RV Tech Library - Propane Furnaces

Furnaces in an RV aren't all that different than a gas-fired forced air furnace in a residential application. They use 12 volt power to run the blower and provide ignition rather than 120 volts, but the principle is the same. They burn gas in a combustion chamber which gives off heat. This heat is then blown through the duct work by a blower fan and cooler air is returned to the furnace via a cold air return duct. RV furnaces do have "chimneys" of sorts, but it's just a stainless steel vent cover on the side of the RV where the hot exhaust gasses escape.

The propane furnace has a number of safety features built in. The electronic ignition will spark at the appropriate time to ignite the gas and begin combustion. If there is not adequate combustion airflow through the burner a sail switch will sense this and shut down the furnace to prevent overheating of the combustion chamber in the event the intake air is clogged or restricted by mud dauber nests or whatever else "built" a home in there. If the combustion chamber gets too hot, a high limit switch will shut down the burner until it cools. And, of course, the furnace only runs in response to a request for heat from the RV's thermostat. The fan blower is not controlled by the thermostat. The fan will start once the heat exchanger has warmed up a bit in order to not blow cold air on you at first. Later, when the burner has stopped, the fan will continue to run until the heat exchanger cools down.

Propane furnaces are found underneath the floor in the RV, generally in a non-storage basement area. The ductwork from these furnaces runs up into the coach and outlets either in the floor, very low along interior walls, or a combination of both. Propane furnaces also provide basement heat to keep the holding tanks and water lines from freezing. Air conditioning and heat pump units are found on the roof and use a separate ductwork system. These ducts are found in the roof of the RV and do not feed the basement storage area therefore running the heat pumps to keep the water system from freezing is a fruitless effort. You must run the furnace to prevent the water system from freezing.

Most RVs now use a common thermostat for both heating and cooling. This thermostat can control both the LP furnace as well as the air conditioning and heat pumps (sometimes referred to as "electric heat"). Many do have interlocks on them so that you can't run one heat pump and one furnace but there are lots of variations on this so, like they say, "your mileage may vary". Most thermostats will automatically kick in the propane furnace when you select heat pumps if the temperature differential between the thermostat set point and the actual interior temperature is greater than 5 degrees. Once you reach the set point, the furnace will stop and the heat pumps will take over the rest of the heating tasks. If it's too cold, and the heat pumps can't handle it, the furnace will kick in again if the 5 degree differential reappears.

BURNER FAILS TO IGNITE AND BLOWER FAILS TO RUN
Condition	Solution
No Electrical Power to the furnace	Reconnect or replace power source
Thermostat defective	Replace thermostat
Thermostat wires broken or shorted	Replace wire or wires
Circuit breaker defective or tripped	Reset circuit breaker and check amp draw from motor according to furnace's specifications
Blower relay defective	Replace relay
Wire off motor or relay	Reconnect wire
Bad ground	Clean and secure grounds
Blower motor defective	Replace motor
BURNER RUNS BUT FAILS TO IGNITE
Low voltage	Correct Power Supply
Gas pressure incorrect	Set pressure to minimum of 11" W.C. with all appliances running. Replace regulator if not obtainable.
Furnace ground wires not secure	Clean and secure grounds
Air intake restricted	Clean air intake
Motor running slow	Correct polarity
Combustion air wheel loose	Check voltage, if 12 VDC while running, replace motor
Exhaust blocked	Clean exhaust
Sail switch defective	Reposition and tighten
Limit switch defective	Replace switch
Edge connector on circuit board dirty	Clean with pencil eraser
Circuit board defective	Clean plug contacts. If still defective, replace (check on board tester when possible)
Gas valve defective	Replace valve or valve coil
Main burner orifice blocked	Clean main burner orifice or replace
High tension lead wire defective	Replace wire
Electrode out of adjustment	Adjust electrode (don't crack the porcelain)
Electrode defective or cracked	Replace electrode
Obstructed burner head	Clean burner head
BURNER IGNITES BUT IGNITION SYSTEM "LOCKS OUT" AND TURNS BURNER OFF
Low gas pressure	Set pressure to minimum of 11" W.C. with all appliances running. Replace regulator if not obtainable
Exhaust blocked	Clean exhaust
Combustion air loose	Reposition wheel and tighten
Electrodes out of adjustment	Adjust electrodes to proper specs
Electrode defective	Replace electrode
Circuit board defective	Clean plug contacts. If still defective, replace.
Flame sensor wire between electrode and circuit board defective	Replace wire
Air leakage at gaskets	Replace gasket
Defective heat exchanger	Replace heat exchanger
SOOTING (caused by lazy yellow flame)
Low gas pressure	Set pressure to minimum of 11" W.C. with all apliances running. Replace regulator if not obtainable
Low voltage	Correct power supply
Air leakage at gaskets	Replace gaskets
Combustion wheel installed backwards	Reposition wheel and tighten
Blockage in heat exchanger or burner head	Clean or replace
Faulty motor	Replace motor
FAN RUNS CONTINUOUSLY WITH THERMOSTAT "OFF"
Defective thermostat	Replace thermostat
Shorted thermostat leads	Replace wiring
Defective relay	Replace relay
LIMITING = BURNER CYCLING ON AND OFF BLOWER RUNS CONSTANTLY WITH THERMOSTAT ON
Furnace over fired	Set gas pressure to a minimum of 11" W.C. with all appliances running. Replace regulator if not obtainable. Also, check main burner orifice, it must comply with the furnace's specifications.
Restricted return air supply	Make sure return air meets furnace's minimum requirements
Restricted or insufficient discharge ducting	A) Ducting must meet minimum specifications B) No excess ducting or unnecessary bends C) All closeable registers must be fully open and unrestricted
Defective limit switch	Replace limit switch
BLOWER SHUTS OFF AT SAME TIME BURNER SHUTS OFF
Wired wrong	Correct wiring
Faulty relay	Replace relay
BLOWER VIBRATES OR IS NOISY
Motor mount loose	Tighten motor mounting bracket
Damaged blower wheel	Replace blower wheel
Motor shaft bent	Replace blower motor
INSUFFICIENT HEAT
Furnace under fired	A) Set gas pressure to 11" W.C. with all appliances running. Replace regulator if not obtainable. B) Check burner orifice. It must comply with furnace's specifications. C) Check ducting and return air according to furnace's specifications.

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