2018 Furnace - Atwood/Dometic model AFSD20141 is short-cycling.I tried the following everything:Jumped the thermostat jumped the high temp limit switch jumped the sail switch removed discharge vents to assure no blockages checked return air for obstructions made sure battery fully charged (I have NOT yet checked the gas pressure) I turned the main gas valve to almost closed to lower gas pressure (as evidenced by reduced stove flame) - that corrected the furnace for short-cycling, but only for one day. MAJOR CLUE TO THE ISSUE: Without fail, when I remove the exterior panel and vent tube, it works fine. The INSTANT I start to replace the panel, the burner shuts off. Is it possible/probable, when I start to put the cover on, the change in airflow disrupts the flame under the electrode and causes the burner to shut off? Would the absence of a strong flame immediately shut off the burner?

How short is the short cycling? If in seconds it can be the flame sensing not working right. If minutes it can be the limit switch not working right.There are a couple of furnace guys on here who might be along to answer your questions.

It cycles every few seconds. I've jumped the high limit switch and the sail switch, but the problem persists.

Seconds suggests the fame sensor not sensing flame long enough. If that is because not enough air to support a flame, unless door open then perhaps the burner area is plogged up. Can happen with wasp nests, eg.

Could it be a bad flame sensor?

agiannetti wrote:Could it be a bad flame sensor?Newer furnaces use the circuit board somehow along with the single probe and neg sparker for flame sensing. Older ones used a third probe that was the flame sensor. I suppose it could be a fault in the board to make for "bad sensor". I am not a furnace tech.Besides what you have already done in the OP, if not done, IMO do a complete burner clean out as what to try, including the orifice. Check the wire connections on the blade terminals are tight for the probes too.We need the furnace pros to jump in here! :)"The older boards used a separate flame sense electrode, also known as remote sensing. Now they sense the flame through the spark gap, aka local sensing. You can sub a local sense board for remote, but not vice versa. -- Chris Bryant "

Furnace Short-Cycling | Good Sam Community

dougrainer
Nomad
Mar 29, 2020
I turned the main gas valve to almost closed to lower gas pressure (as evidenced by reduced stove flame) - that corrected the furnace for short-cycling, but only for one day.

This is NOT going to really lower the gas pressure. It will LOWER the VOLUME of LP gas to the system. The ON/OFF LP tank valve just turns the LP on and OFF and has no relation to LP pressure. What you did was, cause a lack of LP volume and that caused low volume of LP to the system. Doug
agiannetti
Explorer
Mar 29, 2020
I took the furnace apart in order to look at the burner. Everything, including burner is clean (as it should be, it's only 2 years old). I did notice a couple grains of debris (the size of grains of sand) in the screen filter in the gas line right where it connects to the main gas hose. I cleaned out the debris, put everything back together and it's working as normal, even with the restrictor piece inside the exhaust vent. However, I'm not convinced the problem won't return since it has worked for short periods of time recently.
mich800
Explorer
Mar 29, 2020
dougrainer wrote:
mich800 wrote:
Do RV furnaces use an inducer motor/fan?

If you are talking about the Fan pressure inducer like in a Gas HOME furnace---NO. The Sail Switch is used instead of a Pressure inducer on RV furnaces. Doug

Thank you. Did not want to suggest something that was not applicable. I had to replace the inducer motor on home furnace last month. Even though it was operating it was turning too slowly and not giving enough draft to keep from tripping the flame roll out.
dougrainer
Nomad
Mar 30, 2020
Well, you have NOT answered my questions. Have you pulled the burner orifice and made sure it is not restricted? Doug
agiannetti
Explorer
Mar 30, 2020
Doug - Yes, see my prior post. I pulled the burner out of the heat exchange, no obstructions. Ran cable through heat exchange tubes, it was was clear, electrode was clean. After I put it back together it ran fine. However, as I suspected, later in the day I tried again and short-cycling continued. I hope it's a gas regulator issue, it's my last hope. Taking it to shop Tuesday.
agiannetti
Explorer
Mar 30, 2020
Also, to answer your question, system NEVER shuts down, just short-cycles.

Regarding that restrictor inside the exhaust vent tube, which decreases the inside dimension of the tube to about 1/2" diameter, do you know it's purpose? When it's removed, no more short-cycling. Can I just remove that restrictor permanently?
wnjj
Explorer II
Mar 30, 2020
agiannetti wrote:
Also, to answer your question, system NEVER shuts down, just short-cycles.

Regarding that restrictor inside the exhaust vent tube, which decreases the inside dimension of the tube to about 1/2" diameter, do you know it's purpose? When it's removed, no more short-cycling. Can I just remove that restrictor permanently?

My wild guess is the restrictor is there to help with the efficiency. Too much flow out the exhaust is sending heat directly outdoors.
JaxDad
Explorer III
Mar 30, 2020
agiannetti wrote:
Also, to answer your question, system NEVER shuts down, just short-cycles.

Regarding that restrictor inside the exhaust vent tube, which decreases the inside dimension of the tube to about 1/2" diameter, do you know it's purpose? When it's removed, no more short-cycling. Can I just remove that restrictor permanently?

To my mind a constant short cycle is indicative of the high-limit sensor kicking in, it hits high temp, shuts down, cools off and repeats.

You need to check first the obvious things like restrictions on air flow inside the rig’s distribution system, that would keep too much heat in the furnace itself. Even a blower turning slower than spec. would do the same thing.
smthbros
Explorer
Mar 30, 2020
agiannetti wrote:

Regarding that restrictor inside the exhaust vent tube, which decreases the inside dimension of the tube to about 1/2" diameter, do you know it's purpose? When it's removed, no more short-cycling. Can I just remove that restrictor permanently?

It sure seems like you have narrowed the problem to combustion ventilation. Sounds like the flame is smothering itself. One other thing to check is that that draft inducer blower wheel is secure on the motor shaft. As you may know by now, the draft inducer and air circulation blower share the same motor.
As far as removing the restrictor, due to liability issues, you may want to contact the manufacturer.
BFL13
Explorer II
Mar 30, 2020
agiannetti wrote:
Also, to answer your question, system NEVER shuts down, just short-cycles.

Regarding that restrictor inside the exhaust vent tube, which decreases the inside dimension of the tube to about 1/2" diameter, do you know it's purpose? When it's removed, no more short-cycling. Can I just remove that restrictor permanently?

Check the space between the two tubes to see if it that space is restricted by something that should not be there. Closing the outer door may shut that part off, and opening the door may let enough air through?

The exhaust vent tube in my 1990 8531 does not have a "restrictor" for the inside tube or in the space between it and the outer tube.

It is two tubes one inside the other with a space between the inner and outer for air to pass through. (Incoming air I guess ????) When the furnace is on, the hot air coming out of the inner tube exhaust is very hot. I always think that is such a waste of heat, but no way to capture it.

The only restriction inside the inner exhaust tube is a small metal part where a sort of bracket holds the two tubes together.

Here is a photo of the inner end of the tube assembly from my spare 8531

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