im sorry but i dont know what brand of furnace i have.
so heres the problem... a couple of days ago i noticed my furnace was blowing cold air so i went to check to see if the pilot was out... well i have and automatic ignitor. so i carefully followed the instructions for resetting the furnace. nope nada. so i check to see what the led light was saying... 4 blinks = open limit switch. well thats so very helpful(not). after doing some research i found the limit switch. and cleaned it all up nice and shiny. replaced it. nope same problem. so i figured it was broken, this was friday... found a wholesaler that has them, they werent gonna give me one... since im just a person but they sold me one. yay.. run home replace it. plugged that puppy in ... same problem. well, looky there THERE ARE 4 OF THE DARN THINGS!!! ok now what!!! only bought one. dont know if the place will sell me another 3 and its too late anyways they are closed. now its saturday. i have a friend come look at it... he checked the fuses. dont know why the led said limit switch. well he pulled one out and it looked weak not blown but weak. put it back in and hurray everything is great!!! gas lit up... heat coming out. whoo hoo. yeah that lasted 15 minutes. go back down same 4 blinking lights!!! so he ran and bought some new fuses... put one in and walla heat again... but again... very briefly. and the same 4 blinks!!! (they haunt me). now im sure i could sit and turn everything off and unplug the fuse put it back in and get 15 minute increments of heat!! but that would get old fast. also my furnace is 13 years old and i just changed the filter... i admit the old one was pretty gross and could have cause the problem.
can anyone help ... its cold

NorthernHeat

There is allot I must be missing in your post. What limit? Location of it? The main limit might just need replaced.

specter22pgm

sorry if i was vague... i have 4 limit switches one on top, bottom, and both sides of the metal box surrounding where the gas and heat comes from (sorry dont know the technical names) they all connect in a chain connected with purple wires ie. -----o------o------o-------o------. are there more limit switches? another person told me about a main limit switch and said it would be located on the wall of the furnace...but i have yet to find another... if one of these 4 are the main how do i tell which one??

thank you for your quick response... hope to hear back from you soon.

Airwren

Limit switches are there in case something is going wrong, although sometimes limit switches go wrong themselves (although I doubt it). with out more information the only advice I can give is NEVER BYPASS A LIMIT it most likely will lead to a very dangerous situation. You will probably need to find which limit is going off and why.

labman

Check those purple wires. The furnace sends power out to the limit switch, and if it is closed, it comes back. If it gets lost in a short, not only does it not come back, it may blow a fuse. There could be another limit switch too. Limit switches have different functions in different furnaces. Besides signaling excess temperature, they can indicate it was up to operating temperature.

It could be one of the limit switches works fine at first, but then shorts out after the 15 minutes. Try replacing each of the 4 one at a time with the new one.

NorthernHeat

The main limit looks nothing tike the others (they are commonly called roll-out limits). The main limit is usually a 1.5" x 2" plastic or fiberglass plate with 2 wires going to it. Many manufacturers use red wires for the main limit. It will have a temperature rating on it commonly, say 160-20 this means opens at 160 degrees and closes at 140. Main limits commonly fail if they become fatigued (opens and closes too often) Be sure you keep filters clean and air flow unobstructed.

letmetellu

The limit switch that Northern Heat described is located on the firewall of the furnace. I am not sure is this is the one that you replaced or not. But on one of the wires connected to this limit there is a small fuse ( it looks like a small fire cracker). These fuses are heat sensing fuses and some times will open if there is a down burst of wind in you flue system. Once they open they must be replaced. A test that you can make to check this fuse is to use a small wire and jumper from on side of the fuse to the other end of the fuse, or you can use a voltage meter to check it, contact one probe on the metal part of one end and the other probe on the other metal end, if you get 24 volts the fuse is bad. You can check the limit the same way one probe on each side of the limit and if you have 24 volts the limit is bad. Never jumper across a limit to make it operate the furnace only jumper acrose to do a test.

specter22pgm

[quote=NorthernHeat]The main limit looks nothing tike the others (they are commonly called roll-out limits). The main limit is usually a 1.5" x 2" plastic or fiberglass plate with 2 wires going to it. Many manufacturers use red wires for the main limit. It will have a temperature rating on it commonly, say 160-20 this means opens at 160 degrees and closes at 140. Main limits commonly fail if they become fatigued (opens and closes too often) Be sure you keep filters clean and air flow unobstructed.[/QUOTE

thank you for your answer... thanks to everyone!!! i called an HVAC tech and he came and cleaned the limit switch that NorthernHeat described. works great now!!!

NorthernHeat

I hope he replaced the limit, I've never known cleaning one to repair it.