Furnace Drains

One important aspect of condensing furnace installation is sometimes overlooked – the furnace drain. A 90% condensing furnace makes a lot of water – comparable to an air conditioner evaporator. All that water needs somewhere to go. In most installations, there will also be an air conditioning evaporator and a drain for it. It is natural to just run the furnace drain into the air conditioning drain, but that can lead to problems. The positive air pressure inside the evaporator cabinet can create a slightly positive pressure on the evaporator drain. If the furnace drain is connected directly to it, this positive pressure can push back on the furnace drain. On some furnaces, this can create a positive pressure at the vent draft switch, causing it to open and shut off the furnace. One solution is to run completely separate drains for the furnace and air conditioner. If the air conditioner and furnace are connected to a common drain, you need to leave an air gap between the furnace drain and the common drain so that pressure cannot back up into the furnace drain and vent. If you are using a condensate pump, make sure it is rated to handle furnace condensate. Because furnace condensate is slightly acidic, some condensate pumps are specifically NOT approved for use with furnace condensate. In case you are concerned about the acidity, you can breathe easy. It is about the same as a carbonated beverage. However, some codes early on required that the condensate from condensing furnaces be neutralized before going into a sewer system. So manufacturers produced acid neutralizing filters which were basically large PVC containers filled with rocks (usually limestone or marble). The acid reacts with the rocks, which have a basic ph that counters the acid. I don’t believe most places require this any longer, but I am not certain. Another issue to watch is the drain outlet. If a gravity drain ices up or is blocked with snow, the draft switch can trip and shut off the furnace. Also, remember that the furnace really does make a lot of water. Try not to dump this water in a place that will cause problems – like right over a sidewalk.