Carrier Model 396 Gas Furnace - Fan cycles at end of cooldown cycle.

Reply

  #1  
Old 12-29-16, 10:35 PM
Member
Thread Starter
Join Date: Dec 2016
Location: USA
Posts: 2
Received 0 Votes on 0 Posts
Carrier Model 396 Gas Furnace - Fan cycles at end of cooldown cycle.

My gas furnace works fine except at the end of the cycle (cycle length seems normal, fan stops when air seems fairly cool). When the fan shuts off, within a couple seconds it comes back on for a couple seconds, then repeats this 2 to 3 times (mostly 3 small cycles). Any ideas?

Carrier/Night&Day Model 396, from 1990, Natural Gas.

I tried some trouble shooting, and noticed that the red and black wires for "fan speed" were reversed on the main board. I had a handyman fiddle with another issue about 10 years ago, perhaps he swapped them. But it seems what I did was change from the heater fan speed being on "high setting 1", the setting normally reserved for cooling, back to the medium low (setting 3) which seems to be the default for heating. The problem didn't change. Sometimes before i noticed the motor seemed to surge pretty hard with the quick turn-on during the cycling, but now that i changed the fan setting it was more gentle, but still repeated the 3 short cycles. I can live with this problem, but i'm concerned it may burn out the motor prematurely with the extra cycles.

Thanks,
Paul
 
Sponsored Links
  #2  
Old 12-30-16, 05:48 AM
Member
Join Date: Oct 2015
Location: Ct.,USA
Posts: 1,174
Received 28 Votes on 24 Posts
All gas furnace blower motor controls have a cool down feature by design to allow the blower motor to continue running for an adjustable period after the thermostat is satisfied because the residual heat in the furnace is hot enough to still warm the area that was calling for heat. If the cool down feature is a mechanical device with turn on/ turn off setpoints, it opens a contact removing power to the blower motor at the turn off setpoint. However, if the residual heat is enough to raise the heat in the mechanical device's blower turn on setpoint, the blower mower starts and runs until the turn off setpoint is reached. This cycling continues until the residual heat no longer affects the turn on setpoint. The solution is to widen the differential between the turn on and turn off setpoints. The setpoint adjustments are included in the operation literature for the mechanical device.
Newer furnaces have an electronic timer that is adjustable for the cool down period and do not have the cycling problem the mechanical devices have. Hope this helps.
 
  #3  
Old 12-30-16, 12:38 PM
Member
Thread Starter
Join Date: Dec 2016
Location: USA
Posts: 2
Received 0 Votes on 0 Posts
I'm thinking it may be something else. I looked over all the paperwork i have, and it states that the heat relay stops the fan approx 105 seconds after flame is off, and no hint on any adjustability. Also i tried running with the filter removed (assuming a much higher flow rate), and got the same 3 extra cycles. Also i just fixed the fan speed wires which were switched and had the fan on hi which is for "cool" setting (i dont' have cooling). So actually that should have made it worse, but no change. I suspect the relay hangs at shut-off, is that possible? Another symptom...in the past when i opened the cover, i hear a click and the fan shuts off, and when i put the cover back on i hear another click. Now when i put the cover on, i hear a rattling for a moment, like the relay is clicking a bit....wonder if the relay is going bad? So if i can figure out how to make the fan cycle longer i'll do it, but suspect it's not going to solve it, i don't think it's a latent heat issue?

Obviously a relay problem is not a quick/easy fix so want to try other things first. So key question, on my unit, how do i extend fan cycle, and/or how do i change the fan-on temp setting? My manual doesn't give hint to either option. Carrier/Night&Day Model 396 JAW048065ABFA or JAW048065 Series B 80,000 BTU/hr nat gas. Approx 1990.

Or should i just ignore this and assume 3 short cycles at end of normal cycle won't hurt the fan motor? I don't like this option too much.
 

Last edited by nofear4by4; 12-30-16 at 12:53 PM.
  #4  
Old 12-30-16, 06:06 PM
Member
Join Date: Oct 2015
Location: Ct.,USA
Posts: 1,174
Received 28 Votes on 24 Posts
Since the relay is controlling the blower, itís coil must be getting voltage from the thermostat or the 105 second timer each time it cycles. Can you determine the source of voltage when cycling by using a voltmeter or disconnecting the sources (one at a time) and operate the furnace. You should have a schematic of the wiring and understand what you are doing as there is a potential for electrical shock if one is not careful.
 
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
 
Ask a Question
Question Title:
Description:
Your question will be posted in: