By the WY&G, Inc. team: licensed HVAC contractor, CSLB C-20 #1044922, over 25 years and thousands of customers in the Santa Clarita Valley and Orange County.
Before the first cold night
- Put in a fresh air filter. The one that ran all summer is full.
- Switch the thermostat to Heat and set it 3 to 5 degrees above room temperature. If the screen is blank, replace the batteries.
- Make sure the furnace door or panel is closed tight. Many furnaces have a safety switch that will not let them run with the door off.
- Check that the furnace switch (it looks like a light switch, near the unit) is on, and the breaker is not tripped.
- Test your carbon monoxide alarms. Every home with a gas furnace should have working CO alarms near the bedrooms.
Do this on a mild afternoon, not at 10 PM on the coldest night. If something is wrong, you will have time to schedule a visit.
Normal on the first start
- A dusty or slightly burning smell for the first 10 to 20 minutes.
- A short delay before warm air comes out. Most furnaces run the blower only after the heat exchanger warms up.
- A click or two, then a soft whoosh as the burners light.
Not normal: stop and call
- Smell of gas or rotten eggs. Do not touch switches. Leave the house and call SoCalGas or 911 from outside.
- CO alarm goes off. Get everyone outside and call 911. Do not run the furnace until it is inspected.
- Burning smell that lasts more than an hour, or an electrical or plastic smell. Turn the system off.
- Clicks, lights for a few seconds, then shuts off, over and over. Very often a dirty flame sensor. See flame sensor and ignition repair.
- Blows cold air the whole time. The burners are not lighting or keep shutting off. See furnace blowing cold air.
- Loud bangs on start up, squealing or grinding. Delayed ignition or a blower motor problem.
Why a furnace that worked last winter fails now
A furnace sits unused for six months. Flame sensors collect a light film, igniters get brittle, and on high efficiency furnaces the condensate drain can dry out or clog, which trips a pressure switch. Many systems also share the drain with the AC, so a drain that clogged over the summer can stop the heat too. These are usually short repairs. A cracked heat exchanger is the serious one, and that is why a yearly heating tune-up includes a careful look at it.
Heat pumps are a little different
If you have a heat pump, the outdoor unit runs in winter too. It is normal for it to steam and go quiet for a few minutes while it defrosts. It is not normal for it to blow cold air for long periods or for thick ice to stay on the outdoor coil. See heat pump blowing cold air.
