Gas and Oil Home Heating Furnaces - Temp. Limiter Switch

How does the temperature limiter switch work? Is it an actual on- off switch, or is it more of a sensor that tells the relay to open? My temp. switch is a flat little panel that has 2 wires coming from it. What is the best way to test the temp. switch? I was using a volt meter but Im not sure what kind of voltage readings I should get. Thanks.

You mentioned a flat little panel. Which is probably fastened with two screws to furnance envolope. If you remove both screws and pull it, you will find almost 6" of two probes sticking inside.

The functions of the limit switch as follows:

The first one is, when the limit switch gets hot, it sends the signal to the control board to power up the blower at low speed.

The second one is, if the blower doesn't come on, the limit switch (high limit)sends the signal to close the gas valve.

You should be reading 24 V DC, before the blower turns on.

I hope this info will help you start with.

By the way, I got a lots of help from the experts in this forum to troubleshoot and fix my furnace.

Good Luck, Ravi

Ravi, you are refering to a fan limit control. These always have atleast three wires, and are usually refer to as a box.

What the OP is refering to is your standard limit switch.

Ravi was right in that if you take it out you will find a probe of some sort stickout into the airflow.

On these two wires you should read 0 volts if it is not too hot. If you read voltage (usually 24v) across this when the unit is cool then either the connections are bad, or the limit switch is bad.

If you read voltage after the unit has been running you have to acertain that there are no other issues before you can condemn the limit switch.

What are the symtoms the unit is presenting?

Steve, below is a copy of my original post. It seems that when the furnace runs more than 15 minutes straight it will shut itself off before the thermostat temperature is reached. Then it has trouble restarting unless I turn the furnaces power off and wait 10 minutes. Then it starts fine and runs fine unless the 15 minutes is used before the set thermostat temperature is reached.



.Im having a problem with my furnace. It is a Bryant Induced-Combustion furnace. The furnace has been shutting off before reaching the set temp. The draft inducer motor seems to run the whole time the furnace is working. When the draft inducer motor stops thats when the burner flame goes out. After the burners shut off, the main blower still goes for about 1 or 2 minutes before shutting off. As soon as the main blower shuts off the draft inducer motor starts again, the pilot will light, and then the burners will light but after a few seconds the draft inducer motor stops again and the burner flame goes out. This process repeats itself over and over. It seems like the problem has something to do with the Draft Inducer motor. When the furnace shuts off before the thermostat setting is reached it is the Draft Inducer motor that is shutting off first, then the burners go out. The only way I can get the furnace to work for any extended period of time (15-20 minutes)is by shutting off the main power switch on the side of the furnace and keep it off for about 10-15 minutes and then turn it back on. The furnace will light fine and work for about 15-20 minutes but still never getting above 63 degrees. (Its been about 16-20 degrees outside the last couple of days.) Does anyone have any ideas why my furnace wont say lit and why it has problems re-lighting? Thanks

So it's YOU! You started a new thread in regard to that same furnace problem? In hopes they don't chide you anymore about you saying how it is the limit? Oh, oh, I hope the forum gods go easy on you. :)

I started a new thread because I needed to know how to test the limit switch.

Put a volt meter across the limit. When the burner goes off it will show 24 volts IF limit is open

Ravi at the top of this thread refers to looking for 24 V DC.

Are you looking for 24 volts DC or 24 volts AC?

ohyeah

Ravi at the top of this thread refers to looking for 24 V DC.

Are you looking for 24 volts DC or 24 volts AC?

ohyeah

24 volt ac ..Ravi is a little confused on limit switch & fan switch functions.
Limit switch---is a safety device. does not let heat exchanger exceed predetermined temp
fan switch-controls blower off/on

How about an agreement on terminology: it's ac, unless the board has converted some of it to DC, like for one of the relays (like my old board), but usually the voltage going through the string of switches/safeties are AC, since that is what the transformer outputs to the controls.

More importantly, if a switch is closed, and still connected, putting each lead on each terminal of the switch while voltage is going thru it will show zero, but if the switch is open, it will show (usually) 24VAC. Or if one lead of the VOM is touching ground, if the switch is open, touching the other lead to each terminal of the switch will show voltage to only one terminal, if closed it will show voltage to both. So "across" means "parallel.

To put the leads in "series" will require removing one the terminal wires from the switch and inserting the meter between the switches now empty terminal and wire that was removed--useful for current reading if your meter allows for series AC amperage as some do (most all allow for DC amps). In "series" means "in line with".

I would caution some of the newbie posters, esp those that just got into diagnosing and now feel like qualified experts (read Ravi) about advising without careful consideration. It's possible to at least blow a meter if you put it on the wrong setting, or at least get no reading and a wrong diagnosis simply because of the wrong setting.
comments/corrections??

Hi Mitchella,

Good post. I am new to trying to diagnos my furnace problem. I am not new to diagnosing in general but I am weak on electrical stuff and using my meter. I do find getting a clear specific description of one's diagnostic replies somewhat difficult.

Terminology is definitely a problem. Some call different devices by different names. Others aren't specific with their wording. I'm one who needs very specific information for me to understand it. If you want allude to or imply or speak in generalities please check with my wife.

Don't get me wrong I really appreciate this forum and hope to gather some answers to my questions. I have not graduated to the expert level yet although I have learned a lot in a short time. To the contrary I realize I know way less than I need to on this subject.

>> More importantly, if a switch is closed, and still connected, putting each lead on each terminal of the switch while voltage is going thru it will show zero, but if the switch is open, it will show (usually) 24VAC. Or if one lead of the VOM is touching ground, if the switch is open, touching the other lead to each terminal of the switch will show voltage to only one terminal, if closed it will show voltage to both. So "across" means "parallel.

I understood all of the above except for the last 4 words but don't tray to explain.

I don't understand why I don't get any current reading when I set meter to 50 ac and touch one probe to ground and one to either terminal of the pressure switch (and other devices) when the switch is either open or closed. I know the switch opens/closes from checking continuity accross the terminals when furnace is on and off.

This is with the control board flashing 4 flashes indicating "thermal protection device open" when furnace is on.

Can you or anyone comment on this or is it difficult to determine?

Thanks
Ohyeah

Possibly the thermal protection device is prior to the rest of the devices you are check so voltage is not reaching that far.


mitchella, when you say: "Or if one lead of the VOM is touching ground, if the switch is open, touching the other lead to each terminal of the switch will show voltage to only one terminal, if closed it will show voltage to both."

This is not always true. depends on wht you are checkin. if you are checkin the switch of a electric heat (240v) sequencer then you would get a reading of 120v from each terminal to ground with the switch open or closed.

The only way to check a switch is to check voltage acroos it and you should read 0 when closed, and applied voltage when open.

by across I mean that you don't have to unhook the wires, and you put one lead of your meter on each side of the switch.

or alternately, unhook the wires and check for continuity through the switch.
DO NOT CHECK FOR CONTINUIYT WHEN POWER IS APPLIED.

Might want to heed your own advice.

Dear Shank,

Point taken, please keep in mind we were talking about 24VAC and limit switches and gas furnace here; I profess no knowledge of electric heaters/switches (at least not yet, as I only have gas heat). But I hope in this limited (no pun intended) case we were addressing, I was right?? I DONT'T give out too much advice, as I see the importance of learning from the masters (how's that for obsequious deference?) I was more concerned about some of these posts using the word series instead of across or in parallel for instructions! I'll exit this "hot" topic now, but keep those posts coming!

Good day, sir.

PS (not for you, Shank): continuity means using the resistance/ohm meter, not a voltage reading.