Water Heater Running Constantly? It Could Be a Hidden Leak

Your water heater is supposed to cycle: heat the tank, rest, heat again when you use hot water. If you notice it firing far more often than it used to, or a tankless unit that kicks on when nobody is using hot water, pay attention. One of the more common reasons in Texas slab homes is a leaking hot-water line under the floor. This guide explains why, how to check it, and the other explanations to rule out first.

How a hidden leak keeps your heater busy

Every gallon of hot water that leaves the tank is replaced with cold water from the main, and the heater has to bring that new water up to temperature. When a hot-water line under the slab is leaking, hot water escapes into the soil continuously. The tank keeps refilling with cold water, and the burner or elements keep running. You're paying for the water and for the energy to heat it.

In tankless systems, the unit fires whenever it senses flow. A hot-side leak creates a small, constant flow, so the heater may turn on at odd times or never fully shut off.

Signs that point to a hot-side leak

A water heater that runs constantly is one clue. These make the case stronger:

• A warm spot on the floor, often between the water heater and a bathroom or the kitchen
• A higher gas or electric bill along with a higher water bill
• Running out of hot water sooner than you used to
• The sound of water near the heater or along a hallway when nothing is on
• A water meter that moves with every fixture off

The test you can do in 15 minutes

You can confirm whether the leak is on the hot side with nothing more than two valves and your meter.

1. Turn off every fixture and appliance, including irrigation, the ice maker and the softener.
2. Watch the water meter's leak indicator. If it's moving, there's a leak somewhere.
3. Find the cold-water inlet valve on top of the water heater. It's on the pipe labeled cold, or the one that feels cooler. Close it.
4. Watch the meter again for 10 to 15 minutes.

If the meter stops when you close the water heater's inlet, the leak is on the hot-water side of your plumbing. If it keeps moving, the leak is on the cold side.

Before closing that valve for longer than a test, turn a gas heater to pilot or vacation mode, or switch off the breaker for an electric heater. Many manufacturers advise against running a heater that can't refill.

Other reasons a water heater runs a lot

Not every busy heater has a slab leak behind it. Rule these out:

Sediment buildup

Texas water is often hard, especially in the Hill Country, San Antonio and West Texas. Minerals settle at the bottom of tank heaters and insulate the water from the burner, so it takes longer to heat. You may hear popping or rumbling. Flushing the tank can help.

A failing thermostat or element

On electric heaters, a bad element or thermostat can make the unit run longer or heat poorly.

The heater is undersized or old

A tank that's too small for the household, or one that's near the end of its life, will run more and recover slowly.

A leaking relief valve

The temperature and pressure relief valve can drip if pressure is high or the valve is worn. Check the discharge pipe for moisture. Water going down that pipe is water the heater has to replace.

A recirculation pump

Homes with a hot-water recirculation system keep hot water moving through the lines so it's instant at the tap. If the pump's timer is set to run all day, or the check valve fails, the heater runs more. That's not a leak, but it can look like one.

A dripping hot faucet

A faucet that drips hot water all day adds up. Check every tap.

Cold weather

In a hard freeze, incoming water is colder and the heater works harder. Seasonal changes are normal; a sudden jump isn't.

Why hot-water lines leak under slabs

Hot-water lines expand and contract every time they heat and cool. Over years, that movement can wear copper where it rubs against concrete, gravel or a sleeve. Hot water also speeds up some kinds of corrosion. Add Texas clay soils that swell and shrink with the seasons, and the hot lines under a slab take a lot of stress. It's no surprise that hot-side leaks are among the most common slab leaks plumbers find.

How a plumber pinpoints the leak

Once the hot side is confirmed, a leak detection plumber traces the hot line's route from the water heater across the floor with an electronic line locator. Thermal imaging is especially useful here: the leak shows up as a bloom of warmth centered on one point. The plumber then confirms the spot acoustically, listening through the concrete for the hiss of escaping water. When both methods agree, the location is marked on your floor.

Detection typically costs $150 to $600 and takes two to four hours. You get a written report with the line, location, readings and photos. Our Our Process page covers each step.

What happens to the water heater afterward

Once the leak is repaired or the line is rerouted, the heater should go back to normal cycling. If it doesn't, have it inspected. A heater that has been running nonstop for months may have built up sediment or worn components. It's also worth confirming the expansion tank (if you have one) is working, since a hot-side leak sometimes hides pressure problems.

How to limit damage while you wait

• Keep the water heater's cold inlet closed if you can manage without hot water, with the heater in vacation mode.
• Watch for moisture in flooring near the warm area and keep the room ventilated.
• Take photos of any damage and note the date you first noticed the problem.
• Call your insurer early if you plan to file a claim. Many Texas policies cover water damage from an accidental discharge, and our FAQs explain what's typically covered.

Energy costs add up

A hot-side leak costs you twice: once for the water, once for the gas or electricity. In summer, it can also add heat and humidity to the house that your air conditioning then has to remove. If your energy bill and water bill both crept up at the same time, that's a pattern worth noticing.

When it's urgent

Call the same day if water is coming up through the floor, a large area is warm or wet, or the meter is spinning fast. For a smaller leak, book detection within days. Hot-water leaks under slabs don't fix themselves, and the longer they run, the more soil and flooring they affect.

Gas and electric heaters show it differently

How you notice the problem depends on the heater. On a gas tank, you may hear the burner light more often, see the flame through the sight glass when no one has used hot water, or notice a higher gas bill. On an electric tank, there is no flame or sound, so the clue is usually a jump in the electric bill, or the breaker area feeling warm. Tankless heaters only fire when water is flowing, so if a tankless unit lights up or its flow indicator shows a trickle while every tap is off, hot water is leaving the line somewhere. That makes tankless homes easier to diagnose: the heater itself is a flow meter. With any type, note how often it runs overnight. A heater that keeps cycling between midnight and six in the morning, when nobody is using water, deserves a closer look at the hot-water lines under the slab.

A water heater that never rests is a common clue in the 1990s and 2000s homes of Cedar Park, Allen and Mansfield.

The bottom line

A water heater that never seems to rest is often the first sign of a leaking hot-water line under the slab. Test it with the meter and the water heater's inlet valve, rule out sediment and the relief valve, and have the line pinpointed before anyone opens the floor. If you're in Texas, find your city to book slab leak detection.