SAN ANTONIO GUIDE
When a gas furnace or heat strip stays cold
The thermostat clicks over to heat and the hallway stays the same temperature it held all evening. That call may be the first time the furnace has been asked to light since the cooling season took over. Mild winters make a long idle stretch normal, until a cold night arrives and the burners or the heat strips have to work. Some houses keep a gas furnace in a closet or in the attic beside the cooling coil. Others warm the air with electric strips inside an air handler that spent the summer only cooling. The symptom is the fan, a click, or silence, and rooms that never warm.
Months of cooling, then one cold call
A gas furnace can sit with the burners closed while the air conditioner carries every hour of the long season. Dust settles on the flame sensor and around the burners. That is a reason a technician looks there on the first cold morning. It is not a reason to unbolt the burner compartment and scrape parts yourself. Condensate from the cooling coil uses its own drain. A summer water problem and a winter ignition problem can share one attic and still be separate failures.
A closet furnace in an older King William or Monte Vista house does not offer the same working room as an attic furnace above a Stone Oak hallway. Southtown and the Pearl area sometimes tuck equipment into a converted space where combustion air is tight. Alamo Heights, a separate city, has the same mix of closets and attics. Say where the unit is, and say whether you hear an inducer start, a click, and then nothing.
What the light-off sequence is checking
A typical gas furnace runs a sequence rather than simply opening a valve. The control calls for heat, a draft inducer runs, a pressure switch has to prove the vent, an igniter glows or sparks, the gas valve opens, and a flame sensor has to prove that a flame is actually there. If a step fails, many furnaces lock out and flash a code on a small light. If you can see that light without removing a panel, write the blink pattern down. Do not jump a pressure switch or tape a rollout switch to force a fire.
The blink pattern is a clue for someone with the furnace manual, not a parts order placed from the driveway. A cracked heat exchanger, a failed igniter, a coated flame sensor, and a gas valve that never opens can all end with no flame. Confirming that a familiar shutoff handle is parallel to the gas pipe is as far as a homeowner check should go, and only when you already know that valve. If the house smells of gas, skip the handle and leave.
Heat strips when there is no gas flame
Heat pumps and some all-electric houses warm supply air with electric heat strips in the air handler. On a mild night the heat pump may carry the whole load and the strips never energize. On a colder night the strips are the backup. If the blower runs and the air at the register stays at room temperature, the strips may not be coming on. A technician checks the breaker, the sequencer or relay, and the heat-kit connections. That cabinet also holds high voltage. It is not a panel to open for a curious look.
A strip failure can feel like no heat even when the outdoor unit is a separate machine. Note whether that outdoor unit runs when the thermostat calls for heat, whether anyone has switched the thermostat to emergency heat, and whether a breaker marked for the air handler or the heat kit is tripped. Resetting a breaker once, after you are sure nothing is burning or buzzing, is a reasonable note to give the technician. Resetting it again and again is how an electrical fault becomes a fire risk. Leave it off.
What a rare freeze asks of idle equipment
Hard freezes are uncommon in this humid subtropical climate, and most winters stay mild, but a real freeze does show up. The February 2021 freeze is the recent event people still name: equipment that had sat idle was asked to run while the weather was far colder than a normal year. This page does not recount harm from that week. It does say that a furnace or a heat strip you have not heard in months deserves a supervised test on a mild day, not a first trial at dawn when the house is already uncomfortable.
CPS Energy is the city-owned utility for both electricity and gas. A cold snap can stress either supply. With no power, a furnace control cannot finish its sequence, and heat strips cannot warm a register. If neighboring houses are dark as well, start with the outage rather than with the igniter. Gas emergency numbers still come from the bill. A forecast from the National Weather Service is the place to see a front coming, not a post that invents a tally of broken furnaces.
Gas odor and carbon monoxide
Smell gas and the project is over. Leave the building, do not flip switches, and from outside call the gas utility emergency number printed on the CPS Energy bill. Do not hunt for the leak, and do not go back inside until the utility or the fire department says the building is clear. A furnace that will not ignite and a house that smells like gas are not a troubleshooting sequence.
A carbon monoxide alarm, or headache, dizziness, or a flu-like feeling that eases outdoors, means get everyone outside and call emergency services. Do not air the rooms and go back to bed. Do not stay to watch the burners. Any fuel-burning furnace or gas water heater can produce carbon monoxide even when heat is a small part of the year. Read carbon monoxide safety before you treat a no-heat morning as an ordinary repair. While you are listing symptoms, say whether the same equipment failed to cool in summer; warm air from the vents is the complaint the long cooling season keeps producing.
Work that stays with a technician
Opening the burner box, cleaning a flame sensor, replacing an igniter, and testing a gas valve are technician tasks. The homeowner side of the line is a filter, thermostat batteries, and debris around the outdoor unit once power is confirmed off, which is the split described in what to do yourself and what to leave. Do not add refrigerant to a cooling circuit because the furnace failed, and do not tape down a safety switch on either machine.
What a no-heat visit costs depends on attic or closet access, the part that failed, whether the call is after hours, and whether the repair stays in the controls or involves the gas train. Those factors, without dollar figures, are laid out in what changes a repair bill. Ask the technician to name the failed step in the sequence, the parts they ruled out, and whether the heat exchanger was inspected or only the controls. A blinking light is not yet that explanation.
Questions when the heat will not start
Can I clean a flame sensor to get the furnace to light?
Leave the flame sensor in place. Reaching it means opening a burner compartment on a fuel-burning appliance, and a bent sensor or a disturbed wire can create a new failure or a safety problem. Write down any blink code you can see without removing panels, confirm you do not smell gas, and let a licensed technician do the cleaning and the combustion check on the same visit.
Are heat strips the same thing as a gas furnace?
No. Heat strips are electric elements in the air handler. A gas furnace burns fuel and needs a vent, an igniter, and a heat exchanger. Some houses have both a heat pump and strips, with no gas furnace at all. Others have a gas furnace for heat and a separate air conditioner for the long cooling season. The data plate, or a technician who has opened the right cabinet, is how you tell which one you have.
Should heat be tested before a cold front arrives?
Yes, on a mild day, with someone who can shut the system down if something smells wrong or the burner lights and then rolls out. Waiting until the morning of a freeze means the first test happens when you most need the heat. A short supervised run also tells you whether the thermostat is still set to heat and whether a breaker tripped during the months the furnace sat idle.
What if the furnace is in a hot attic and the night is freezing?
Attic equipment is a common South Texas arrangement, and the attic can still be the furnace location on a cold night. Access may be harder if the hatch is tight or the platform is awkward, which matters for a technician carrying tools. Do not climb into a dark attic to reset controls if you are unsure of the footing or the wiring. Note any water stains from the cooling season so the visit covers the drain as well as the burners.
Does a mild climate mean carbon monoxide is unlikely?
No. A gas furnace that runs even a few nights a year, and a gas water heater that runs all year, can both produce carbon monoxide if they vent poorly or the heat exchanger fails. Mild winters do not retire that risk. An alarm on each sleeping level, and the habit of going outside when it sounds, matter more here than the number of freezing nights on the calendar.
Sources (checked October 6, 2026)
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