Your furnace is meant to turn on and off a couple of times per hour. When the room reaches the temperature on the thermostat, the furnace should cycle off. Once the temperature drops a degree or two, the furnace should cycle back on. When your furnace is switching on and off every couple of minutes instead of 2 or 3 times per hour, this is generally a sign that something is wrong. Here's a look at some common causes of frequent cycling and what you can do about them.
Clogged air filter.
This is a very common cause of frequent cycling, and luckily, it is easy to fix. When your furnace air filter becomes too clogged with debris, air cannot pass through the unit properly. The furnace falsely senses that the room is warm enough and turns the unit off. Then, it registers that your home is too cold, which causes it to switch on again.
The first thing you should do whenever your furnace starts cycling too frequently is change the filter. Pull it out of the slot between the furnace and the return air duct. Take it to the hardware store to ensure you buy the same size, and slide the new filter into place. Then, remember to change your filter every month, going forward. If your furnace still cycles too frequently, then consider the other possibilities below.
Poorly located thermostat.
Thermostats should not be placed near windows or doors. If your thermostat is near a drafty window, a cold air might blow in, causing the thermostat to read a lower temperature and switch your thermostat "on." Once the draft stops, the real temperature of the room is registered again, and the furnace switches off. This cycle can repeat itself endlessly, especially on a windy day.
If your thermostat is near a window, door, or other drafty vent, contact your HVAC technician and ask them to move it to a better location. This is not a project you should attempt yourself; doing so involves working with electrical wiring, which requires a permit or license in most jurisdictions.
The furnace is too large.
If your filter is clean and your thermostat is properly placed, another possibility is that your furnace is too large. A too-large furnace will often heat up certain portions of the home very quickly, while others stay too cool. As soon as the furnace switches off, the heat dissipates and the temperature evens out. The overall temperature of the home drops, triggering the heater to kick back on -- and then it unevenly heats the home again. This cycle repeats itself. The frequent cycling is not good for the furnace; it puts additional wear and tear on it.
Some inexperienced HVAC technicians may accidentally install furnaces that are too large. Another possibility is that the last homeowner made the false assumption that a larger furnace is always better and demanded that the HVAC contractor install too large a furnace against better judgement. If you suspect your furnace is too large for your home, the best way to handle the issue is to call an HVAC technician other than the one that installed the furnace. Ask them to take a look at your system and offer their opinion as to whether or not the furnace is too large.
If the furnace is new and under warranty, you may be able to exchange it for a smaller model at a discount. If it is older, your options are to pay a bit more to replace it prematurely or live with the short cycling for a few more years. Replacing the too-large furnace immediately may be a bit costly up front, but it will save you money on your energy bills since operating an over-sized furnace is very inefficient.
If none of these issues are to blame for your furnace's short cycling, the other possible cause is a corroded flame sensor. This is an issue your HVAC technician can check for when he or she comes out to look at the size of your unit or placement of your thermostat. A flame sensor should be able to be replaced quite affordably. For more information, check out websites like http://www.customcomfortinc.com/.