How to Check for a Hot Water Slab Leak (Without Any Tools)


Warning Signs of a Slab Leak

Do you suspect a slab leak? Maybe you’ve heard a quiet hissing sound coming from your faucets when they are turned off. Maybe there is a warm spot on the floor that you’ve noticed for the last couple of weeks. Whatever the reason, if you think that you have a slab leak, this article will give you a quick and easy way to test for a hot water slab leak without any tools. Read on! 

A slab leak is a leak on one of the pressurized, copper water pipes that run underneath the concrete slab foundation. In Dallas, and Texas overall most homes built after 1960 feature a poured concrete slab foundation. Unfortunately leaks under a concrete slab foundation are more difficult and expensive to repair than other foundation types. Older homes in DFW could have a pier and beam foundation, this test can also be used and the repair is much more straight-forward.

If your home was built after 2010, there is a much higher likelihood of having plastic water lines called PEX which are run through the attic instead of the copper water lines under the slab. This would protect your home from a slab leak and provides easier access to plumbing.

Prerequisite of the Test

The only thing you need in order to perform this test is a working shut off valve on your water heater. If the water heater has an old gate valve (round-style handle) that hasn’t been operated in years, then there is a good chance it won’t work properly. If the water heater has a nice ball valve (lever-style handle), then this test will be easy.

The first thing you need to do is to make sure that none of the faucets are dripping. Check all of the sink faucets and tub/shower faucets to make sure they are turned off all the way and not leaking. Don’t worry about the toilets or outdoor faucets, because they only have cold water running to them. If you have any running or dripping faucets, fix those first before testing. Finally, make sure that everyone else in the house is aware that you are about to test the system. They should not turn on any faucets until you are done.

Performing the Test

Once you’ve verified everything is off and there are no leaks, go to your water heater. If there is a recirculation pump, make sure it is unplugged. Next, simply turn off the cold water supply valve on the water heater. (It will be to the top right of the water heater when you are facing it.) Make sure it is all the way off. Wait for about 2 minutes and quickly turn the valve back on. If you hear/feel water rushing through the valve as you turn it on, then you probably have a slab leak. If there is no sound, then you probably do not. You can repeat this test and wait longer than two minutes if the result was inconclusive.

Another very common symptom of a hot water slab leak is a hot spot or warm spot on the floor, so check for those around the house at the same time. You can use your hands or (even better) an infrared thermometer to narrow this down.

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How This Test Works

By shutting off the valve and making sure all of the hot water fixtures are not leaking, you created a closed-system. If there are no leaks, the pressure will stay the same and there will be no flow through the valve when you turn it back on. If there is a leak, then over that 2-minute period the pressure will drop. Then when you turn the valve back on, water will noticeably flow through the valve to make up the water that was lost.

Think of it like heavy traffic at a stoplight. Have you ever been in traffic so bad that when the traffic light turns green you just have to sit there because there is no room for you to keep driving? The traffic light is the shut off valve. The road past it is the hot water piping system. As long as traffic is backed up by the accident, there is no traffic movement regardless of whether the light is red or green. If, however, one lane opens up and “leaks” the backed-up traffic, then when the light turns green again (valve open) there will be traffic movement.

In a normal system, there are no hot water pipes buried in the yard. So if one of the pipes is leaking, it will either be below the slab or inside the house. If it was inside the house, then you would probably have noticed water damage long before now. Using this reasoning, we can deduce that it is very likely one of the hot water pipes below the slab is leaking. Combine that with other evidence like a warm floor, high water bill, hissing sounds, etc… it is pretty conclusive.

The Cold Water System, Cross-Overs, and Other Considerations

By itself, this test is not fool-proof. It is possible to have a cross-connection between the hot and cold systems at another place (like a shower valve). If this situation exists, then just shutting off the cold water inlet valve at the water heater won’t completely isolate the hot system and the test probably won’t work.

This kind of tool-less test is possible on the cold water system, but it is much more complicated. Because you have water lines that run through the yard and others that serve the irrigation system, performing this test at the water meter (or even just watching the meter flow indicator) may indicate there is a leak somewhere, but it won’t narrow it down to a slab leak. (Read more about isolating the irrigation system here in this article.) If you have a good ball valve where the main water line enters the house, you can do this test on the cold system too, but be aware of false-positives such as slowly-running toilets, etc…

Next Steps on Fixing a Slab Leak

If you still think you have a slab leak and want to know more about repair possibilities, check out this article on the four different ways to repair a slab leak.

Also, please give us a call and talk to our master plumber. He will be able to step you through different scenarios and pricing for your particular situation over the phone.

At Legacy Plumbing, we specialize in slab leak repair. There is not a one-size-fits-all approach. Every house is different and your preferences matter. We have refined the different methods of slab leak repair and are experts at finding the best way to make the repair for you and your home.



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