Originally Posted by Old-Biscuit
When Hi temp switch opens Fan continues to run until heat exchanger cools down, HI temp switch closes and flame is reestablished until Hi temp switch opens again.....repeat
Hi temp switch doesn't completely shutdown/lock out furnace
The above quote is true. However, and isn't there always a 'however'
, there is an exception to the statement quoted!
The Suburban Service and Training Manual makes this statement,
"If during the heating cycle, the limit switch circuit opens and remains open for 5 minutes, the module board will go into lock out and shut down the blower motor. If this occurs, the thermostat will need to be reset for the furnace to operate.
My motorhome exhibited the same problem you are experiencing. I determined that the burner was cycling off and on with the High Limit Switch. The limit switch is an electro-mechanical device that is not designed for repeated operation. The switch is supposed to stay closed for the life of the furnace. It should only open if something is wrong.
In my experience the limit switch would cycle the flame off and on simulating normal furnace operation. Eventually the limit switch would wear out and intermittently start hanging open for >5 minutes and the furnace would shut down.
The root cause of my furnace problems was not the furnace. It was the installation of the furnace
. The coach designer did not put in enough outlet registers. This caused the furnace to over heat and the limit switch shut the furnace down. The flame would then cycled off and on based on the limit switch.
My solution was to add 3 4" round wall registers to supplement the floor registers the coach manufacturer installed.
Attached is the Suburban Service and Training Manual. Go to page 20, there you will find minimum ducting specifications for most Suburban furnaces.
You could have plugged registers, blocked cold air return, mountains of stuff plugging the fan, etc. Anything that inhibits air flow can cause High Limit Cycling.