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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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Re: Furnace keeps tripping off on limit switch. replaced...

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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Re: Furnace keeps tripping off on limit switch. replaced...

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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Rool out switch keeping tripping

assume you mean roll-out switch on a is a temperature sensitive safety control intended to limit the temperature in the vestabule or burner area. if it keeps tripping, it's because something is causing the temp to get too high. a dirty filter would be first suspect. it could also be an inoperative blower, a dirty evaporator, or a blocked flue or vent pipe. check all these things out and you will likely find your problem..

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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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Hi, yes the thermostat feed does come off the furnace from the transformer. That's where it gets it 24 volt supply, 120 volts is stepped down to 24 volts for the controls. If it trips the main breaker, it may be a matter of the wires that feed the breaker are loose and this will cause the breaker to over heat and trip. Shut it off and tighten the wires.The breaker may also be weak. This is a high voltage problem. So, anything that runs off of this breaker can cause it to trip, like the blower motor may be drawing to much amperage. Make sure there is no lint build up on the motor or in the windings or on the blower wheel also. My guess would be the breaker is burned or wires are loose creating enough heat to trip it at times. This is where you will need to start. Keep me posted. If you have a amprobe you can check the rated amps on the data plate of all motors and see what they are drawing under a load. If not, you will have to see whats getting hot, breaker, motors ect. Keep me updated as you will be able to find the problem. Like I said, my bet would be the breaker is going. You can also kill power and pull the breaker to look for any hot or burned spots on it. Concentrate there first.
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I will give you my best suggestions based on the information that was provided. It sounds like the blower motor is trying to come on several times unsuccessfully and then the flame is shutting off. If this is the case, here are my best guesses.
  1. If the blower motor has a run capacitor, the capacitor could be bad and the motor is trying to start but is pulling too many amps and tripping out on the internal thermal overload. You will need a multimeter that is capable of checking microfareds to determine if the capacitor is bad. Capacitors are usually will commonly see a marking of 5uf, 7.5uf, 10uf, etc... Sometimes all you need is a new capacitor. These are not very expensive.
  2. The blower motor itself may be bad.
  3. If there is a fan switch, it could be bad. A fan switch has a set of contacts that are normally open that will close when the switch heats up and send voltage to the blower motor. These will eventually wear out and act erratically if at all.
  4. The motor may not be getting the proper voltage, this too can cause the motor to pull too many amps.
  5. The most likely reason the flame is shutting off after the blower fails to come on is because the furnace is tripping on the high limit switch. This switch typically cuts the 24 volt power to the gas valve.
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Sounds like the limit switch is not bad ...... its just doing its job. You need to determine why he furnace is overheating. Are your filters clean? Is your blower wheel clean? Is your blower motor running fast enough? It might be set on to slow of a speed. The blower motor capacitor could be bad. Are all of your air vents in the house open? Are your return air registers blocked? Is your duct work properly sized? Put a thermometer in the return air duct. Note the temp. Now stick the thermometer in the plenum. Note this temp.. Subtract the high temp from the low temp. This number should fall within the temp. rise rating found on the data plate inside the the furnace on the same sticker as th model and serial numbers. If your temp. is above the suggested temp rise. you have an air flow problem. If it is within the suggested temp rise listed then your high limit control should be replaced.
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