Q: In a current remodeling project, the clients need to replace their gas boiler. Does it make sense to upgrade to a condensing boiler?
A: Foster Lyons, an engineer and building-science consultant, responds: Because condensing boilers are 10% to 12% more efficient than equivalent non-condensing models, you’d think this question would be easy to answer—just figure out fuel costs and the cost of the equipment and run the numbers, right? But it’s not quite that simple, because of the differences between the two types of boilers.
With a traditional (non-condensing) boiler, the exhaust gases are very hot, typically around 400°F. Those hot exhaust gases are immediately sent up a chimney and take a lot of thermal energy with them, which limits the energy efficiency of traditional boilers.
A condensing boiler, on the other hand, has components in the flue system that transfer some of that thermal energy from the hot exhaust to the water that is being heated. In the process of transferring that energy, the exhaust gases cool enough to cause condensation of the water vapor from those gases, hence the name “condensing boiler.”
The exhaust from a condensing boiler is typically around 100°F—much cooler than the exhaust from a traditional boiler. In short, compared with traditional boilers, condensing boilers take a greater percentage of the energy inherent in the fuel and transfer that energy to the water being used for heat.
However, this increase in appliance efficiency adds a variety of complications. First, the controls for condensing boilers are more complex. 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.
Second, the liquid condensate that is generated in the exhaust heat exchanger needs to be drained off somewhere, which usually requires a reservoir and a pump of some sort (see photo, top). Third, the condensate liquid has a pH in the range of 3 to 5 (not as acidic as lemon juice, but more acidic than milk and about the same as tomato juice). That means the exhaust heat exchanger—and anything else the condensate liquid may touch—needs to be chemically resistant to acid. Stainless steel or aluminum-silicon alloys are the materials of choice. Also, because of this high acidity, the exhaust cannot exit through the same masonry chimney that is being used for the existing boiler without an acid-resistant liner.
Fourth, because the exhaust from a condensing boiler is relatively cool, it’s not particularly buoyant. It doesn’t go up a chimney very easily, like the 400°F exhaust from a traditional boiler does. So the exhaust needs to be pushed out with an exhaust fan. These added components boost the cost of condensing boilers compared with that of non-condensing boilers with the same output.
In addition to the fuel-efficiency benefit, the exhaust flue for a condensing boiler doesn’t need to be masonry or metal. It can be made from ABS, PVC, or CPVC pipe with a high-temperature rating (see second photo). Because these less expensive materials can be used, gases from a boiler are commonly exhausted through a sidewall or rim joist rather than through a chimney, which can make a condensing boiler a good option for new construction. In your replacement scenario, the clients need to weigh the potential long-term fuel savings against the immediate added cost of upgrading to a condensing boiler.