Atwood furnace problem

[clintstimson]

I have an Atwood 8531-iv furnace that is giving me an airflow/limit switch fault light. Is there any way to increase the blower output on these models??? The furnace will come on and run for about 5-8 minutes and then heat shuts off (blower continues to run) after cooling for approx. 2-3 minutes furnace re-fires and repeats cycle. Sail switch and limit switch appear to operate normal.If I pull off the outside cover and open up the shroud around the burners the heater will run for approx. 20 min. and heat normally. leading me to believe blower output is to low???? I'm fairly new to this so I hope I'm asking in the right spot.

[chasfm11]

It is possible that you have a real problem and that the safety switches are doing their job.
- how are your batteries? The fan runs off the batteries and if they aren't putting out proper voltage, the fan will run slow, causing either the sail switch to drop out or the burner area to overheat, shutting down the gas valve.
- is there any blockage in the output vents to the RV? It may not be visible if it inside of a duct. While the furnace is running, consider checking in front of each duct for air flow.
- My Atwood has bushings, not bearings in the motor. A couple of years ago, it started to squeal and run slower. I took it apart, got some more oil in the bushing and it was better for a while. The fix was to replace the motor. It isn't a terrible job but you do have to take the gas valve out which means that you have to shut off the propane to the whole RV and bleed off the lines first.

We have two 8525-IV so I always have another unit to compare. In your case, it might be helpful to hear another 8531 just to compare the noise from the fan speed. While it is possible that you have a flaky limit switch or sail switch, it sounds like air flow is your problem.

[clintstimson]

My ducts appear to be clear. However, I will check them again. I will check the Batt. voltage. I did not know the blower operated off the battery's. We have 2 six volt bats. in series. I'm not sure that was original configuration as we bought the trailer used. Thank you I will investigate that path.

[shipwreck1]

i would just call Atwood service, they are always ready to help and that will take out the guesswork.

[RV Wizard]

I have ran across this before and I would bet you have either a bad high limit thermostat or improper air flow. If you have the installation manual, you will find that there is a minimum of number of ducts that must be connected to the unit. You must also have a good return vent for the unit to not cycle off on the high limit. I have had to add another duct to get the proper airflow across the heat exchanger to correct an insufficient air flow problem. You might diagnose by removing one of the blank duct plates off the furnace and see how long it will run. The other possibility could be a faulty high limit thermostat. You would have to be measuring the temperature at this thermostat to determine if it is correct without just trying a new one. If it is weak, this usually means that it has had to cycle on and off, again, due to high temperatures which could be the result of low air flow.

[Clifftall]

Quote:

Originally Posted by shipwreck1 i would just call Atwood service, they are always ready to help and that will take out the guesswork.

574-264-2131

cliff

[clintstimson]

I contacted Atwood service and they instructed me on how to test the high limit switch. It showed a limit fault. However, I opened up an extra duct plate and with the outside cover off the heater ran for over an hour and heated normally. I put the outside cover back on and the heater shutdown and cycled after 8 mins. I am going to follow rvwizards advice and see if I can get this fixed finally.

[Gary RVRoamer]

Note that opening up a duct outlet as you did takes some load off the fan motor and lets it spin up better. Your problem still could be simply that the motor is not able to sustain rated speed under load, i.e. weak motor or low voltage to the motor. The fact that it does run with reduced load shows the sail switch is working ok and the high limit switch as well.

Is this problem recent or has it existed since you owned the rig? If it is not new, you may just need to optimize the ducting by removing excess flex hose, eliminating loops & kinks, etc. That can improve air flow quite a bit. Those flex hoses are restrictive to begin with and slopping installation can make them much worse.

[clintstimson]

The heater has worked this way since we bought it 2 years ago. I was unaware it was not working properly until I complained to another Rv'r how annoying these heater's are(Ignorance is bliss.) cycling like this.
I looked at the hoses and there is a lot of excess in them. I'm going to cut them down and try to run them in the most direct route. I am looking to add an extra duct outlet on the cover I removed. Thanks for the input.

Also, I said I spoke to the atwood folks, and they told me how to test the fault. They had me check the voltage on the limit switch wire after the fault. He said if there is no power then it's a bad limit switch??? Am I wrong in thinking the limit switch did what it was supposed to do, and it should not have power after it opened for an overheat???

[RV Wizard]

The limit switch should open at a given temperature. I can't remember exactly what that temperature is but it is stamped on the T'stat. I want to say it is around 225* F. Straightening out the duct work might allow enough air flow where you won't need additional venting. It all depends on the BTU rating as to the size of return and number of vents needed to ensure good air flow across heat exchanger.