Copyright Goodheart-Willcox Co., Inc. Chapter 3 Boilers 51 mentioned earlier, this situation is not good for lower- and medium-efficiency boilers using conventional metal flue vents. Condensing boilers extract additional heat by adding a second heat exchanger to the system, Figure 3-18. The flue gases pass through the secondary heat exchanger after they leave the primary heat exchanger, extracting more heat from the burned fuel and increasing the efficiency of the boiler above 90%. When this additional heat is removed by the secondary heat exchanger, part of the combustion gases condenses into liquid, releasing latent heat into the boiler’s water. (Recall from Chapter 1, Human Comfort and Heat Transfer that latent heat is the amount of heat required to change the state of matter without a change in temperature.) This secondary heat exchanger is made of noncorrosive stainless steel, which resists the effects of flue gas condensate. Condensing boilers do not use metal for their flue vents rather they use plastic piping such as PVC (polyvi- nyl chloride) or PP (polypropylene), which resist corrosion caused by acidic condensate from the products of combustion, Figure 3-19. Because of this, con- densing boilers may be vented through the wall, roof, or existing chimney. Drain connections also attach to their heat exchangers for condensate removal. 3.5 Modulating Boilers In order to match the heat output of a boiler to the corresponding load on a building, the industry has developed what is known as modulating boilers. This type of boiler is capable of automatically changing its heat output by varying or “modulating” the input of fuel and air used for combustion, Figure 3-20. The CODE NOTE PP versus PVC Pipe When installing a condensing boiler, note that more states are requiring the use of polypropylene (PP) instead of polyvinyl chloride (PVC) as vent piping material. PVC will not withstand higher flue gas temperatures compared to polypropylene. PP piping may be more expensive than PVC, but it does have more flexibility when it comes to flue pipe installation, as it does not need to be glued. Stainless steel plates Piping connections Threaded studs for mounting Xylem Inc. Figure 3-18. A cutaway view of a secondary heat exchanger used in a condensing boiler. Goodheart-Willcox Publisher Figure 3-19. Because condensing boilers have acidic flue gases, vents must be polyvinyl chloride (PVC) or polypropylene (PP), which is shown here.