Two types of boilers can be better than one — especially when trying to minimize operating costs over the course of a heating season. By combining a large mass, noncondensing boiler with a high-efficiency condensing boiler — the essence of a hybrid system — contractors can ensure that the heating system is running at optimum efficiency.



To understand the impact of a hybrid system, it is essential to remember the primary functionality of noncondensing and condensing boilers. Noncondensing boilers are designed to run at higher return water temperatures — above 130 degrees — to help maximize longevity. The average efficiencies of noncondensing boilers range from 80 to 85 percent thermal efficiency.

Condensing boilers reach their maximum efficiency — upwards of 95 percent — when the return water temperature is below dew point.

Looking at the peaks and valleys of the heating season, noncondensing boiler peak efficiency aligns with high-temperature applications that typically occur over the core heating season months of December through February. Condensing boilers are more aligned with the milder heating season shoulder months of October, November, March, and April.

Running condensing boilers in high return temperature applications reduces their operating efficiency to marginally higher than noncondensing boiler designs.

By combining noncondensing and condensing boilers in a hybrid system boiler plant, users can reset building set points, reduce fuel consumption to lower operating costs, and create comfortable, consistent building temperatures.



With an understanding of the hybrid system concept, contractors may be curious as to why they can’t achieve the same results by using an all high-efficiency boiler plant. To answer this, it is important to consider the benefits and not just the functions of a hybrid boiler plant.

When looking at boiler replacement applications, installing a hybrid system makes sense in existing, noncondensing replacement boiler applications. Most often, there are at least two high-mass, noncondensing boilers in older boiler plants. Installing a high-efficiency condensing boiler in place of one of the failing high-mass boilers now enables the efficiency benefits of the hybrid system. Further, this approach enables an incremental step in financing the overall boiler replacement capital outlay over time.

New build applications can be costly endeavors. In short, condensing boilers usually cost more — upwards of 30 to 40 percent more — than noncondensing boiler systems because of the non-corrosive heat exchanger materials required. A hybrid system is more cost effective to purchase upfront and often achieves the greater portion of the energy savings of replacing an entire boiler plant with high-efficiency condensing boilers.



Inherent to their design, noncondensing boilers have a long product life because of their thicker cast iron design and wider water pathways. Installing a hybrid system ensures a longer life of the boiler plant dedicated to the coldest heating season months.

Consider this example. A hybrid system could be utilized in an elementary school with two large cast iron boilers, one of which needs to be replaced after many years of dependable operation. The cast iron boiler is replaced with a Weil-McLain Evergreen™, SlimFit™, or new SVF™ high-efficiency condensing boiler to match the heating requirement. The advanced Weil-McLain Unity™ control allows the assignment of an alternating lead/lag boiler plant, utilizing the new condensing boiler as the primary boiler over the heating season shoulder months and the existing noncondensing boiler for colder days.

Combining a cast iron boiler with a high-efficiency condensing boiler can reduce wasted energy, lower utility costs, and protect the environment with a smaller carbon footprint — all while extending the life of the existing heating system at a lower acquisition cost.

The use of hybrid systems is becoming more and more common, with engineers and building owners evaluating the heating efficiencies and other benefits of going hybrid in their current and new build heating systems. Commercial buildings such as medical facilities, apartment complexes, churches, schools, colleges, nursing homes, and corporate campuses are using hybrid system boiler designs more frequently.

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