When the heat won't come on during a Houston cold snap, the cause is usually small: a thermostat still in COOL or with a dead battery, a tripped breaker or blown low-voltage fuse, a clogged filter, a dirty flame sensor on a gas furnace, a failed igniter, or a heat pump that hasn't switched to auxiliary heat. Houston heating systems sit unused for eight or nine months, so the first 35°F morning of the year is when weak parts show up all at once. Diagnostic fees commonly run $75 to $150, and flame sensor cleanings and igniter replacements typically land in the low hundreds. Check the thermostat, breaker, and filter first. If you smell gas, leave the house and call the gas utility before calling anyone else.
Gas furnace or heat pump? The checks are different
Find out which one you have. If the outdoor unit runs when you call for heat, you have a heat pump. If only the indoor unit runs and there is a gas line to it, you have a gas furnace. Many Houston homes have a gas furnace in the attic with the AC coil sitting on top of it.
Gas furnace checks: thermostat on HEAT with fresh batteries, breaker on, furnace door switch fully closed, filter clean, and the gas valve on the furnace turned parallel to the pipe. If the furnace lights for a few seconds and shuts off, the flame sensor is dirty, which is the most common first-cold-morning failure. If it clicks but never lights, it is usually the igniter.
Heat pump checks: the outdoor unit should run and the air at the vents should feel mildly warm, around 90°F to 100°F, not hot like a furnace. Below about 35°F the heat pump alone struggles and the electric auxiliary heat should engage. If the thermostat shows AUX or EM HEAT and the air is still cold, the heat strips or their breaker have failed.
Why first-cold-morning failures are so common in Houston
Systems here heat for a handful of weeks a year. Flame sensors build a film of oxidation over a long idle summer in a 130°F attic, igniters get brittle, and low-voltage fuses that survived a hurricane season's power flickers finally give out on the first heating call. Half the city discovers the problem the same morning.
The 2021 freeze and the occasional hard cold snap since have taught Houston that heat matters more than the mild winters suggest. Heat pumps that were sized for cooling can struggle in the 20s, and homes without working auxiliary heat get cold fast. Those events also created contractor backlogs that lasted days.
Corrosion plays a part near the bay. In Pasadena and League City, salt air corrodes gas valve terminals, control board connections, and heat pump contactors that never get exercised until winter. A system that cooled fine in September can refuse to heat in December for a reason that has nothing to do with the furnace itself.
When to call and what a repair typically costs
Call now if you smell gas, if the furnace keeps cycling on and off, if a carbon monoxide detector sounds, or if the house is dropping below 60°F with an elderly person or infant inside. For a gas smell, leave first and call the gas utility's emergency line, then a contractor.
Typical costs: diagnostic commonly $75 to $150, flame sensor cleaning often $80 to $200, igniter replacement typically $150 to $400, low-voltage fuse and board check in the low hundreds, heat strip replacement for a heat pump commonly $300 to $800, and a control board $400 to $900. A cracked heat exchanger means replacement, not repair.
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