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Furnace keeps tripping off on limit switch. replaced limit switch still trips, blower motor seems to work fine.

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  • johnbo Dec 12, 2007

    The limit switch is for 250 degree it is working right I have laser thermoter reads it is trpping at this temperature Conventional furnace not sure how old it is

  • johnbo Dec 12, 2007

    The Intertherm furnace model MGHA 070 keeps kicking off on the limit switch replaced switch same thing. The switch goes off at 250 degrees. The blower seems to work fine. This is a conventional furnace

  • johnbo Dec 12, 2007

    I jumped the fan switch and it still kicks off on the limit switch. i didn't think the anticipator would cause the limit switch to go off.

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You have a problem with either the fan sw differentials
setting or the system thermostat,either the fan is not
cyling on at the right time or is cyilnt off too soon If the
fan sw is with the limit sw set the differential according
to recommendations, if the fan is on timer set it for
longer, check the heat anticipation setting on tstat
with the number on gas valve also. jump the tstat
red to white & place a clamp on ammeter around
wire jumper or use a VOM set to amp
& run amps thru it red to white set heat aneicipator
to match the number on the gas valve
If you have dual fan/limit switch set to recomendations
or whatever the recommended differential is

Posted on Dec 12, 2007

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  • Eugene Merritt Dec 12, 2007

    Anticipator in combination with the tstat sw
    turn heat on and off via the gas valve

  • Eugene Merritt Dec 12, 2007

    If you jumped; the fan switch the fan should have came on
    and cooled the plenum before the limit switch cut the heat '
    off (thermostat should turn the heat off before the limit sw
    does(limit sw is a safety control ) check the heat anticipator
    heat anticipator in combination wilth the gas valve and thermostat
    control heat output
    [

  • Eugene Merritt Dec 13, 2007

    If the heat anticipator can not be set to
    match the setting on the gas valve then
    the thermostat is defective most
    honeywells are at .8 ... 800ma but
    set according to the gas valve current
    rating for anticipaor

  • Eugene Merritt Dec 13, 2007

    Current flowing thru the thermostat resistor (heat anticipator )
    generates heat that causes the metal coil to warp and open
    the thermostat contacts turning off the gas valve If there is
    not enough current flowing thru the resistor (coil of wiire)
    not enogh heat is generated and the heat stays on longer
    before breaking the thermostats contacts in your case
    thermostat is not shutting the heat off the llimit swithch
    is so if nonone has changed the setting of the heat anticipator
    then the thermostat is defective (current create heat the greater the
    current the greatet the heat the less the current the less the heat)
    That how the heat anticipator can cause the limit switch to trip

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It's always possible that the limit "dial" switch is sticking leading you to believe that it's only at 150º.

How old is the furnace and when was it last cleaned.

Posted on Dec 12, 2007

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You need the model number of the furnace to determine what the flashing light is telling you. Many times the part that looks to be at fault is not the true fault in the furnace. A slow running blower could cause the limit switch to trip. The circuit board tells you that the limit tripped, but the real cause is the motor or clogged filter.

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Make sure that there is not a manual limit that has tripped and needs to be reset. Roll out limits, and check primary limit. On some models if a limit is tripped the blower starts to run right away.
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sounds like the furnace control (low voltage) circuit is open, or in other words you either have a fuse blown or a limit switch that needs to be reset. Most newer furnaces have a small knife like fuse, usually 3 or 5 amp located directly on the circuit board. Limit switches that have a manual reset are usually located in the fire box just above the burner. There may even be a limit switch located on the blower assembly. If you fuse was blown, you may need to find out what caused the short in the circuit or it will do it again. If the limit switch was tripped, you need to turn the furnace back on and watch the flame to see if there is "roll out". It it continues to trip or blow fuses you need to have the unit serviced.

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Last week my furnace went out. My landlord called a furnace technican and he said that the limit switch was fried. So he replaced the limit switch and did a tune up of the furnace. It took 2 days for the...


Hello, a limit switch will open when the furnace overheats due to restricted airflow, check the blower wheel make sure it is clean and also make sure the blower motor is running at good speed and not dragging. Sometimes a weak capacitor can slow down a blower motor to the point that the limit switch will open. Not sure what kinda tune up the tech did but you dont replace a limit switch untill the cause of the limit switch opening is found.

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How do you test a pressure switch and other limit controls?
This is for people who are experienced with electrical equipment and the use of a Volt Ohm meter. *Please never by pass a pressure switch or limit control. The pressure switch and limit switches are there for your safety. You would test by setting your volt meter to "Volts AC" and test the rollout switch by placing one meter probe on one terminal of the rollout switch and your other meter probe to a good ground. Have the red probe touching the top terminal of the rollout switch and the black probe touching a ground (body) of the furnace. You should be getting 25.85 volts which means the rollout is good on the top terminal. I would next test the bottom terminal of the rollout by touching the red meter probe to the bottom terminal on the rollout switch and the black terminal to ground. If I get 25.85 volts on the bottom terminal the rollout is good. If I do not get any voltage on the bottom terminal then the rollout has tripped and can be reset (if equipped) by pressing in on the little button or replaced. If your rollout switch is tripped you probably have a stopped up heat exchanger or a leaking heat exchanger. I would recommend calling in a service technician to find out why the rollout switch tripped. If you have an open limit control either the furnace has over heated or the limit has gone bad. Problems that would make a limit open up would be dirty filters, dirty evaporator coil causing a restriction in the air flow or a slow blower motor (check the capacitor on the blower motor to make sure it is good). A weak blower motor capacitor will cause the blower to run slow and eventually fail. Pressure switches, and limit switches can be tested with a meter in the same manner. With the furnace calling for heat you can test each of the terminals on the pressure switch to ground to make sure the pressure switch is closed. You should be getting between 24 to 28 volts from each terminal to ground if the pressure switch is closed. If the pressure switch is open you either have a stopped up vent, drain line (if you have a condensing furnace) or bad pressure switch.

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Most, if not all, furnaces have high limit switches that keep a constant check on the temp inside the furnace or air handler. On most systems, once this high limit switch has been made (got to hot), the limit switch breaks control voltage to the heating device (strips, gas, fuel, etc.etc..). The blower motor continues to run until the high limit has dropped in temp and reset itself.

On some models, the high limit is a manual reset, meaning you need to reset the switch yourself or shut the unit off and then back on to reset it.


Check all filters and replace as necessary. Check the coils and fan blades for dirt build-up and cleanliness. Check motor belt (if applicable). If all these fail to solve the problem, then your issue either lies within the blower motor itself not moving enough air for proper heat transfer. Or the high limit switch itself is failing.

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