what's going on here? idling fan?

Reply

  #1  
Old 06-12-12, 06:31 AM
Member
Thread Starter
Join Date: Jun 2012
Posts: 21
what's going on here? idling fan?

My 3-year old indoor Lennox a/c system works great. But in the last few days of high temps, it's started doing something I don't recognize. In between cooling cycles, when the compressor is off, there is a faint rushing/humming sound coming out of the air handler. Sounds as if the fan is running slowly. Sort of idling. Constantly. There is a very small amount of air (ambient temperature) coming out of the vents, so that's probably indeed what's happening. Why is this? I hear this noise long after the compressor has cycled off. I guess I expect that when a cooling cycle is complete, and the heat exchanger has warmed back up, the fan would eventually shut off entirely.
 
Sponsored Links
  #2  
Old 06-12-12, 09:23 AM
Member
Join Date: Aug 2008
Location: Texas, California
Posts: 1,443
Can you remove one panel and check to see if that fan is running ? slowly ? As I understand, the blower should end either right away or end within 3 or 4 minutes.
 
  #3  
Old 06-12-12, 09:54 AM
Member
Thread Starter
Join Date: Jun 2012
Posts: 21
OK, thanks. I have a better handle on it now. When I look inside at the main fan (the fist-sized motor) is NOT running. There is something else that sounds like a fan in the box below it. That boxs looks like it has something to do with the gas heating, but I don't know how to get into it.

Now, significantly, it looks like SOMEONE set the "FAN" control on the Breaburn thermostat to "ON" instead of "AUTO". When I set it back to "AUTO" that fan noise stops. So this may just be a user error, but I'm still a little puzzled about what the noise is, if it's not the main fan. Is there another fan in there?
 
  #4  
Old 06-12-12, 10:08 AM
Forum Topic Moderator
Join Date: Oct 2002
Location: Hamilton County, Ohio
Posts: 4,296
Upvotes Received: 1
It was the " main fan" that was running. What you saw with a fist sized motor was the inducer fan. This only runs when the furnace calls for heat.

In the box below is the main blower fan with a motor closer to cantalope size. It may be a multiple stage blower and runs at a low speed when the thermostat is in the on position until the thermostat calls for for heat or cool and then kicks up to full speed. In the auto position it will not run until the thermostat does it's heat/cool call.
 
  #5  
Old 06-12-12, 11:23 AM
Member
Thread Starter
Join Date: Jun 2012
Posts: 21
Ah, many thanks! It's a bit funny that the thermostat "fan" control applies to this inducer fan instead of the main blower, but ...

What exactly does this inducer fan do? Does it have to do with getting air to the combustion chamber, rather than circulating air in the house? I did notice that when the smaller fan was running, there was a small amount of output from the vents. So that fan is, in fact, blowing through the ducts.
 
  #6  
Old 06-12-12, 11:44 AM
Gunguy45's Avatar
Super Moderator
Join Date: Dec 2007
Location: USA
Posts: 21,105
No HVAC Pro....but this is the way I understand things....

Normally if a stat fan control is set to "ON" the main fan (air handler) comes on at full speed. Now as he said...if this is a fancy newer multistage furnace...the air handler may run at a slower speed for air movement and dehumidification.

The air handler will normally have a large squirrel cage fan that moves a lot of air. Just like when the A/C kicks on. The inducer is a very small motor and fan that is only powered up when there is a call for heat (and I guess some time afterward to evacuate remaining fumes). When the stat calls for heat...the first thing is the inducer kicks on to assure good combustion air flow.....once that is established..the gas turns on and the burners ignite. When the control circuits sense that the burner is up to temp...the air handler fan kicks on. After the stat is satisfied....the burners shut off...a bit later the air handler will shut off when the heat exchanger has reached the right temp. I "believe" the inducer stays on for a bit after the burners shut off...but I don't know how thats controlled.

The inducer fan should have no effect on airflow through the vents....unless something (ducting) isn't connected correctly. The inducer vents outside.

You can see what is actually making the noise by turning the stat to heat then bumping it up to 80 degrees or so. If the noise you hear starts and the the burner ignites...it's the inducer and something doesn't seem right. It shouldn't do anything in "cool" mode.
 
  #7  
Old 06-12-12, 02:03 PM
Member
Thread Starter
Join Date: Jun 2012
Posts: 21
OK, I have a Lennox G60UHV, which looks like this

http://www.atchvac.com/files/1512/84..._4cut_0501.jpg

The "combustion air inducer" motor is the one at the top. That one isn't moving. When the FAN control is set to AUTO, I hear the noise from somewhere below that, like in the middle. It's probably coming from the blower squirrel cage fan, which is at the bottom.

Looks like what's happening is that if the furnace or a/c compressor aren't on, and the FAN control is set to "ON'. it just runs the fan at a low speed to provide miniscule circulation. When the compressor turns on, the fan automatically kicks into high gear. So what "ON" means is that the blower is always going to be on, but just ramped way down when there is no heat to pump. What "AUTO" means is that the blower only comes on, in high gear, when there is heat to pump.
 
  #8  
Old 06-12-12, 02:08 PM
Member
Thread Starter
Join Date: Jun 2012
Posts: 21
Sorry, I meant to say "When the fan control is set to "ON" I hear the noise from somewhere below that, like in the middle."
 
  #9  
Old 06-12-12, 02:27 PM
Gunguy45's Avatar
Super Moderator
Join Date: Dec 2007
Location: USA
Posts: 21,105
Possibly the slow movement helps to circulate air and equalize temps...but it's not like any system I ever saw...granted..there weren't that many. Fan ON always kicked it to high and ran constantly.

Does the outside unit do anything during these times?
 
  #10  
Old 06-12-12, 03:38 PM
hvactechfw's Avatar
Member
Join Date: Jan 2011
Posts: 6,245
the V in your model number indicates you have a variable speed blower which when there is a call for fan only it will run at slow speed.
 
  #11  
Old 06-12-12, 04:10 PM
Gunguy45's Avatar
Super Moderator
Join Date: Dec 2007
Location: USA
Posts: 21,105
Thanks Kevin....that explains a lot.

One question....WHY? Just for temp equalization?
 
  #12  
Old 06-12-12, 04:16 PM
hvactechfw's Avatar
Member
Join Date: Jan 2011
Posts: 6,245
basically, also it is for noise reduction. Most homeowners want their system quiet and EFFICIENT to just turn the air over in the home.
 
  #13  
Old 06-12-12, 04:40 PM
Member
Thread Starter
Join Date: Jun 2012
Posts: 21
*That* is handy to know, because that's exactly what's happening. I guess the point is to provide some circulation, without strong breezes that you'd get from the regular speed. I was never told that by the salesman/installer.

Now that I look it up, we're talking "two-stage variable-speed". I gather the "two-stage" means two heat levels.
 
  #14  
Old 06-12-12, 04:41 PM
hvactechfw's Avatar
Member
Join Date: Jan 2011
Posts: 6,245
yes, it is a 2 stage furnace
 
  #15  
Old 06-12-12, 04:43 PM
Member
Thread Starter
Join Date: Jun 2012
Posts: 21
Yes, this is supposed to be a very quiet system. Now that I look into it, it looks like the power level on the blower fan ramps up and down gradually. That cuts down on the "whump" you get from most systems when they start up.
 
Reply

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread
Display Modes