Carbon monoxide comes up constantly in conversations about furnaces, boilers, and water heaters. Every gas-burning appliance carries some risk if it isn’t venting properly, and that fact gets mentioned in passing almost everywhere it’s relevant. What rarely gets covered is the fuller picture: where CO comes from in a Western Montana home, why homes here carry more of this risk than milder climates, and what protects you beyond a detector on the wall.
With heating season starting in earnest and homes sealing up tight against the cold, this is worth understanding in full rather than picking it up in fragments across different appliance conversations.
Why Missoula Homes Carry More of This Risk Than Warmer Climates
Carbon monoxide is produced whenever fuel burns: natural gas, propane, oil, or wood. In a properly functioning system, combustion happens completely and the byproducts vent safely outside. The risk appears when combustion is incomplete or when venting fails, and both of those failure modes are more relevant in a place like Missoula than somewhere with a short, mild heating season.
The first reason is simply how many combustion appliances a typical Western Montana home runs simultaneously. Many homes here have a gas or oil furnace or boiler for primary heat, a gas water heater, and in a lot of older homes and rural properties, a wood stove as supplemental heat. That’s three separate combustion sources under one roof, each with its own venting path and its own way of failing.
The second reason is how tightly Missoula homes are built and sealed to survive a real winter. Good insulation and air sealing keep heating costs down, but they also mean less natural air exchange with the outside. A home that leaked air freely would dilute a small CO release before it became dangerous. A tightly sealed home doesn’t have that safety margin, which is exactly why mechanical ventilation and properly functioning combustion equipment matter more here than in leakier, milder-climate construction.
The third reason is the length of the heating season itself. A furnace or boiler that runs six or seven months a year has far more operating hours, and therefore far more opportunity for a venting or combustion issue to develop, than one that runs a few weeks in a mild climate.
Where It Comes From
- Furnaces and boilers.
The heat exchanger separates the combustion process from the air or water that circulates through your home. A crack in that heat exchanger, which develops from years of thermal expansion and contraction through repeated heating cycles, allows combustion gases to mix with the air your system distributes. This has no visible symptoms from outside the equipment, which is exactly why it’s caught through inspection rather than observation. - Gas water heaters.
Any gas-powered water heater vents combustion byproducts, and a water heater installed incorrectly, with inadequate venting, or with a flue that’s become blocked poses the same risk as a furnace with a compromised heat exchanger. Water heaters are sometimes installed in smaller, less ventilated mechanical spaces than furnaces, which can compound venting problems if they occur. - Wood stoves and fireplaces.
A genuinely Montana-specific risk factor. Wood stoves produce carbon monoxide as a normal combustion byproduct, and they rely entirely on proper flue draft to carry it outside. A flue that’s partially blocked with creosote buildup, damaged, or improperly sized for the stove creates conditions where CO can back up into the living space instead of venting. Homes running a wood stove alongside a furnace or boiler have two independent combustion and venting systems that both need to be functioning correctly. - Attached garages.
Vehicles left running in an attached garage, even briefly with the garage door open, can push CO into the connected living space, particularly in a tightly sealed home where pressure differences can pull garage air inward. - Portable generators.
During a winter power outage, a real possibility in parts of Western Montana, a generator running in a garage, breezeway, or too close to windows and doors is one of the most common sources of serious CO poisoning nationally. Generators need to run entirely outdoors, well away from any window, door, or vent.
The Symptoms People Mistake for Something Else
Carbon monoxide poisoning produces symptoms that overlap heavily with common winter illnesses, which is part of why it goes undiagnosed until it becomes serious. Headache is typically the first symptom, followed by dizziness, fatigue, nausea, and confusion as exposure increases.
The pattern worth recognizing: symptoms that improve when you leave the house and return when you’re back inside. Multiple people in the same household feeling similarly unwell at the same time, particularly if a pet is also acting sick or lethargic. Symptoms that seem to track with when the furnace, boiler, or wood stove is running.
If you notice this pattern, don’t wait to investigate. Get everyone outside into fresh air, and if symptoms are significant, call for medical attention and mention suspected CO exposure specifically, since it requires a particular type of blood test to confirm.
What Detectors Do and Where They Fall Short
Every home with any combustion appliance needs working carbon monoxide detectors, ideally on every level and near sleeping areas. That part isn’t in question. What’s less understood is how detectors fit into the broader safety picture.
Detectors have a service life of roughly five to seven years. The sensor inside degrades over that period regardless of whether the unit still powers on and passes its self-test button. A detector that’s been on the wall for a decade may simply not respond reliably anymore. Check the manufacture date on the back of every unit in your home and replace anything past that window.
More importantly, a detector alarms because CO is already present in your living space at a concentration high enough to trigger it. That’s a critical backstop, but it means the actual prevention happens before that point: through equipment that’s properly maintained, correctly vented, and inspected on a schedule rather than left to run indefinitely between problems.
What Gets Checked During an Inspection
A heating system inspection that addresses CO risk directly involves more than a visual look at the equipment. Heat exchanger inspection, often using a camera to examine interior surfaces that aren’t otherwise visible, checks for cracks or separation that allow combustion gas leakage into circulated air.
Combustion analysis measures the actual composition of flue gases, including CO levels, to confirm the burner is operating within specification rather than just assuming it from how the system sounds or looks.
Flue and venting inspection confirms that combustion gases have a clear, unobstructed path outside. This matters for furnaces and boilers and is equally critical for wood stoves, where creosote buildup in the chimney is a distinct maintenance item from anything on the mechanical heating system.
Garden City’s furnace repair and heating service visits include these checks as standard practice, not an upsell, because a system that looks and sounds fine can still be producing dangerous CO levels without any way for a homeowner to know without instrumentation.
For water heaters, a similar venting and combustion check applies, and it’s worth including as part of any water heater service or inspection rather than treating the water heater as separate from the home’s overall combustion safety picture.
What to Do Before the Season Gets Underway
Test every carbon monoxide detector in the home and check the manufacture date on the back. Replace anything more than seven years old regardless of whether it still passes its test button. If you run a wood stove, have the chimney inspected and cleaned before the season if it hasn’t been done recently. Creosote buildup is a fire risk as well as a venting risk, and both matter.
Never run a generator in a garage, breezeway, or near any window or door, even temporarily. Keep it fully outdoors and well away from the house.
If your furnace, boiler, or water heater hasn’t had a professional look in the past year, scheduling that before the coldest months arrive is the most direct way to catch a developing issue while it’s still a straightforward repair rather than an emergency.
Garden City Plumbing and Heating has served Missoula and Western Montana since 1985, working across the full range of heating equipment common in this region, from older boilers to modern furnaces to the water heaters running alongside them. Schedule a heating system inspection with Garden City today or call (406) 481-3548 before winter settles in, and get a clear answer about how your home’s combustion equipment is performing.
A tightly sealed, well-insulated home is exactly what you want for a Montana winter. It’s also exactly why the equipment producing your heat needs to be running the way it’s supposed to.