Cast-Iron Vs. Stainless-Steel Gas Boilers

Cast Iron

• Cast-iron boilers are durable and efficient, and some models are known to last 35 years or more. The cast-iron material is used for the heat exchanger component in the boiler. The heat exchanger is the section of the boiler where warm water is held until a faucet is turned on, thereby signaling the boiler to deliver hot water through the plumbing system to a kitchen sink, bathtub, shower or appliances such as washing machines or dishwashers.

Stainless Steel

• Stainless-steel boilers are made of pressed steel and are therefore slimmer and smaller than are cast-iron boilers. Like cast-iron boilers, stainless-steel boilers are an integral appliance for delivering hot water through a home’s plumbing system. Tankless water heaters and boilers are made of stainless steel or aluminum and not out of cast iron. As a result, you can wall-mount a stainless-steel tankless water boiler because it’s made from lightweight materials. Tankless units heat water on demand, instead of storing water, as is the case with conventional tall and cylinder-shaped storage water heaters and boilers made from cast iron.

Similarities

• All boilers work with the furnace and thermostat of a home’s heating system. The thermostat signals the boiler to produce hot water, which in turn signals the boiler’s pilot burner, gas valve, circulator and vent damper to begin operating. Both cast-iron and stainless-steel boilers have stainless-steel burners over the flame that heats the water inside gas-fueled boilers. Gas-fueled boilers have vent dampers, which connect to the home’s chimney to exhaust fumes and gas by-products. Electric-powered cast-iron and stainless-steel boilers do not require burners or vent dampers. They use heating elements to warm the water inside of the boiler instead of burners and do not require vent dampers and chimney connections because they do not emit gas by-products.

Differences

• Cast-iron boilers have lower AFUE ratings than do stainless-steel boilers. Their ratings go from 80 percent -- to meet minimum requirements by the U.S. Department of Energy -- to about 87 percent. On the other hand, stainless-steel boilers start at 80 percent, and condensing and tankless boilers have AFUE ratings as high as 98 percent. A gas-fueled cast-iron boiler can build up condensate, which is a liquefied version of gas by-products, if the temperature inside of the boiler drops below 131 degrees Fahrenheit. Condensate can corrode a conventional cast-iron boiler. As a result, some homeowners opt for boilers made of stainless steel to prevent this problem, whether they are conventional tall storage tank boilers, non-condensing boilers or tankless stainless-steel boilers. Because cast iron is corrosive, conventional boilers are more prone to rust and collect sediment, requiring regularly scheduled maintenance to empty and clean the sediment buildup.