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Old 11-13-2012, 12:52 PM   #1
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Need help with furnace short cycling

We were staying in the LY last night and found that the bed room furnace was short cycling. On for 5 minutes and then off for 2 and then back on. Room eventually obtained set temp, but obviously there's something wrong. First time we've used the furnace for any time since we purchased the LY. The furnace is an Atwood 8516 Series II.

After some research I found that the most common causes were:
Obstructed fresh air intake
Obstructed ducted air exhaust
Obstruction in the burner chamber
Weak limit switch
Low LP pressure
Slow blower fan
Low DC voltage

I did some initial troubleshooting and found that the unit has sufficient voltage when running and LP pressure is good. I also did verify that the shutoff is due to the over temp limit switch. All interior vents are open and the unit doesn't change operation when the outside door is open so I'm betting it's not a fresh air intake issue.

This leaves me with either a weak limit switch, an obstructed burner chamber, marginal ducted air or a slow blower fan. I did check the recommended square footage of the duct work v/s the MFG. recommended volume and found that AS installed marginal amounts of duct work with only two 4" lines with several 90 degree bends. They recommend that the 16k btu furnace have at least 24 square inches and I currently have just over 25. It's also possible that I have a mud dobber infestation in the burner chamber deep enough that I can't see it through the exhaust port. Haven't checked the burner assembly as of yet.

Here's what I'm planning so far.

Replace limit switch (figure it's good to do since it's been cycling and now weak)
Add one additional 4" duct (Nice to have one in the hallway anyway)
Inspect and flush burner assembly


What I was wondering is, how do you clean out the burner chamber?

I was thinking, since I have to remove the fan and burner assembly to replace the limit switch, I'll have a good opportunity to clean out the burner chamber.

Does anyone see a problem with simply running a hose into the burner inlet and let the water flush out the exhaust?

This should dissolve any of the mud dobber nests and flush them out if there in there. I then plan to use compressed air to blow out any of the remaining water. After that I can fire up the furnace and dry out any remaining water. Just hate to have to remove the furnace if I don't have to.

Any thoughts?
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Old 11-13-2012, 01:02 PM   #2
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I don't know about the water. I have thought of doing that but I would not try it while it is installed. You are going to need to dry it out with compressed air before the ignitor will work. There is a sight glass on mine that lets you view the burner. You might try running without the front cover which will remove any restrictions on the exit side. If that cures the problem maybe you have a restriction in the lines somewhere. There should be a screen over the intake and it is wise to put one over the exhaust because dirt dobbers will go through the exaust and make nests.

Perry
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Old 11-13-2012, 01:24 PM   #3
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The first thing I would look at is the thermostat. Disconnect the 2 thermostat leads and dross them. If the furnace does not short cycle that's the problem. Buy a cheap thermostat anywhere in which you can adjust the hi low off o temps. It does not have to be 12v or for an RV. If it is for 24v it will work fine.
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Old 11-13-2012, 02:45 PM   #4
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delimiter

take a close look at your thermostat; inside of it you will find a delimiter control, which is adjustable. You can set your furnace to run longer and then stay off longer. You can eliminate short cycling, but you need to experiment a little bit to hit the sweet spot-keep a record of where it is set by default, and where you move it too, and whether or not you wish to make the run time longer, etc, etc. I think you will be happy with the changes.
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Old 11-13-2012, 03:08 PM   #5
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Your year may have the same thermostat as my 91. In which case I would also say look at the thermostat with the cover off while it cycles. If it is the thermostat that is causing the cycle you will see the switch make and brake as the heater goes on and off.

I placed a small piece of match cover paper between the 2 parts of the element that come up from the spring coil to increase the separation between them. This increased the hysteresis of the stat mechanically.

Some trial and error will be required to get the stat where you want it so it does not over heat during a cycle.
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Old 11-13-2012, 03:31 PM   #6
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Guys, this is not a thermostat issue. I’ve verified that the thermostat contacts are still closed and the over temp limiter is causing the shutdown before reaching room temp. The units burner cycles on/off/on/off because of the over temp limit switch. The fan never turns off while doing this. I've verified this by measuring the voltage path and the break going to the ignition/propane valve board is at the limit switch. Voltage passes through the thermostat then through the sail switch but is stopped at the output of the limit switch. Once the burner box cools off it fires back up. Eventually the room gets warm enough to turn the thermostat off.

I ran the unit with the front cover off and it did not change a thing. The unit is either overheating or the over temp sensor is weak. I'm going to replace the over temp sensor as routine, but since I have to disassemble the motor and burner assembly to do this I was just wondering if I could flush out the burner box at the same time to eliminate the mud dobber infestation possibility. I do plan on using compressed air to blow out any water still left in the burner assembly before I put the burner back on.

Any issues you guys can see by doing this? As far as I know the burner box is simply a zigzag of metal conduit pipes starting at the burner and exiting at the exhaust. There are no electrical assemblies inside this pipe. The pipe should be sealed from the inside of the air box or CO would be allowed into the interior of the motor home. CO alarm never went off.
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Old 11-13-2012, 03:44 PM   #7
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If it doesn't blow out with air, I don't know if water will flush it any better. In any case, it isn't going to hurt anything... get it as dry as you can, and within 5 seconds of the furnace coming on it will be bone dry.

The temp limit switch is hard to diagnose other than replacing it. Without sticking a probe in to get the actual temp.

If it is getting too hot you are dealing with an airflow issue (if the sensor is good). Once you clean the burner, make sure you are getting a nice clean flame, solid blue and not waving around. Is the fan pushing a good amount of air out?
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Old 11-13-2012, 04:45 PM   #8
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The fan seems to be pushing good air, but I'm not sure it's to spec. The main cabin furnace seems to blow a little more air, but that's a 35K BTU unit v/s the 16K that feeds the bedroom. I get air out of the exhaust so it's not blocked off completely. It may just be restricted assuming that there are any dobbers in there at all. I did smell the distinct odor of burnt insect critter when I fired up the furnace. Nothing like burnt mud dobber nest to make your day. I figure that the water will desolve the mud that the dobber nests are made of. Maybe run water through the furnace, let it set for a few minutes to soften the nests and then run water through again to flush the little buggers out.

Changing out the motor is the last thing on the list if the other things don't fix it. To replace the motor would cost about $90. The limit switch was only $15 so it was a no brainer to just replace it as first step. Like you said, it's tough to diagnose without a temperature probe. I did verify that the over temp limit sensor was opening up after running for about 5 minutes.

I'm thinking that I can do all the cleaning of the motor assembly and tubes while I was changing out the limit switch (located behind the blower wheel against the back of the air box). I also figure that adding a third ducted vent will add additional airflow through the system. This should cool the burner box better. As I calculated, the amount of ducted air space AS installed is marginal at best with only two 4” vents that take several 90 degree turns and have at least 10' runs each.

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Old 11-13-2012, 04:53 PM   #9
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Quote:
Originally Posted by wmarsha View Post
take a close look at your thermostat; inside of it you will find a delimiter control, which is adjustable.
Keep in mind not all thermostats have a delimiter. If you get a new thermostat make sure it has a delimiter.
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Old 11-13-2012, 06:10 PM   #10
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We had a new fiver once (before we figured out how good AS are) that did the same thing. It was under warranty, and the solution suggested by Dometic and the trailer builder was to add an outlet. It did fix the problem.

The only question in my mind is that my trailer was new, so the dirt-dobber idea may need more investigation........


Good Luck,

JD
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Old 11-13-2012, 08:48 PM   #11
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Quote:
Originally Posted by 92landyacht View Post
Guys, this is not a thermostat issue. I’ve verified that the thermostat contacts are still closed and the over temp limiter is causing the shutdown before reaching room temp. The units burner cycles on/off/on/off because of the over temp limit switch. The fan never turns off while doing this. I've verified this by measuring the voltage path and the break going to the ignition/propane valve board is at the limit switch. Voltage passes through the thermostat then through the sail switch but is stopped at the output of the limit switch. Once the burner box cools off it fires back up. Eventually the room gets warm enough to turn the thermostat off.


You've done your homework.


Quote:
I ran the unit with the front cover off and it did not change a thing. The unit is either overheating or the over temp sensor is weak. I'm going to replace the over temp sensor as routine, but since I have to disassemble the motor and burner assembly to do this I was just wondering if I could flush out the burner box at the same time to eliminate the mud dobber infestation possibility. I do plan on using compressed air to blow out any water still left in the burner assembly before I put the burner back on.

Any issues you guys can see by doing this? As far as I know the burner box is simply a zigzag of metal conduit pipes starting at the burner and exiting at the exhaust. There are no electrical assemblies inside this pipe. The pipe should be sealed from the inside of the air box or CO would be allowed into the interior of the motor home. CO alarm never went off.
Usually, with older furnaces showing this problem, it is because of impending failure of the blower motor bearings causing a loss of RPM. Less often it can be caused be insufficient voltage at the blower motor caused by cumulative losses in the wiring. Check the voltage right at the blower motor while operating, if below 12.0 you have a wiring problem. Otherwise chances are the motor bearings are shot and you'll have to choose between replacing the motor and replacing the furnace.

Airflow is proportional to the square of RPM with these so a small RPM drop is enough to cause overheating.
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Old 11-13-2012, 10:18 PM   #12
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That was along my thinking too. I just figured that replacing the limit switch would be required as well since it has may have been cycling for some time before I purchased the unit last year.

I've checked the voltage all the way along the chain. The voltage never drops below 12.5V with the blower and burner running.

I guess I'll just do what I planned and if the problem is still present then I'll replace the blower motor too. I could replace the furnace for around $460 (found a web site that offers the Atwood 8520-IV for $419 plus shipping) but I'd like to see this one fixed if possible. It looks in good shape and fires reliably so fixing this issue should not be too difficult and should cost much less to get running. All though the new furnace is tempting.
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