Condensing Furnace Drains

With the abundance of condensing furnaces, drain problems are no longer limited to the summer months, but are now a year round concern. Although the drain on a condensing furnace is a relatively small detail in the overall scheme of things it can shut a system down if not run properly. Normally, there are two furnace drains: one for the condensing heat exchanger and one for the vent. Some furnaces combine then inside the furnace while others require the installer to take care of that. The vent should slope towards the furnace so any water condensing inside the vent runs back to the drain. This also prevents water dripping out the vent and creating an ice dam. The drains need to be trapped, but only once. After the drain leaves the trap the pipe should never rise. Sometimes sags in PVC drain lines cause unintended secondary traps. Secondary traps will keep the water from draining out, creating a mess. Many manufacturers now provide a manufactured trap. If a furnace has a built in manufactured trap you should not add another one. Multi-poise condensing furnaces pose a special problem: you have to know which end is up (literally) to know how to position the trap and drain. Often these furnaces come configured for upflow installation but must be reconfigured for downflow or horizontal installation. Make sure the drain gets moved to the right location for whatever position the furnace is installed in.  In general, you should not run the air conditioning condensate drain and the furnace condensate drain into a common line. The positive pressure from the coil can travel through the drain to the furnace drain and cause the vent safety switch to trip. It is OK to run both into the same condensate pump basin, so long as it is open to the air and not sealed tight. If you use a condensate pump, make sure that it is rated for furnace duty. The condensate from furnaces is moderately acidic and can eat up some pumps that are not designed to handle furnace condensate. If the drain will run through unconditioned space that may drop below freezing, it will need to be wrapped with a heat tape to prevent it from freezing. If the furnace is located in an area which can be damaged by water overflow, such as an attic, it will require a secondary drain pan underneath the furnace. Finally, remember water runs down hill. The drain should slope away from the furnace until its outlet.