By Dan Vastyan
Steam systems are not what they used to be, or at least so it would seem to some homeowners. The fact of the matter is that many retrofit steam boilers are not lasting as long as their predecessors under certain conditions – especially those built after 1992. Naturally, it begs the question, “Why?”
Regardless of brand, a steam boiler that fails shortly after a decade of service is not unheard-of. Homeowners, realizing that the original unit in their basement had lasted over half a century, are left perplexed and feeling slighted. Of course, they look to the installer for answers.
Maybe you have experienced this. It’s true, steam boilers today do not always last as long as the units of yesteryear. But upon deeper investigation, it’s no giant mystery.
Much has changed since the glory days of steam. While these systems can still be very comfortable and efficient, there are a few new factors to consider. Changes in efficiency requirements and water quality have proven to be game changers in regard to the service life of steam boilers.
Steam boiler sections tend to wear out at the water line. All steam boilers have this issue; it’s simply a matter of how long it takes to wear them out.
The water line is the flash point, where water instantaneously turns to steam. This creates a hammering effect on the section wall. Constant wetting and drying at and above the water line causes the cast iron to rust. If left in place, this rust can actually improve resistance against further corrosion, but if it’s removed by chemicals in the water, corrosion continues. As this cycle continues over and over, the boiler is “eaten” from the inside.
Today, boiler designs and poor water conditions have both proven to accelerate this process.
Trading durability for efficiency
If you’ve lowered a new steam boiler into a basement and hauled the old one out, you’ve experienced firsthand the main difference between the two; weight.
The 1992 DOE regulations – which for the first time mandated that steam boilers must meet or exceed 75% AFUE – turned the world of cast iron boilers on its head. Old boilers were big, heavy units with thick sections and much lower efficiency levels. New boilers are smaller and lighter, and required by law to have higher efficiencies. Today, 82% AFUE is required for gas fired steam boiler.
To conform to new efficiency codes, all manufacturers of steam boilers have been forced to lighten their cast iron sections. This increases heat transfer and lowers standby heat loss, but inherently reduces the service life of the boiler. This is especially true in situations where units are exposed to chemicals that weren’t always present 50 years ago.
Water conditions have changed over the years as well, and not for the better. This is truer in some places than others. Increased use of road salt, fertilizers, etc., has raised the salt and chloride levels in well water. Chlorine, fluorides, and other chemicals are added to municipal water to ensure safe (potable) consumption. These chemicals all speed up the corrosion inside a steam boiler.
Initially filling a boiler with water containing these chemicals is harmful, but the bigger issue stems from make-up water. Under normal conditions, make-up water is added to replenish minimal vapor loss released from functional air vents, low water cut-off maintenance and flushing wet condensate return piping.
The single most important factor that impacts the life of a steam boiler is the amount of fresh water added to the boiler during operation. Fresh water brings dissolved minerals, salts and oxygen into the boiler, greatly accelerating corrosion. Let’s face it; the piping and components in most steam systems are aging, too.
In an aging, poorly maintained steam distribution system, excessive make-up water is required to compensate for losses due to leaks and faulty air vents that allow steam vapor to escape during the entire steaming cycle. Unless recognized and fixed, this perpetually introduces more chemicals to the inside of the boiler.
The process of producing steam in a boiler doesn’t remove chlorides or other contaminants from the boiler water. As make-up water is added, the concentration of contaminants increases.
If you’re called to replace a steam boiler – whether it’s 10 or 75 years old – it requires more attention than what a water boiler retrofit might need. Seeing to these details will ensure longer service life for the new equipment. When you pipe the new unit, be sure to include a Hartford Loop to maintain proper water level in the boiler.
Pressure check the system or install a water meter to check for the use of makeup water. It’s extremely important to locate and repair all system leaks prior to installing a new boiler in the existing steam system. Preferably, all steam vents should be replaced as a part of the retrofit.
It’s also advised to test the acidity of the make-up water. The pH level of the water in a steam system should remain between 8 and 10. The chlorine content should also be checked.
In most installations, steam boilers don’t require water treatment for protection. However, water in certain areas of the country is unsuitable for use in steam boilers due to high levels of chlorides, total hardness, or low pH.
- When testing makeup water, the following are a good rule of thumb:
- Chlorides should be less than 30 mg/L (ppm)
- Hardness (as CaCo3) should be less than 100 mg/L (ppm)
- pH should be between 8.0 and 10.0
Make-up water that doesn’t fit these standards must not be used in a steam boiler and requires treatment prior to use. Consult a qualified steam boiler water treatment company such as Rhomar Water Treatment, Inc.
Dan Vastyan is a journalist who has covered the plumbing and HVAC industry exclusively since 2010, with a special focus on hydronics. Manufacturer training, personal research and time spent with a pipe wrench in hand have given him a deep technical understanding of residential and commercial systems alike.