Natural gas flue gas starts to condense at roughly 130 degrees F, so a condensing boiler needs low return water temperatures to work in its high-efficiency range, above 90 percent. Stainless steel or aluminum heat exchangers resist the acidic condensate, which is drained through a neutralizer, and the cool exhaust is vented in plastic pipe. Most are modulating, which is why techs call them mod-cons.
Condensing boiler's only hitting 86 percent because the return is coming back at 150, the reset curve's set way too hot.
A condensing boiler on a system designed for 180 degree water rarely condenses; the payoff comes from low-temperature emitters and outdoor reset.
Also heard as
- mod-con
- condensing
Related terms
Outdoor reset
A control strategy that lowers hot water supply temperature as outdoor temperature rises, following a set curve.
Modulation
Varying a burner's firing rate continuously between low fire and high fire to match the load.
Flue gas
The hot exhaust that leaves the boiler after combustion, made up mostly of nitrogen, carbon dioxide, water vapor and leftover oxygen.
Delta T
The temperature difference between supply and return water in a hydronic system.