Luxaire High-Eff Failing on Second Stage

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Old 02-03-19, 07:19 PM
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Luxaire High-Eff Failing on Second Stage

Hello all, first question on this forum, hopefully someone can confirm my theories.

I have a Luxaire High-Eff furnance (didn't grab the model but i don't believe it's relevant once i explain the situation) that, after an extremely cold snap up in Ontario, Canada (-25 Celsius for a day or two) has begun shutting down/failing when the furnace goes into Stage 2 of heating. Error code is indicating airflow problem. Performed the following diagnosis:
  1. Checked inlet and outlet vents for ice buildup/debris (including disconnecting the outlet vent from the draft inducer fan outlet to visually check in the draft inducer. All was clear.
  2. Filter was changed about 3 weeks ago, still looks perfectly clean. Not suspecting an issue.
  3. Checked all three pressure switches (draft inducer, low stage, high stage) with ohm meter and all three closed with less than 2 ohms resistance upon draft inducer startup. Seems all three are functioning normally.
  4. Disconnected all hoses and blew compressed air through them (while completely disconnected from pressure switches to prevent damage). All hoses were clear of any obstruction.
Now where things went off the rails:

​​​​​​5.Checked the voltage across all three pressure switches and only found 24V across draft inducer pressure switch and low stage pressure switch, no voltage at the high stage pressure switch though.

Should there always be 24V across the high stage pressure switch as well, or is there a possibility the control board is fried? That's the only conclusion i can come too. Checked all the wiring and everything is good, less than 5 year old furnace so i doubt its a wiring issue.

Any input (confirmation of my suspicions or alternatives) are appreciated.
 
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Old 02-04-19, 05:29 AM
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Hi, if you are getting voltage across the pressure switches that means they are open, a wiring schematic would be helpful.
Geo
 
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Old 02-04-19, 06:20 PM
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Is the error code an open limit or open pressure switch?

They're two very different things. One concerns combustion/venting air and the other, temp of indoor air moving around the heat exchanger.

What's the full model number of the furnace?

Is the problem new? Have you experienced this furnace running in extreme cold before?

You'll see 24v across the high pressure switch unless it's running on high fire.

To check ohms properly, you have to isolate safety switch from circuit.

You can get it running on high fire right away for testing purposes by jumping W1 and W2 on the board. Normally, either a timer brings on the second stage after x minutes or a 2-stage thermostat does it, requiring you to wait or bump the thermostat way up to bring second stage on.

(The 2-stage stat is preferable but right now the concern is getting the unit running properly, so don't worry about which you have for now)
 
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