Suburban SF-35VHQ Short Cycling

[wmcgaughey]

Our furnace is short cycling. Thermostat set to turn the furnace on, furnace fan starts, burner starts, runs for approximately 10 seconds, burner shuts off, cycle starts over. Power to the switch on the furnace is 12.4 volts, when the burner ignites, the voltage drops to 11.6 then starts a decline to around 7 volts, burner shuts off, voltage returns to 12.4, burner reignites, cycle starts over. Sail switch is fine, gas supply is fine, trying to determine what is causing the load change that is tripping the flame off.
2024 GDS 390RK

[ncitro]

I know on the water heater the ignitor does double duty as the flame sense as well and shuts down the unit if it does not sense the unit has lit. My guess is something similar in the furnace, whatever they are using to detect its lit is the issue. That or the board if the gas supply is good as you state. How does the flame look when lit? It should be steady blue. If not there could be a spider nest in the combustion chamber?

[TucsonJim]

It looks like your voltage drop is the issue. Are you running on batteries, or shore power and the converter?

Jim

[wmcgaughey]

Flame is clear blue, flame sensor could be the issue, just can’t figure the reason for the voltage drop when the burner is firing.

on shore power. Battery voltage is 13.6

[howson]

Flame is clear blue, flame sensor could be the issue, just can’t figure the reason for the voltage drop when the burner is firing.

on shore power. Battery voltage is 13.6

I think the question at this point is if the voltage drop is caused by something in the furnace or external to the furnace.

What I'd try is wiring a temporary 12V power source independent of the RV to the furnace and see if it runs as expected or if it still short-cycles.

If the furnace cycles with the external power source, the problem is with a component in furnace.

If the furnace runs fine -- and when hooked back up to the RV cycles again -- there's a wiring problem external to the appliance.

Howard

[Jlawles2]

I say check the connections on the power supply to the furnace. Even pull the furnace fuse and check it (odd thing but someone had an issue and the fuse had micro corrosion at the connection resulting in a similar issue).

The dropping in voltage is typically an indication in either too small of a power supply wire that is heating up over time and increasing resistance (lower voltage), or a bad connection that is doing the same.

[Country Campers]

Have the battery checked at an auto parts store or battery store to see if it is healthy or not. Even though it is new or shows a proper voltage the severe drop in voltage may mean a faulty battery. Also battery connections should be tight and clean.

Brian

[wmcgaughey]

Thank y’all for the good advice. Still testing. Will let you know how it turns out.

[wmcgaughey]

Tech says he bench tested for 30 minutes, furnace worked fine. He did adjust the flame sensor/igniter so hoping that was the fix. Gap was too wide? Still don’t understand how that caused the voltage drop unless the control board is designed to lower the voltage to drop out the gas valve shutting down the flame??

[howson]

Tech says he bench tested for 30 minutes, furnace worked fine. He did adjust the flame sensor/igniter so hoping that was the fix. Gap was too wide? Still don’t understand how that caused the voltage drop unless the control board is designed to lower the voltage to drop out the gas valve shutting down the flame??

Removing the furnace from the RV and testing externally is standard practice. Effectively the same test (in this case) as hooking up a different external power source to isolate the problem to the camper or RV.

No, the control board does not lower the voltage. The gas valve is either commanded on or off.

If the problem does not come back, Jlawles2 (Joseph) may have nailed the issue (post 6). The tech removing and reconnecting wires to the furnace may have done the trick.

Please post at least one more follow-up in a few days if the problem does not repeat so we know it's fixed.

Howard

[wmcgaughey]

Installed the furnace, secured the unit to the deadwood that was installed by GD that wasn’t being used. Tech feels the backend of the furnace, (end closest to the inside of the basement), wasn’t secured and would have excess movement when traveling that may have affected the setting on the flame sensor.
Said all that to say the unit now works. No new parts, ductwork was secured to the furnace where it had fell loose. Basically a really poor install.