RV Furnace Trouble Shooting

Before attempting to trouble shoot your furnace be sure you understand the sequence of operation. Also see definitions.

This list is a guide only, of the more common causes of furnace failure, it is by no means complete.

Electronic Ignition Models (DSI)

Before going through the list below, these items should be checked.
Gas Pressure should be 11" of water column.
Power supply, DSI furnaces require 10.5 VDC to 13.5 VDC to operate.
Ducting, check for the correct number of unrestricted ducts and outlets.
Check the return air path, be sure it meets the required minimum sq. in.
RV furnaces are not approved for use with filters.

	Blower doesn't run Possible causes: no power thermostat Warning: Never cross the thermostat wires with the thermostat connected. You will destroy the thermostat. dirty contacts bad on/off switch broken or shorted wires bad thermostat bad time delay relay or thermostat relay You have time delay relay if there is normally a delay from the time you turn the thermostat up to the time the fan actually comes on, usually around 30 seconds. Instant on blower = thermostat relay loose or broken wire or poor ground tripped circuit breaker or blown fuse - not all furnaces have either of these bad motor
	Blower runs - no ignition Possible Causes: low voltage bad fan motor bad gas supply or low pressure exhaust or intake vents obstructed bad or dirty sail switch bad limit switch bad or out of adjustment electode bad electrode wire bad gas valve dirty burner bad control board NOTE: the design of electronic ignition furnaces is such that once the thermostat closes and calls for heat the blower will run until the thermostat is satisfied, the battery runs to low to run the motor, the motor wears itself out or the thermostat is shutoff.
	Burner ignites - goes out after a few seconds Possible causes: low gas pressure intake or exhaust vents obstructed dirty burner electrode(s) out of adjustment, dirty, or defective bad control board
	Sooting - usually shows as a black streak above the exhaust vent. Possible causes: dirty burner - common with older models that use a cast iron burner low gas pressure intake or exhaust vent obstructed bad fan motor
	insufficient heat Possible causes: low gas pressure poor gas supply - crimped gas line, under size tanks in below freezing temps.

This is generic and applies to most DSI model furnaces.

	Thermostat calls for heat.
	Thermostat relay or time delay relay is energized and allows power to the blower.
	Blower motor runs up to speed.
	Sail switch closes at 75% of rated blower speed.
	Power goes through the limit switch to the ignition control board.
	The control board begins trial for ignition, opening gas valve and initiating ignition spark.
	Burner ignites.
	Thermostat reaches setting and opens.
	Burner shuts down.
	Fan runs briefly to cool furnace.

Definitions:

sail switch - micro switch with an arm that extends into the blower air stream which closes when the blower reaches approximately 75% of its rated speed. Also called an air prover switch.

limit switch - safety switch, a normally closed switch that opens if it gets to hot, opening turns off power to the gas valve and igniter board

fan switch - a normally open switch that closes at a preset temperature, causes the furnace to run for a short time after the thermostat opens, allowing the furnace to cool down.

time delay relay - same function as fan switch, only has its own heater

igniter electrode - similar to a spark plug, 2 versions, 3 probe (remote sense) or 2 probe (local sense)

ignition control board - when powered, initiates gas valve opening and spark sequence which lasts approximately 7 seconds. Older models are 1 try, thermostat must be cycled off for at least 10 seconds before another ignition cycle is attempted. Newer boards are 3 try, will attempt to ignite 3 times at approximately 60 second intervals.

gas pressure - LP gas pressure must be 11" of water column (6.25 oz per sq. in.), checking and adjusting requires a manometer.