Without a regular maintenance plan, furnaces can experience all kinds of problems. When your furnace fails, the most common behavior is for it to blow with no heat. This your furnaces way of telling you it's a in a "lockout mode".
When the furnace tries to start up but fails to, it pretty much gives up and locks out. The result is your furnace blowing air cool air and leaving you with a cold home.
So the real question is, why is it going into lockout?
Lockouts are for your own safety. It's a way to tell you that "Hey, something not right with me, I'm going to turn myself off so I don't hurt anybody".
The two most common reasons are that your furnace is getting too hot or that it's not able to vent the smoke it produces adequately.
The furnace's job is to produce heat and then push that heat throughout your home. However, if produces too much heat or isn't able to circulate that heat throughout your house, then a "high limit switch" will detect the excessively high temperatures, and yes, shut it down into a lockout mode.
Reasons why it's overheating include:
- Too much gas traveling through the gas valve
- Clogged filter restricting airflow, therefore, allowing heat to build up within the furnace
- Clogged air conditioner just above the furnace which will also restrict airflow and allow heat to build up within the furnace
- A weak or intermittent working blower motor. No air movement means all the heat stays in the furnace
- A cracked heat exchanger (Yikes!)
There are a few more but those are the most common reasons.
Venting Is Restricted
Where there's smoke, there's fire; and your furnace is no exception. Your furnace produces a smoke, or the technical term "flue gases" that need to be vented out.
This is an essential step, as improper venting can be deadly. That's why if the furnace is unable to vent the flue gasses properly, it will go into a lockout.
The way your furnace determines if the venting is adequate is by using an inducer motor (pulls air) to pull the vent gasses out and using a "pressure switch" to ensure the pressure the inducer motor is producing is adequate.
It's the failure to activate the pressure switch that triggers the lockout.
Failure to close the pressure switch include:
- Failed pressure switch
- Clogged venting
- Weak or completely failed ventor motor
- Water in the ventor motor due to a clog in the drainage system
- Clogged/cracked primary or secondary heat exchanger
Independent Part Failures
Sometimes it's not a matter of safety that triggers a lock but rather just but a part simply failing. These independent failures will prevent the furnace from coming on:
- Failed or dirty flame sensor
- Failed HSI
- Failed gas valve
- Failed control board
Ultimately when a furnace goes into lockout and just blows air, it means something is wrong. The possible list is extensive, and even though we've listed the majority causes, there are still more.
The goal of this article was to help you understand the specific reasons for lockout and allow you to speak more specifically with the technician once he finally determines what the problem is.
Best of luck with your furnace diagnosis and remember, if you're looking for professional furnace repair, call AccuServ!