A story made the rounds online a few years back that I still bring up on jobs, because it says more about carbon monoxide than any pamphlet I could hand a client. A renter kept finding post it notes around their apartment in handwriting that was not their own, one reminding them to save their documents, another claiming their landlord would not let them talk. Convinced they were being watched, they set up a webcam and asked the internet for legal advice. It took a stranger, an engineer reading through the poster's old messages, to notice one detail that mattered. The apartment had no windows. He had once nearly poisoned his own family with carbon monoxide from a stove left on aboard a boat, and he recognized the pattern. He suggested a CO detector. It read 100 parts per million. The notes had been written by the renter, asleep, hallucinating.

I tell that story because carbon monoxide rarely looks like what people expect. It does not announce itself. And the tighter a house gets, the more that quiet, easy to miss buildup becomes something I have to test for on every single job, not just the obviously old ones with a rusty furnace in the corner.

Why Tightening A House Changes The Math On Combustion Appliances

What Backdrafting Actually Is

A natural draft water heater or furnace has no fan pushing exhaust gas up its flue. It relies entirely on the buoyancy of hot gas rising, the same principle that makes smoke drift upward from a candle. That works fine as long as nothing in the house is competing for air. The moment the house pressure drops low enough, that weak, unforced draft can reverse, and combustion gas including carbon monoxide comes back down the flue and into the room instead of leaving through it. That reversal has a name, spillage, or backdrafting, and it is more likely in a tighter house for a simple reason. A leaky house lets outdoor air seep in from a hundred small gaps, which keeps pressure closer to balanced. A tightly sealed house has fewer of those gaps, so when something pulls air out, the pressure drop concentrates instead of dissipating.

The Exhaust Fans That Outmuscle A Chimney

The thing most often strong enough to cause that pressure drop is not exotic. It is a kitchen range hood, a bathroom fan, or a clothes dryer, all of which use a real motor to move air, something a natural draft appliance does not have. Running one of these while a natural draft water heater is firing can be enough to pull its exhaust back down the flue. A less obvious version of the same problem shows up when a home upgrades one appliance and forgets about the other one sharing its chimney. Swap an old furnace for a high efficiency model that vents out the side wall, and the water heater's vent, now alone in a flue sized for two appliances, can lose the draft strength it used to share the load with.

The Test Nobody Fully Trusts, But Still Runs

Energy auditors run something called a worst case depressurization test, running every exhaust fan in the house at once to see whether a combustion appliance still drafts under that pressure. Researchers at Lawrence Berkeley National Lab have pushed back on how much that specific test actually tells you, since a house can fail it on a windy day and pass it on a calm one, and the industry's own two major certifying bodies do not even agree on what level of depressurization should count as a failure. What nobody disputes is that the appliance draft test and the CO reading taken directly in the flue are worth doing every time, on every job, regardless of what the depressurization number says. Wind matters more than people expect here too. A chimney cap uses passing wind to help pull gas up and out, so carbon monoxide poisoning is actually more common on still, windless days than on gusty ones, which runs against most people's intuition about when a chimney would work worst. One auditor's field story sticks with me: a family dog lying across a bedroom heat register with the door closed pulled that room down to negative 11 pascals, enough to threaten backdrafting, and no scheduled test would ever have caught it, because the cause was a dog, not a fan.

The Detector You Have Is Probably The Wrong Kind

Standard UL rated carbon monoxide detectors are built to alarm only at levels high enough that a healthy adult could already be showing symptoms, because the devices were deliberately calibrated that way years ago after low level models caused too many false alarm calls to fire departments. That means the detector bolted to your hallway ceiling right now may say nothing at all while carbon monoxide sits at a level low enough to cause the chronic headaches, fatigue, and confusion doctors sometimes misdiagnose as flu or fatigue for months, but high enough to matter. A low level CO monitor, the kind that displays an actual number instead of waiting to scream, catches that range instead. If your bedroom sits anywhere near a natural draft appliance, or you have ever wondered whether that recurring headache clears up the moment you step outside, that number is worth having.

What This Means For Air Sealing And Insulation Work

If a home has any natural draft or atmospherically vented appliance, combustion safety needs to be checked before air sealing or insulation work starts, not after. Tightening a house that already has a spillage prone water heater or furnace does not create the problem. It concentrates a problem that was already there. The fix is not complicated, even if it costs money. Sealed combustion and power vented equipment removes the mechanism entirely, since the exhaust path is fully closed off from the living space and, in some models, pushed out mechanically instead of relying on buoyancy. A heat pump water heater or an air source heat pump for space heating goes a step further and removes combustion from the equation altogether. None of this is optional once a house gets tight enough. It is the price of admission for airtightness done right, not an upsell.

Where This Leaves Me

I do not think twice about pausing an insulation or air sealing job to deal with a natural draft appliance first, even when the homeowner just wants the energy upgrade they called about. The story that started this post did not happen in an old, leaky building. It happened in an ordinary apartment with no windows, and the poisoning went on long enough to produce hallucinations before anyone thought to check.

What I check on every job now, before anything else touches the building enclosure: