Not having hot water in a bathtub can be frustrating, especially when other plumbing in the bathroom is working just fine. A number of problems can cause a tub to lack hot water while the nearby sink works fine. Some of these potential problems require the skill of a plumber to repair; others you may feel comfortable troubleshooting by yourself. Before calling a plumber, investigate the possible problems that may be causing your tub to produce only cold water.
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Several problems may cause only your bathtub to lack hot water, including a defective faucet, a water heater malfunction, a clogged pipe and a hidden leak.
A Defective Faucet
A defective bathtub faucet may be the cause of the problem, particularly if you turn the faucet handle to open the faucet and no water comes out. If this happens, it probably means that something is broken between the stem and the seat of the valve that controls the flow of water. This is a pain, but it's relatively good news because a broken faucet is a cheaper and easier fix than some of the alternatives.
If you're willing, you can replace a broken tub faucet on your own. Turn off the water supply and remove the faucet using an appropriate-size wrench. Carefully measure the hole where the faucet was installed and purchase a faucet that corresponds to this size. Replace it by following the assembly instructions.
Water Heater Malfunction
Bathtubs require a lot of hot water. If you're getting plenty of cold water but it's not heating up, a malfunctioning water heater may be the source of your problem. If you have an electric water heater, one or more of the heating elements may not be working properly, causing water to flow cool or lukewarm. The heater's thermostat may need to be replaced. Or you may just need to make an adjustment, such as relighting the pilot light in a gas heater or restarting an electric water heater.
Another common problem in both electric and gas water heaters occurs when the dip tube has broken off inside the tank, allowing hot and cold water to mix. An issue with the tub or shower mixing valve may also be preventing hot water from mixing with the cold. Hire a service provider to take a look at the heater if you suspect this to be the problem.
A Pipe Is Clogged
Another possible reason why your tub is not getting sufficient hot water is a clogged pipe. This is especially likely if you live in an old home with pipes that have been there for decades. Old pipes sometimes get clogged with mineral deposits due to chemical imbalances in the water. This can block hot water from reaching the bathtub faucet, even though the sink's hot water faucet is working fine.
A clogged pipe may be the culprit if you replace the faucet and still have no hot water. Hire a plumber to flush the pipes and return them to a normal functional state.
A leak somewhere in your hot water pipe can cause the hot water to flow poorly or not at all. If you notice water damage and stains on the ceiling or crawl space below the tub, you almost certainly have a leak. Leaks are harder to identify when the tub is on the first floor of a slab-foundation home or apartment. Have a plumber take a look at the system if you suspect this to be the cause of the problem.