Hot Water Line Leaking Under the Slab: Warm Floor, Water Heater That Won't Rest

Signs it's the hot side

A warm patch on tile or wood. Often between the water heater and a bathroom or the kitchen, since that's the path the hot line takes. Bare feet notice it first, and some owners notice the dog has picked a new favorite spot.

The water heater runs more than it should. You hear it firing or recovering when nobody has used hot water. It's refilling to replace what leaks out.

A higher gas or electric bill along with the water bill. You're paying to heat water that goes into the ground.

Hot water runs out sooner, or takes longer to arrive. Some of it is escaping before it reaches the tap.

A hiss under the floor that's louder after hot water use. See running water under the floor.

What to shut off right now

Close the cold-water inlet valve on top of the water heater. That stops the heater from refilling, so the hot side can't keep pushing water into the slab. Cold water keeps working everywhere.

Then check the meter. If its leak indicator stops moving, you've confirmed a hot-side leak and stopped it for now. For the heater itself, follow the manufacturer's instructions for a heater with its water supply shut; many gas models have a vacation or pilot setting, and electric heaters may need the power off. If you're unsure, leave the gas and power alone and say so when you call.

How a hot line leak is found

Hot leaks are usually the easiest slab leaks to locate. Closing the heater inlet proves the side. A thermal camera shows the heat bloom through tile or thin flooring. Acoustic listening confirms the exact spot. The full process is on slab leak detection.

Repair options for the hot line

• Spot repair through the floor over the marked spot, if the pipe around it is sound.
• Reroute of the hot branch only, run through the attic. This is common, because the hot line is often the only one failing. See slab leak reroute.
• Whole-house repipe if the cold lines are the same age and showing their own problems. See whole-house repipe.

Damage to watch for

Hot water under a floor can warp wood, loosen tile, swell baseboards and feed mildew. Photograph anything you see for insurance. Drying out flooring and drywall is usually a separate water damage company, so mention the damage when you call and to your insurer. What to do first has a checklist.

Westminster houses and hot lines

In many single-story Westminster houses the water heater sits in the garage or a utility closet, and the hot line runs under the slab to the kitchen and bathrooms. A leak anywhere along that run can warm the floor above it. The main slab leak repair page covers the other kinds of slab leaks in the area.

Questions callers ask

Can I still use cold water?

Yes. With the water heater's cold inlet closed, the hot side stops refilling but cold water still reaches every tap and toilet.

Why is my floor warm but there's no water?

Hot water can leak into the soil and gravel under the slab for a long time without surfacing. The heat rises through the concrete before any moisture does.

Is a warm floor always a leak?

No. Radiant floor heating, ductwork running under the slab in some homes, or afternoon sun on tile can warm a floor too. A warm spot plus a moving meter is the combination to act on.