Is a warm spot on my floor a slab leak?

Most Phoenix-area homes sit on a concrete slab, and the hot water lines to some rooms run through or under it. When one of those lines leaks, hot water soaks into the gravel and concrete below, and the slab above the leak warms up. The spot tends to be small, often a foot or two across, and it stays put.

Cold water lines leak too, but they don't warm anything, so a warm spot points to the hot side. That includes the hot line itself, and a joint or fitting near the water heater. It doesn't rule out a cold-line leak elsewhere in the house, and it doesn't mean the leak is directly under the warm spot. Water follows the easiest path through soil and gravel, so the wet area can sit a few feet from the break.

The Arizona Geological Survey explains why slabs in the desert are under stress: "The resulting changes in soil volume can cause considerable damages to homes, sidewalks, pipelines, and streets." Soil that swells and shrinks with monsoon rain and summer drying moves pipes and joints over the years. If you'd like the full background, why slab leaks are common in Arizona covers the causes.

Before you assume the worst, run through the harmless explanations.

• Sunlight. A floor next to a west or south window can be warm for hours in the afternoon and cool by bedtime. A leak doesn't follow the sun.
• A water heater, dryer or oven nearby. Heat passes through walls and into the slab near a closet or kitchen appliance.
• Ducts or an air handler. Where ducts run close to the slab, hot air in summer can warm the surface above them.
• A recirculating hot water loop. If your home has a recirculation pump, hot water cycles through the line and the pipes stay warm on purpose. See why a recirculating pump can heat your cold water lines.
• Hot pipes near the heater. The U.S. Department of Energy notes that "Heat traps prevent heated water in a storage tank from mixing with cooled water in pipes, a process called thermosiphoning." Without a trap, warm water can drift into the lines near the heater when nobody is using it.

The tell is timing. A patch that is warm only when the sun hits it, or only when an appliance is running, is almost certainly not a leak. A patch that is warm in the early morning, at night and with the A/C running is a much better suspect.

You don't need tools. You need about 30 minutes and a quiet house.

Test 1: the meter. Turn off every fixture, the ice maker, the irrigation controller and the washer. Then check the meter. EPA's version is simple: "Check your water meter before and after a two-hour period when no water is being used. If the meter changes at all, you probably have a leak." If it doesn't move, the warm spot has another cause. If it moves, keep going. Our page on how to do a water meter leak test shows how to read both meter types.

Test 2: the hot side. With the meter still moving and all fixtures off, close the cold water supply valve on top of the water heater. It's the valve on the pipe that feeds the tank. Watch the meter for 10 minutes.

• If the meter stops or slows sharply, the leak is on the hot side: the water heater's connections or a hot line. That fits a warm spot.
• If the meter keeps moving at the same rate, the leak is on the cold side, or ahead of the house valve. Then the warm spot is probably a coincidence, and what to do when the meter still moves with the main valve off is your next read.

Open the valve again when you're done.

Two more clues help. Put your ear to the floor over the warm spot in a quiet moment. A faint hiss or trickle that you can't place is worth noting. And check the bill: EPA suggests looking at a cold month, because "If a family of four exceeds 12,000 gallons per month, there could be serious leaks." Our page on a high water bill with no visible leak shows how to read one.

A hot water leak under a slab doesn't heal, and it gets worse in a few ways. Water keeps running, so the bill goes up. Hot water is being heated to replace what leaks out, so the water heater runs more than it should. And the ground under the slab stays wet. EPA notes that "It is important to dry water-damaged areas and items within 24-48 hours to prevent mold growth." Wet soil and a warm, damp slab under flooring give mold what it needs. If the floor is very hot, stay off it barefoot and keep kids and pets away until a plumber has checked it.

If the meter is moving and you think a hot line is leaking, you can close the main valve to stop the loss. You'll have no water in the house, but you'll stop the damage while you arrange a diagnosis. If you're not sure where the valve is, see where the main water shutoff valve is in a Phoenix home.

The next step is finding the exact spot. Plumbers use electronic listening equipment and heat sensing to mark the break before anything is opened, which is why guessing and jackhammering is not the way to start. How plumbers find hidden leaks explains the methods. Once the break is located, the repair options are usually a spot repair or rerouting the line through a wall or attic, and how a slab leak repair works in Phoenix compares them. Water damage coverage is its own question; does homeowners insurance cover a slab leak? covers the basics.

HQ Plumbing & Air offers leak detection so the source is pinpointed before repair work starts. We're licensed and insured, we give you a free estimate, and you get the price up front before work starts. If water is running and you can't stop it, that's an emergency, and we're available 24/7.

Here's the short version. A warm spot is a clue, not a verdict. If the meter moves with everything off and shutting the water heater's cold valve slows it, treat the spot as a hot water leak and get it located. If the meter sits still, look at the sun, the ducts and the appliances.