Lennox gas furnace G24M4-75A-4

FacilitiesGuy

Hi,

  I've got an issue with a Lennon G24M4-75A-4 gas furnace.  It shuts down after about 5 mins of operation and flashes a "limit switch open" code.  The limit switch is the original model and has number starting with 175 on the front, (assuming that is temp in degrees F).  The furnace will heat the space, but it takes forever.   The furnace was installed in 1995. 

It sounds like an air flow issue to me so I did the following;

   I measured the air temp at the limit switch at 208 degrees Farenheit (taken with a probe inside the hole where the limit switch mounts)  

  I pulled the filter out, (its changed every month) and pulled the supply fan to check it out.  It is operating fine and checked the blower wheel-all good, set screw is tight and the bearings look and feel great.  I blew off any dust on the blades/housing.  Measured 2.8 Amps at yellow motor lead under full load and installed.

  I cut an opening into the plenum to check for an obstruction at the coil.  I found nothing obviously wrong, except for some minor dust.  I vacuumed and then cleaned the coils regardless.

  I measured duct flow pressure before and after the coils and the results were as follows: 1.20 in Wg (before) & 1.15in Wg (after coils). 

  Next I measured air flow around the building, there are 8 ducts all on one level.   The max 41 cfm and min was 33 cfm.  (all louvers open)  Adding these up, gives me a value of 302 CFM.  This seems unusually low.

  I then tested pressure at the farthest leg of the ductwork and found 1.00 in Wg pressure just before the ceiling grill.

  Can you help me get the fan curves for this unit &/or offer any advise.  I was on Lennox's website and only found installation guides.

 

lchmb

sounds like

your looking in the wrong direction. If the limit switch is opening that is on your combustion side. The reason you cannot heat the building is it dropping out the flame and the blower remains on moving cool/cold air. I would be amping out the inducer motor after a long run, cleaning the tube's for the pressure switch and checking for obstructions in your condensate lines to start....If all of those check ok, check your venting for obstructions (bee's nests) and look in the air intake...

JaredM

too little air

Your external static numbers are off the chart.   The furnace is not moving enough air and your temp rise shows it.    It sounds like you need more ductwork.



That furnace has a 4 ton blower, which means it will move 1600 cfm at  an external static of .5"  wc.