“Why Won’t My Furnace Turn On?” 5 Things to Check

January 14, 2015

Dead.

That’s how you’d describe your furnace. It just won’t turn on and you’re freezing your butt off. Before calling a furnace repair person, check these 5 things first. 

1. Check your thermostat

Is your thermostat set on “heat”?

Are you sure? Children (or ghosts) may have switched it to “off” or “cool” by accident. Check just to make sure.

If that didn’t do it, turn the temperature setting 5 degrees higher than your room temperature.

Still not working? Then...

2. Check the furnace’s power switch

Your furnace has a power switch (aptly named “furnace switch”) that looks JUST like a light switch.  Someone may have switched it to “off” by accident.

Look on your furnace or on a wall near the furnace for this switch:

Image source: Jenn Durfey, flickr.com

“Up” usually means “On” and “down” usually means “off.”

If it’s down, flip it up and see if your furnace now has power.

3. Check the furnace’s circuit breaker

Darn, the above didn’t work?

Your furnace may not be getting any juice because the furnace’s circuit breaker tripped. (Yes, even a gas furnace uses electricity and has a circuit breaker).

Go to the breaker box and find a circuit breaker labeled for the furnace (look on the circuit box door for labels if nothing is labeled next to the switches.) 

If the furnace’s circuit breaker switch is on “Off,” switch it to “On.”

If the furnace’s circuit breaker switch is in the neutral position (middle), switch it to “Off” then “On.” 

Power should return to the furnace.

Note: If your breaker trips again after you reset it, do NOT keep resetting it. You have an electrical problem that needs fixing. Call a professional heating technician or electrician to diagnose and fix the root of the problem. 

4. Check the air filter

Let’s say the furnace was working just fine then it suddenly shut off.

Your air filter may be filthy and needs changing.

A dirty filter can cause the furnace heat exchanger to overheat, forcing the furnace to shut itself down.

Change the furnace filter (and keep doing so every 1 to 3 months). Out of air filters? Here’s an air filter buyer’s guide.

If your filter isn’t too dirty, then..

5. Clean the drain lines

Fun fact: A high-efficiency furnace (90%+ AFUE) creates and drains out tons of condensation every day.

However, if mold or slime is clogging the condensate drain line, the water backs up into the furnace, causing it to shut down (thanks to the handy dandy overflow shutoff switch).

How to clean the condensate drain line:  

  1. Find the main drain. Look for a plastic PVC pipe near your furnace and follow it back to the furnace until you see a vertical pipe with an opening. That’s the main condensate drain.

  2. Use a wet-dry vacuum to suck out the sludge. Put the vacuum over the vertical opening to suck out the blockage. Use you hand to make sure the vacuum is on the pipe air-tight. 

  3. Pour in some bleach and hot water. If the vacuum didn’t work, pour in bleach into the main drain. Wait 10-15 minutes and then pour in hot (not boiling) water. This should get rid of the mold and sludge.


The above didn’t help? Call a professional

Sadly, if the above DIY tasks didn’t help then you’ll need a professional’s help to fix your furnace.

If you life in the Kansas City area, contact Santa Fe Air Conditioning and heating to get your furnace fixed ASAP.


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