[obarrett]

Hi Folks,

My furnace inducer blower goes on and stays on without igniting the furnace.
The thermostat will not turn off the furnace, I need to physically switch it off.

Brand: Nordyne
Model: L1RA 090N 16B

The circuit board does emit a single red led flash every few seconds:

In the manual this indicates:

Limit Circuit Open or External Load On "W"

So I began some testing:


- continuity tests on the two flame rollout switches were successful while the furnace was off
- voltage tests were successful when furnace was on and they are both closed (around 24 volts on both sides of the posts, while the furnace was running)

- continuity test on the Pressure Swtich shows continuity when I manually (with my mouth) suck the pressure out of the tube
- continuity test fails on Pressure Switch when I blow into the tube
- voltage test FAILS on the Pressure Switch when the furnace is running, no voltage on either post

- I jumped the two pressure switch wires, eliminating the switch and no change, furnace does not ignite, 1 single flash on the LED.

- I removed the pressure switch and did a voltage test directly on the two orange wires going into the pressure switch, this show 0 volts on both wires.


So... my questions:

- what does "Load on W" mean? Is this the W wire from the thermostat?
- no voltage is going to the blower switch, does this sound like circuit board could need replacing?
- what else should I look at?


This issue first popped up 4 days ago...
After jumping the limit switches and doing some tests, banging some things around and some cussing, a and beer the furnace began functioning properly.

It worked fine for the next 3 days, cycling on and off as expected with the thermostat.

Last night it reverting to its bad behavior and I can not figure it out...

Why am I not getting power to the pressure switch?


Thanks in advance folks

[mygirlsdad77]

Okay, so you tested the flame rollout switches, but did you test the high limit? This would be my next step.

[obarrett]

Thanks mygirlsdad77...

Long story short, its fixed.


Short story long... when I tested the voltage on the high limit it was fine on both posts (but looking back, this is when the furnace decided it wanted to work intermittently)

After reading your post, I removed the high limit yesterday, and did a continuity check...
It was fine.
I then had a *** moment.

During the continuity test, I began moving the sensor on the limit and that is when I lost continuity...

So depending on how the limit switch was sitting, it worked or not worked...

I tossed in a new high limit and we are back in business...
Wifey is happy...
My 4 year old son thinks I can fix anything...
And my dog isn't giving me bad looks anymore..


Thanks folks... great resource here..


One last question... the furnace is about 7 years old.. is replacing a high limit switch typical maintenance, or is there a bigger issue?

[mygirlsdad77]

Limits do fail from time to time. Normally they trip from overheating due to lack of air flow caused by a number of things such as dirty filter, plugged secondary heat exchanger(if 90+furnace), plugged ac coil on top of furnace, weak blower motor or bad motor capacitor, constricted supply or return grills, etc. Also a tripping high limit can sometimes be caused by a cracked heat exchanger or high gas pressure. However, like I said, they do just fail sometimes. Since the new limit has fixed the problem, I would say you are good to go. Just watch to make sure the furnace is operating normally and I think you've got the problem solved. Good job and let us know if any further problems arise.