Water heaters: Minnesota Journeyworker Plumber Practice Questions

What safety device must every storage-type water heater have?

A T&P relief valve (or separate temperature and pressure valves) opens before the tank can overheat or overpressure and explode.

At what maximum temperature must a water heater's temperature relief function open?

The temperature relief is set at no more than 210°F, below the 212°F boiling point at atmospheric pressure, so the stored water cannot flash to steam.

What is the highest pressure setting allowed for a water heater's relief valve?

The pressure relief may not be set above the working pressure marked on the heater (typically 150 psi).

Where must the temperature-sensing element of a T&P relief valve be?

The hottest water rises to the top, so the sensing element must be in the top 6 inches of the tank to react in time.

What size must the T&P relief valve discharge pipe be?

The discharge pipe is full size (at least the valve outlet size) all the way, so the valve can relieve its rated flow.

May a shutoff valve be installed in a T&P relief valve discharge pipe?

A valve in the discharge line could be closed and disable the relief valve, so none is allowed.

Which T&P discharge pipe installation violates the codes?

The discharge pipe must drain by gravity with no traps or upward runs; a rise creates a trap that holds water, can freeze and slows relief.

Why must the end of a T&P discharge pipe NOT be threaded?

A threaded end invites a cap or plug that would block the relief path. The codes require a plain end.

Under the UPC, which termination is prohibited for a T&P relief valve discharge?

The UPC lets relief discharge end only through an air gap into the drainage system or outdoors, 6 inches to 2 feet above grade and pointing down. A crawl space is neither, and a failing valve there would go unnoticed.

Which material may NOT be used for a T&P relief valve discharge pipe?

Discharge piping must handle 210°F water; PVC is rated for cold water only. Copper, CPVC, PEX and galvanized steel are listed options.

Where is a shutoff valve required at a water heater?

Both codes require a shutoff on the cold supply to the water heater so it can be isolated for service.

When is a drain pan required under a storage water heater?

Both codes require a pan where leakage would damage the building, with a drain piped to an approved location.

What is the minimum size of a water heater pan drain?

Pan drains are at least 3/4 inch and run to an indirect waste receptor, floor drain or outdoors.

A gas water heater is installed in a residential garage. How high must its ignition source be above the floor (ignore the FVIR exception)?

Gasoline vapors are heavier than air and collect near the floor, so the burner and pilot must be at least 18 inches up.

A gas water heater in a garage is listed as flammable vapor ignition resistant (FVIR). What does that change?

The fuel gas codes exempt listed FVIR appliances from the 18-inch rule, but they still must be protected from vehicle impact.