Cast-iron boilers have long been a staple of the North American hydronics market. They’ve been part of the evolution of hydronics technology from steam and open-loop, water-based systems to state-of-the-art systems supplying radiant panels, domestic hot water, snowmelting and pool heating. When properly installed and maintained, they can remain in service for 30 years or more - a statistic very few other appliances can match.
As newer boiler designs have appeared over the last 15-20 years, some practitioners think cast-iron boilers should take their final bow, and gracefully transition from mainstay heat source to museum artifact. I’ve even been asked at seminars why I even bother to make reference to such “dinosaurs.”
I think it’s a huge mistake to write off cast iron as a viable material for boiler construction now, and for many years to come. How can I justify such a statement? Read on.
Oldies But GoodiesBesides long life, cast-iron boilers bring two very desirable characteristics to hydronic systems:
1. Low head loss through the boiler.
2. Thermal mass for operating stability.
In addition to the circulator, hardware is needed to hydraulically isolate a boiler with a high-flow-resistance heat exchanger from the distribution system. This detail is often handled with a pair of closely spaced tees, as shown in Figure 1. According to R.S. Means, the installed cost of two 1 1/4-inch copper tees is about $135.
However, even the $550 in added hardware looks paltry compared to the cost of operating this circulator over its service life. Consider a 200-watt boiler circulator that operates for 3,000 hours each year in an area where electricity costs 14 cents/kwhr. Its current annual operating cost would be:
Assuming a modest 4 percent annual inflation on electrical rates, the operating cost of this circulator over a 25-year service life would be:
So, here’s the choice: Go with a nominal $4,000 total owning and operating cost for this circulator, or eliminate the need for it. The latter is possible when the boiler has a low-head-loss heat exchanger. There are undoubtedly many combinations of heat exchanger geometries and material(s) that could achieve this, and cast-iron sectional construction is definitely one of them.
Massive BenefitsAnother often-underappreciated feature of cast-iron boilers is their inherent thermal mass, which comes from multi-hundred pound heat exchangers filled with several gallons of water. This thermal mass serves as a mediator between the rate at which heat is generated and the rate it’s delivered to the load.
Here’s a comparison between the thermal masses of two small boilers. The total thermal mass of each boiler is estimated by adding the thermal mass of the metal heat exchanger to that of the water in the heat exchanger. Thermal mass is calculated by multiplying the specific heat of each material by the weight of that material. It’s represented in units of Btu/degrees F (e.g., the number of Btus needed to raise the mass 1 degree F).
In this case, the cast-iron boiler provides more than 12 times more thermal mass than the compact boiler. This implies it can “soak up” more than 12 times as much heat for the same temperature rise. This becomes a real benefit in systems with lots of small zones.
About now some of you are saying, “But the low-mass boiler can modulate its firing rate down to 20 percent of full capacity, so its thermal mass doesn’t matter.” This is true to a point - the point where the minimum modulation rate is well above the rate of heat delivery to a highly zoned distribution system.
Consider a highly zoned distribution system where the load is only 5,000 Btu/hr. Perhaps a single panel radiator is operating to warm a bathroom on a fall morning. The boiler supplying the system is rated at 70,000 Btu/hr. to cover the full design load of the building.
Assume this scenario is handled by the on/off cast-iron boiler described previously. Also assume the differential on the temperature controller operating the burner is 25 degrees F. The theoretical burner on cycle would be:
The low-mass modulating boiler, at its minimum firing rate (15,000 Btu/hr.), outputs 10,000 Btu/hr. more heat than is currently needed by the 5,000 Btu/hr. load. Assuming the same 25-degree operating differential, the burner on cycle would be:
Don’t get me wrong; I’m not trying to knock modulating boilers. Instead, I’m trying to demonstrate that “microload” zoning can create loads that are far smaller than what current-generation modulating burners can provide without cycling. Without adding thermal mass in the form of a buffer tank, relatively short burner operating cycles will occur under low load conditions.
Iron+Mod+ConFor decades, cast-iron boilers were operated as on/off devices. Anything that created a circuit between the T T terminals on the limit controller fired the burner at full capacity. Break this circuit and the burner was off. The boiler’s thermal mass “smoothed” the pulses of heat production so significant fluctuations in supply water temperature did not occur. Adding outdoor reset control further smoothed heat delivery and boosted efficiency during partial load conditions. When a cast-iron boiler supplied a low-temperature load such as slab-type radiant floor heating, designers included a mixing device between the distribution system and boiler to protect the latter from sustained flue gas condensation.
Because this scheme worked well, and has been used on millions of installations, one might assume it’s the only way to apply cast-iron boilers in future systems. At least that’s what I thought until a few months ago when I was shown what was billed as a condensing cast-iron boiler. Upon hearing this concept, my gut reaction was: That’s a big mistake - everyone knows that flue gas condensation will corrode cast iron. But the rest of the story, along with some follow-up research, has convinced me this concept is viable and offers some unique benefits.
Here’s a quick summary of what’s going on: Cast iron does corrode when wetted with flue gas condensation. The chemistry between acid and iron is still the same. However, the rate of corrosion can be reduced through design features such as a down-flow heat exchanger with post-purge air flow to drive condensate off the sections at the end of each firing cycle. Independent laboratory testing has confirmed that metal loss rates are such that the cast-iron sections stay within ASME specifications for a projected life in excess of 25 years, even under sustained condensing mode operation.
These findings change the context of the situation. Instead of simply dismissing cast iron as a material that corrodes under condensing mode operation, I feel the relevant questions become:
- 1. Are other
mod/con boilers with stainless-steel or aluminum heat exchangers going to
provide equivalent or longer lives?
2. Will any mod/con boiler installed today be serviceable 25 years from now?
3. Will we have the same fuels available to operate these boilers 25 years from now?
With these thoughts in mind, I think cast-iron mod/cons, with “managed metal loss” design details, are as viable as any other currently available boiler option.
Putting It All TogetherIf you like the concept of a mod/con boiler with plenty of thermal mass and low head loss, you’ll also like the simplicity of system design that such boilers allow. Figure 2 shows one possibility.
The low-head-loss boiler allows for “flow-through” piping design. There’s no need for a separate boiler circulator. The only circulator in this system is an ECM-based, pressure-regulated unit that automatically adjusts speed to maintain constant differential pressure on the distribution system. Think of it as providing cruise control for the hydraulic side of the system. For a typical residential system, this circulator will have a peak power consumption in the range of 35 watts. Under partial load conditions, wattage could drop into single digits.
Each panel radiator has a thermostatic valve allowing for room-by-room comfort control. The boiler has a built-in outdoor reset control that provides cruise control for water temperature. It ensures the boiler operates at the minimum possible temperature to maximize efficiency.
This approach is also “minimalist” in terms of parts count. It’s reminiscent of a time when most residential and light commercial systems used a single boiler and single circulator. But hidden beneath that simplicity are state-of-the-art subsystems for combustion, control and distribution. Together, they provide stable operation, excellent comfort and peak energy efficiency on both the thermal and hydraulic sides of the system. This is about as “green” as a hydronic system gets, at least for the present.
Finally, cast iron is a highly recyclable material. About 90 percent of the cast iron used in new boilers is recycled from old radiators, DWV pipe, fire hydrants and other sources. Part of that new cast-iron mod/con might have even come from the wheel of a railroad car that carried cast iron radiators to a warehouse 50 years ago.
I hope you’re convinced not to write off cast iron as a viable material for future boilers. It may not be in the form of a dry-base, atmospheric design, but when this unique material is combined with the latest combustion, control and distribution system technology, it can still carry the flag for generations of future hydronic systems.
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