No heat, continuous blower (aux limit switch?)


  #1  
Old 01-05-03, 06:53 AM
coldinMA
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Angry No heat, continuous blower (aux limit switch?)

Hi,

This morning our furnace blower is operating continuously, but with
no heat, and a flashing code on the electronic controller's LED:

York Diamond 90 gas fired hot air furnace (upflow I believe)
Model P3URB12N07501C
LED flashes 4 times

This code indicates that the auxiliary limit switch has opened, but I'm
not sure how to find that switch and verify it. The wiring diagram mentions
a primary limit switch, auxiliary limit switch, and auxiliary limit switch on
the blower inlet, but the furnace diagram doesn't indicate where those are,
and I'm not familiar enough with furnaces to even guess :-(

The filter was very dirty when I opened the furnace (very much my fault).
Removing the filter (to improve air flow I assumed) didn't fix the problem.

I shorted the R and W thermostat wires to rule out the thermostat. It
also seems that the LED wouldn't flash an error if the thermostat was at fault.
No change.

Reading other posts seems to confirm that a limit switch is the problem.
Any help finding the limit switches, determining how to test them, and
other suggestions on fixing the problem will be greatly appreciated!
What would they even look like?

Lastly, if it's a limit switch, are these easy to acquire and replace?

Thanks!
 
  #2  
Old 01-05-03, 07:03 AM
bigjohn
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Look around in the blower compartment for a limit with a reset button. It will probably be a round disc looking affair mounted on the blower housing. Most likely, the dirty filter resticted air flow and caused a limit to trip. Your right, the board monitors the various functions in the furnace; it won't indicate a code if the thermostat is not functioning properly.
 
  #3  
Old 01-05-03, 07:21 AM
coldinMA
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Hi,

Thanks for the reply - I'll look in the blower compartment, but
I may have mislead you (sorry):

I found more precise diagnostic info inside the unit as opposed
to the manual (that's what I get for reading the manual).

4 flashes means the "high limit switch" is open. There's no
auxiliary limit switch in my furnace. By tracing the wires, I
found the high limit switch as a square block mounted into
what looks like the hot air output, just above the burner box.
It's flush with the surface, and has two external contacts.

There doesn't appear to be a reset on it. Should I just jumper
it and see if that fixes things? Then I'd know, I think, that it's
just a bad switch.

Many thanks!
 
  #4  
Old 01-05-03, 08:08 AM
bigjohn
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The primary limit is usually automatic reset. Yes, you can bypass it temporarily to see if it makes a difference but don't leave it that way. Better yet, if you have a voltmeter, attach the probes to the terminals, then start the furnace. If you get a voltage reading across the switch, it's open. You might take it out and see if the sensing element is deteriorated. You also may have a blockage in the ductwork or flue which would mean the limit is doing its' intended job. Look at the posts for leeway; he had a blockage in the return air duct and the limit was shutting off the furnace like it was supposed to. The way we figured it out was he would take the door off the the blower compartment and the furnace would run fine. Having the door off gave the furnace more airflow. After you have turned the power off to reset the board/module does the furnace try to start or does it immediately go into lockout?
 
  #5  
Old 01-05-03, 08:23 AM
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Clear the code

Most units require you to clear the code by killing the power to it and then turn it back on after a few seconds. Remove the other side panels if you can, most high limits have a manual reset. Its a bimetalic disc about the size of a quarter with a small reset protrution between the two connectors... the dirty air filter is why it tripped.
 
  #6  
Old 01-05-03, 08:27 AM
coldinMA
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Jumping the terminals caused the furnace to run fine, so we're
onto something!

I'll check the voltage across the terminals and post that.

Running with the blower door removed doesn't help. The behavior
is the same: combustion chamber fan comes on, then the
main blower, and the 4 flashes appear almost immediately on the
LED's. I don't hear the gas valve click or see the burners try to
ignite at all. Sounds like that would rule out a blockage in the
returns?

I did remove the high limit switch and briefly inspect it. Nothing
looked obviously damaged, but I have no reference. What would
it look like if the sensing element was bad?

The flue and clean air intake are both clear - they open to the side
of the house and are easy to check thankfully. I'll double-check.

Thanks for warning me against running with the jumpers in place.
I'm going to try and vacuum out the blower since the filter was so
dirty. Any other thoughts on how to determine if it's the switch itself,
or something else, are appreciated.

Thanks again!
 
  #7  
Old 01-05-03, 08:51 AM
coldinMA
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Hi again,

The weirdest thing has happened: the furnace now seems fine.
I removed the jumpers and quickly retried operation, and it worked
perfectly. So, all I've done so far is:

- Replaced the filter (though running with it and the blower door
removed did not help)
- Removed the high limit switch, tested it with an ohm meter,
found ~100 ohms resistance and reinserted it. Successfully ran
the furnace with this switch jumpered.
- Removed the jumpers and the furnace now runs fine. There's no
voltage across the terminals, so it appears to either be permanently
closed somehow and the (unknown) problem still exists, or the
problem has disappeared.

I'm almost feeling worse! Could removing the switch have dislodged
debris? Or maybe this is an intermittent problem. Any other thoughts?

Thanks as always.
 
  #8  
Old 01-05-03, 08:52 AM
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Schematic

try to read the schematic. It has a legend that tells you the abreviation for the high limit switch. Look at the wires that go to the switch that you jumped. there are numbers on them. get the numbers of both and see if you can see where on the diagram that those two wires meet. This will tell you the name of the switch you jumped. If you have an OHM meter, test the switch out of the unit without the wires attached, it should be closed. If not replace it. There are lots of places in MA to get this, Bell pumps Simons, Johnstone supply, or Heat Incorpoated in Hudson,NH on Flagstone Drive( YORK OEM), what city are you in?
.
 
  #9  
Old 01-05-03, 10:07 AM
coldinMA
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Hi,

The switch measured ~100 ohms when taken out. Since the furnace
is magically working (see my last post) after re-inserting the switch,
now I'm not sure if 100 ohms means open or closed! I'd assume that
it means open, since closed would measure zero resistance, but if the
switch was open, I can't explain why the furnace works now...

If it does turn out to be the switch, anywhere near Burlington MA
or north towards Chelmsford would be great (near to work).

Thanks for your help.
 
  #10  
Old 01-05-03, 11:37 AM
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Parts

Chelmsford, thats not too far from Methuen, Bell Simons Company, 975 Riverside DR (978) 686-9568 or 688-8820, or Lowell Branch at 1278 Gorham St (978)453-5853, Bring the part in with you and they can match it up. 100 OHMS is too much, replace it. MSRP will be less than $25.00 I think., Burlington is near Woburn which has lots of supply houses on Boston Post Rd Industrial area Try Johnstone Supply, 781-933-8506, hope that helps. I'm not sure if Johnstone sells to the public, I think they do, I know Simons does.
 
  #11  
Old 01-05-03, 11:42 AM
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Parts

Chelmsford, thats not too far from Methuen, Bell Simons Company, 975 Riverside DR (978) 686-9568 or 688-8820, or Lowell Branch at 1278 Gorham St (978)453-5853, Bring the part in with you and they can match it up. 100 OHMS is too much, replace it. MSRP will be less than $25.00 I think., Burlington is near Woburn which has lots of supply houses on Boston Post Rd Industrial area Try Johnstone Supply, 781-933-8506, hope that helps
 
  #12  
Old 01-05-03, 01:14 PM
bigjohn
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I agree. The limit is a switch and 100 ohms resistance is too much. The switch contacts may have some oxidation/corrosion buildup on them. It need replacing- good catch.
 
  #13  
Old 01-06-03, 04:10 PM
coldinMA
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Thanks very much for your expert help and suggestions on where
to buy the replacement part! Since the current limit switch is
probably living on borrowed time, I'll get the new part in there
right away.

Can't tell you how much I appreciate your advice. This is a
wonderful forum!
 
  #14  
Old 11-26-07, 05:37 AM
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Your Help ENDURES and is appreciated!

Same problem, different flavor heater. I'd searched too many places before I dumbed onto this post. I'd identified the LCD code, but couldn't define it. I'd decided my problem was the High limit switch, and was trying to find out if I can TEMPORARILY by-pass the high limit just to diagnose. DoItYourself IS a superior find with wonderful posts. Thanks! They endure time.
 
  #15  
Old 01-26-08, 09:51 AM
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Red face A big thanks - you guys are brilliant!!

Same scenario happened to me this am.... scratched my head, said a few colorful words... then decided to google. You guys pointed me in the right direction, problem was the "other" limit switch mounted on the "burner box/Ventor".

It appears this switch measures the pressure in the burner box and on the burner motor (not blower motor)? but was stuck open and would not reset with the normal power cycle. I pulled the tubing off and gave a few love taps, plugged them back in and it appears to work again.

I'm curious and defer to your expertise - My unit is about 8 years old, running flawlessly, until today. Is this a switch that I should expect to change? What could cause this pressure switch to trip? I am going away on business for 2 weeks leaving my wife and 2 young boys at home (both unenthused at the prospect of it happening again).

My sincere grattitude to all you guys (and gals) that contribute.

Jevad
 
  #16  
Old 01-26-08, 11:16 AM
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If a person lives in the part of the country that experiences extremely cold temps and you plan to be out of town and no one to tend to the house, it is always a good idea to have someone drop by to see if there is still heat in the house. Newer type furnaces have more parts to go wrong.

The pressure switch diaphram inside may have a pinhole leak in it. It could be either end of the vacuum tube is not as secure as what it should be. It could have bad contacts that sporadically make good contact in it, sometimes, then soimetimes not.

There could be a reason why the pressure in the furnace is not quite up to snuff and may be always just on the cusp of working or not working: condensate water not draining properly, bad condensate trap (your can readily check these things out and try to blow out any water by disconnecting drain hose and blowing out to have it all come out toward the floor, and also blow the other way back into the furnace. Then refill the condensate line trap with water. Pay close attention to if there was an initial obstruction that you ended up clearing out); exhaust or intake of air for furnace slightly partially obstructed/restricted; draft inducer fan slipping (but be doubtful unless you heard terrible racket coming from it); carboned up insides of furnace (kind of rare I think unless flames have had history of burning quite yellow over period of time).
 
  #17  
Old 01-26-08, 07:09 PM
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which switch exactly

There are many switches on gas units and depending on their location will determine what their function is. Try to determine what they are attempting to do. Then when they trip, you don't say oh well , replace the switch. Rather you should be asking yourself is this switch doing what it was designed to do? Namely open and cut off the supply of gas if the stack should become restricted? Or maybe open the gas supply circuit if the flame rolls out (backwards) of the combustion chamber into the gas valve area. Another would be the proving switch for the exhaust if its power vented. Then there is the high limit switch or as some call it, the fan failure switch (depending on the manufacturer). The trick here, is to identify the open switch, and then verify the conditions that opened it.
 
  #18  
Old 12-21-09, 05:55 AM
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Thanks

Same thing happened to us, we have a Frigidaire 80, and the filter was so dirty it must have tripped the high limit switch. I had a single flash on the indicator light. After turning off the power to it I took the switch out, examined it (looked good, no corrosion), put new filter in and then jumpered the high limit switch. I just held the wire jumper there and turned the power back on. The single flash turned into a solid light, then the flame light and then I removed the jumper, bingo, heat.

Now before doing that procedure I turned on and off the furnace a few times hoping to reset the issue. No dice.

So my question is, is that high limit switch on it's way out, do i need to replace that?

It looked good, it was clean, no frills or wear and I turned the furnace on and off once and it works fine. This all happened last night, now it's morning and we have heat.

any advice?

BTW, this post got my heat working, we are a few days before Christmas and I have a 6 and a half month pregnant wife. THANK YOU VERY MUCH!
 
  #19  
Old 12-21-09, 06:30 AM
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So will it run now or not, since you put in that filter?

Whenever you test, it is best to first do one procedure, then see if that fixes things, rather than take 2 (or more) actions at once - because then you do not know what action really helped the cause?

If a limit switch trips - then it means it is working, at least in the sense that it can switch. Although it is possible that it could lose it's calibration and trip at some lesser temp than what it used to. Jumpering will not verify that sort of condition. You would have to insert a temp probe (like those 1/8th inch diameter, 6 inch long probe, 1 inch dial thermometers, you can buy at a national chain tool store or grocery stores.) into the heat exchanger right below or above(you'd have to drill that size hole - you are drilling into the blower air part of the exchanger and not into where the burning gas is, by doing so. Only deril to dewpth to pucture th rough the neceassary sheetmetal and no more), and insert probe to the same depth as the limit switch sensor to actually know. Then read what your thermometer says and compare to what the rating is on the high limit switch. If the high limit rating was 200-40, that means it is supposed to shut off at 200F, and come back on again 40 degrees less at 160F.
 
  #20  
Old 12-21-09, 01:15 PM
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furnace is working

ecman51, yes the furnace is working with the new filter.

I actually have a cooking probe that I will try when I get home. However I may need to back pedal and put the old filter back in to test if it shuts off... The furnace air flow is working very well now and we are getting better circulation. So with the new filter I don't think it will reach a temp of 200 to determine if the current high limit switch does still work.

So is that all the High Limit Switch does, tests the tempature?
hmmm guess that would make sense hence the name
 
  #21  
Old 12-21-09, 01:31 PM
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Originally Posted by sauceSquatch

So is that all the High Limit Switch does, tests the tempature?
hmmm guess that would make sense hence the name


This man is ready to be employed as a furnace repairman!

You will remember a couple of days ago someone had a plugged AC condenser coil, but the "repairman" recommended new ductwork to the tune of $1000.
 
  #22  
Old 02-05-11, 02:59 PM
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Thumbs up York Diamond Series

Excellent advice; helped me out.

In Texas you can't just go out and purchase some HVAC parts w/o a 'license'. I found a place online, partsguy.com (or similar) back in 2008 when I moved into this old place and needed to replace the furnace control board. The ignition sequence was the issue the first time around and replacing the control board did the trick. The igniter was crapped out too, so I replaced that and sourced it locally, too.

The onboard diagnostics on the new board from '08 reflected that the unit was not operating properly w/ an initial red flash, and the high limit or low limit switch failed or the ducting was not right.

Called the aformentioned partsguy.com and chatted a short time and got a new board at the old price from they folks and paid through the freezing nose this time to have it overnighted. The new control board, this one ALSO superceeded to a new part from the last go 'round, came in and I got it installed only to find that the unit was working properly, green flash, followed by the pesky 4 flashes indicating the low or high limit switch.

With frustration running high, I googled my version of the question and came up with this thread and my frustration was magically transformed into elation as I found TWO limit switches in series, the one at the top of the fan exit needing to be reset.

i reset the uppermost 'switch' and pressed the safety switch and delightfully observed the proper ignition sequence and the amber LED flashed it's 'call for heat, all's well" signal. Thanks for the help.

Now to the meat of my question. The furnace control failed while we were experiencing "rolling blackouts" and I suspect that the initial problem was a surge or spike when the main line was reconnected with the power grid and my question is:
Do we need to protect modern furnaces with electronic controls with surge protectors and is a standard computer equipment surge protector robust enough to handle the current draw of the main and combustion chamber fans?

Thank you all in advance for your opinions.
 
  #23  
Old 10-23-11, 12:31 PM
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open high limit switch

Thanks for this thread. My furnace had 4 red light flashes indicating a open high limit switch. My filter was dirty... hadn't replaced it for 7 months but have a new one in. I was getting 24v on the limit switch but it still was not igniting but the fan was blowing. Pulled the wires off the limit switch - black plate about 2"x4" and reattached. I guess this reset the switch. Turned the power off for 20 min but it didn't reset the switch. Now it is working.

This thread saved me some money and a service call that I'm sure would have recommended some replacement parts.

Thanks.
 
  #24  
Old 10-23-11, 12:33 PM
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Originally Posted by spgrc
Thanks for this thread. My furnace had 4 red light flashes indicating a open high limit switch. My filter was dirty... hadn't replaced it for 7 months but have a new one in. I was getting 24v on the limit switch but it still was not igniting but the fan was blowing. Pulled the wires off the limit switch - black plate about 2"x4" and reattached. I guess this reset the switch. Turned the power off for 20 min but it didn't reset the switch. Now it is working.

This thread saved me some money and a service call that I'm sure would have recommended some replacement parts.

Thanks.
those switches are supposed to auto reset. If it stuck once then it should be replaced.
 
  #25  
Old 10-23-11, 03:51 PM
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Ok, it looks fairly simple to replace - only 2 screws. There is a part number printed on the switch. This is the first time it happened but I'll replace it. It's weird now that it is working, when the blower is on but the furnace is not ignited, the voltage on the limit switch is 0V AC. I'm assuming there is voltage only when the furnace is ignited. The furnace is an Enviro Plus 90 RPJ II - model GUK100N16A.
 
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Old 10-23-11, 05:34 PM
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That is an ICP product. Across the terminals you would see 0v if the switch was closed and the furnace was powered. You would see 24V if the switch were open and the furnace was powered.
 
  #27  
Old 10-23-11, 07:27 PM
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I think the limit switch is shot - fortunately it appears to be closed (0v) regardless whether it is the blower only, blower and ignited or no blower/not ignited. When I had the problem, the reading was 24v... can't seem to get the 24v reading anymore.
 
  #28  
Old 10-24-11, 04:18 AM
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no, it is reading properly.... IF the witch were open (indicating the unit was overheated) it would read 24V across the terminals.
 
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Old 10-24-11, 07:55 PM
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Oh, I see. Thanks hvactechfw!! I'm learning a bit about this furnace. I'll keep an eye on it and if it trips next time I'll replace it. But I'll definitely make sure I replace the filter every 3 months if not sooner. Thanks a bunch.
 
  #30  
Old 02-02-13, 03:00 AM
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Fixed--Thank You!!!

This forum was a huge time and money saver!! I have a York Diamond 90 furnace that is about 13 years old. I lost heat in the middle of a 10 F day outside. The blower was running, but no warm air. With the temp inside down to 60 and falling fast, I dug up the manual and learned how to check the LED light for an error code. It flashed 4 times meaning that an auxiliary limit switch had opened due to dirty filter, bad blower motor, etc. My filter was very dirty, so I cleaned it. Tried restarting, but only the blower came on again. LED still flashed 4 times.

I looked around for the quarter sized switch with the red button. No luck. I tried turning off furnace power for 10 minutes to allow a reset. Still no luck.

Then I saw the discussion in this forum about the switch above the burner box. I found the switch/sensor that was flush with the hot air output. It is black and made by Texas Instruments. It had 2 leads coming out of it. I simply unplugged one of the leads and plugged it back. Like magic, everything was fine and the furnace started right up.

My question--why didn't cutting power reset the switch if unplugging the leads did? Do I have a bad switch?

Thanks. I think I just saved a $150 emergency service call!!

I
 
  #31  
Old 02-02-13, 11:31 AM
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>


Usually such switch have a manual reset. They must be pushed to reset them.

Also, such limit switches are usually there to prevent an unsafe condition from occurring. If you have repeated episodes of that switch opening, have it inspected by a competent repairman.
 
  #32  
Old 02-02-13, 12:37 PM
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Unfortunately, just like coldinMA said earlier in the thread, there is no manual reset. I had to unplug the lead to get it to reset.

I have read in other forums since I wrote this that people have solved the problem by gently tapping the switch. It is supposed to automatically reset, but sometimes it sticks open. I think I might have bumped it enough during the unplugging to get it to close.
 
  #33  
Old 02-03-13, 02:19 PM
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Umm. This last issue illustrates the limitations of diagnosing problems over the internet.


First, the latest poster shopped around for a thread he thought was relevent to his problem, rather than starting a new thread.

That's usually a mistake. If people were really good at diagnosing furnace issues, they wouldn't be here.

By supposing and discussing an auxilary limit switch problem, I was focussed on that as a possible issue.

Instead, what appears to have been the problem was a high temperature limit switch that was opening and failing to close by itself ---sticking open.

That's quite a different issue.


The moral of this story is that people would be wise to START A NEW THREAD when they have a problem with which they need help.

When you do that, describe in detail the sequence of events that is happening with your furnace. That is FAR more likely to lead to a prompt and accurate diagnosis of a problem than burdening people with your guesses about what the problem may be.
 
 

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