Boiler Heat Exchanger: Aluminum vs Cast Iron vs Stainless Steel

As you shop for boilers and read their specifications, you’ll see three materials used in heat exchangers: cast iron, aluminum and stainless steel. The heat exchanger is the part that transfers heat from the combustion gases into the water that is being circulated by the boiler, so it is a crucial part of the design.

Let’s look at the pros and cons of cast iron, aluminum and stainless steel heat exchangers and when each material is used.

Pros and Cons of Cast Iron Heat Exchangers

  • Pros: Cast iron is inexpensive and it handles temperature changes quite well. It is also very durable, and boilers manufactured with cast iron heat exchangers have some of the longest warranties. Limited lifetime and 20-year warranties are offered on top cast iron models from Peerless Boiler, Utica Boilers and other boiler makers.
  • Cons: There are three concerns with cast iron. It is the heaviest of the three materials. Secondly, it cannot be used in condensing boilers for reasons specified below in the advantages of stainless steel. Finally, cast iron’s heat transfer efficiency is lower than the other two materials.
  • The bottom line: Most floor-mounted, standard-efficiency, affordable gas and oil boilers are built with cast iron heat exchangers.

Pros and Cons of Aluminum Heat Exchangers

  • Pros: While used the least of the three materials, it has the advantage of being lighter and more efficient in transferring heat than cast iron and less expensive than stainless steel.
  • Cons: Aluminum doesn’t transfer heat as well as stainless steel, and it cannot be used in condensing boilers.
  • The bottom line: Aluminum heat exchangers are found in boilers in the 90% to 94% efficiency range like the Utica UB90-100 IV 90% boiler and the Weil McLain Ultra Gas S3 CT with models from 92-94% efficient.

Pros and Cons of Stainless Steel Heat Exchangers

  • Pros: Stainless steel is the only material of the three that can be used in condensing boilers. These boilers capture additional heat by condensing the moisture in exhaust gases to be ultra-efficient. The problem is that flu gas condensate is very acidic and quickly destroys cast iron and aluminum heat exchangers. Stainless steel is lighter than the others, so it is popular for wall-hung gas boilers.
  • Cons: Stainless steel is the most expensive material, and is rarely used in non-condensing units, so its efficiency benefits aren’t utilized. Secondly, because of the acidity of condensing boiler flue gases, warranties are typically 15 years or less and are often pro-rated after five to seven years.
  • The bottom line: Some studies show that condensing boilers do not have the durability of cast iron boilers, and so the money saved through energy efficiency might be lost through having to replace the equipment sooner, as the president of US Boiler argues in this interesting letter to the EPA’s Energy Star team.
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