SAN ANTONIO GUIDE

Fuel-burning appliances still matter in a cooling climate

The air conditioner has been the machine you hear for months, the gas furnace has barely fired, and it is easy to treat combustion as a visitor that missed the season. The water heater did not miss it. Neither did a furnace that sits in a closet or an attic until a cold night asks it to light. Carbon monoxide is a reason to leave the building, not a reason to stand at the appliance and investigate. Mild winters do not retire that rule.

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

A long cooling season does not retire the risk

San Antonio’s climate is humid subtropical, with a long stretch of cooling and winters that are usually mild. Hard freezes are uncommon and real. The February 2021 freeze is a known example of weather that pushed idle heating equipment into service. This page does not count injuries or damage from that week. It says that any fuel-burning furnace is still a fuel-burning furnace on the nights it runs, and a gas water heater runs in August as well as in January.

Attic and closet equipment make the appliances easy to ignore. You do not walk past a basement boiler on the way to the kitchen, because that layout is not the local one. The furnace may share space with the cooling coil above the ceiling. The water heater may be in a garage or a hall closet, still venting, still combusting, while the condenser carries the season. Forgetting them is the mistake the climate invites.

Alarm, symptoms, and leaving

If a carbon monoxide alarm sounds, get everyone outside and call emergency services. Do not hunt for the source, do not open windows as your project, and do not silence the alarm and go back to sleep because the cooling is still running and the house feels normal. A headache, dizziness, or flu-like symptoms that ease once you are outdoors belong in the same response. Fresh air making people feel better is a clue to leave, not a clue to search the attic.

Do not try to decide which appliance is at fault while people are still inside. Furnace, water heater, a vehicle left running in an attached garage, a grill, or a generator can all be involved, and you will not sort that safely in a hallway. Come back inside only when emergency responders say the building is clear. Then a technician can inspect. The order is people, then outdoors, then the call, then the equipment.

Gas odor is a different emergency

The smell of gas is not a carbon monoxide alarm, and it is not a thermostat problem. Leave the building, do not flip switches, and from outside call the gas utility emergency number printed on the CPS Energy bill. CPS Energy is the city-owned electric and gas utility. The number belongs on that bill even though the utility has a public website. Do not hunt for the leak with a flame, a light switch, or a second tour of the house to fetch a pet you can instead ask responders to know about.

A furnace that will not light can be a repair on another day. A furnace that will not light in a house that smells of gas is not a repair you start. Stay out. Neighbors and the utility need the building empty more than they need your notes on the inducer motor. After the gas incident is over, write down what you smelled and when, so the heating technician is not guessing from a cleared house.

Furnace, water heater, and what never comes indoors

Have a technician inspect a gas furnace that soots, that smells hot or metallic when it runs, or that you were told has a cracked heat exchanger. Do not keep testing it on cold mornings to see if the smell was a fluke. The water heater deserves the same honesty: rust at the draft hood, a burner that rolls out, or a vent that has come apart is a shutdown item, not a waiting item. Combustion air and venting are the technician’s subject. They are not a Saturday adjustment.

Never run a generator in a garage, a patio enclosure, or indoors, even with a door cracked. Never use a charcoal grill or a gas oven as heat. Those are classic ways carbon monoxide gets into a house during a rare freeze or an outage, when people are improvising because the furnace sat idle and then failed. Keep generators outdoors and well away from doors, windows, and soffit vents. If you are unsure the distance is enough, it is not enough. Move it farther or shut it off.

Alarms, and what a cooling visit does not cover

Install alarms the way the manufacturer describes, and read the national guidance cited on this page about placement near sleeping areas. This article does not cite a local code section and does not pretend to be the fire marshal. If you want the local expectation, ask the fire department or the inspector who already knows your address. Test alarms, and replace units that are past the date printed on them. A chirp for a low battery is not the same sound as an alarm. Learn the difference on a calm day.

A cooling-season service visit looks at airflow, condensate, and the outdoor unit. It may happen in the same attic as the furnace and still never include a combustion check. Do not assume the sticker on the air conditioner means someone inspected the heat exchanger. The shape of that cooling visit is described in what a cooling visit covers. Ask for combustion and venting as their own line if heat is in the scope. A thermostat that will not stop calling for heat, or that you no longer trust at night, is a separate fault in thermostat troubleshooting.

After the house is cleared

When responders release the building, the next person should be someone who can work on fuel-burning appliances and can say whether the furnace, the water heater, or both are safe to run. Ask what they inspected, what they shut down, and what they did not open. A waved detector in the hallway for a minute is not a heat-exchanger inspection. Hiring questions, including the state license for air conditioning and refrigeration work when the cooling side is involved, are in what to ask before you hire.

Cost follows the same honesty. An alarm event can end in a repaired vent, a condemned furnace, or a finding that a car was left running in the garage and the appliances are sound. Those are different jobs. Factors that move a bill, with no prices invented here, are in repair cost factors. None of those factors matter until everyone is outside and emergency services have been called. Do not shop for a quote from inside a house with an alarm still sounding.