Troubleshooting Lennox Elite Furnace

Reply

  #1  
Old 05-22-19, 11:32 AM
Member
Thread Starter
Join Date: May 2019
Location: USA
Posts: 4
Troubleshooting Lennox Elite Furnace

Hello,

I have a Lennox EL180UH090E furnace / blower connected with Honeywell THM5421R eim and a THX9421 thermostat. While messing around to see what it would take to upgrade to an Ecobee, I accidently shorted the blue and red wire at the thermostat. it made a quick spark and wires quickly seperated.

Went to attic and all looks ok. the EIM doesnt seem to be showing any error codes. Lights are green. The control board on the furnace is showing the red "heartbeat" blinking light, which means no error code.

However, the blower seems to be barely running. It spins up when I turn on the fan through the thermostat but at very low speeds. If using in cooling mode, the outside unit turns on. However, due to low blower speed, there is barely any air coming out of vents.

Any thoughts on what could be the issue?

Thanks
 
Sponsored Links
  #2  
Old 05-22-19, 12:44 PM
Bob14525's Avatar
Member
Join Date: Jun 2013
Location: USA
Posts: 1,462
Likes Received: 8
The red wire is usually the 24VAC coming from the transformer, while the blue wire is usually the Common (ground) wire, which is the other side of the transformer output. Since you shorted the red to blue, you probably effectively shorted your 24V transformer. Often, there is a fuse in the transformer primary which blows if the output is shorted, to prevent burning out the transformer. Ideally, if you have a voltmeter, check the voltage between the red and blue wires. It should read approximately 24-27VAC. If the voltage is much lower than that, you either blew the transformer fuse (if you have a fuse), or the transformer itself.

What seems odd is that the fan is working at all if the 24VAC is missing or is very low. If you don't have a voltmeter and there is a Harbor Freight store nearby, you can pick up a meter there for under $10.
 
  #3  
Old 05-22-19, 01:02 PM
Member
Thread Starter
Join Date: May 2019
Location: USA
Posts: 4
Thanks for a quick response. I just measured the voltage acrsso the blue and red wire. Got 28 volts so I guess it must not be the transformer. any thoughts on what it could be? Perhaps the honeywell EIM THX5421R or thermostat?

Going to bypass the EIM and install an ecobee straight onto the control board to see if its the thermostat.

Thanks
 

Last edited by kashg334; 05-22-19 at 02:42 PM.
  #4  
Old 05-22-19, 01:30 PM
Bob14525's Avatar
Member
Join Date: Jun 2013
Location: USA
Posts: 1,462
Likes Received: 8
I'm also a little surprised also that the control board isn't showing an error. However, not knowing how the control board is powered (it may have it's own transformer or power supply), it's difficult to say why it doesn't show an error. I would suggest that you double check the voltage between the red & blue thermostat wires before buying a new transformer. If it is significantly lower than 24VAC, then try buying a new one.
 
  #5  
Old 05-22-19, 02:43 PM
Member
Thread Starter
Join Date: May 2019
Location: USA
Posts: 4
Hello, just checked the voltage and shows 28 volts. will bypass the EIM and thermostat by installing a new ecobee right onto the control board to see if that solves the issue. Any thoughts in meanwhile?

Thanks
 
  #6  
Old 05-22-19, 03:55 PM
Member
Thread Starter
Join Date: May 2019
Location: USA
Posts: 4
ok, so the ecobee is working well. However, the fan speed feels significantly lower compared to the downstairs unit. This wasnt the case prior.
 
  #7  
Old 05-22-19, 04:46 PM
Bob14525's Avatar
Member
Join Date: Jun 2013
Location: USA
Posts: 1,462
Likes Received: 8
Sorry, I don't have any idea why the fan speed would be significantly lower than it used to be. Hopefully, Pete or Houston will be by, as they are professionals, I'm just a homeowner.
 
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