Section 2 · Water Heaters
Water Heater Repair Guides
Water heaters come in two main types. A tank heater keeps water hot in storage, while a tankless unit heats water only as it flows. The first checks differ, so name your type first, then use the problem table and the upkeep notes on this page.
Choose your appliance
- Tank Water Heater
A tank water heater keeps a full tank hot, with cold water entering low and hot water leaving from the top. Sediment, a worn anode rod and the T&P relief valve explain many of its problems. Start with the problem table, then use the upkeep schedule to add years to the tank.
Tank, tankless or heat pump: how each one heats
A tank, or storage, water heater keeps a supply of water hot at all times. Hot water leaves from the top of the tank as cold water enters at the bottom. Gas tanks use a burner, and electric tanks use upper and lower heating elements.[1] Because the tank stays hot around the clock, it loses some heat even when no one uses water. An insulation blanket can reduce that standby loss.[1]
A tankless water heater heats water only when you need it, so it has no standby loss.[1] Rinnai describes the sequence for its own units. The flame lights and heats two heat exchangers. Water preheats in the secondary exchanger, reaches its set temperature in the primary one, and the flame turns off when water use stops.[2] The unit needs a minimum flow to start. Rinnai gives 0.4 to 0.6 gallons per minute to ignite and 0.26 gallons per minute to keep running.[3]
A heat pump water heater moves heat from the surrounding air into water held in a tank.[4] When demand is high, it switches to standard electric resistance heat, which is why some people call these units hybrids.[4]
Knowing your type changes the first check. A tank heater can leak at a fitting or lose its gas pilot. A tankless unit may not fire when only a trickle of water flows, and many models report faults as a code on a display. The table below starts from the symptom, so you can match your heater to the right row.
Water heater safety basics
If you smell gas near a water heater, follow the emergency steps at the top of this page. The A. O. Smith manual adds one point: do not use any phone inside the building.[5] Call 911 first, then your gas company, from outside.[6]
Hot water burns fast. NFPA advises setting the water heater to 120°F to lower the risk of scald burns.[6] To check it, run the hot tap for 3 minutes and test the water with a cooking thermometer. If you lower the setting, wait a full day before you test again.[6]
Gas water heaters that are broken or vented badly can produce carbon monoxide.[6] Exhaust from any fuel-burning appliance must vent outdoors through a clear, open path.[6] If a gas pilot keeps going out or is hard to light, do not keep relighting it; NFPA says to have a fuel gas professional relight it.[6]
Common water heater problems and the first check
Each first check below is safe for an owner. Do it before you call, and note what you found for the plumber.
| Symptom | First check | Guide |
|---|---|---|
| Water on the floor near a tank | Close the cold water inlet valve to the heater if water is spreading, then find the wettest spot.[5] | water heater leak causes and fixes |
| Leak that seems to come from the bottom | Dry the floor and check the drain valve first, since it sits near the bottom of the tank. | — |
| Gas pilot keeps going out | Relight it once by the manufacturer's instructions, and call a gas professional if it goes out again.[6] | pilot that keeps going out |
| Tankless unit gives no hot water | Open the hot tap fully, because a tankless unit needs a minimum flow before it will fire.[3] | — |
| Code on a Rinnai display | Write the code down exactly as shown before you reset anything. | — |
| Unsure of the heater's age | Find the rating plate and note the model and serial numbers.[5] | — |
| Big repair on an older tank | Compare the tank's age with the 10 to 11 years a 2007 NAHB study gives tank heaters.[7] | fix-or-replace calculator |
Each heater type has its own page with parts, a diagram and deeper checks: tank water heater problems and tankless water heater problems.
Upkeep that adds years to any water heater
Tanks collect sediment. One A. O. Smith gas manual recommends draining and flushing the tank every 6 months and checking the T&P relief valve on the same schedule. It also says to pull the anode rod for inspection every 3 years.[5] Your own manual may give other intervals, so check it first.
Tankless units collect scale and lime too. Rinnai says to flush its units at least once a year to keep them free of scale and lime. The right timing depends on your water quality.[8] It says a professional installer should do the flush, and that the in-line screen filter should be checked and cleaned now and then.[8]
Lower the temperature if it is set high. University of Maryland Extension says 120°F is enough for most households and can cut water heating energy use by 6 to 10 percent.[1] Heat pump models can use up to 60 to 70 percent less energy than standard electric resistance heaters.[1]
Plan for replacement. A 2007 NAHB study puts gas tanks at about 10 years, electric tanks at about 11 and tankless units at more than 20.[7] Write the install date on the heater when it goes in, so the next decision is easy.
Sources
“Home Energy: Water Heating” · accessed
“Tankless 101” · accessed
“Frequently Asked Questions (Professional)” · accessed
“Instruction Manual: Residential Gas Water Heaters, Power Vented Models (100341945)” · PDF · p. 1 (what to do if you smell gas), pp. 39-41 (periodic maintenance) · accessed
“NFPA Educational Messages Desk Reference, 2026 Edition” · PDF · §3.1.3, §3.6.5, §9.2.1, §9.2.3, §19.3.1 · accessed
“Study of Life Expectancy of Home Components” · PDF · Table 1, p. 10 and p. 14 (water heater rows) · accessed
How this page was made
This page was drafted with AI assistance from the manufacturer and safety sources listed above, then checked against those sources in an AI-assisted review. No licensed technician reviewed it. Last reviewed . Our methodology