In the winter of 2014, homeowners in North Carolina reported an upsetting problem: their furnaces were shutting off in the middle of the night on the coldest night of the year. The reason? Condensate drains on the high-efficiency condensing units were freezing where the drain pipes terminated outside the house. With the pipes blocked, the condensate backed up into the furnaces, where safety switches shut the units down.

The issue came as a surprise in North Carolina, where winters are relatively mild, hard freezes are rare, and high-efficiency furnaces are a relatively new factor in the market. On a national scale, however, condensate freeze-ups aren't a new thing. They've been reported by homeowners and contractors from Atlanta, Georgia, to upstate New York — see, for example, this 2010 conversation at Green Building Advisor ("Frozen condensate overflow line"), this thread at the pro heating contractor forum HVAC-Talk ("Condensation line keeps freezing"), and this thread from the DIY forum ("How can I prevent my HVAC condensate drain line from freezing?").

There are many approaches to solving the problem. Some people advise cutting the condensate line off flush to the outside wall and pitching the line steeply down toward the outside, to keep it free and clear so it won't freeze. But that's risky: what if condensate drooling down the side of the house damages the siding and finishes? Other people recommend equipping the outdoor line with a heating element to keep it above the freezing point — which has the obvious down side that it expends at least some of the energy that was recovered by condensing the furnace exhaust in the first place (and introduces another potentially failure-prone gizmo into the system).

Then there's the obvious answer: drain the furnace condensate into the sewer line instead of pumping it outside the house. In North Carolina, code officials responded to last winter's troubles by amending the building code to allow just that: furnace condensate lines in the state can now be routed into the home's sanitary drain. But that solution is not without its own hazards — which has prompted some municipal wastewater managers to complain about the new rule, as station WRAL has reported (see: "'Green' furnace law still needs tweaking, some say," by Monica LaLiberte).

What's the problem with draining condensate into the town sewers? Simple: because of the chemistry of gas combustion, the fluid is corrosive. The hot fire in a gas furnace or boiler is a reaction between natural gas (or sometimes propane) and oxygen — the units burn methane (CH4) or propane (C3H8) and oxygen (O2) to produce H20 (water) and CO2 (carbon dioxide). But the hot flame also sucks in atmospheric nitrogen, which makes up 70% of the atmosphere, and the fire is hot enough to force the nitrogen to react with oxygen (even though that is an "endothermic" uphill reaction that absorbs heat rather than releasing it). The resulting NO2 (nitrogen dioxide) and NO (nitrous oxide) dissolve into the water vapor in the exhaust stream to form nitric acid.

In old-fashioned low-efficiency gas burners, all that gas — the CO2, the water vapor, and the nitrogen products (known collectively as "NOX"), went up the hot chimney into the atmosphere. They polluted the air, but they didn't damage the house. But that was a waste of energy: gas burners that didn't condense the water out of the exhaust stream can't rate much higher than 80% efficiency at best, and about 65% was more typical. To squeeze 90% or more of the heat energy out of a gas flame, you have to condense the water vapor out of the exhaust into liquid form and recover the heat of evaporation from it.

It turns out, however, that when you condense out the water vapor, the nitrogen products — the NOX — come along with it. What you end up with isn't clean water — it's a dilute solution of nitric acid.

Acidic concentrate directed into a basement floor drain has eaten away the drain's brass grille. Under the floor, the condensate could be attacking cast-iron drain pipes or reinforced concrete.
Mike Bernasconi Acidic concentrate directed into a basement floor drain has eaten away the drain's brass grille. Under the floor, the condensate could be attacking cast-iron drain pipes or reinforced concrete.
That acid can do damage, explains Mike Bernasconi, a veteran plumbing and heating contractor who markets a condensate neutralizer called "Neutra-Safe," made in New Hampshire and Massachusetts. "

That acidic condensate that comes out of these condensing appliances will damage pretty much any type of metal that it comes in contact with," Bernasconi told JLC. "It will damage some plastic too, although plastic is pretty impervious to acidic solutions. If you have a septic system, it's possible to kill all the bacteria in the septic system. If you dump it outside, it will kill plants, grass — anything that it comes in contact with."

The 14-point pH scale of acidity runs from 0 at the far acidic end to 14 at the far alkaline, or basic, end, with pH 7 denoting a neutral solution. Furnace condensate typically ranges between 3.0 and 5.0, but it can get stronger: in a YouTube interview with hvac trainer Bruce Marshall of Emerson Swan, Mike Bernasconi says he has measured condensate as low as 1.8 or 2.0 — in part because of the effect of chemicals added to the fuel gas by utilities (see: "Neutra-Safe Condensate Neutralizers | Emerson Swan"). (The pH scale, by the way, is a logarithmic scale, so a fluid with a pH of 2 has ten times the acidity of a solution with a pH of 3.) 

Draining the condensate into house drains can damage the plumbing in older homes, explains home inspector Richard Aiello of I-Spy Home Inspections in Winthrop, Massachusetts. Even if cast-iron drains inside the house have been replaced with PVC plastic that the acid can't harm, he points out, "often there are there still old cast-iron drain lines in the basement, and the drain line that runs from the from the trap or the clean-out to the street is cast-iron, and all the pipes that are in the ground outside under the street are old cast-iron pipes." Furnace and boiler condensate can corrode those pipes. And if the fluid sits in a pipe or a trap and evaporates, it becomes even more concentrated — and more corrosive.

Plumbing codes everywhere address that issue. Before you can drain any corrosive liquid into the house drains, you have to neutralize it. The market has responded by creating an assortment of "condensate neutralizers" like the units Neutra-Safe sells — basically, acid-resistant plastic tubes filled with a mineral, typically limestone (calcium carbonate), that reacts with the nitrates in the effluent flowing through the tube and reduces the fluid's acidity to milder levels. Below, Steamboat Springs, Colorado, plumbing and heating contractor Nathan Marshall demonstrates the simple process of attaching a neutralizer to the mechanical room wall and coupling it to the drain line (YouTube link: "Condensate Neutralizer Installation").