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if it is shutting off abruptly the limit is either wired wrong or set wrong. If the fan comes on and blows the flame out there is a hole in your heat exchanger which can result in fire or carbon monoxide poisoning. Have an experienced tech take a look at it to be safe.
Here's my best shot.
Check the temperature settings for the fan switch.
The low OFF setting should be about 100 degrees.
The ON setting should be around 150 degrees and this is the setting that "makes" the ON contacts.
The HIGH LIMIT setting should be about 180 degrees.
If these are set, can you use a meter to see if the FAN ON contacts are closed?
Lastly, it appears that one hot line should go to one side of the blower and the other hot line should go to the bottom FAN terminal with the other side of the blower being fed by wiring from the top FAN terminal.
Good Luck. Gary
If this is the old style limit/fan switch. Turn power off
remove the wires and label where they got disconnected from and wire the new one in. Make note of the settings on the old limit switch so you can
adjust the new one. The limit switches come in
different lengths so get the right one.
If it is a flat bimetal disc, then I think you have only to
wires that have to be connected so hook up same
to wires from the old one. With Bimetal disc fan
limits you need to make sure you get the same
temperature reading as the old one
As a first check, There should be 2 gauges and a pressure switch with histeresis.
You need to set the tank pressure switch above the regulated output pressure.
The tank pressure will fluctuate between the upper limit and lower limit set in the compressor switch.
The outlet regulated pressure should be below the lower of the switch settings or you will get a varied outlet pressure.
Try setting the tank pressure lower limit higher on the switch - above the outlet pressure required.
Always stay within the tank maximum rating.
If things are nor working properly, it may be the outlet regulator (make sure it is emptied of condensate daily) or a fault with the compressor switch if the tank pressure drops too low before starting.
If the compressor is on more often than usual, then it may need service or repair to worn parts. This is particularly true for vane compressors.
Sounds like the fan switch on the furnace is set to low. Turn power off to furnace, take door off of furnace look for a fan switch, it may be a fan - limit together. The limit settings should be approximately 160° to 200°, and the fan settings should be about 100° to 140° set it to 120° it it's already set there then you probably need to replace it.
It sounds like the limits are out of adjustment. With the door down, rotate the down limit until it clicks. Then raise the door manually, and set the up limit until it clicks. You should be able to hear the switch 'click' when it activates. Pay attention to which way the plastic nuts are moving on the screw going up and down. That way you know which way to turn them to set it.
I am providing some information, but I encourage you to have a furnace technician look at your furnace. The reason is; the limit switch trips when there is a problem that may present a fire or combustion hazard. I've seen these switches go bad, but not very often. Usually they are tripping because the plenum is sooted up and excessive heat builds in the combustion chamber. This is common in older furnaces that have pilot flames- the flame runs all year and soot deposits in the plenum, plugging the air flow through the combustion chamber after years of use.
The limit switch is a safety control switch located on the furnace just below the plenum. If the plenum gets too hot, the limit switch shuts off the burner. It also shuts off the blower when the temperature drops to a certain level after the burner has shut off. If the blower runs continuously, either the blower control on the thermostat has been set to the ON position or the limit control switch needs adjustment. Check the thermostat first. If the blower control has been set to ON, change it to AUTO; if the blower control is already on AUTO, the limit switch needs adjusting.
To adjust the switch, remove the control's cover. Under it is a toothed dial with one side marked LIMIT; don't touch this side. The other side of the control is marked FAN. There are two pointers on the fan side; the blower goes on at the upper pointer setting and turns off at the lower pointer setting. The pointers should be set about 25 degrees apart. Set the upper pointer at about 115 degrees Fahrenheit and the lower one at about 90 degrees Fahrenheit.