DENVER GUIDE

Carbon monoxide, combustion appliances, and a tighter house

The alarm in the hall is sounding, or everyone woke with a headache that faded on the porch. Carbon monoxide is a combustion problem, not a comfort problem. Furnaces, boilers, water heaters, fireplaces, and ovens can make it. The first move is outside. The search for the source comes later, with the people already out.

By Mike Maziarz · Sources checked October 6, 2026 · How we write these guides

When the alarm sounds

Get everyone outside, including anyone who says they feel fine. Call emergency services from outside. Do not hunt the basement for a cracked exchanger, a loose flue, or a blocked vent while the alarm is still the loudest fact in the house. Do not pull the batteries to win silence and then stay. Silence is not the same as clear air.

A headache, dizziness, nausea, or flu-like symptoms that ease outdoors belong in the same exit even if no alarm has sounded yet. Alarms fail, get unplugged, and reach an end of life. Symptoms in more than one person, or in a pet, that improve in fresh air are a reason to leave, not a reason to start coffee and see. Tell the dispatcher what you felt and what fuel-burning appliances the house has.

If you smell gas as well, you still leave, and you also use the emergency number on the gas bill from outside. Carbon monoxide itself does not smell. A rotten-egg odor is the fuel, a separate warning that can show up in the same hour. Xcel Energy is the name many local gas bills use. This page does not print the digits.

Which appliances are in the conversation

Anything that burns fuel in or beside the living space belongs on the list: the furnace, a boiler, the water heater, a fireplace, a gas oven, a clothes dryer, an attached garage. A car warming up in a closed garage is a combustion source aimed at the house. Do not idle a car in the garage 'just to warm it' with the house door shut or open. The door to the kitchen is not a seal.

Forced-air furnaces and boilers both show up in Denver. You do not need to know which is more common on your block to take an alarm seriously. You need to know which one this house has, and whether a water heater shares the mechanical room. Tell that to the firefighters or the technician. A radiator on the wall means look for a boiler. Supply registers mean look for a furnace or an air handler. Both can share a building with a water heater that also vents.

Outdoor cooking and camping gear stay outside. A generator stays outside, far from windows and from the fresh-air intake of a high-efficiency furnace. A generator running in a garage, even briefly, is how an outage becomes a poisoning.

Tight houses and older brick

A house that has new windows, sealed can lights, and a careful weatherstripping job holds heat. It will also hold combustion products if a vent spills or an exchanger fails. Newer infill in Central Park and a renovated brick house in Cherry Creek can both be tight for different reasons. Age of the brick is not a clearance.

Combustion air has to come from somewhere. A louver in a mechanical-room door, a duct to the outdoors, or the volume of an open basement may be part of how the appliance was installed. Do not cover those openings with cardboard because the room felt cold in January. If you do not know whether an opening is combustion air, ask before you block it. The heating and cooling overview is the map of closets and appliances. It is not a permit to modify the air the burner uses.

Backdrafting is a word for smoke or exhaust that should have gone out and came in instead. A fireplace that puffs into the room when the furnace starts, or a scorched or sooty draft hood, is a note for a qualified person. It is not a reason to prop the appliance and keep burning. Wind on a Front Range night can be part of that story. The fix is not a guess you make with the flue apart.

Alarms are not thermostats

A carbon monoxide alarm and a thermostat live on different jobs. The thermostat asks for heat. The alarm watches the air. A blank thermostat screen does not mean the air is safe, and a quiet alarm does not mean the furnace lit correctly. The thermostat troubleshooting page stays with modes, schedules, and blank screens. Do not use it as a substitute for an alarm in the hall.

Place and replace alarms the way the manufacturer describes, and follow the local fire authority where it adds a rule. This guide will not invent a mounting height or a code section. Units print an end-of-life date. When that date arrives, or the alarm chirps a fault the instructions do not clear, replace the unit. A chirp you have learned to sleep through is a disabled detector.

Test alarms with the method on the device, not with a running car or a blocked flue. Do not 'test' the furnace by partially covering the vent. If you rent, tell the owner about a missing or expired alarm in writing and still leave if it sounds or if symptoms show up.

After the all-clear, the heating question

Emergency crews may ventilate and check the air. When they release the house, the fuel-burning appliances that were running still need a qualified look before they become the heat again. Ask who is cleared to restart them. Restarting everything yourself to 'see if it happens again' is how you repeat the exposure.

A technician should be willing to explain what they inspected: vent joints, the burner flame in words, the heat exchanger if that was the concern, and whether elevation setup came up for a furnace or boiler. You are owed the explanation. If they want to replace the appliance, the reasons should be the defect they can name. The cost factors page is about the work, not about bargaining a safety shutdown into a discount.

Hiring questions belong before the next ordinary visit, not shouted from the porch. The questions before hiring include permits, what the person will inspect, and whether they will talk about combustion air. Use them when you are choosing. Use the exit when you are symptomatic.

Habits that are not a substitute for the exit

Keep vents and intakes clear of snow, nests, and stored lumber. Keep a combustion closet from becoming a warehouse. Schedule a look at fuel-burning equipment before the heating season if you can. None of those habits cancel an alarm that is sounding now. Prevention is what you do on a quiet afternoon.

EPA and fire-safety pages describe how carbon monoxide behaves indoors and why alarms matter. They are references, not a remote technician. NFPA's home material and the Red Cross page on poisoning are there so the exit steps are not a local legend. Local firefighters still own the emergency.

Write down, when the house is calm, which appliances burn fuel and where the alarms are. Put the gas-bill emergency number where you can find it without going back inside. The list is short on purpose.