Why is Steel Better than Cast Iron?

STEEL VS. CAST IRON

Which is better for building boilers out of, Steel or Cast Iron? Why would you choose a Steel boiler instead of Cast Iron one? Which direction is the industry headed in? All good questions! I hope to provide the answers.

Slowly, the industry is turning away from the cast iron heat exchanger... Even more so in Europe. Here in the States we see that most manufacturers have already made the switch on larger industrial and commercial equipment. It is easy to see why when you burn 2-5 gallons of oil an hour, when efficiency and life expectancy translate into lost profits, and when simple physical weight and size are issues. Check the codes and you will find that Cast Iron is not ASME compliant on systems that run steam at pressures greater than 15 psi or 250 degrees F. It just isn't strong enough. Steel is up to 50% stronger than cast iron, and that is why it is demanded by the industry requirements in these larger systems.

Unfortunately for the mass market, the major manufacturers are still resistant to make the switch to steel at the domestic level for several reasons. They have something that everyone is used to, they don't want to have to retool all of their factories, retrain all of their employees, and redesign everything they do. If it were cost effective for them to switch to steel, you know that they would. Instead, they redesign what they have, throw some silicone into the cast iron mix to try to force it to be more resistant to thermal shock and corrosion, and hope that someone doesn't come out with a serious domestic steel boiler.

Oops, too late...

Thermal Shock

"The branch that does not bend in the breeze will break in the wind, Little Lotus Blossom..."

Sage advice from the Orient, and it suits our first topic well.

Thermal shock is a situation where the traditionally hot water in a boiler is suddenly displaced by substantially cooler water, usually from the return piping, or perhaps fresh city or well water.
As you probably already know, most metals will expand and contract based on the temperatures they are confronted with. When a metal is rigid, like cast iron, and it isn't given that opportunity to flex, (or to "bend in the breeze") it will crack.

The end result will be cracked pressure vessels, cracked cast iron sections, and a host of other problems. It should be noted that 60% of cast iron boiler failures are a result of cracked sections, usually due to thermal shock.

Studies have shown that steel is more resistive to these failures than cast iron. Steel will heat up rapidly, it expands and contracts easily, and it is highly resistant to constant temperature fluctuations.
Why do you think it is almost universally used in large commercial and industrial boilers?

Price
You will find that the Benjamin line is about evenly priced with most of the Cast Iron stuff you see out there. We may sometimes be $500 to $1000 less or so depending on who you buy from, and what you are comparing it to. As for the CC500 and FS140, there just isn't anything else out there to fairly compare them to... That simply demonstrates how unique and revolutionary their designs are!

Longevity
The Benjamin OB1000 comes with a 20 year warranty. The first 5 years are very straight forward. After that it is pro-rated for the following 15 years based on current selling price at the time of failure. (Please read the actual warranty so you are clear on what we are talking about.) This type of warranty is basically an industry standard.

THE DIFFERENCE BETWEEN OURS AND THEIRS?
Most of the cast iron boiler manufacturers will indeed provide you with a similar warranty on their heat exchanger sections, however if you really read their coverage, it does not include the gaskets between the boiler sections. This is the weakest link in their chain, and they completely ignore it. If you have a failure there in three years the price to repair it, and it will be significant, will be a haggling point between you and your installer – good luck.

As far as the Benjamin line is concerned... We ask: "What is a boiler section gasket?" We don't have them. Every Benjamin unit features a solidly welded one piece steel construction design.

Corrosion
Corrosion is a major concern if you are running the lower water temp. returns and lower stack temps common in todays radiant heat systems.

It has been noted by others that many of the cast iron heat exchanger failures that are credited to "Thermal Shock", really do need to give way in part to corrosion.

When fuel is burned, some of the products of oil combustion turn out to be sulfur and water vapor. Usually this isn't all that much of a problem for the boiler, because they go right up the chimney due to their higher temperatures. However, when a radiant heat system (that runs lower return water temps) first fires up after a significant thermostatic change, or when a "Cold Start" boiler system fires, it can chill these flue gases down and actually condense the water vapor inside the boiler. As the water vapor forms it pulls the sulfur in the gas along with it, and you have accidently formed sulfuric acid on your heat exchanger. Granted, it doesn't usually sit there for long as it will eventually get "baked" off as the temperature of the boiler comes up, but it does happen, and it's aggressive impact is significant.

It doesn't take a rocket scientist to see a forthcoming failure when you take a section of the boiler that is already under stress from "thermal shock" and you subject that stressed section to sulfuric acid. Unfortunately for Sulfuric Acid, "Thermal Shock" usually gets all of the credit for the failure.

Little can be done to combat this as all boilers are subject to it at various times, but wouldn't you feel better knowing that your boiler is made of steel instead of cast iron? Studies have documented that the densely compacted uniform structure of steel is significantly less vulnerable to deep pitting corrosion than cast iron.

Efficiency
Cast iron boiler manufacturers claim to have comparable high efficiencies. And they may, at least for the first few weeks after installation. Like it or not, the combustion gasses from fuel oil are dirty from day one. The longer the system operates between tune-ups and cleanings, the worse they get. Slowly, and at a very small level at first, soot will start to build up on the heat exchanger. This will insulate it and cause an inefficient heat transfer from the combustion gasses thru the heat exchanger into the water. And, it only gets worse as time goes by. The solution should be simple. Clean the heat exchanger. The problem with cast iron boilers, is that they typically have tight passages and heat transfer pins. Their sectional designs make it near impossible to get a cleaning brush into, around, and behind all of the nooks and crannies. No matter how smooth the casting is, cast iron is dimpled, and soot will collect, and stick to, those dimples. Good luck getting all of it out. Unfortunately for cast iron boilers, even when they are cleaned on a routine basis, it is near impossible to return them to their original efficiency rating. In time, they will only get worse.

Conclusion
I do want to stress that the most important part of a good system is the design. The boiler is just the locomotive at the front of the train. Make sure your heating contractor is someone you have confidence in, is someone that you trust, and is someone that you believe knows how to design a proper system.

I hope that the points that I have made above have brought to light why a cast iron boiler isn't the best alternative. A lot of people are really sold on cast iron, and a lot of contractors that I talk to seem to like them too. I believe that they have been duped by a slick marketing campaign by an 150 year old industry standard, and that it is only a matter of time until everyone will be making the switch to steel.

Benjamin has over 30 years of experience, and a complete line of solid, real-world tested products on their side. Any type of competition from the cast iron boiler manufacturers doesn't really exist yet. This allows me to pose a question to you...

Which side of the future do you want to be on?