Copyright Goodheart-Willcox Co., Inc. 52 Hydronic Heating: Systems and Applications action of a modulating boiler is similar to that of a gas pedal on an automobile. As the outdoor temperature drops, the heat load on a building increases. The modulating boiler responds to this change by increasing its heat output through increasing the fuel and air input used for combustion. Conversely, as the outdoor air temperature increases, the modulating boiler responds by decreasing its heating output, thus saving money on the building owner’s heating bill. Modulat- ing boiler controls are typically only found on condensing boilers, Figure 3-21. This is due to the fact that as the boiler’s output decreases, the hot water return temperature decreases, therefore increasing the chance of flue gases reaching their dew point. These combination modulating and condensing boilers are known in the industry as mod/con boilers. The rate at which a modulating boiler can throttle its output is known as the turndown ratio. To understand this concept, think of the minimum and maxi- mum firing rate, or Btu output, of a modulating boiler to correspond to 0–100%. Because today’s combustion technology only allows for a minimum firing rate of 20%, the best turndown ratio in the industry is currently 100:20 or 5:1. A mod/con boiler that has the capability of modulating between 100% and 30% would have a turndown ratio of 3:1, and so on. Some boilers operate using a two-stage temperature control, also known as high and low fire control. This control is a fixed Btu burner output, which usually equates to 100% of the maximum Btu output capacity at high stage and A B Goodheart-Willcox Publisher Figure 3-20. A—A floor-mounted boiler. B—A wall-hung modulating boiler.