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.

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.