A family friend recently told me of “issues” with their home heating system – “when it runs, the lights get halos around them from all the soot in the air.”

I couldn’t believe it when I heard this, and immediately warned him that the condition likely is making his family sick and could easily kill them. No joke.

Carbon monoxide poisoning is the “leading cause of accidental poisoning deaths in America” – it’s colorless, odorless and quickly affects your brain, knocking you out then killing you. In lower amounts, it can lead to chronic headaches, dizziness, depression, nausea and a host of other problems.

As we tighten up our homes to make them more energy efficient, we have to be particularly vigilant about keeping any combustion devices in the home tuned up and properly vented.

Let’s look at this in more detail….

Potential sources of carbon monoxide

Anything the burns fuel can generate Carbon Monoxide (CO) because CO is a byproduct of incomplete combustion. With perfect combustion, you get Carbon Dioxide, water and some by-products that vary depending upon what you’re burning. But with imperfect combustion, you get some CO. And, since almost no combustion is perfect, pretty much every combustion system generates some CO. But in general, oil systems produce more CO than gas systems because it’s much harder to keep them perfectly tuned.

Remember that ANY combustion system can produce CO. Fireplace, stove, oven, water heater, boiler and furnace all need to be properly vented. Even candles produce CO in that beautiful, yellow flame, even though it’s in very small quantities.

How a furnace breaks

A common source of household CO is the combustion furnace. When a furnace runs, the combustion process is in a sealed chamber. Air is blown past this chamber on the other side of an air-tight piece of metal, called a heat exchanger.

Normally, the air in your house is isolated from the combustion chamber, so any harmful fumes go up your chimney or out the vent flue.

Over time, the metal in the heat exchanger is stressed or deteriorates and can crack or develop holes. When this happens, you have no choice but to replace the heat exchanger or the entire furnace. To run a furnace with a broken heat exchanger is to play Russian Roulette.

Other ways a combustion system can poison you

I started this post with a photo of the flue from an oil burning boiler (boilers make hot water for radiators or water heaters). The opening is where a barometric damper used to be. Under normal situations, a chimney works by the principle of hot air rising. As the chimney lining warms up, so does the air, causing a constant flow of exhaust fumes up and out the chimney. The process starts with the hot fumes coming from the combustion chamber. Seems pretty simple, right?

Have you ever made a fire in the fireplace and had it “backdraft” – causing your living room to fill up with smoke? The same thing can happen with your heating system.

In order for the chimney to work, the air in the chimney has to be hot enough to actually rise  up and out. It also has to heat the air inside the chimney enough for the process to start. This gets harder when it’s cold outside. Additionally, the air rising has to be forceful enough to overcome other things in your house trying to suck air out, like bathroom and kitchen ventilation fans. There’s a delicate balance that gets harder to maintain as the house gets tighter. This is why combustion problems are getting more common. Old houses were leaky and it was very easy for a flue to work because fresh air could come in through all the cracks in the house. People also didn’t have Viking stoves with massive ventilation hoods!

Now, imagine your heating system sitting in your basement. Most systems still have flues like the one shown in the photo above. They force air through the combustion chamber and then expect it to rise up and out the chimney. But when it first starts up, the chimney might be cold. That cold air acts like a plug, stopping up the chimney. So the barometric damper is supposed to slam closed to minimize the amount of exhaust fumes that enter the house. Even a working damper will allow some fumes to come in (notice the soot stains around the joints of the flue). Once the flue/chimney is warmed enough, it will create a constant airflow up and out. That’s why you’ll notice the damper swinging inward. (Note – this is a rough description of what is happening. The damper also helps to increase the efficiency of the system by controlling the airflow. Google it if you want to learn more).

The old-fashioned way of venting combustion devices is really incompatible with a modern, tight, energy efficient house. This is why professional energy auditors are so against using these types of systems. They’re simply too easy to screw up.

Modern high-efficiency systems are much safer because they seal all the combustion gases away from your home’s air supply. Units like this Peerless PureFire are so efficient that they use sealed plastic tubing for venting the combustion gases. The system take combustion air from outside the house through one port, and blows the exhaust gas out the second port. Unless the system breaks, there’s no way for the combustion gas to mix with your home’s air.

In addition to being safer, these systems are much more energy efficient. The reason they can use plastic pipe is because the system extracts so much heat from the combustion process that the exhaust is merely warm, not hot. So it’s not unusual for these systems to be 20%-30% more efficient than their conventional counterparts.

While this all may sound like an advertisement for a new heating system, it’s just building science. Old systems were designed for use in homes that were leaky. They took liberties with the designs that don’t work in a modern home. So if you renovate your home, you can’t just add insulation and air seal – you may have to replace your heating system. You might also find that your fireplace doesn’t “work right” any more. Your home is a complex system of interacting parts. Turning on a fan in one room causes pressures to change in other rooms. Running the fireplace can actually cause your heating system to back-draft. Likewise, your furnace can cause your fireplace to back-draft. It’s enough to boggle the mind.

Why leaky ducts are so important

If you’ve read my articles for long, you know I have strong opinions about leaky ducts. Now I’m going to explain why, in addition to wasting energy, leaky ducts can actually make you sick or kill you.

In a perfect duct system, air gets sucked in through the “return” air system from various areas of the house. It then gets blown out through the “supply” air ducts and provides exactly the same amount of air back to the house. But consider what happens if the supply and return air amounts aren’t the same in each room – especially the rooms where you have your heating system.

Suppose you suck more air out of the basement than you supply to it. Well, that air has to come from somewhere, so it’s going to try to suck down the flue to your water heater and heating system. It’s going to suck from outside. It’s going to try to suck from other places around the house. But if your basement door is closed, the basement is going to be “depressurized” by the air getting sucked out.

When that happens, your conventional heating system can back-draft. A sealed combustion system, like shown above, won’t be affected by this, but most people have conventional systems. So depressurizing the basement is a really bad thing.

Now, combine this with your laundry room. Lots of people put the dryer downstairs with the heating system. A dryer sucks even more air out of the basement when it runs. So if you have imbalanced air to your basement and your dryer is running, you have double-trouble.

Even if your ducts appear proper, there’s a good chance that they’re extremely poorly made in the basement. There’s a common practice, called “panned returns”, where the duct system consists of sheet metal or cardboard nailed or stapled to the ceiling joists in the basement. Rather than actually use air-tight ducts, they save a few dollars using this dangerous and totally inappropriate construction practice.

It doesn’t take a building scientist to figure out that this is stupid, unhealthy and dangerous. What happens when the blower runs? It’s going to suck air from the basement through all those cracks. What does that do? It depressurizes the basement and can cause your heating system or water heater to back-draft. Stupid. Stupid. Stupid.

Why is this unhealthy? Besides the fact that you might be sucking exhaust fumes back in to your house, you’re sucking in whatever else is in your basement. Fumes from cans of paint. Mold. Mouse “stuff”.

Unfortunately, all duct leaks aren’t as obvious as the one shown above. Let’s dig a little deeper. You know that a lot of duct systems run through the walls and the attic. What happens if your supply ducts are leaking into these spaces?

If the system is “losing” air to the outside, it has to replace this from somewhere, and that somewhere is outside. So any duct leaks to the outside cause the house to become depressurized which in turn creates conditions that could cause back-drafting.

How can you tell if this is happening? It’s difficult without sensitive test equipment. With the right equipment, you can just turn on the blower, and you’ll immediately see if the house is becoming depressurized and to what extent. But without it, here are some tricks.

Try putting that shrink-wrap plastic on some of your leaky windows. Then, observe which way it bows when you turn your blower on. If you have central air, you can test it too. If the plastic bows inward, that means that room is being depressurized. If it bows outward, then the room is at a higher pressure.

The problem here is that each room is different. You might have higher pressure in some rooms but not where it’s needed. However, it’s a start that can help to show you how balanced your system is.

The dangers of industrial kitchen fans

If you did a fancy renovation of your kitchen, you may have installed a high capacity range hood. These things can move 500 to 1500 cubic feet of air per minute (CFM). For comparison, a bath fan moves about 100 CFM. And we all “know” bigger is better, so you probably opted for the master chef version, pumping out 1500 CFM. Well, for a home kitchen, that’s about the stupidest thing you can do.

Yes, you need some ventilation for your range. But very few people need this much capacity. And in a tight house, an industrial fan can easily cause your fireplace or heating system to back-draft. Easily. The thing is, it’s a balance. You need enough to exhaust the moisture and gas from the range but not so much to cause back-drafting elsewhere in the house. That’s yet another reason why you want sealed combustion heating systems and water heaters. It’s one less thing to think about.

I’ll stop here. This has gotten too long. Just be aware – your house is a delicate system that needs to be kept in balance. You’ have a handful of things to keep your eyes out for. And remember, if you ever start feeling the symptoms of CO poisoning, get fresh air and have your house checked out. Your life could depend upon it.