Atwood Furnace Blower Motor Low Air Output

[Bill R]

I tried a search first for my issue and did not find anything.  May be my search capabilities are lacking so apologies in advance.

Since purchasing this coach 2 years ago, I have had what I think is below normal low flow air output from my Atwood 8520-IV 20,000 BTU furnace.   The unit operates as expected, starts up very quickly and does not cycle off until the thermostat set point is met.  Just not the flow I would expect.  This unit provides heat to the bathroom and bedroom of the coach.  However, the front Atwood 8531-IV 30,000 BTU furnace puts out significantly more air output.  The temp output for the 8520 gets to 120F with a 65F in.  The front 8531 puts out 155F air with the same 65F in.   Also, the exhaust gas outside has a significantly more noticeable stronger fume odor from the 8520 exhaust versus the 8531 exhaust.  What I would describe as an incomplete combustion smell. 

I am going to list all of the troubleshooting items that I have performed below and then why I think the blower motor needs to be replaced.  

- I have inspected all of the inside duct work to make sure there are no obstructions.  With the exception of some pieces of broken tile in the ductwork from when the tile was replaced, there are no obstructions.  Also the number of vents and SF area of ventilation is well within the spec of the installation manual requirements.

- I have made sure the outside combustion exhaust port is clear.  Initially I did find some mud dobber nests, but they have been removed and I have scoped the exhaust tube and it looks very clean now.

- I have removed the burner and it is in good shape.  Also inspected the burner tube exhaust section with a scope and it is clear.

- I have measured voltage to the motor.  It is at whatever the battery voltage is.  12.3V plus.  

- I have measured the amps when the motor is running.  The FLA for this motor is 4.6 Amps.  I was measuring 3.45 Amps.  

- I have measured the RPM of the motor.  It is running around 1,500 RPM.  I cannot find the spec for RPM's of this motor anywhere.  I have talked with the current OEM distributor, talked with Dometic tech support, and Googled and still not able to get an RPM spec.  I do find that the 8531 motor spec it 2,250 RPM.

My theory is that the motor has slowed down over 16 years of use causing decreased ventilation flow and will also cause a lower combustion fan flow to the burner and in turn lower flame temp and incomplete combustion.

I can easily purchase the replacement motor for around $75.  But before I spend the money and even the time to replace it, I want to see if anyone has had the same experience and did a motor replacement fix the issue?  Also looking for thoughts from all the brilliant minds in this forum even if you haven't had this problem.

[tmw188]

What’s the other FLA on the other furnace? Maybe its wired to the stat to run at a different speed, if it has that ability? Did you check the plenum where the 4” take offs are attached? 

Edited October 22, 2023 by tmw188

[Bill R]

31 minutes ago, tmw188 said:

What’s the other FLA on the other furnace? Maybe its wired to the stat to run at a different speed, if it has that ability? Did you check the plenum where the 4” take offs are attached? 

The FLA for the 8531 is 7.6 Amps.  I measured that also and it was showing 5.3 Amps when running.  They are different motors between the two units.

Yes, checked all 4" plenums.  All clear.

[jacwjames]

Have you checked the routing of the duct work themselves, maybe there is a restriction/pinched point.  

Also, I have sections of duct work that drop down into the basement and then back up.  I found that a mouse(s) had chewed some holes in the section that ran in the basement.   The passenger side of the compartment on my fresh water tank opens and I found the leaks when I was working on something else.

[Bill R]

10 hours ago, jacwjames said:

Have you checked the routing of the duct work themselves, maybe there is a restriction/pinched point.  

Also, I have sections of duct work that drop down into the basement and then back up.  I found that a mouse(s) had chewed some holes in the section that ran in the basement.   The passenger side of the compartment on my fresh water tank opens and I found the leaks when I was working on something else.

Jim - Yes I have checked the duct work, but I have not scoped the plenum that goes under the floor.  I will do that.  However, I did remove the distribution box header so there would be no air restriction on the supply side, and the outlet temp still stayed at around 120F.   There is in addition to the plenum under the floor three 4 inch ducts that are very short runs.  Two to the bedroom and one to the bathroom.  I can see in those and there are no restrictions.

So I am 99% sure that this is not a distribution restriction issue.  

 

[Bill R]

On 10/21/2023 at 9:13 PM, jacwjames said:

Have you checked the routing of the duct work themselves, maybe there is a restriction/pinched point.  

Jim - Here is a pic of the supply duct.  You can see clear to the furnace discharge plenum.  No restrictions.

Does anyone have an Atwood 8520-IV 20,000 BTU furnace.  I would love to know what your inside register discharge temperature runs.

 

[jacwjames]

What about the output ducts as they go to the registers?

[Bill R]

Jim - The scope is in the output duct looking to the discharge plenum.  I removed the floor register and put the scope in going towards the furnace plenum discharge.  I also scoped the duct from one floor register to the other floor register.  There was no blockage as I ran the scope through the output ducts.  Sorry for the confusion.