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rick gorman 08-14-2020 10:17 AM

2012 31RSE Winslow - SH 35 Suburban furnace cycling on high limit switch. Have replaced limit switch, cleaned fan, replaced burner, propane regulator (wc set at 11in.) rerouted ducts. Temperature at bathroom register reaches 180 degrees after about five minutes. High limit switch opens then closes in about a minute, repeat. Draw on fan motor 6.5 amps. The basement door is open to insure return air is not the problem and none of the ducts are restricted. HELP PLEASE

Old-Biscuit 08-14-2020 05:55 PM

180*F at bathroom register....How are you measuring that air temp?
**SH 35 uses a 160*F High Temp Limit Switch

Basement door open?
So furnace is pulling in OUTSIDE air vs RETURN Air from INSIDE the trailer
Furnance should pull Inside air thru Return Air Path

'Limiting' (operating on High Limit)
Overfiring due to HIGH LP System Pressure or oversized/damaged burner orifice
Obstructed Return Air Path....minimum of 55 sq inch

Restricted Duct(s).....
*rerouted ducts? Four 4" minimum required with at least 56 sq inch
Motor maps 6.5A----SH35 with 3" motor 8.2 amp draw with 375 CFM Discharge
**motor running slow/air flow low/12VDC Voltage Low??

Heat Exchanger obstructed/low air flow thru it....
Heat Exchanger can be cleaned but must be completely removed from furnace/everything disconnect........hgih pressure air blown thru OR even run water thru it/drain/blow out with air

rick gorman 08-14-2020 09:13 PM

Good information and questions, I also wondered why the limit switch was not tripping sooner, as I looked at the furnace, the limit switch is located at the front of the furnace and as the fan blows the air past the heat exchanger itís almost a direct passage through the heat exchanger through that center duct and past the limit switch, that center duct does not get nearly as hot as the one above or below it.
I changed five of the ducts in order to eliminate sharp turns, my thinking on the return air was to keep the basement door open to make sure the furnace was getting enough return air, the duct temp measured with a digital probe with the door open or closed is the same. Changing the ducts to allow better airflow did make a difference. It used to cycle on the high limit switch after about 3 to 4 minutes of operation and then after three times it would go into lock out so Iím making improvements but there is still something thatís not right, i have checked voltage, but will check again, do you think 6.5 amps indicates a problem? Donít have a way to measure cfm, the bathroom register that gets the hottest (180) and the shortest duct (18Ē) seems to have great air flow. 🙂

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