RV Furnaces have come a long way in the past 40 years, from the basic metal box with a burner, a pilot and a thermostat valve, to forced air pilot type models, to the modern electronically controlled models- even a 2 speed, 2 stage model, offering a high and low fan and variable flame.
While the basics of the flame safety systems are the same as in the water heaters, the furnace contains more safety devices, which can affect operation. Let’s look at them now…
The flame sensing used in the furnace is identical to the flame sensing used in the water heater- indeed most circuit boards are interchangeable (though not all- new models use a “Fan Control” circuit board, which turns off the blower motor if the furnace fails to ignite).
Looking at a typical schematic diagram for a furnace, we will find a couple more devices in the power line to the circuit board. The power that energizes the circuit board (ignition module) has to travel through the Thermostat, a hi temperature Limit Switch ( which shuts the power if the furnace over heats), and a Sail Switch (also called an ‘air prover’ switch), which will not close until the blower is running at at least 75% of its full speed, ensuring that there is enough for the flame to burn, and enough air moving across the heat chamber to keep it from getting too hot. Then the power goes on to the circuit board connector.
Being mechanical connections and switches, each of these devices has the potential to cut the voltage a small bit- and indeed, there is usually some voltage drop through all of these. The main circuit board needs above 11 volts to operate- ideally above 11.5, so especially running off battery power losing just a little bit of voltage across each of these items can mean the difference between cold and hot air.
Another factor is that people will sometimes replace a single part, eliminating its voltage drop, the furnace will run, but they really will not have fixed the problem- just delayed it for a bit- another component might have a larger voltage drop, or there could be a bad connection. The easiest way to test these items is with a volt meter- testing across the item- in other words, set your voltmeter to a low voltage range and put one lead on either side of the sail switch, thermostat or limit switch. Any voltage reading you get will be a voltage drop.
Often these voltage drops are caused by the quick connector hooking the wire to the device, and not the device itself, so many times simply removing the wire and reconnecting it will fix the problem.
A few thoughts to wrap up- be aware that a thermostat can pass 9 or 10 volts to the furnace, which will be enough to activate the fan relay, but not enough to activate the ignition control board, so just because the blower fan runs, you cannot count out the thermostat as the problem for “blowing cold air”-
A multimeter is a useful and necessary tool in virtually every RV appliance troubleshooting procedure, and most RV furnace problems (though certainly not all) are in the control/safety systems, and can safely and easily be repaired by many people.
I have not gone in to the LP side of RV furnaces- working on the LP system takes more knowledge, both of safety procedures and of common practices, so it’s outside the scope of this post, but most times working on the LP system is not needed to diagnose and repair an RV furnace.