Why do they put these here

[1stanner]

Hi Went to a house today and found a disc limit switch factory mounted on a venter motor. It had kicked out. I took a temp reading after I reset it and it only got about 112 degrees behind the disc. What is the real purpose of a limit on the ID fan. Also is it any indication at all of a cracked heat xchr. The unit ran perfectly after I reset it. The customer says he had to reset it about 3 times in the last 2 years, and like 4 times in the last week. Probably just a weak sw. But I don't know why they put these discs here. The flue gas was at normal temp and the CO reading at the exhaust was 19 ppm.
Kinda funny but he had a bad hose connected to the pressure switch from the ID fan and took a McDonalds straw and inserted it into the tube and onto the Pressure switch. I obviously fixed it. I cant see that being a problem could it. The switch opened and closed fine. Not sure if it really would have anything to do with the limit kicking out. Thanks for any info

[freonrick]

do they have a fire place? I have seen where a house is tight and they light the fire place and cause a neg. air pressure in the house cause all the windows are closed. then the furnace comes on and is down drafting tripping the limit at the draft inducer. also look for anything in the furnace room that could cause a neg. pressure.

[fatmomcarl]

did u ohm the switch ? u sure it was nothing lol last winter a dude from my company thought the same thing .. because boss was out of state on vaca i was in charge & took a 10 pm call that CO detectors were goin off like crazy .. the whole effin town was there as far as police, fire & all .. hahaha what a dikkerd he felt like ... one thing that kinda was dumb on the peoples part is on the co detector when it goes off the light flashes & indicates " move to new location" .. they put the co detector outside & stayed in there .. lucky to be alive .. i dont remember the ppm but wheew are we ( cuz of him ) lucky not to have gotten in trouble .. anyway i am sure u know what ur doing so i hope u dont catch a body ... Go Steelers !!!!!!!!!!!!!

[1stanner]

the 19 ppm was when I climbed onto the roof and checked the exhaust gas. Not in the house

[Paul Pippin]

I am concerned about you climbing on the roof to get your CO reading that should be taken at the appliance not on the roof. As for the switch tripping you might want to see if the indoor fan is shutting down or dropping out on occasion.

[otto]

A tight house + new bathroom fart sucker=negative pressure. Used to see it all the time with spillage from the water heater (Hot water heater for those of you so inclined).

[Advanced Response]

We need to know the make of the furnace to elaborate more on the problem... Was this a bryant or carrier 80% with the manual reset spill switch? If so,, I have never seen one trip without just cause.. We need to know specifics if this unit was in a closet in the garage, utility/laundry room? I would be looking for a negative pressure situation in the room or the home.. Possible return air leaks in the closet or room..

Need more information..
Good luck
J

[tinknocker service tech]

is it a masonary chimmey. it may be blocked off enough to slow down the draft and tripp the switch. inducer maybe slowing down after running awhile and tripping the switch
most of the switches have a temp setting on them see what it is and what you are putting out after twenty minutes

[htrguy]

Originally posted by 1stanner
What is the real purpose of a limit on the ID fan. Also is it any indication at all of a cracked heat xchr. The unit ran perfectly after I reset it. The customer says he had to reset it about 3 times in the last 2 years, and like 4 times in the last week. Probably just a weak sw. But I don't know why they put these discs here.

They're for blocked flues.
The solutions and hints have been layed out by the experienced techs here. Negative pressure from fireplaces water heaters sealed closets (lack of combustion air) dead bird/squirrel in the pipe (another reason to check combustion at the unit).
By the way, I hope the McDonalds straw was not the permanent fix for the old hose!

[Edited by htrguy on 02-02-2006 at 12:31 PM]