I have replaced the open limit switch on my furnace four times in the last four years. I just replaced it today, again and the furnace will not kick in unless I turn off the thermostat and restart it. After several attempts it will start, but will run fine a couple of times then I will do the same thing, again. I service man came last year and cleaned it up, but does not know why the limit switch is burning out. All vents are open and returns are open. Air filters are changed monthly for the same reason. I don't know know what else to do. Any suggestions?

You should test for gas manifold pressure. The furnace could be overfiring which would would kill the high limit in short order. You will need a special pressure gauge for that purpose. Set the pressure at the perscribed setting on the furnace name /information plate attached to the furnace.

So, the limit switch is for gas? There must be more than one type of limit switch, then.

On my furnace, it says that if the limit switch pops a lot, to call in a repair person.
Sounds like you did the right thing in calling someone in - perhaps you can call in someone else - does your gas provider offer free house calls/diagnostics, for example? This is a busy time of the year in the northern hemisphere for hvac folk, no doubt.

This past summer I replaced my plenum and the feeds to make them more balanced and to cover a new room addition. At the same time, I replaced what appeared to be asbestos lined ducts with flexible, insualted ones. I liked the solid ducts, and had them wrapped in insulation, but some of it got wet, and my plenum was starting to rust. I wonder if limit switches sense the load (back pressure?) on the heated air side, too? In other words, if you close up all the vents, the heated air has no where to go, and the limit switch keeps the furnace from blowing up or some such by shutting off the burners?

Probing in the dark,
chuckb

The limit switch cuts off the power to the gas valve, The reason it cuts the power off to the gas valve is because the gas valve might be adjusted to high allowing too much gas pressure to enter the combustion chamber. Since the furnace is not designed for a higher gas pressure and the increased BTU output it trips the safety limit.

The consensus of the information that was provided in the post and the fact that this control has been replaced annually for the last four years indicates that to high of gas pressure more than likely is the culprit causing the limit to trip to often and finally to stop working all together.

Most service people never think out of the box and that is why his service person just kept replacing the limit switch every year and told the customer he did not know why the limit kept going bad.

There is a reason for parts to fail. Some fail because there useful service life has past. Other parts fail because of a unusual problem that is interconnected with them causing the failure.

During the normal course of furnace ownership a high safety limit never has to be replaced. They usually last the life of the furnace unless there is a very unusual problem causing there demise which usually consists of to high a furnace temperature and excessive tripping.

This Dragon program is great. Been using it for over a year now. I just love the fact that I just speak into a microphone and the typing magically appears on the screen. Sure save my pinky's from all the typing.

I've heard of the dragon program. Does it work in reverse, too?

I see that it does not do grammatical testing (I think ms word does this annoyingly at times) - in distinguishing between there and their. Perhaps it flips a virtual coin.

How do you edit - does it have special control words/barks?

I understand your limit switch - I could not find such in the legend of my furnace schematic - perhaps it's mechanical with no electrical hookups and that's why it does not show up in the schematic.

I've heard of too high an incoming water pressure, but gas, too?

Makes sense, if you think about it, but would it not make more sense to put in a pressure regulator valve instead of just a switch?

Enjoy,
chuckb

There is a pressure regulator valve in the gas valve. It is adjustable and if adjusted to high it causes the problem. Usually this shows up when a conversion from natural gas to propane or visa versa takes place.

OK, I'll go with that.

I'm still not sure what to do about the auxuiliary limit switches in my unit. One is self resetting (the one that is somewhere behind the blower), and the other I've been manually resetting. Not sure what they sense/limit.

I'm considering just replacing it and seeing what happens. With burners exposed, the heater works great, but I'd rather have the front cover on and have it work great.

I found this on another forum:
The purpose of a limit switch is to shut your gas valve off if your furnace were to over heat, the limit has nothing to do with the thermostat.
The limit switch is wired in series with the 24 volt control circuit of your furnace.
There are two wires connected to the limit switch, when the furnace is cool enough the switch closes which connects the two wire together and allows the burner to come on. If the switch is bad or the furnace is to hot the switch might remain open and the burner won't light.
You need to observe your furnace in the heating cycle to see exactly what is and what is not starting or stopping.
Check the wiring diagram on the inside of your blower door and locate the limit switch/switches in your furnace. You can jump around the limit switch to test it.


It sounds like the aux limit switch is to keep the furnace from overheating. Since the switch trips when there is NO heat (at startup before igniter comes on), I'm suspecting that the swithc is at fault and will look for a replacement.

What do you think?
chuckb

Sounds like I wrote it.

Jim?
Jim Barnhart - Heating, Air Conditioning, Fridge, HVAC - Home Appliances - Home/Garden - AllExperts.com (http://www.allexperts.com/ep/696-26038/Heating-Air-Conditioning/Jim-Barnhart.htm)

So, I replaced my 215degree F limit switch and things seem fine.

Learned a lot, and a lot more to be learned,
Thanks,
chuckb.

Sounds like he still didn`t attempt to check or regulate the gas pressure,, another year,another limit switch

That's OK the furnace will last only a few more years from the abuse.

Well, I may be in the same boat - it turned out that my replacement 215 degree fahrenheit aux limit switch needed a manual reset after the initial euphoria. I'd done a thorough cleanout of the exhaust flue in the area immediately around the switch (roofers had dropped some debris - redwood twig, nails, etc.) but am temporarily running it with the front panel leaning against the unit, but not quite closed.

I don't know what trips the switch - but will check exhaust pipe roof cap.

Clueless in CA,
chuckb
Oh, I'd recently instaled new grills dampers, with filters. I suppose as a test I could try running it w dampers and grills totally out of the system, but am running out of ideas.