Gas and Oil Home Heating Furnaces - 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!

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.

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!

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?

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.:)

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!

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.

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?
.

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.

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.

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 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.

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!

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.

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

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).

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.:thinker: