The water heater is one of the most-flagged items in any home inspection, and the same three mistakes show up week after week. Most homeowners never look at their water heater until it stops making hot water. But the problems we find usually have nothing to do with hot water. They're about safety: pressure, combustion gases, and fuel supply.

Whether the house is brand new or 40 years old, here are the three water heater mistakes we write up most often, why they matter, and what to do about them.

Mistake #1: A bad TPR valve discharge line

The single most common water heater defect we find is an improperly installed discharge pipe on the temperature and pressure relief (TPR) valve.

The TPR valve is the brass valve near the top of the tank with a lever on it. If the water inside gets too hot or the pressure climbs too high, it opens and dumps water so the tank doesn't fail. A water heater with a failed or blocked relief path is essentially a pressure vessel with no safety. That's how tanks rupture.

The valve itself is usually fine. The problem is the pipe connected to it. Here's what we see most:

• Wrong material. PVC or other pipe not rated for the temperature. Hot discharge water can soften or burst it.
• Reduced size. The discharge pipe must be at least the full size of the valve outlet, typically 3/4". We find 1/2" lines and pinched-down fittings.
• Running uphill. The line should drain by gravity. Water that sits in a trapped or uphill section can freeze or corrode the valve shut.
• Threaded or capped end. The end must be plain, never threaded or capped, so no one can plug it.
• Bad termination. Lines that end in the attic, inside a wall, or too high above the floor where scalding water can spray someone.
• Tied into the pan drain. Some local jurisdictions allow it, but often it hides a leak or puts the discharge somewhere unsafe.
• Missing altogether. Yes, we still find valves with no discharge pipe at all.

What to do: Look at the pipe coming off that brass valve. If it's white plastic, smaller than the valve outlet, runs uphill, or ends somewhere you can't see, have a licensed plumber correct it. It's usually an inexpensive fix with a big safety payoff.

Mistake #2: Improper venting on gas water heaters

A gas water heater burns fuel, and the exhaust, including carbon monoxide, has to leave the house through the vent. When the vent is installed wrong, those gases can spill back into the home or overheat nearby framing.

The venting problems we flag most often:

• Poor slope. Horizontal sections of the vent connector need to rise toward the chimney or flue, at least 1/4" per foot. Flat or downhill runs slow the draft and let exhaust spill out.
• Loose or disconnected joints. Sections that have separated, are missing screws, or were never fastened. We often see gaps right above the draft hood.
• Clearance to combustibles. Single-wall vent pipe generally needs 6" of clearance to wood and other combustibles. Double-wall (B-vent) generally needs 1". We find vent pipe touching roof decking, attic framing, and stored boxes.
• Missing or damaged draft hood. The hood on top of the tank is part of how the appliance drafts. It shouldn't be crooked, crushed, or removed.
• Backdrafting. Scorch marks or rust around the draft hood are clues that exhaust is spilling instead of venting.
• Bad termination. Missing rain caps, caps too close to the roof, or vents that end inside the attic.

What to do: If you have a gas water heater, every home should have working carbon monoxide alarms. Look for discoloration around the top of the tank and gaps in the vent pipe. Any venting defect is a job for a licensed plumber or HVAC contractor, not a weekend DIY.

Mistake #3: Gas supply problems

The third mistake sits at the bottom of the tank: the gas line feeding the burner. These are easy to overlook because the water heater still lights and runs fine.

What we find most:

• No sediment trap (drip leg). A short capped pipe stub should sit just before the appliance connection to catch debris and moisture in the gas line before it reaches the burner controls. It's required by most codes and manufacturer instructions, and it's missing on a large share of the units we inspect, including in newer homes.
• No accessible shutoff valve. There should be a gas shutoff in the same room, within reach of the appliance, so the gas can be turned off without shutting down the whole house.
• Flexible connector problems. Appliance connectors that are kinked, too long, routed through a wall or the cabinet, or older uncoated brass connectors that should be replaced.

What to do: Follow the gas line from the wall to the water heater's control valve. You should see a shutoff valve and a capped vertical stub just before the connection. If either is missing, or if you ever smell gas, have a licensed plumber address it.

Honorable mentions

These didn't make the top three, but they show up on reports almost as often:

• Drain pan issues. Water heaters in attics or above finished space need a pan with a drain line routed to a safe location. We find pans with no drain, cracked pans, or no pan at all.
• Garage installs not elevated. Gas water heaters in garages generally need the ignition source raised at least 18" off the floor and protection from vehicle impact.
• Missing bonding. Hot and cold water lines should be electrically bonded, which is often missing.
• Age and corrosion. Most tank water heaters last roughly 8 to 12 years. Rust at the base, on fittings, or water in the pan means it's living on borrowed time.

The bottom line

None of these mistakes are exotic. They're the result of rushed installs, DIY repairs, and details that get skipped because the water heater "works." But a water heater that works isn't the same as one that's safe.

If you're buying a home, these are exactly the items a good inspection should catch before closing, while there's still time to negotiate repairs. If you already own your home, take five minutes this week to look at your relief valve line, your vent pipe, and your gas connection.

Want a second set of eyes? SFR Inspections provides licensed TREC home inspections for buyers, sellers, and investors. Contact us to schedule your inspection!