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Old 01-08-2013, 09:58 PM   #1
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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!
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Old 01-09-2013, 07:11 PM   #2
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Lennox 80MGF 3/4-100A limit switch trips


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 .

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