If your home has a hydronic heating system -- a unit that produces heat from a central boiler -- you probably have baseboard heaters or cast-iron radiators if your home is older. Radiators are notoriously noisy as they warm up, and can sound like monsters to the young ones in your household. Sounds emanating from the boiler itself sound just as spooky, and are often the result of sediment, air or excessive temperatures.
Popping and Crackling
One of the most common causes of noises from boilers -- as well as hot water heaters -- is sediment on the bottom of the tank. The sediment, typically lime, traps water underneath it, and when the flame comes on, it overheats this water, which turns to steam and creates a crackling or popping sound. The steam may also thump against the side or top of the tank as it rises. Since lime is alkaline, the best way to get rid of the deposits is to flush the tank annually with a mild acid such as phosphoric acid.
When the burner on a gas-powered boiler switches on, you may hear a sizzling sound accompanied by a small pool of water under the boiler. While there may be a small leak in the boiler drain that causes water to seep around the burner, condensation on the underside of the boiler can also cause the problem. When the flame comes on, the condensation droplets or seepage quickly boil, resulting in the sizzling sound. If the problem is condensation, the sounds stop once the boiler heats up. If it's seepage, the noise should stop when you tighten the drain valve.
Loud banging sounds may be the result of thermal expansion of the tank walls as the water inside heats up. It could also be caused by water hammer, and if so, is most noticeable when water is added to the system through the cold water inlet. Installation of a pressure-relief tank next to the boiler can correct thermal expansion, and you can stop water hammer by installing water hammer arrestors in the water supply pipes. Either condition can damage the pipes and the boiler's tank if allowed to persist.
Aluminum Anode and Hard Water
Both gas and electric boilers and water heaters may have an anode rod to prevent corrosion on the inside of the tank. If the water in your area has a high enough concentration of lime, and the rod is made of aluminum, a reaction between them can produce excessive amounts of aluminum hydroxide, which generates the crackling or gurgling sounds in the tank. Replacing the aluminum anode rod with one made of magnesium corrects this problem. If the tank has a buildup of lime on the bottom, flush it before making this switch.