Heat exchangers are the equivelant of the radiator in your car. The difference is the radiator transfers heat to the air and a heat exchanger transfers heat to sea water. Most heat exchangers are made of tube bundles where the sea water travels through the tubes and the engine coolant travels around the tubes. Exchangers can suffer all the problems that your car’s radiator does, plus some additional ones. The two most important items in the periodic maintenance list for heat exchangers is to keep the sacrificial zinc in good shape and to change the engine’s coolant regularly.
The zinc is provided to reduce corrosion in the heat exchanger. It functions by corroding away, “sacrificing” itself for the good of the copper in the exchanger. The zincs may need replacing every year or as often as every two months, depending on the environment and how the boat is used. It is better to replace the zinc before it needs it, rather on letting it dissolve away all together. If the zinc dissolves too far, you will not be able to remove the zinc portion from the threaded pipe plug. This isn’t a big deal, but it just adds a little cost. Also, if the zinc wears away unevenly, it can corrode at the base, letting the rest of the zinc fall into the heat exchanger. Not all exchangers have a sacrificial zinc as they are made of cupranickel alloy and don’t require the protection.
The coolant in your engine has a designed life expectancy. For “standard” ethylene glycol, it is two years and for “long life” propylene glycol antifreeze it’s five years. These are “projected” lifetimes and depending on the engine/service may too long between replacement. We use the term coolant because “antifreeze” is much more than it’s name. It has components that stop corrosion, lubricate pumps, and increase the boiling point of the coolant. Once the corrosion inhibitors get tired, then the coolant will begin to corrode parts of the cooling system, starting with the least noble metals, like aluminum. The seal in the water pump can fail if not lubricated properly. Coolant often contains “silicates” that can collect in the heat exchanger as it gets older, clogging it.
One of the most common problems is clogging of the sea water side of the exchanger due to foreign debris. The type of debris can vary from seaweed, to sand, to zebra mussels. Almost all heat exchangers have access plates that can be removed to clean out the debris. There are also calcium deposits that over time can block off the tubes or coat the tubes reducing the heat transfer. Acids can be used to remove the deposits, but it can eat through the tubes as well. Be sure to pressure test the exchanger after cleaning with acid. If you have replaced the sea water pump impeller and it had lost some of it’s blades, then most likely they have traveled through the hose to the heat exchanger. The pieces can lodge against the tubes and reduce the water flow.