INDIANAPOLIS GUIDE

Carbon monoxide and fuel-burning heat

The alarm in the upstairs hall is chirping a pattern you have never heard, or a headache that felt like a cold eases as soon as you step onto the porch. In a house with a gas furnace, those are not mysteries to solve in the basement. Carbon monoxide is a gas you cannot see or smell. Fuel-burning heat, a water heater in the same utility room, a blocked vent, or a car left running in an attached garage can all put it in the air. This page is the leave-the-house rule and the ordinary habits that belong around a furnace. It is not a way to find the leak yourself.

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

What carbon monoxide has to do with the furnace

When fuel burns completely, you get heat and a vented exhaust. When it burns poorly, or the exhaust cannot leave, carbon monoxide can spill into the rooms. A gas furnace is a common heating appliance in Indianapolis, so the basement or closet where it sits is part of the story. So is the metal or plastic vent, the chimney on an older house, and the outdoor termination on a high-efficiency unit.

You will not confirm carbon monoxide by sniffing. It does not smell like gas. A gas odor is a different emergency, the fuel itself, and it also means leave and call the number on the utility bill from outside. Carbon monoxide can be present with no odor at all. That is why the alarm exists.

Other fuel-burning appliances in the same house matter. A water heater, a fireplace, a gas dryer, or a stove can be involved. Do not decide which one it was while you are still indoors. The first job is people and fresh air outside, not a tour of the utility closet.

Alarms are the tool, not a furnace sensor

Install alarms the way the manufacturer describes, which for most homes means outside sleeping rooms and on each level where people spend time. A single detector sitting on the furnace in a Broad Ripple or Irvington basement does not watch the bedrooms. Follow the instructions for height and for places to avoid, such as a dead-air corner or a spot that is always dusty and humid.

Press the test button when you do the fall heat check, and read the date on the back. Alarms do not last forever. The date is the instruction. A chirp that is the low-battery pattern is a reason to replace the battery or the unit, and then to make sure the new sound is the test sound, not an alarm you have been ignoring.

Know the difference between a test chirp and a real alarm before you need it. Read the pattern in the booklet once, on a calm day. If you are unsure in the moment, treat the sound as real.

Symptoms that mean get outside

A carbon monoxide alarm means get everyone outside and call emergency services. Headache, dizziness, or flu-like symptoms that ease outdoors mean the same thing, even if no alarm has sounded yet. Nausea and confusion belong in that list of reasons to leave. Do not hunt for the source. Do not open a panel to look at the flames. Do not silence the alarm and sit back down to see if you feel better.

Pets count. If the alarm is going and you feel fine, you still leave. Meet outside, away from the door, and do not send someone back in for a phone if the house is the problem.

People sometimes describe the feeling as a cold that vanishes on the porch and returns in the kitchen. That pattern is a clue to tell emergency services and, later, a technician. It is not a clue to experiment with. Write it down after you are outside, if you can do that from memory.

What not to run and what not to block

Do not heat the house with a gas oven, a grill, a camp stove, or a generator. A generator running in a garage, even with the door open, can fill a house. Do not warm a car in an attached garage with the garage door up and the house door unlocked. Exhaust finds its way in. Those are winter habits that show up in the same season as furnace trouble, and they are separate from a cracked heat exchanger.

Do not block a furnace vent, a chimney, or the intake and exhaust pipes on the side of the house. Snow, ice, a bird nest, or a cover left on after painting can stop the exhaust. If you can clear snow from the ground without climbing, do that on ordinary days. If an alarm is already sounding, skip the yard project and leave.

Do not bypass a rollout switch or tape a furnace door to keep heat coming after a scare. A safety that tripped is information. The visit guide is for after the house is safe: what a technician should explain, and what you should have written down. It is not a script for staying inside a house that just alarmed.

After everyone is outside

Call emergency services from outside. Tell them there is a carbon monoxide alarm or that symptoms eased outdoors. Follow their instructions about when the house may be entered. A heating company does not replace that call. The utility does not replace it either. Emergency services first.

When you are allowed back, or when a technician is allowed in, the question is why combustion products were in the air. A blocked vent, a failed heat exchanger, an appliance that was backdrafting, or something that was running that should not have been. Ask for that finding in writing. Do not accept a filter change as the whole answer to an alarm.

The hiring questions still apply once the house is cleared: insurance, a written finding, and a refusal to jump out safeties. After-hours timing can change a quote, and the quote page lists those factors without numbers. None of them outrank the order to stay out until the house is cleared.

Habits on ordinary days

On a day with no alarm, you can still reduce the chance of a surprise. Keep the furnace closet or basement path clear so vents and the front of the unit can be seen. Have the burners and the vent looked at during a fall heat check. Replace the filter so the furnace is not overheating and shutting down in odd ways. None of that diagnoses carbon monoxide. It keeps the appliance in the condition a technician can actually inspect.

If the thermostat is blank or the furnace will not start, fix the call for heat only in the ways that do not involve opening the burner box. The thermostat guide is that short list. A furnace that will not light is not an invitation to prop the door open and run it anyway.

Tell everyone who lives in the house what the alarm sounds like and where to meet outside. A guest on a January night should not have to guess. The meeting spot is part of the heating system, in the only way that matters when the alarm is the thing that speaks.