This article was originally published in the May/June 1998 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.


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Home Energy Magazine Online May/June 1998


The Uncurable Soot Mystery

by Zolton Cohen

Home inspector Zolton Cohen uses a carbon monoxide (CO) detector to check out a residential gas furnace. Low CO readings and an absence of soot made him look elsewhere for the source of the mysterious soot.
Cohen checks a water heater. In Donna and Jim's home, the water heater was producing CO and had rusty flakes piled up around the vent. Unfortunately, curing the water heater did not eliminate the soot.
Cohen checking garage door jambs. In Donna and Jim's home, the garage door showed soot passing in from the outdoors. But the malfunctioning minivan wasn't the ultimate culprit, either.
Falling in love can be a wonderful thing. It's what produces marriages, kids, all the good things in life. But if we start falling in love with our own theories, we can get into trouble.

On a recent investigation, I discovered two causes of carbon monoxide (CO) and soot stains in a home. Once I found those first two problems, along with reasons to rule out the other likely sources of pollution, I was blind to the actual culprit. I fell in love with my first two discoveries, and that led me down the primrose path.

As a home inspector with a syndicated home maintenance column and an on-line interactive homeowners' forum, I hear quite a bit from homeowners with problems. But in December, 1996, a young couple came up with an that inquiry I had to investigate in person. Donna and Jim had lived in their eight-year-old home for about five years. In the six months before they called me, they had started noticing black stains on the walls, ceilings, and carpets. The stains were even getting onto the clothes in the closets. They also had elevated levels of carbon monoxide in the house that kept setting off their CO alarm.

My initial guess was that the problem probably had to do with a hole or crack in the heat exchanger of their eight-year-old gas forced-air furnace. A year later, I found out I was right. But over that year, other major problems in the house threw me, and several others working on the case, off the track.

The First Search When I got to the house, Donna showed me the awful-looking stains on her carpets, ceilings, and walls. The patterns of dirt followed the framing buried beneath the drywall, and showed up as soot lines underneath the doors and along the hallway walls in the second story.

The hallway dirt marks might have been due to the common practice in this area of panning the second-story floor joists to create a (leaky) cold-air return system for the upstairs. The carpet in this area was filtering the dirt from the air as it got sucked into the return inside the floor. But this didn't explain where the soot--and the CO--was coming from.

Donna showed me her previous two furnace filters. Black soot had entirely covered the white material. She said they had each been in service only two weeks.

On my CO meter, general house air registered 2­4 parts per million (ppm) upstairs and down. Looking in the downstairs utility closet, I found that the power-vented, direct-vent furnace had a small leak by the vent fan housing that registered 178 ppm on start-up. But there wasn't a whit of soot around this leak, nor was there any on the air conditioner coil in the plenum above the furnace. The flue had no blockages, and everything was venting normally.

The water heater, though, was a big polluter. The burner on the 50-gallon power-vented unit was piled high with rust and debris, and its vent fan came on seven or eight seconds after the burner fired. Soot and rust flakes covered the top of the heater inside the metal cage, and there were black soot particles on top of a nearby plastic water softener tank. The water heater spit out 162 ppm of CO on start-up before the fan came on.

To rule out other pollutant sources, I asked about lifestyle habits. Donna said she never burned candles or oil lamps, rarely cooked on her gas range, and never used the gas fireplace. The gas clothes dryer was venting normally. I checked the pilots on the dryer and range; both were burning normally and showed no signs of soot.

I was pretty sure I'd pinned the problem down. I was in love with my discovery. I told Donna to have the water heater and furnace serviced and to give me a call if she had any more problems.

Donna called back a month later. She'd had the rust vacuumed off the water heater burner and had had the leak in the furnace fixed. She had called the water heater manufacturer, who had told her the fan delay on the vent fan of the water heater was normal, and that the water heater couldn't be contributing to the problem if it was burning cleanly. Still, her furnace filters were turning the color of coal. I went back to look around.

Returning to the Scene Not much had changed since my first visit. Same CO levels, same black marks. This time, I focused on moisture, thinking the black marks could be mold or mildew. But the 40% relative humidity in the house was not enough to create a living mold environment, and there was no visible sign of mold in the attic.

Although the water heater was still contributing CO to the house air, it was much reduced, occurred only on start-up, and lasted for only a few seconds. Not enough of a source, I felt, to produce that level of buildup throughout the house. I left, frustrated, but not as frustrated as Donna and Jim, both of whom were near tears.

This January, about a year after Donna had first called, she called again to invite me to a meeting at her home. After losing patience with the company that had installed the furnace--when their half-dozen trips to her house had produced no results--she had gone straight to the top. She managed to convince the furnace manufacturer to send an investigator from Texas to check out the house and furnace, and to address her concerns. He met us at the house, along with the furnace installer and the local furnace representative.

This gentleman looked like he knew his stuff. For four hours, he paced back and forth, took samples of the soot on Donna's walls, cut out pieces of the furnace filters, and collected some of the furnace condensation in a glass jar. He used a digital manometer to measure the pressure differences between the house and the outside. He walked outside the house, sampled the rocks by the foundation, looked up into the trees, wiped tissues along the house siding. It was a thorough investigation that turned up absolutely nothing. And he was convinced that the furnace could not be the source of the problem.

Surprisingly, no one took the furnace apart.

The Culprit Found? The furnace installer and I turned on our carbon monoxide detectors. There were 4 or 5 ppm of CO in the house air that day. The installer wandered out into the attached two-car garage and found between 18 and 22 ppm.

I shut the door between the garage and house, and I could feel cold air flowing into the house. The furnace investigator's manometer confirmed that air from the garage was bleeding into the house. That leak wasn't surprising: the stack effect was in full force. The attic scuttlehole cover alone was probably leaking enough air to depressurize the house. Could the garage be the source of the CO in the house? And if so, how did that relate to the sooty stains?

Looking carefully at the jamb of the garage door, I found soot stains on all three hinges. It appeared that air was carrying soot into the house. But where from?

By chance, I wandered around behind two vehicles parked in the garage. Glancing down, I noticed that Donna's 1997 sport utility vehicle had a clean tailpipe, while the tailpipe of Jim's van was caked with black soot. I stuck my finger in there and came out with a black, powdery mess. The furnace rep wiped a tissue inside the pipe for later lab analysis. Jim later told us that the van had a spark plug that kept fouling out, and that he usually backed the van into the garage when he came home from work.

At last, we had the three conditions for indoor air pollution: a pollution source, a route into the living space, and a driving force to carry the pollution. Apparently, the van's exhaust loaded the garage air with soot and CO each morning. Over the course of the day and night, this air was entering the house through the faulty door seal, thanks to the pressure difference between house and garage.

As I've since learned, automobile engines are perhaps the most potent source of CO in or around a house. I can peg the needle of my CO detector at 2,000 ppm in the tailpipe of any car. In comparison, the furnace exhaust on Donna's house registered 20 ppm.

When homes leak, they often draw in air from the garage. Steve Klossner, of Lakeland, Minnesota, studied unexplained CO alarm soundings. On average, homes in his study drew 25% of their make-up air from the garage, and some drew as much as 85% from this source. In the case of Donna and Jim's home, the air was carrying not just CO, but soot as well.

I suggested that Donna and Jim park their cars outside for a while to see if that cured the house. I was in love again, this time with a novel explanation for soot in a home.

Case Complete? Did my beloved theories bear out? Not exactly. Donna didn't contact me for over a month. When I called her for an update, she dropped a bombshell.

She hadn't been satisfied with my findings, or those of the furnace investigator. And she was still getting some soot buildup on her furnace filters. So she hired an independent furnace installer to completely dismantle the furnace. In the heat exchanger, he discovered a soot-caked hole the size of Donna's little finger. She later told me this was the second heat exchanger that had failed in this furnace.

This case will probably never be completely solved. Today, Donna has a new furnace (courtesy of the manufacturer), clean filters, and no CO in the house air. The hole in the heat exchanger was clearly a problem, as it may have disrupted the combustion enough to cause CO and soot production. None of the other dozen or so cracked heat exchangers I've found (out of the probably 400 I've inspected so far on home inspections) contributed even one ppm of CO to the house air.

All the same, I have learned that falling in love can be a good thing, in the right time and place. But not when conducting home performance investigations.

Zolton Cohen is a building inspector and journalist in Kalamazoo, Michigan. He can also hosts an on-line forum for home questions at


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