The Water Heater Job You Can Do and the Part You Can't

Water pooling under the tank means the tank lost. That one isn't a repair. Everything else probably is.

Pilot won't stay lit on a gas unit? That's usually a thermocouple, about fifteen bucks, ten minutes with a wrench. Electric unit gone lukewarm, or hot for four minutes and then arctic? That's a heating element or an upper thermostat, twenty or thirty dollars, and a multimeter tells you which. Water that smells like a struck match is a spent anode rod, not a dead tank. Before you spend a thousand dollars, spend twenty minutes ruling out the cheap stuff.

But if it's seeping from the seam at the bottom, or the pan has standing water in it and the connections up top are bone dry, the steel is done. Glass lining cracks, the tank rusts from the inside out, and there's no patch for that. Plan the replacement now, while you can choose the day.

Find out how old it's before you shop

The label on the side has a serial number, and almost every brand hides the build date in the first four digits. Search the brand name plus "serial number date code" and you'll decode it in a minute. Rheem uses month and year. A.O. Smith uses year and week. If it's twelve years old and you're already on the floor with a flashlight, you're not repairing it, you're buying time.

While you're down there, write down three things: capacity (40 or 50 gallon, printed on the label), fuel type, and vent style. Vent style matters more than people expect. A plain sheet metal hood with a pipe going up into the ceiling is atmospheric. A plastic pipe running out a sidewall with a fan you can hear is power vent, and that one has a control board, a blower and a pressure switch. Power vent is a plumber's job unless you do this for a living.

Measure the door too. A 50-gallon tank is a little over five feet tall and won't corner into a 30-inch closet the way the old one came out when the house was framed around it.

What the swap actually looks like

Shut the gas at the appliance valve, or kill the breaker and verify it dead with a meter. Close the cold inlet. Run a hot tap somewhere in the house until the water cools off, because 140-degree water on your forearm will end your afternoon.

Hose on the drain valve, other end outside or into a floor drain. Open a hot faucet upstairs to break the vacuum, then open the drain. This is where it goes wrong. A tank that's never been flushed has a cone of hardened sediment sitting right over the drain port, and you'll get a trickle. Crack the cold inlet open for two seconds to blast it loose. If that fails, pull the drain valve out entirely, put a wide pan under it, and let the sludge come. A wet/dry vac earns its keep here.

Then disconnect the water lines, pull the vent connector, unhook the gas, and walk it out on a two-wheel dolly with a ratchet strap. Empty, a 40-gallon gas tank is manageable for one strong guy and awkward for two. Full, it's another four hundred pounds of water. Don't try to shortcut the draining.

New tank goes in, level, on a pan if there's finished space anywhere below it. Install the heat trap nipples the right direction, arrows matter. New flexible connectors, not the old ones, because corrugated stainless work-hardens and you get one good bend out of it. Thread sealant on tapered pipe threads only, never on a flare or a compression face.

Electric guys, read this twice: fill the tank completely and purge the air before you turn the breaker on. An element energized in air burns out in seconds. You'll be back at the store before dinner.

The parts nobody tells you about at the register

The unit isn't the whole list. These are the items that show up on an inspection and never in the box.

• Temperature and pressure relief discharge pipe. Full size, no reductions, no valve, no threads on the end, terminating six inches off the floor where you can see it dump. Copper or CPVC. Never plug it. That valve is the reason your water heater is a water heater and not a missile.
• Thermal expansion tank. If there's a pressure reducing valve or a check valve on your main, your plumbing is a closed system, and heated water has nowhere to go. You need one, pre-charged to your street pressure. Around fifty dollars and thirty seconds of work.
• A real shutoff on the cold inlet. If the existing one is a corroded gate valve, replace it with a quarter-turn ball valve while the system is drained. You will thank yourself in nine years.
• Sediment trap on the gas line. A short capped drop below the tee. Cheap, required most places, catches scale before it wrecks the gas valve.
• Elevation and impact protection in a garage. Many jurisdictions still want gas units raised and bollarded where a bumper can reach them. Ask your inspector, not the internet.

And pull the permit. It's usually modest money, the inspector catches the thing you missed, and an unpermitted water heater has a way of surfacing during a home sale at the worst possible moment.

The vent test that decides everything

On an atmospheric gas unit, the exhaust leaves because hot air rises. That's the whole system. It works until your house fights it.

Here's the test, and it's not optional. Close the exterior doors and windows. Turn on the clothes dryer, both bath fans and the kitchen hood. Fire the water heater burner and let it run a minute or two. Then hold a smoking match or an incense stick at the edge of the draft hood. The smoke should get sucked up and in. If it rolls out into the room, you have spillage, and that's combustion gas entering your house.

Stop there. Call a plumber or an HVAC contractor. Backdrafting is how people die quietly in their sleep, and it's more common in tight modern houses than anybody wants to admit.

Where I stop and pick up the phone

I'll do a like-for-like swap in an open basement with an existing gas valve and a clean vent. I won't touch these:

Anything in an attic or on a floor above finished living space. A leak up there doesn't cost you a tank, it costs you a ceiling and a floor. Anything that requires modifying gas piping, changing fuel type, or jumping capacity. A water heater sharing a common vent with the furnace, because sizing that chimney correctly is engineering. Power vent, direct vent, tankless, or a heat pump unit with condensate and a dedicated circuit. And any job where the main shutoff won't actually shut off, because now you're doing two jobs at once with water running.

When you do call, ask for the quote itemized: unit and model number, permit, haul-away, expansion tank, new shutoff, pan, connectors, labor. Three quotes. The cheap one that skips the permit line isn't cheap, it's borrowed.

One more thing before you close the utility room door. Put a working carbon monoxide alarm on that level of the house, and a battery-powered water sensor in the pan. Twenty-five dollars, both of them, and they're the only two things in the room that will wake you up.