| Furnace |
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We
recommend that only qualified technicians work on furnaces.
Do It Yourself at your own risk. You are responsible for all safety precautions
and leak checking.
Before attempting to trouble shoot your furnace be sure you understand the sequence of operation. Also see definitions.
Many people find that the slender spaces beside their forced air furnace is ideal for storing such items as cookie sheets, cutting boards, even paper or plastic bags. In addition to the obvious fire hazard, I would like to point out another dangerous situation that could result from this practice.
The air intakes (for the fan that supplies return air to the furnace blower) are located on either side and the top of the metal furnace housing. These air intake slots draw air from the surrounding area and the blower fan forces this air past the sealed combustion chamber in the furnace and on to the warm air duct work. If this intake air flow is restricted, (with loose plastic bags, aforementioned cookie sheets, etc.) the fan motor speeds up. (the same effect is observed when the intake hose of a vacuum cleaner is plugged off).
There is a separate combustion air system, sealed from the inside of the coach, that feeds fresh air to the burner assembly inside the furnace. This system draws in outside air to allow the proper combustion of the fuel gas and then forces the exhaust products out again. The fan for this system is powered by the same furnace motor that runs the blower, therefore it speeds up at the same rate.
The increased air flow in the combustion chamber alters the burn characteristics of the flame - producing a leaner, hotter flame. This overheating can lead to failure of the furnace safety devices, damage to the electronic components of some models or in the worse case a cracked combustion chamber. Exhaust products, including CARBON MONOXIDE, will now be forced into the living area !!!! CO is odorless, colorless and extremely toxic - a silent killer.
Please check your furnace compartment for air flow restrictions. If you have any doubts or concerns regarding your furnace's condition, have it inspected by a licensed, qualified RV gas technician. A carbon monoxide detector installed in your unit is a very inexpensive life insurance policy.
Forced Air Furnace: Component not complicated. There are three circuits involved. Time delay relay (an electrical switch). Turn furnace on, nothing happens for 20 seconds. Takes leftover hot air out of furnace after flame goes off. Limit switch -- heats up, switch opens. Cools down, switch closes. A safety factor, it says let's shut the furnace off rather than burning the coach down. Sail switch (air prover switch) sits in windstream. If fan doesn't turn fast enough, it will shut down furnace. Thus if you have low voltage, you'll also have a slow fan, and it will be the cause of it not lighting.
Thermostat is a metal coil which is temperature activated. Turn up thermostat, power to time delay relay. 20 secs later, sail switch closes, circuit board has second delay built in -- blows out any leaked propane thru exhaust. After 20 secs, high voltage spark 20-80,000 volts -- opens gas valve and creates spark.
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.
This list is a guide only, of the more common causes of furnace failure, it is by no means complete.
Pilot Light Won't
light or stay lit.
Make sure the thermocoupler is positioned properly in the pilot flame.
A common problem is a bad regulator at the propane tank. A simple test will
indicate if this is the case. Light all the stove burners and look at the color
of the flame. The flames should be blue with little or no yellow color. If the
flame does not change color then the regulator is probably working. A bad
regulator could also cause problems with the hot water heater.
Fan doesn't run
and no heat.
If the fan will not start you should first check the thermostat. Remove cover
and look for the "anticipator" adjustment. It will be an adjustable
control with a sliding contact over a straight bare wire or a a bare wire wound
about an insulating material. If the fan will not start set the temperature to
maximum and then move the anticipator slider while listening for the fan to
start. Be sure to wait long enough... it nomally takes our furnace 30 to 40
seconds for the fan to start once the thermostat sends a signal.
If the fan starts after you move the slider then you have probobly found the
problem. In this case you may find a slider position near the original position
that will work reliablily. If your thermostat anticipator adjustment uses the
straight wire design and the wire lies directly on the plastic housing then you
should look to see if the wire has sunk into the plastic. This wire produces
heat and causes the wire to sink into the plastic and the slider no longer makes
contact. It may be necessarry to replace the thermostat.
Fan runs but no heat.
If your furnace fan starts you can assume that the thermostat is working.
Possible problems are insufficent air flow through the furnace or a bad
regulator at the propane tank.
A furnace contains an internal sail switch, that senses the air flow. If the air
flow is not sufficient then the switch will prevent the furnace from igniting
and the fan will run but you will get no heat. Check to see if any heat
registers are closed or blocked. Some furnaces will not tolerate even a partual
closure of a heat register. A low voltage condition may cuase the fan to run too
slow to activate the sail switch.
If you have an electronic ingnition check to see if the two contacts are
touching or are too far apart. They should be about 1/8 aprart.
Turn on thermostat -- zip.
First, do you
have power? Check wires going to the furnace (not just the battery). Maybe a
fuse problem? And fuses can be in some strange places.
If you have power to furnace
Fan runs, but continues to blow cold air. Thermo
works, time delay works -- both did their jobs. Maybe sail switch, low voltage,
pet hair in air wheel, etc all cause insufficient air flow. Furnace is designed
to keep blowing cold air. Thermostat says keep running, but board has gone into
lockout. Turn it off and back on again. Wait a couple of seconds, and board will
recycle and try again.
Takes long time to heat up a coach
Limit switch
will shut down gas when furnace gets too hot --- even if coach isn't well heated
yet. Biggest problem is the ducting. for each 10K BTU, need one 4" duct.
Too often ducting is inadequate, and contains a too many restrictions (90 degree
turns, etc). Manufacturers unfortunately don't engineer this well. But RVers can
be sloppy, and cover either duct or return air. If air flow blocked, motor will
run faster, since it isn't having to do it's work. Can adversely impact air/fuel
mixture.
"Furnace doesn't work"
If this is all
you tell the service tech, it's not good info -- will cost $ to diagnose. Tell
the service tech all the circumstances. Low power supply is often the culprit.
You may have been dry camped for 3 days, but tech guy has you plugged in to
shore power, and when he tries the furnace it works fine. You'll save dollars by
telling the service folks ALL the circumstances involved in any appliance
failure.
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.
NOTE: This online manual is a work in progress and is subject to change at any given time. It is also a generic peace of work and is meant only as a general guide. Each Caravan/RV is distinct and individual in it's own right, do not take anything written here literally as it may or may not apply to the exact Caravan/RV you will be using.