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 > Furnace Short-Cycling

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agiannetti

Orange, CA

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Posted: 03/26/20 10:04am Link  |  Quote  |  Print  |  Notify Moderator

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?

BFL13

Victoria, BC

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Posted: 03/26/20 11:58am Link  |  Quote  |  Print  |  Notify Moderator

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.


1991 Oakland 28DB Class C
on Ford E350-460-7.5 Gas EFI
See Profile for House electronics set-up.

agiannetti

Orange, CA

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Posted: 03/26/20 01:16pm Link  |  Quote  |  Print  |  Notify Moderator

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

BFL13

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Posted: 03/26/20 01:25pm Link  |  Quote  |  Print  |  Notify Moderator

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.

agiannetti

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Posted: 03/26/20 02:12pm Link  |  Quote  |  Print  |  Notify Moderator

Could it be a bad flame sensor?

BFL13

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Posted: 03/26/20 02:29pm Link  |  Quote  |  Print  |  Notify Moderator

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! [emoticon]

"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 "

* This post was edited 03/26/20 02:44pm by BFL13 *

agiannetti

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Posted: 03/29/20 12:54pm Link  |  Quote  |  Print  |  Notify Moderator

Update, burner works fine when vent tube is removed. As soon as vent tube is replaced, burner shuts off. I remove some sort of airflow restrictor inside the vent tube that reduces the tube to about 1/2" diameter, burner works fine. I'm sure the restrictor is there for a reason so I will replace it, but just another clue to the problem. I'm going to remove the furnace and inspect the burner assembly.

BFL13

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Posted: 03/29/20 01:04pm Link  |  Quote  |  Print  |  Notify Moderator

I have an older furnace so I don't know what a vent tube is. On my 8531 you don't have to take out the furnace to get at the burner assembly. It is a PITA where the screws are way back in there. You need a magnetic screwdriver to get the screws back in!

With such a new furnace, unlikely to be faulty parts. Wasp nests possible though.

mich800

Pontiac, MI

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Posted: 03/29/20 02:15pm Link  |  Quote  |  Print  |  Notify Moderator

Do RV furnaces use an inducer motor/fan?

dougrainer

Carrolton, Texas

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Posted: 03/29/20 04:01pm Link  |  Quote  |  Print  |  Notify Moderator

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

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