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Old 11-30-2020, 09:31 PM   #1
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Question about an Atwood Furnace Model 8520

Have dual furnaces and both the same. Getting a no air flow light the fans never kicks on at start up on both units. Check powers and grounds to both units all ok. Any help would be appreciated thank you
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Old 11-30-2020, 10:40 PM   #2
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DC Power from Fused source is AT furnace all the time...same DC source goes to one side of thermostat

When Thermostat Closes DC goes back to circuit board to trigger DC power to FAN

What is interesting is that you say BOTH will not start
Are they controlled from Same thermostat?
What thermostat?

What 'power & grounds' did you check?
Checked using voltmeter?

What Year/Model is RV?
Does the DC for furnaces go to A/C Unit control module then to T-stat?

Here is the wiring for the Furnace ...but RV wiring is an unknown

Could be the On/Off CB switch (On furnace) could be tripped
Could be bad wiring connection at circuit board
Could be bad circuit board
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Old 12-01-2020, 04:54 PM   #3
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Forgot to add the wiring diagram so here it is

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Old 12-02-2020, 11:00 AM   #4
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Quote:
Getting a no air flow light the fans never kicks on at start up on both units.

You are referring to the single flash of the internal diagnostic light? AFAIK, that is intended to be for a high limit switch trip, which is often caused by reduced circulation air flow. It should only happen once the burner has ignited, but you say they don't light at all and the fan never runs?


If the fan doesn't come on at all, the problem is in the circuits that Old Biscuit described. Follow up on his questions & suggestions to determine what is happening.
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Old 12-02-2020, 11:56 AM   #5
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Originally Posted by Gary RVRoamer View Post
You are referring to the single flash of the internal diagnostic light? AFAIK, that is intended to be for a high limit switch trip, which is often caused by reduced circulation air flow. It should only happen once the burner has ignited, but you say they don't light at all and the fan never runs?


If the fan doesn't come on at all, the problem is in the circuits that Old Biscuit described. Follow up on his questions & suggestions to determine what is happening.
It is a DUAL Error Code
Limit Switch/Airflow

STANDARD 3 TRIES FOR IGNITION CONTROLS
DIAGNOSTIC CHART
FAULT LED INDICATION LOCKOUT
*Internal Circuit Board Failure Steady On, No Flashing =Hard
*Limit Switch/Airflow Problems 1-Flash With 3-Second Pause=Soft
*Flame Sense Fault 2-Flashes With 3-Second Pause===Hard
*Ignition Lockout Fault 3-Flashes With 3-Second Pause === Soft (1hr.retry)
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Old 12-02-2020, 06:48 PM   #6
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Thanks for the help. Check all the powers with ground directly on batts and check voltage from diagram using a voltmeter. Found t stat sending voltage to board but no output, also could here relays buzzing on boards. Double check ground connection and continuity all ok. Check with previous owner and found out they had hook up batts charger wrong and overcooked batts and they exploded in box. The water heater wasnít working also and I replaced the control in it to get it to work. Believe all of this related. Thank you for all the help it was very helpful.
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Old 12-03-2020, 11:22 AM   #7
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Originally Posted by LeeA View Post
Thanks for the help. Check all the powers with ground directly on batts and check voltage from diagram using a voltmeter. Found t stat sending voltage to board but no output, also could here relays buzzing on boards. Double check ground connection and continuity all ok. Check with previous owner and found out they had hook up batts charger wrong and overcooked batts and they exploded in box. The water heater wasnít working also and I replaced the control in it to get it to work. Believe all of this related. Thank you for all the help it was very helpful.
Sounds like furnace boards took a shot also.....
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