HVAC Symptoms: What Your System Is Telling You
Something off with the heat or the AC? Start with what you notice. Each sign points to a likely cause, an urgency level and the service that fixes it, so you know whether to act right away, call soon or bring it up at the next tune-up.
Smells That Can’t Wait
A rotten-egg smell means natural gas. Get everyone out without flipping switches, lighting anything or using a phone inside, then call the gas utility or 911 from outside. A carbon monoxide alarm gets the same response.
For a hot electrical or burning-plastic smell, switch the system off at the thermostat. If the smell keeps growing or you see smoke, get out and call 911.
Now, Soon or at the Next Tune-Up
Every sign gets one of three urgency levels. Use them as a guide, and trust your gut when something seems wrong.
- Emergency Now: a safety risk. Switch the system off or leave the house, as the sign calls for, before you think about the repair.
- Call Soon: something has failed or is about to. Book a repair and run the system as little as you can until it’s fixed.
- Can Wait for a Routine Visit: a comfort or cost problem with no safety risk. Note it and bring it up at the next tune-up, or book a visit when it suits you.
Summer changes the math. No cooling on a Houston afternoon in July, with a baby, an older adult or someone sick in the house, is more than a comfort problem: treat “Call Soon” as urgent and get them somewhere cool while you wait.
Symptom Checker
Pick the sign that fits best to see its likely cause, how urgent it is and the service that fixes it.
Choose what you notice, then press Show the Likely Cause.
Each Sign, Its Cause and Its Fix
Most symptoms have more than one possible cause, from a clogged filter to a failed part. Testing on the visit, not guesswork, settles which one it is.
| What You Notice | The Likely Cause | How Urgent It Is | The Service That Fixes It |
|---|---|---|---|
| Warm air from the vents | A clogged filter or dirty coil, a failed capacitor or contactor, or low refrigerant from a leak | Call Soon | AC Repair to test the parts and find any leak |
| No air at all from the vents | A blower that won’t run: a tripped breaker, a dead motor or capacitor, a failed control board or a float switch over a full drain pan | Call Soon | AC Repair, or Heating Repair in winter |
| Ice on the indoor coil or copper lines | Too little airflow (a clogged filter, closed vents, a weak blower) or low refrigerant | Call Soon; switch cooling off until then | AC Repair once the ice has melted |
| Water near the indoor unit | A clogged condensate drain line, a rusted or cracked drain pan, or a coil melting off its ice | Call Soon; switch off if water reaches a ceiling | AC Repair to clear the drain or fix the pan |
| Burning smell | Dust burning off the burners or heat strips on the first cold day, or an overheating motor, wire or capacitor | Emergency Now for an electrical smell; Can Wait for a Routine Visit for dust that burns off | Heating Repair, or AC Repair if the cooling was on |
| Gas smell, like rotten eggs | Natural gas escaping from a loose fitting, a valve or the furnace’s gas connection | Emergency Now: leave the home, then call the gas utility or 911 from outside | Heating Repair, once the utility says it’s safe |
| Rattling or banging noise | A loose panel or screw, a stick in the outdoor fan, or a cracked fan blade or blower wheel | Call Soon for banging; Can Wait for a Routine Visit for a light rattle | AC Repair for the outdoor unit, Heating Repair for the furnace |
| A boom when the furnace lights | Delayed ignition: gas builds up for a moment before it lights, often from dirty burners or a weak ignitor | Emergency Now: switch the furnace off and leave it off | Heating Repair before the heat runs again |
| No heat | A thermostat set wrong or out of batteries, a tripped breaker, a dirty flame sensor, a cracked ignitor, or a heat pump’s reversing valve stuck on cooling | Call Soon, sooner with a hard freeze coming | Heating Repair for the furnace or heat pump |
| Some rooms hot, others cold | Leaky, crushed or disconnected attic ducts, too few return grilles, blocked vents, or a system sized wrong for the house | Can Wait for a Routine Visit | Ductwork sealing, repair or resizing |
| Energy bills higher than last year | A dirty filter or coil, low refrigerant, duct leaks into the attic, or an aging system that runs longer for the same comfort | Can Wait for a Routine Visit | Maintenance first, replacement if the system is worn out |
When Two Signs Show Up Together
Symptoms often travel in pairs, and the pair narrows things down.
- Ice and warm air: the coil froze, so air can’t pass through it. The real fault is the airflow or refrigerant problem behind the ice.
- Water and ice: the drain may be fine. The water is melting ice, and the freeze is what needs fixing.
- Uneven rooms and high bills: leaky ducts are a strong suspect, since cooled air escapes into a hot attic before it reaches the rooms.
- Short run times and a muggy house: a system that starts and stops every few minutes may be oversized, low on refrigerant or overheating, and it never runs long enough to pull the moisture out.
- Repeat breakdowns and rising bills on an older system: that’s the point where system replacement is worth weighing against one more repair.
Safe Steps While You Wait for a Repair
Once the visit is booked, a few simple moves keep the house livable and keep a small fault from growing.
How do I thaw a frozen coil while I wait?
Switch cooling off and set the fan to On. Moving air melts the ice faster, usually within a few hours, and towels around the indoor unit catch the extra water. Don’t chip or hose the ice off: the coil’s thin fins and tubing bend and puncture easily.
How do I keep the house comfortable in the meantime?
In summer, close the blinds on the sunny side, run ceiling fans and skip the oven and dryer in the afternoon. In a cold snap, close off rooms you aren’t using and keep space heaters on the floor, three feet from anything that can burn. Never heat the house with the oven or a grill.
What if water is dripping from the ceiling under the attic unit?
Switch the system off at the thermostat so it stops making water, catch the drip and move furniture and electronics out of the way. A soaked ceiling is heavy, so keep people out from under it. The drain line or the drain pan needs attention before the cooling runs again.
Can’t Find Your Symptom?
Systems don’t always fail by the book. When you book, describe what you see, hear or smell in your own words, even if it sounds odd. The diagnosis on the visit sorts out the rest.