Goodbye, Old Friend: Mod-Con Boilers are Replacing Cast Iron Units

Jan. 1, 2009
I was very busy this summer. The golf clubs didn't see much action, but the sales order pad did, and the trend was quite specific. Increasingly, cast

I was very busy this summer. The golf clubs didn't see much action, but the sales order pad did, and the trend was quite specific.

Increasingly, cast iron boilers are being replaced with multiple modulating-condensing (mod-con) boilers. In the mid-50s, when many of the boilers replaced this summer were new, natural gas was going for 7 cents per therm.

Today energy prices are causing a surge in high-efficiency boiler replacements. A 40- to 50-year-old cast iron boiler is perceived as an energy waster rather than an old reliable friend.

I can't argue with the mathematics of the savings that a mod-con can provide in most heating applications. This is especially true where I live, along the “Sweet Tea Line” in Southern Ohio. Winter temperatures here remain above 32F for most of the winter.

Modulating. With most winter days above 32F, that means heat loss is at least 50% less than on the coldest days of the year. Less heat loss equals less gas input required. This is the “modulating” benefit of the mod-con boiler. The mod-con adjusts its percentage of input in relation to the heat loss.

Natural gas prices are now well over a dollar per therm instead of a dime. Besides modulation, today's boiler has to squeeze out every bit of the combusted fuel.

Condensing. When flue gases condense, or change state from a gas to a liquid, that latent heat gets absorbed by the water in the heat exchanger. Instead of flue gas heat going up the chimney, its heat is now applied to the system water.

The most important factor in determining efficiency is return water temperature. The lower it is, the greater the condensation will be, resulting in higher efficiency. Throttle the system flow rate to get water temperature drops of up to 30 degrees for decreased return water temps. Typically, flue gases condense at temperatures up to 135F. Whenever the return temps are below 135F, the efficiency will be at least 87%. Water temperature control now becomes important.

Reset control

Cast iron boilers require water temperatures of at least 140F to prevent damage from condensation. Mod-con boilers don't require any minimum water temp. When the outdoor temperature is 32F, an operating water temp of 130F or so is typical for a radiator or baseboard system. The return water temp will be around 20 degrees less, resulting in efficiencies in the mid 90s. If the majority of the heating season is 32F or above, real savings can be achieved.

Multiple mod-cons

Mod-cons operate most efficiently at their lowest fire. To take advantage of that fact, I use multiple mod-cons controlled by a staging control that brings all the boilers on low fire first. The concept is called parallel modulation.

While bringing the system water temperature up to its target temperature, the staging control modulates each boiler's fire at the same rate. Only on the very coldest day will any boiler get to 100% fire. Most of the season, they'll be at 50% fire or less.

Hybrid systems

In colder climates, or when available, a standard efficiency boiler can be integrated into the strategy. Run the mod-cons when the target water temperature results in flue gas condensation, typically around 155 to 160F. At higher target temps, bring on the cast iron.

One of the summer change outs was a hybrid. A school system wanted higher efficiency. When I visited the job, I found two cast iron boilers. One was more than 30 years old and the other about 5 years young. I proposed, “out with the old, and in with the new.”

Three mod-cons would operate as the first stage, on their own parallel modulation staging control, while the existing cast iron boiler is brought on as the second stage. The school administrator loved the idea and the contractor's quote. The system is installed and ready to go.

Use your imagination to adapt the above concepts to your replacement opportunities this heating season.

Patrick Linhardt is the hydronics heating division manager for Habegger Corp. — Carrier Division, in Cincinnati, OH. He often lends his sleuthing skills and technical expertise to local contractors in need. To order Linhardt's Field Guide to Steam Heating, visit, or call 513/703-5347.