-water outlet, then document differences between tank and tankless connections in their lab logs.
Materials: Tank-type water heaters (gas or electric, shop demo units), tankless water heater (display/demo unit), T&P relief valves, discharge pipe and fittings, dielectric unions, gate or ball valves, pipe wrenches, Teflon tape, CSST or black-iron gas piping sections (for layout only, not pressurized), safety glasses, gloves, lab log sheets
Check: T&P discharge pipe correct size and termination location per IPC; no leaks at supply or drain connections after pressurizing; dielectric unions present where required — all criteria required to pass.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

NCCER Plumbing Core and Level 1 — Plumbing Fixtures and Rough-In
1. Why is a dielectric union required when connecting copper supply pipe to a steel water heater nipple?
State Journeyman Licensing — Plumbing Codes and Safety Devices
2. The T&P relief valve on a water heater has a ¾-inch outlet. According to the IPC, the discharge pipe must be:
State Journeyman Licensing — Plumbing Codes and Safety Devices
3. Where must the open end of a T&P relief valve discharge pipe terminate?
NCCER Plumbing — Plumbing Fixtures Installation
4. Which of the following is the standard recommended thermostat setting for a residential tank water heater to balance hot-water availability with scalding prevention?
NCCER Plumbing — Safety and Lab Procedures
5. Before a student opens the cold-water supply valve to pressurize a newly connected tank water heater in the lab, what step is required by the lab safety protocol?
NCCER Plumbing Level 2 — Water Heaters
6. What is the function of the dip tube inside a tank water heater?
NCCER Plumbing — Plumbing Fixtures and Equipment
7. A tankless water heater differs from a tank-type heater primarily because it:
State Journeyman Licensing — Workplace Safety and OSHA Requirements
8. During a real job-site water heater replacement, which of the following is the correct first safety step before disconnecting an existing energized electric water heater?
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