Hot water tanks or boilers must be strong to withstand the constant strain of heat and pressure as they heat water from outside the system and pump it through the house. Over time, this strain can create cracks and leaks throughout the system, especially with older models. Fortunately, you have a few options for dealing with leaks before replacing your heat system components.
Joints and Pipes
If a leak is coming from your pipes or the pipe joints, be sure to check what kind of pipes you have. Some pipes have lifelong warranties for non-leaking features, and can be replaced at little or no cost if you are willing to invoke this warranty. Otherwise, try to isolate exactly where the leak is coming from. Try looking at the pipes when they and the water are cool, because hot water tends to evaporate quickly and can be difficult to spot.
If the leak is coming from a joint, use a wrench and tighten the joint, being careful not to overtighten; simply increase the joint seal. If the leak seems to be coming from the pipe itself, you can either replace the pipe or try to repair it. If you want to repair it, buy some pipe sealing outer liner and seal off the section of the pipe. Unscrew the nut and loosen the joint until you can slide the pipe out slightly. Wrap the pipe in the leakproof liner and then replace it. You can also buy internal leak sealers, solutions that you can put in your water system and run through for about 24 hours. These are designed to internally patch small leaks and may work if you have low pressure but cannot find a leak source.
Hot Water Tanks
If the leak is coming from the tank itself, try to isolate it. If the drain valve is leaking, this is an easy fix -- tighten the drain valve at the top of the tank or replace it completely if the leak does not go away. If there seems to be condensation around the vent, check to make sure the vent is clear. If the heating element gasket is leaking, turn off the power, shut down the water and replace the gasket completely.
If the tank has corroded and developed a more integral leak, you will need to replace the tank itself, and it is probably best to call a professional at this point. You will need to shut off both power and water to the entire tank in order for it to be replaced. Be sure you bleed the air from the pipes before you use it again.