Good Sam Club Open Roads Forum: Tech Issues: Furnace Short-Cycling
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 > Furnace Short-Cycling

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dougrainer

Carrolton, Texas

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Posted: 03/29/20 04:08pm Link  |  Quote  |  Print  |  Notify Moderator

The ONLY way at this point is to have the LP pressure checked at the furnace. NOT doing this will just cause problems in diag.
1. Removing the exhaust tube decreases the separation of exhaust and fresh air intake.
2. So, when short cycling, the system NEVER shuts down, just stays running with short cycling?
3. You cannot bypass a Sail switch on most furnaces built in the past 5 years. IF you engage the Sail switch BEFORE the control module starts the fan, it will cause ignition problems. The newer Module boards are smart enough to know when the sail switch is closed before fan start up. Older Furnace boards you could keep the sail switch closed and the furnace would operate correctly for testing.
4. I would want to take a voltage check on the output side of the sail switch to see if it loses its 12 volt signal for a few seconds and then restores the signal which would cause short cycling. Which would mean a bad sail switch
5. This also points to a air volume restriction. Doug

dougrainer

Carrolton, Texas

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Posted: 03/29/20 04:11pm Link  |  Quote  |  Print  |  Notify Moderator

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

Orange, CA

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Posted: 03/29/20 05:08pm Link  |  Quote  |  Print  |  Notify Moderator

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

Pontiac, MI

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Posted: 03/29/20 05:28pm Link  |  Quote  |  Print  |  Notify Moderator

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

Carrolton, Texas

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Posted: 03/29/20 07:37pm Link  |  Quote  |  Print  |  Notify Moderator

Well, you have NOT answered my questions. Have you pulled the burner orifice and made sure it is not restricted? Doug

agiannetti

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Posted: 03/29/20 10:29pm Link  |  Quote  |  Print  |  Notify Moderator

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

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Posted: 03/29/20 10:39pm Link  |  Quote  |  Print  |  Notify Moderator

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

Cornelius, Oregon

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Posted: 03/30/20 01:18am Link  |  Quote  |  Print  |  Notify Moderator

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

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Posted: 03/30/20 06:47am Link  |  Quote  |  Print  |  Notify Moderator

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

WI

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Posted: 03/30/20 11:25am Link  |  Quote  |  Print  |  Notify Moderator

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.

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