This article was written by HVAC/furnace technician Benoît Mongeau. Thank you, Ben.

High efficiency (or 90%, or condensing) furnaces use a set of two heat exchangers to retrieve more heat from the combustion products than their mid-efficiency counterparts. Because of this, they generate flue gases much colder than those of a mid-efficiency or natural draft unit. That not only completely changes the way the furnace has to be vented (I will talk about venting specifically in a later tip), but a lot of condensate is also generated, which we'll focus on. This water comes from two sources: moisture, which was already present in the combustion air, and the combustion process itself, as the hydrogen atoms from the natural gas molecules (methane, CH₄) combine with oxygen to form water. Now, as technicians, you don’t need to know this part, but if you’re a bit into chemistry, here’s the basic chemical equation:

CH₄ + 2 O₂ + heat = CO₂ + 2 H₂O

That equation means that in perfect combustion, for every molecule of CO₂ you produce, two water molecules are also produced. That adds up to a lot of water vapor.

For the furnace to work properly, that condensation needs to be drained out. Otherwise, it would accumulate inside the heat exchanger, inducer, and venting, impeding proper gas/combustion product flow. Most furnaces will have at least two internal drains, typically one for the heat exchanger and one for the vent, usually at the inducer outlet or on the inducer housing.

The secondary heat exchanger outlet is sealed inside a plastic part called the collector box, designed to collect the condensate and drain it.

All condensate drains go into a trap.  The condensate trap is absolutely mandatory for a high-efficiency gas furnace. Since the drain taps into the exhaust system, leaving it open to the air would allow for a potential exhaust/flue gas leak in the living space, which is a big no-no. Additionally, the inducer motor would suck air through the drain if it weren’t trapped, which could affect combustion and would prevent proper drainage. Keep that in mind; if you ever add an extra drain (off a tee on the venting, for example), you will need to TRAP it, always.

The only downside to the trap is the potential for blockage. The trap needs to be cleaned out regularly, and that should be done during every maintenance. Rinse it out, and make sure water flows through the trap properly from all its ports. If there’s any poor flow, fill it up and blow through it a few times to get the dirt out. Hotter water helps with stubborn blockages. The need for regular cleaning also means that drains should be installed as much as possible in a way that allows for the trap to be easily removed. I highly recommend using clamped flexible hoses for the drain, used as close to the trap as possible. Avoid hard-piping the whole drain, as it will be impossible to remove and clean out the trap.

To ensure proper drainage, here are the proper practices:

• Make sure every component that produces condensate is sloped towards the drain. That means slope the venting down towards the furnace (typically a ¼’’ slope per foot of length, minimum). Also, slope the furnace itself! Look in your installation manual. Most manufacturers will call for the furnace to be installed with a slight forward pitch to allow condensate to drain from the heat exchanger.
• Slope the drain line itself, obviously. Avoid double trapping and vent the drain after the trap to prevent airlocks (you can learn more about the disadvantages of double traps by reading this tech tip).
• Avoid running the drain in an area where it could freeze. That includes running it under the natural fresh air inlet if there is one.  

Finally, note that furnace condensate is acidic, and some states/provinces/countries may require the condensate to be neutralized prior to draining.

—Ben