My Furnace Runs Every 5 Minutes. Why?

Is your furnace running too often? If the furnace in your home is turning off and on every five minutes then you have a problem with your furnace, generally termed short-cycling.

Short-cycling is when the furnace keeps turning off before the set heat or air conditioner temperature can be reached inside the home. This can be caused by something as simple as a misplaced thermostat or as dangerous as a blocked exhaust vent.

Below I am going to cover the most common reasons for a furnace to short-cycle and some basic steps you should take in order to correct the problem.

Misplaced Thermostat

Believe it or not, the location of your thermostat can play a big role in the overall performance of your HVAC system. A misplaced thermostat can cause inaccurate temperature readings which can then, in turn, cause your furnace to turn off before the air has actually reached the set temperature.

Bad Locations

1. Above or below a vent

If your thermostat is placed above a floor vent, or below one in the ceiling, then it is going to heat up faster than the air in the house which will lead to a furnace short-cycle.

2. Around windows

If it is mounted near a window that receives hours of sunlight every day it can cause the thermostat to have an inaccurately high-temperature reading which makes the furnace turn back off too soon.

3. Near Lamps

This only applies to situations where the lightbulb used is a traditional bulb and not an LED. Traditional bulbs put off large amounts of heat which can interfere with the thermostat. Make sure that there is not one around the thermostat.

4. Next to a furnace or other appliances

Sometimes a thermostat will be mounted on the outside of a wall that houses a furnace or a water heater. Appliances like these are designed to heat up and may interfere with the accuracy of the temperature reading on the thermostat.

Ideal Location

The ideal location for a thermostat to be installed, to both ensure peak accuracy in temperature readings, and to avoid a short-cycling furnace, is in a central and open area with plenty of airflow.

Blocked Exhaust Vent

Since furnaces use combustion to function you have to have an exhaust running outside to prevent any harmful gasses from filling up your home. If the exhaust vent becomes clogged then it will trap hot air inside the furnace – causing it to overheat and short-cycle.

This is usually caused by some type of critter or bird roosting inside the vent and obstructing the airflow. Due to the potential carbon monoxide risk, if this is the case, then the necessary steps to clear it out should be taken immediately.

The exhaust will either be located on the roof of your home or it may be exiting through a wall. If you aren’t comfortable cleaning out the vent yourself, then you should contact an HVAC technician.

Dirty Furnace Filter

Failure to replace your furnace’s filter can lead to a lack of air flowing into the furnace. Without enough air, the furnace will overheat, and the high-limit switch will do what it was made to do and shut the furnace off.

The rate at which a filter dirties will depend on a variety of factors including, but not limited to, local climate, local air quality, and whether or not there are pets in the home.

Although yours may need to be changed sooner, you should never allow the number of days between filter changes to exceed 90. I recommend you check your filter every month until you figure out the optimal times between changes.

The furnace is too Large

If the furnace installed in your home is rated for more square footage than the home has then you can expect the unit to short-cycle. This happens because the furnace heats the living area too fast, causing it to turn off and on far more than it should.

An HVAC setup with too large of a sized furnace not only wastes energy, but it causes noticeable temperature swings which can lead to discomfort in your home.

In order to determine if your home has an oversized furnace, you should contact a locally licensed HVAC technician. They can help you determine the right furnace size for your home and help you get it installed.

Bad Flame Sensor

The flame sensor is a precautionary piece of equipment in place to ensure raw gas does not leak into your home. It does this by ensuring a flame is lit when the furnace turns over. If the sensor is broken it could be sending false signals to the furnace – telling it to shut off when in actuality there is nothing wrong.


A furnace that turns on every five minutes is clearly indicative of an underlying issue. Some of the most common issues are a bad thermostat location, blocked exhaust, dirty filter, or an oversized furnace.

Many of these problems can be troubleshot and fixed DIY style, but if you don’t feel comfortable in your ability to fix the problem don’t hesitate to contact an HVAC technician so you can ensure the job is done right the first time.