Why Does My Furnace Keep Turning On and Off?

February 25, 2014

Got a fickle furnace that frequently turns on and off? It’s an annoying problem during cold Northern California winters.

We call this problem “short cycling”, and you should take it seriously because it can:

• Cause a premature furnace breakdown: Constantly turning on and off puts extra strain on the furnace. This leads to a premature breakdown with expensive repair costs.
• Increase your energy bills: Imagine turning your car on and off for every mile you drive. That’s a wasteful way to drive. Similarly, your furnace needs time to run to work efficiently.

So, what’s causing this issue?

Short cycling: 3 common causes

There are 3 common reasons why a furnace keeps turning on and off:

1) Dirty air filter

This is most common cause of short cycling. A dirty air filter restricts air from getting into your furnace. That’s no good. Your furnace’s heat exchanger needs that air blowing over it or it overheats, causing the furnace to shut down.

Change (or wash, if you have a washable type) your air filter if it’s dirty. You should change or wash it at least every 30 days. Use our guide to buying air filters if you’re out.

2) Thermostat issues

There are 2 thermostat issues that can cause short cycling:

• Malfunctioning: If the thermostat isn’t working properly, it can cause short cycling. Have someone repair or replace it.
• Improper placement: If the thermostat is placed near a heat source (direct sunlight, heat register, etc) the thermostat sensor thinks you home is too warm and turns off the furnace.

3) Furnace is too big for your home

A furnace that’s too large for your home heats your home too quickly and shuts off. When your home cools off, the process repeats—furnace quickly heats the home, home cools off. Ad nauseum until the furnace breaks down.

This is the worst-case scenario since there’s not much you can do but get a new, properly sized furnace. You’ll need a contractor to come out to your home and perform a heat load calculation to see if your furnace is oversized or not.

But start with checking your air filter first. That’s the easiest and cheapest solution you can do yourself.

Contact us if you need any help and are in our service area.

Service Champions, your local heating and air conditioning company, serves San Jose, Sacramento, East Bay, and the surrounding areas. For more information on any of our products or services, contact us online.