vertical runs. The vent configuration in Figure 3 was shown to us by a A Failing-Furnace Mystery by J.D. Ned Nisson A very high-efficiency condensing furnace was installed in the basement of a single-story house. The two-inch PVC flue pipe was run up into the attic, horizontally about 15 feet, then up and out through the roof. When the furnace was fired up, it ran for a while but then shut down. After a period of time it could be fired up again, but then would shut down again. The manufacturer, seeking to avoid a bad reputation for the appliance, replaced the furnace with a new unit but the problem persisted. What was wrong? The new, high-efficiency, directvent and induced-draft