When heating or cooling quits, the first question is not what broke but whether anyone is in danger. A gas smell, a carbon monoxide alarm, smoke or a hot electrical smell are safety problems: you act first and leave troubleshooting to others. A furnace that won’t start or an AC blowing warm air is a comfort problem, and a few safe checks often find the cause, or at least tell the technician where to start.
Safety comes first
You smell gas, a rotten-egg or sulfur smell. Leave the house now and take everyone with you. Do not switch lights, the thermostat or appliances on or off, do not use a phone inside, and do not look for the leak. From outside, well away from the house, call 911 or PG&E’s gas emergency line. Once the gas side has been made safe, the furnace or other heating equipment should be checked before it runs again; our 24/7 line takes gas-odor calls.
The carbon monoxide alarm sounds. Get everyone, pets included, outside into fresh air and call 911 from there. If anyone has a headache, dizziness, nausea or unusual drowsiness, say so on the call. Do not silence it and stay; do not go back in until responders say it is clear. The furnace, water heater and any other fuel-burning appliance should be checked before they are used again.
Smoke, or a hot electrical or plastic smell. If you can do it safely, turn the system off at the thermostat and at the breaker, and leave it off. Visible smoke or flames: get out and call 911. A faint dusty smell for the first few minutes of the season’s first heating cycle is usually dust burning off the furnace; a smell that stays or grows is not.
Water through a ceiling under an attic unit: turn the system off, keep people away from wet light fixtures and catch what you can.
Five safe checks before you call
A few minutes, no tools, no opened panels.
- Thermostat. Is it set to Heat or Cool, with the setpoint past the room temperature? A blank screen often means dead batteries. A schedule or vacation hold can override what you just set. With a heat pump, frost on the outdoor coil and a burst of steam during a defrost cycle are normal.
- Breakers and switches. Most central systems have one breaker for the indoor furnace or air handler and one for the outdoor unit. A tripped breaker often rests in the middle; push it fully off, then on, and reset it only once. Also check the service switch near the furnace. It looks like a light switch and is easy to flip by mistake in a hall closet or garage.
- Filter. A clogged filter can make a furnace overheat and shut itself off, or let an AC coil freeze. If you see ice on the indoor unit or the large copper line, set cooling to Off and the fan to On for a few hours to thaw it.
- Condensate switch. Air conditioners and high-efficiency furnaces make water. A float switch in the drain pan or line stops the system when the drain backs up. If the pan holds water and you can reach it safely, turn the system off and empty it with a wet-dry vacuum or towels. If it fills again, the drain is clogged further down.
- Outdoor unit. Clear leaves and stored things crowding it. Leave its panels and the disconnect box shut.
What not to do
- Don’t reset a breaker more than once, tape it on or swap in a bigger one.
- Don’t keep power-cycling a furnace. After repeated failed ignition tries, many furnaces lock out for a reason.
- Don’t bypass a float switch, limit switch or door switch to get the system running.
- Don’t run an AC that has ice on it, or one whose outdoor unit hums while the fan stands still.
- Don’t add refrigerant from a store-bought can.
- Don’t heat the house with a gas oven, the stovetop, a grill or a camp heater, and never run a generator indoors or in the garage.
Getting through the wait
No cooling in a hot spell. Close blinds on sun-facing windows early, before the rooms heat up. Run fans, drink water, skip the oven and spend the afternoon in the coolest room, usually the lowest and shadiest one. Open windows once it is cooler outside than in, often after sunset; cool nights in Portola Valley can bring a house down quickly. If the heat is affecting someone older, very young or unwell, go somewhere air-conditioned.
Smoke days. Keep windows shut even when it is warm, and if you have a portable air cleaner, run it in the room where people sleep.
No heat on a cold night. Layer clothing and blankets, close off unused rooms and gather in one. Plug space heaters straight into a wall outlet, keep them three feet from anything that burns, and never leave them running unattended or while you sleep.
When to call the 24/7 line
| Situation | Who to call | Meanwhile |
|---|---|---|
| Gas smell | 911 or PG&E from outside, then our 24/7 line | Stay out until cleared |
| CO alarm | 911 from outside, then an equipment check | Stay out until cleared |
| No heat on a cold night, with children, older or ill people at home | Our 24/7 line | One warm room, space heater used safely |
| No cooling in a hot spell, with someone vulnerable at home | Our 24/7 line | Shade, fans, the coolest room |
| Breaker trips again after one reset | Our 24/7 line | Leave it off |
| Water through a ceiling | Our 24/7 line | System off, catch the water |
| Runs, but noisy, weak or uneven | Office hours | Fresh filter, note when it happens |
| One room uncomfortable, the rest fine | Office hours | Check that its registers are open and clear |
Office hours are Monday to Friday 7 AM to 7 PM, Saturday 8 AM to 5 PM and Sunday 9 AM to 3 PM. Same-day service is available, and the day’s schedule decides how soon a technician can come. For how after-hours visits work, see emergency heating repair and emergency AC repair.
What to have ready
- What happened, in order: sounds, smells, when it stopped and what you already checked.
- Any blinking light on the furnace control board, often visible through a small window in the furnace door. Count the flashes.
- A photo of the label with model and serial numbers on the indoor and outdoor units, and any warranty papers.
- Clear access to the furnace closet, attic hatch or side yard, with pets shut away.
Every visit starts with a diagnosis. Before a larger repair you get the findings, parts, labor and options in writing, so a late-night decision does not have to be rushed.
Making the next one less likely
Many emergencies start as parts that weaken for months: a capacitor, an igniter, a slow drain. A check before the heating season and another before the first hot spell catch many of them; a maintenance plan puts both on the calendar.