Furnace Not Turning On? 8 Things to Check Before You Call for Repair

The first cold night of the season has a way of finding out which furnaces skipped their check-up. In Corona, Menifee and Rancho Cucamonga, a lot of heaters sit unused from spring until November, so it's common to flip the thermostat to "Heat" and get nothing but silence, or a fan blowing cool air.

The good news is that some "broken" furnaces are really a simple setting, a tripped switch, or a clogged filter. Here are eight safe things you can check yourself before you schedule a service call, plus the signs that mean it's time to stop and call a professional.

Safety first: If you smell gas (a rotten-egg smell), don't touch any switches. Leave the house and call your gas utility or 911 from outside. Everything below assumes there's no gas smell.

1. Check the thermostat settings

It sounds obvious, but this one catches a lot of people, especially after a summer of running the AC.

• Make sure the system is set to Heat, not Cool or Off.
• Set the temperature a few degrees above the current room temperature.
• If the fan is set to On instead of Auto, you may feel air blowing that isn't warm yet. Switch it to Auto.
• On a smart or programmable thermostat, look for a schedule or "away" mode that's overriding your setting.

2. Replace the thermostat batteries

Many thermostats run on AA or AAA batteries. A blank or dim screen, or a low-battery icon, can mean the thermostat can't tell the furnace to start. Fresh batteries are an easy fix worth trying first. If your thermostat still acts up afterward, it may need repair or replacement.

3. Look at the circuit breaker

A gas furnace still needs electricity to run its blower, igniter and controls. Find the breaker labeled "furnace," "FAU" or "HVAC" in your electrical panel. If it's tripped, switch it fully off and then back on. If it trips again, leave it off and call a technician, because a breaker that keeps tripping points to an electrical problem.

4. Check the furnace power switch

Most furnaces have a switch nearby that looks like a regular light switch, often on the wall next to the unit or on the side of it in the garage, closet or attic. It's easy to bump off by accident, especially if the furnace shares a closet with storage. Make sure it's on.

5. Replace a dirty air filter

A clogged filter is one of the most common reasons a furnace shuts itself down. When airflow is restricted, the furnace can overheat, and a built-in safety switch will turn the burners off to protect the system. You might notice the furnace starts, runs briefly and then stops.

Pull the filter out and hold it up to a light. If you can't see light through it, replace it with the same size (it's printed on the frame). This is also a good habit to keep up through the winter, especially if you have pets or live near dusty construction.

6. Make sure the furnace door is closed tightly

Furnaces have a safety switch behind the front panel that keeps the system from running when the door is open or loose. If the panel was removed to change a filter and not snapped back on firmly, the furnace won't start. Reseat the panel so it's flush and secure.

7. Check that the gas supply is on

If your other gas appliances, like the stove or water heater, are working, your gas service is fine. Next, look at the gas valve on the line going into the furnace. The handle should be lined up with the pipe (parallel), which means it's open. If it's crosswise, it's closed. If you aren't comfortable with gas lines, skip this step and call a pro.

8. Look for a blocked vent or condensate issue

On high-efficiency furnaces, white PVC pipes carry exhaust outside and drain water away. If an outside vent pipe is blocked by leaves, a bird's nest or debris, or if the condensate drain line is clogged, the furnace may shut down as a safety measure. Clear anything you can see outside the house, but don't try to take apart pipes or the drain trap yourself.

When to stop and call a professional

Some problems aren't do-it-yourself territory. Call a licensed HVAC technician if:

• You smell gas or notice a burning or electrical smell.
• The breaker trips again after you reset it.
• You hear loud banging, screeching or rattling when the furnace tries to start.
• The furnace clicks over and over but never lights.
• It starts but shuts off after a few minutes, even with a clean filter.
• Your carbon monoxide detector goes off. Get everyone outside first, then call for help.
• You've worked through the checklist and it still won't turn on.

At that point the problem is often a part like the igniter, flame sensor, pressure switch, control board or blower motor. A technician can test these safely and tell you whether it's a straightforward repair.

Repair or replace?

If your furnace is older and has needed several repairs lately, it's fair to ask whether fixing it again makes sense. A good technician will walk you through the condition of your system, what the repair involves and what replacement would look like, and let you decide. If a new system is the better choice, Icee Hot offers financing through Service Finance (subject to credit approval) to help spread out the cost.

How to keep it from happening next winter

Most no-heat calls in our area come right when the weather turns, after months of the furnace sitting idle. A fall furnace tune-up lets a technician clean and test the burners, igniter, flame sensor, safety switches and airflow before you need the heat. Pair that with regular filter changes and you'll lower the odds of a cold surprise.

Furnace FAQ

Why is my furnace blowing cold air?
Check that the thermostat fan is set to Auto rather than On. If it is and the air still isn't warm, the burners may not be lighting, which can come from a dirty filter, a dirty flame sensor or an ignition problem.

How often should I change my furnace filter?
Follow the filter manufacturer's recommendation, and check it monthly during heavy use. Homes with pets, allergies or lots of dust usually need more frequent changes.

Is it safe to reset my furnace myself?
Resetting the breaker or power switch once is fine. If the furnace keeps shutting off or the breaker keeps tripping, stop resetting it and call a technician, because repeated resets can hide a real problem.

Do I need a furnace tune-up if I live in Southern California?
Yes. Even with mild winters, furnaces sit unused for most of the year, and dust, worn parts and loose connections build up. A yearly tune-up before the cold weather helps the system start reliably and run safely.

Need help with a furnace that won't turn on?

If you've tried these steps and your home is still cold, the family-owned team at Icee Hot Air Conditioning and Heating can help. We handle furnace repair, tune-ups and replacement for homeowners in Corona, Menifee, Rancho Cucamonga and nearby communities across Riverside, San Bernardino and Orange counties. Call 951-427-9117 or request service online.