Running time of furnace

My sister-in-law's furnace has a problem. On a recent visit, I noticed the furnace would come on, run for about 30 seconds, then shut off. It would only stay off for about 75 seconds before coming on, again for only 30 seconds. This is a constant thing, 30 seconds on, then 75 seconds off, etc,etc. She said it was checked, and she was told she needed to add a cold air return vent.That doent sound right to us. My hubby and I think its the thermostat,IE: wired wrong or just plain defective.We think she will end up needing a new furnace if it continues to run this way, Any thoughts as to what could be going on here? The furnace is somewhat new, the house was built in the 60's.

 

There are actually two things that come on and go off in a typical furnace: 1) fire; 2) fan/blower. When the thermostat tells the furnace it wants heat, the fire comes on ... and when the heat exchanger gets hot the fan/blower tries to cool it off by sending the heat into the house ... and the fan/blower will continue to do that for as long as the fire is burning and/or until the thermostat is satisfied and the heat exchanger is again cool. So then ...

You are hearing the fan/blower come on and go off as it cools the heat exchanger (by sending hot air into the ducts), and the fan/blower does that as controlled by the fire/fan "limit switches" ... and there is your problem. This could be a matter of simply setting the fan limits correctly, and it is extremely unlikely that there is anything wrong with your thermostat or that even the furnace needs replaced.

Do a web search for "fan limit settings" and you will likely end up here:

http://toad.net/~jsmeenen/blower.html

 

Hi, I had the EXACT same problem with a 2 year old furnace and all it was, was that the flame sensor needed cleaning. It didn't even look dirty, but after gentle cleaning. POOF, no more problem.

 

By adding a cold air return vent are you meaning it has none? Because if the furnace is pulling air in from the basement and then blowing it through the ducts, that may be just what she needs. The furnace can only warm the air so much and if the air coming in is too cold then the fan will cycle.

 

Sounds to me it is likely the flame sensor also (if your furnace has one). This will send back to the ignition control that there is flame present. If it's the slightest bit dirty, then it won't sence it properly. Best way to clean it is with an emery cloth.

If you send make and model of the furnace, it will be easier to determine possible causes of the problem.