How to Find a Broken Wire in a Heated Floor

Things break. It’s a fact of life and like anything else, there can be problems with radiant floor heating. A tile setter unknowingly severs a wire with his trowel and continues tiling. A grout knife makes its way through a heating cable. A previous repair from years ago up and fails. A hole is drilled through the floor for a new doorstop, severing the heating wire somewhere between the terminal end cap of the cable and the circuit breaker. 

These are all things that can result in your electric floor heating not working. All is not lost, however, because underfloor heating repair and home wiring is typically a pretty straightforward process. The tricky part is finding the break. Luckily, WarmlyYours has the tools for the job. 

Now, if you're wondering "How do I find a broken electrical wire underground?" the first step is get in touch with us. By contacting a WarmlyYours account representative or a member of our technical support team, you can rent our Troubleshooting Kit (TSK), which consists of a dielectric strength tester (Hipot), a variable AC transformer (Variac), a multimeter (which can be used as a circuit tester or voltage tester), a time delay reflectometer (Shortstop) and of course, some splice kits to repair the break once it has been found. A Thermal Imaging Camera (TIC), which is rented separately, will also be needed to help us “see” through the tile.

The process for electric underfloor heating fault finding is fairly straightforward. Always start with getting the Ohms readings from the floor circuit. If the readings are open or infinite from one core wire to the other core wire, you have a break. If you have Ohms readings between either core wire and the ground sheathing, you have a short circuit. It is possible to have both a break and a short on the same electrical circuit. If the ohms confirm a break, the Hipot will be the tool you will need. If the readings confirm a short (regardless if there is a break or not), you can use the Variac.

The shortstop may come in handy in helping to narrow down an area where the break or short has occurred. This device simply measures the length of wire to where the damage to the wire is, and reports back in length of feet of wire. For example, if you’re testing a project that used TempZone Flex Rolls and the shortstop gives you a reading of 40 feet, we’ll know the damage will be in the first 5 or 6 feet of the heating mat. Knowing this will help to focus our efforts in locating the damage when we work with the Hipot.

But before we get into the processes themselves, here are some tips that apply to both. It helps to have the room as cool and as dark as possible so we can more readily see the heated cable through the tile with the TIC. I usually bring some black garbage bags to put over any windows in the room, if there are no blinds or shades. This will help limit the effects of sunlight on the floor. In fact, all lights should be shut off, especially overhead incandescent lighting as it tends to leave a heat signature of its own.

The Hipot Process

Important: Only a licensed, qualified electrician should carry out with this process.

This process involves sending between 400 to 600 volts through the core wire with the intention of “jumping” the current across to the ground shield of the flooring cable, essentially duplicating the behavior of a sparkplug. Care must be taken with this process because of the potential for electric shock.

Once we’re able to jump the current across these two components of the wire, we repeat the process sending the voltage across every 2 seconds at 2 second bursts. This is tedious work. I usually use a stopwatch or set a timer on my cellphone and try to get into a rhythm—2 seconds on, 2 seconds off, while tapping the wire with the probe in one hand and holding in the reset plunger with the other. I do this for 10 minutes. After 10 minutes of charging the wire, grab the TIC and scan the floor in search of the hot spot created by the Hipot.

The heat signature will usually be a dime-sized orange dot, only a few degrees warmer than the rest of the floor. If there is no signature present, take Ohms readings again. Sometimes this charging of the wire will cause the core wire to fuse with the ground shield, creating a short circuit. If we have core to ground readings at this point with the ohmmeter we can move on to the Variac process (explained below), if not, charge the wire for another ten minutes. Remember,: every 2 seconds, for 2 seconds, for 10 minutes. Repeat as necessary and take Ohms readings after each cycle. You might be tempted to charge the wire constantly in hopes of speeding up the process. Don’t. Operating the Hipot in this manner can seriously damage the equipment. That could be costly and delay your efforts.

The Variac Process

When compared to the Hipot process, the Variac process is a walk in the park. The risk of electric shock is minimized, as we’re now dealing with low voltage. In the kit, there will be an extension wire with a small portion of exposed wire. This cord is plugged into the Variac, and the loose end is connected to the core wire and ground shield that register an Ohms value or short circuit.

A short circuit is simply a point in the circuit where the core wire is coming into contact with the ground shield. Since this ground shield has a lower resistance, any voltage it encounters will be directed away from the circuit and safely to the house ground. Electricity seeks the path of least resistance and we’re now going to use this to our advantage with the Variac.

With the Variac and extension cord hooked to the core and ground shield, locate the multimeter and set it to read amperage. Place the clamp around the exposed section of wire on the extension cord. With the dial at zero, turn on the Variac and slowly turn the dial up. Try not to exceed 10% of the circuit’s rated voltage (12 volts for 120 volt heating elements and 24 volts for any 240 volt circuit). The multimeter should start to register an amp draw. Amperage should be in the 1.5 to 2 amps range. Make sure this amperage draw is constant. If the amperage goes to zero, the short circuit has been broken and you’ll have to revisit the Hipot process. Try not to put more voltage and a higher amp draw through this circuit either, as that risks breaking the short circuit. Let the system “cook” at 1.5 to 2 amps for about 10 minutes. Get the TIC ready and start scanning the floor. We should start to be able to see the heated wire through the tile at this point. It will look like an orange line in a serpentine pattern weaving back and forth across the floor. Where the orange line stops is where the core wire and ground shield are and you have the found the short.

Now all that’s left to do is break the tile and start chipping away the pieces. Take a hammer to the tile center and use a dull screwdriver to begin prying the pieces. Go slowly and carefully, avoid using any sharp tools as we don’t want to further damage the heating system we’ve worked so hard to try and fix. Then, after you've exposed the damaged wire, you can use a splice kit to repair it (do not use electrical tape). Once that's done and all of your readings come back clear, you can replace the removed tile. 

To speak with a WarmlyYours account representative about renting our troubleshooting kit, use our contact us page and if at any point you have any questions, please don’t hesitate to contact our technical support team at 1-800-875-5285.