Lennox 80MGF 3/4-100A Limit Switch Trips - HVAC - DIY Chatroom Home Improvement Forum

Go Back   DIY Chatroom Home Improvement Forum > Home Improvement > HVAC


Thread Tools Display Modes
Old 01-08-2013, 09:58 PM   #1
Join Date: Jan 2013
Posts: 1
Rewards Points: 10

Lennox 80MGF 3/4-100A limit switch trips

This unit trips the limit switch since the day it was installed (years ago). After an extended run, like after an 8 hrs setting at a 55 deg. F then trying to bring house up to 72 deg. F. It does take longer to trip the cooler the house is when starting re-heat.
Today I removed all filters and register covers, inspected the A/C unit below the furnace (vertical down-draft config.), checked for any duct work obstructions as best I could to insure max. air flow. Then removed the limit switch, front and center , just right of the pressure switch. Leaving the switch hooked up and dangling, I inserted a temp. probe into the hole where switch was and temp. went as high as 225 deg. F. in just a few of minutes but unit did not trip. Haven’t checked the temp. that the switch opens and the info. I could find, it should open at 180 deg. F. Seems to be doing it’s job.
The unit has 1/4" clearance to the drywall on the left side, 1-1/2" to the rear and is open right and front. (+12") The hole in the slab under the unit is wider than the unit, I don’t know about the depth. The original 7-6" dia. ducts under slab that spider away have 4" x 10" register covers. Added was a 8" x 10" attic duct around 30' long with 2-6" x 14" covers. The main 14" return, with 20" x 20" filter, is just a few feet from unit and the added return looks like at least 12" dia. with 2-12" x 12" filters. One per room.
The first heating season after install I had install co. come back out and a second large reputable co. come out as well. Many techs.; no luck. I gave up. So, am I on the right track; too much unit/to little air flow or what? Need to add, fan is at max. speed.


Last edited by JBuff; 01-08-2013 at 10:14 PM. Reason: Duh!
JBuff is offline   Reply With Quote
Old 01-09-2013, 07:11 PM   #2
Join Date: Aug 2012
Location: Canada ontario
Posts: 117
Rewards Points: 75

wow lots of questions ..hmmm well is this unit a brand new install furnace a/c and ducting ? is there sufficent return air ? is the drop on the supply air side too short or wrong type and air dead heading against the block end ? pics may help out if you can .


hvac dude is offline   Reply With Quote

Thread Tools
Display Modes

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

Similar Threads
Thread Thread Starter Forum Replies Last Post
3 way switch layout has me confused crescere Electrical 5 09-08-2012 10:26 AM
lennox g26 door switch gardenman HVAC 4 03-01-2012 07:53 PM
can switch work for two different outlets? jlmran Electrical 8 02-10-2011 12:31 PM
Replace secondary limit switches on Lennox 80MGF? dawg89 HVAC 1 02-05-2010 06:31 PM
Basic Switch wiring question; Dimmer to Toggle switch (Pictures included) inspectator Electrical 8 08-07-2007 01:19 AM

Top of Page | View New Posts


Search Engine Friendly URLs by vBSEO 3.6.1