Can the plastic exhaust pipes run up through the chimney, using it as a raceway?
Only if there is no water heater or other appliance still venting into the chimney. Check the manufacturer’s requirements carefully. The vent pipe must be protected from freezing when run through an unheated space such as a chimney. Figure 1. Schematic of a condensing furnace. How to Avoid Potential Problems People who change from a conventional to a condensing furnace often notice differences. The most common of these is cooler air coming from the registers. (“How can it heat my house if it blows cold air?”) One way of making a furnace more efficient is to blow more air across the heat exchanger and deliver that warm air at a lower temperature. But, even 100deg.F air feels cool if it’s blowing across your skin. Combine that higher flow of cooler air out of the registers with the extended running time for the blower (to wring more heat from the heat exchanger), and you are likely to have a recipe for a dissatisfied customer. Smaller homes with shorter duct runs are especially prone to
