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Old 09-20-2021, 12:29 AM   #1
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Furnace issue

Hi all. This is probably a question that's been answered before but I could not find any.
I have an Atwood 8940-iii-dclp furnace in my 2000 Holiday Rambler. I have replaced every component except the sail switch, fan assembly and heat exchanger and it still short cycles. Has any one had this same problem and what if any solution fixed it?
I am not really wanting to replace with a new unit unless I really have to.
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Old 09-20-2021, 07:53 AM   #2
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Among the items already replaced , was the over temp sensor one ?

Have you had the motor out and cleaned the blower fan ?

Have you had the furnace fully out of the coach for a full cleaning , including the inside of the heat exchanger ?
Helped a friend with short cycling and ended up getting about 3 lbs of mud dauber nest out of the exchanger .
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Old 09-20-2021, 11:12 PM   #3
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Yup changed and cleaned everything except what I listed previously. The only thing I'm thinking it might be is a propane supply problem .

Heat exchanger is clean. As it's the exhaust tube.
I've tried the troubleshooting chart that I have for these units but nothing seems to work.
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Old 09-26-2021, 11:28 AM   #4
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I changed just about everything. Control board, sail switch, electrode and didn’t work for me.

Until I took heating element off and the nozzle top and found the hole I have circled was clogged. Sent the $120 Control board back to Amazon for refund kept the other $10 and $20 parts. Works fine now.
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Old 09-26-2021, 11:43 AM   #5
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Quote:
Originally Posted by Resqmedic02 View Post
Hi all. This is probably a question that's been answered before but I could not find any.
I have an Atwood 8940-iii-dclp furnace in my 2000 Holiday Rambler. I have replaced every component except the sail switch, fan assembly and heat exchanger and it still short cycles. Has any one had this same problem and what if any solution fixed it?
I am not really wanting to replace with a new unit unless I really have to.
Short cycling is due to:
*High LP System Pressure (overfiring)
**Orifice damaged....poking things thru it to clean/oversizing it
*Blocked/obstructed discharge ducting/registers (airflow issue)
*Blocked/obstructed Return Air Path (airflow issue)
*Fan room wheel loose/broken vanes (airflow issue)
*Dirty heat exchanger (heat transfer issue)

Sail switch has nothign to do ith short cycling....if bad there would not be any DC for gas valve/ignition




Joe442
That is the burner orifice.
If clogged then gas flow is obtructed resulting in poor flame/no flame
Not a short cycle situation like OPs
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Old 09-26-2021, 12:00 PM   #6
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What is meant by short cycle?
1) Does it flame for maybe 2-3 minutes off for a few and back on? The older stats you needed to set the anticipator. Not sure how they do it with a digital stat. This is a short cycle.
2) Does it fame and maybe 10-20 fame out, 30-90 seconds the
fan stops , waite a little and try again and again for like 5 try's. Then stops trying until you turn off the stat and turn on (reset). I would clean the flame sensor with steel wool, check the and position of it to the flame. Check the voltage of the 12V to the furnace, and check the flame size. I had this issue and it was junk in the orifice and blocked about 1/3 of the opening. This is not a short cycle, its a failure to to sense the flame, it fails during the cycle.

PS, I was a much slower typer then Old-Biscuit. He is the expert I would go to.
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Old 09-26-2021, 12:14 PM   #7
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Short cycling.....operating on the High temp Limit switch

Fan comes on, flame ignites, then temp gets to high at discharge of heat exchanger/limit switch open---Fan STAYS running
Temp cools at discharge of heat exchanger/limit siwtch cloes/flame reignites
Repeat
That is 'short cycling' (also called 'Limiting')
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Old 09-26-2021, 04:30 PM   #8
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Quote:
*Blocked/obstructed discharge ducting/registers (airflow issue)
*Blocked/obstructed Return Air Path (airflow issue)
I'd bet on one of these two, causing the furnace to cycle on the hi-temp switch
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Old 09-26-2021, 05:05 PM   #9
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If the furnace runs for 30-60 seconds than cycles off then the control board must be getting a signal from the overtemp limit switch. As other posted have stated this is likely due to low air flow in the combustion side of the furnace, due to an obstruction in the inlet or outlet ducting or the combustion side blower wheel being partially blocked. Propane attracts wasps and it's not uncommon for insect built structures to be the root cause. Why so many of us have wasp screens on our propane furnace combustion air inlet and outlet vents.
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Old 09-27-2021, 07:33 AM   #10
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When I had unit apart I checked combustion chamber and vents for blockage. None found. I cleaned the fan hardly dirty.
I've replaced thermostat, (works fine on A/C and heat pump) replaced burner assembly, electrode and valve assy as well as the valve on propane tank has been replaced and the limit switch has been changed twice to rule out faulty limit switch.
I am familiar with the cycles this unit goes through. The only thing I haven't tested yet is voltage at connections. Other than that I still have the less than a minute heat then of and few minutes later heat again and so on.
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Old 09-27-2021, 08:05 PM   #11
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Quote:
Originally Posted by Resqmedic02 View Post
When I had unit apart I checked combustion chamber and vents for blockage. None found. I cleaned the fan hardly dirty.
I've replaced thermostat, (works fine on A/C and heat pump) replaced burner assembly, electrode and valve assy as well as the valve on propane tank has been replaced and the limit switch has been changed twice to rule out faulty limit switch.
I am familiar with the cycles this unit goes through. The only thing I haven't tested yet is voltage at connections. Other than that I still have the less than a minute heat then of and few minutes later heat again and so on.
Quote:
Originally Posted by Gary RVRoamer View Post
I'd bet on one of these two, causing the furnace to cycle on the hi-temp switch
*Blocked/obstructed discharge ducting/registers (airflow issue)
*Blocked/obstructed Return Air Path (airflow issue)
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