1. Hydronics WORKSHOP
John Siegenthaler, P.E.
Don’t Write Them Off
A
s boilers with compact stainless- to pipe boilers and control cast-iron
steel and aluminum heat exchang- boilers so that sustained flue gas con-
Cast-iron boilers ers have entered the North American densation is avoided.
market over the last 15 years, some Same Old, Same Old: For decades,
still have a future. heating pros may be thinking that
the age of cast-iron boilers is all but
cast-iron boilers have operated as on/
off devices. Anything that created a
over. After all, everyone knows that circuit between the T T terminals on
cast iron can’t be used for condensing the limit controller fired the burner at
boilers, right? full capacity. Break this circuit and the
I used to think that way. My rea- burner was off. The boiler’s thermal
soning went like this: The acidic mass smoothed the pulses of heat pro-
nature of condensed flue gases cor- duction into a reasonably stable range
rodes cast iron. Eventually, the scale of output temperatures. When a cast-
created by this corrosion significantly iron boiler supplied a low temperature
reduces heat transfer within the boiler. load such as slab-type floor heating,
It can also eat through galvanized vent designers included a mixing device
piping in a matter of months. We’ve between the distribution system and
discussed this many times in past col- boiler to protect the latter from sus-
umns, and looked at details on how tained flue gas condensation.
Because this scheme worked well
Figure 1 and has been used on millions of
installations, it’s easy to assume that
it’s the only way cast-iron boilers
can be applied. At least that’s what I
thought until I was shown what was
purported to be a condensing cast-
iron boiler.
When I heard the words cast-iron
and condensing in the same sentence,
my gut reaction was BIG MISTAKE.
However, some follow-up research
convinced me that with the right
approach, it is possible to build a
mod/con boiler with a cast-iron heat
exchanger that offers some unique
benefits.
Here’s a summary of the concept.
Cast iron still corrodes when wet-
ted with condensate. The chemistry
between acid and iron hasn’t changed.
However, the rate of corrosion can be
reduced through features such as a
Photo courtesy of Mestek.
down-flow heat exchanger (see Figure
1), spreading the condensation over
a greater surface area, thicker section
walls and post-purge air flow that
drives condensate off the sections at
the end of each firing cycle.

2. Hydronics WORKSHOP
Independent testing has shown loss” design details, are as viable as any 500-pound boiler is not something
metal loss rates are such that the cast- other currently available boiler option. you just pick up and carry into a
iron sections remain in conformance Restoring Old Charms: Besides building. It takes a bit more logistics
with ASME safety factors for more the high thermal efficiency associated and sweat to place one of these heavy-
than 20 years, even under sustained with condensing-mode operation, weights relative to boilers that are
condensing-mode operation. Under cast-iron mod/cons bring two other attached to a wall.
partial condensing-mode operation desirable characteristics to a system: Based on my calculations, a typi-
— more typical of real installations low head loss and high thermal mass. cal cast-iron boiler rated at 75,000
— measured metal loss rates sug- Low head loss is a trait the North Btu/hr.-output has about 12.5 times
gest that the cast-iron heat exchanger American hydronics industry enjoyed, more thermal mass due to its metal
could remain within the ASME spec without much appreciation, until the and water content, compared to a
for upwards of 100 years! appearance of residential-size con- compact mod/con boiler of the same
A Different Narrative: These find- densing boilers. Cast-iron boilers cre- peak capacity. All that metal and water
ings change the context of the situa- ate extremely little head loss, even at content make cast-iron boilers “self-
tion. Instead of simply dismissing cast substantial flow rates. This is a direct buffering” in most installations. Buffer
iron as a material that corrodes when result of their sectional design with tanks are simply not required.
exposed to flue gas condensation, the large internal water passageways. As a Our industry has found work-
relevant questions become: result, we routinely piped them such arounds for the higher head loss and
1. Are other mod/con boilers with that the entire system flow passed low thermal mass of compact boil-
stainless-steel or aluminum heat through the boiler. ers. In most cases we add a dedicated
exchangers going to provide service The appearance of wall-hung boil- boiler circulator and install a hydraulic
lives equal to or beyond those provid- ers with very compact heat exchangers separator or buffer tank. This works,
ed by a condensing cast-iron boiler? ushered in an era in which a dedi- but usually not without added floor
2. Will any mod/con boiler installed cated boiler circulator was typically space and never without added cost.
today be serviceable 20 years from now? required. Although the installed cost of Back To The Future: Consider the
3. Will we have the same fuels avail- this dedicated circulator may be under simplicity of system design shown in
able to operate these boilers 20 years $300, its projected operating cost over Figure 2 on page 28.
from now? a nominal 20-year life is often more The low head loss, cast-iron mod/
Although the final point might than $1,000. That extra cost certainly con boiler allows full system flow to
seem extreme, I think it’s relevant. nullifies part of the fuel savings. For pass directly through the boiler. A sin-
Given the current changes in global better or for worse, designers of many gle variable-speed, pressure-regulated
fuel supplies and environmental legis- compact mod/con boilers conceded circulator provides flow through the
lation, the heating business, as we now low head loss as a necessary sacrifice boiler and the entire space-heating
know it, could look very different two for a compact and light design. distribution system. It automatically
decades from now. Thermal mass was another char- adjusts speed to maintain constant
With these thoughts in mind, it’s rea- acteristic of cast-iron boilers that was differential pressure on the distribu-
sonable to conclude that cast-iron con- often cursed, and yet provided an tion system as the thermostatic radia-
densing boilers, with “managed metal often unrecognized benefit. Sure, a tor valves on the panel radiators open
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3. Hydronics WORKSHOP
and close. For a typical residential point, the boiler returns to outdoor the form of a dry-base, atmospheric
system, this circulator would have a reset control for space heating. design, but when cast iron is combined
peak power consumption in the range This approach is reminiscent of a with the latest combustion, control
of 40 to 60 watts. Under partial load time when most residential and light and distribution system technology, it
conditions, wattage could drop into commercial systems used a single can still carry the flag for generations
single digits. boiler and single circulator for space of future hydronic systems.
The boiler is equipped with an out- heating. However, hidden beneath that
door reset control that ensures it oper- simplicity are state-of-the-art subsys- John Siegenthaler, P.E., is principal of
ates at the minimum possible tempera- tems for combustion, control and dis- Appropriate Designs, a consulting engi-
ture and maximum possible efficiency tribution. Together, they provide stable neering firm in Holland Patent, N.Y.,
for the prevailing load conditions. operation and peak energy efficiency and the hydronics editor for Plumbing
Domestic water heating is treated as on both the thermal and hydraulic & Mechanical. E-mail him at john@
a priority load, the same as with other sides of the system. hydronicpros.com. The third edition of
mod/con boilers. The boiler operates I urge you to not dismiss cast iron his text, “Modern Hydronic Heating,”
at an elevated temperature during this as a viable boiler material for the will be released Jan. 14, 2011. For details,
mode. When the tank reaches its set- present and future. It may not take visit www.hydronicpros.com. PM
Figure 2

4. with
Reprinted with permission from Plumbing & Mechanical, December 2010 © BNP Media