Deciding whether to an upgrade from a noncondensing boiler to a condensing unit seems like it should be a simple math problem. Just plug in the fuel costs, the equipment costs, and the efficiency, and then figure out the payback time. Right?

In The Journal of Light Construction, engineer and building science consultant Foster Lyons explains why the decision isn’t quite that simple. While condensing boilers are 10–12 percent more efficient than noncondensing options, the added efficiency creates new considerations, he says.

“First, the controls for condensing boilers are more complex,” he writes. “Water returning to the boiler (after heating the house) is used to pull the thermal energy out of the hot gases in a heat exchanger. The temperature of that return water can’t be too high; otherwise, the exchange of heat doesn’t happen properly. Maintaining an optimum return temperature requires more controls than on a traditional boiler.”

Lyons also discusses how to manage the liquid condensate and the cooler exhaust air produced by condensing boilers. In new construction, he says, venting condensing boilers can be easier because these units can use less-expensive venting materials. In replacement scenarios, pros and their clients should weigh any additional costs of the upgrade against the long-term fuel savings.

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