How Does an RV Furnace Work?

ONE OF THE GREATEST PERKS of RV camping is reliable cooling and heating. It’s much more comfortable to wake up and prep your coffee in a heated recreational vehicle than to climb out of a wet tent to light your camp stove and try to get a fire going. Just like residential units, RV heating systems require regular maintenance. But how can you service or repair an RV furnace if you don’t know how it works? So let’s answer this commonly asked question: How does an RV furnace work?

At the simplest level, an RV furnace heats air and circulates it using a fan. Most RV furnaces operate on DC power even when your RV is plugged into shore power, allowing them to work at campgrounds or off-grid. But there’s much more to it. So let’s dive into everything you need to know about operating, servicing, and troubleshooting RV furnaces.  

Key Takeaways

• RV Furnace Basics: RVs use either forced-air or hydronic heating systems. Forced-air furnaces burn propane and use a blower to circulate heated air, while hydronic systems heat water and distribute warmth through radiators.
• How a Forced-Air RV Furnace Works: When the thermostat calls for heat, the blower activates. Safety switches confirm proper airflow and temperature, and the circuit board opens the gas valve, initiating ignition. If a flame is detected, the heat cycles normally. If not, the system enters lockout.
• Operation, Maintenance & Troubleshooting: Furnaces rely on adequate propane, proper airflow, clean vents, and correct voltage. Owners should regularly vacuum return-air areas, clear exhaust vents, monitor propane use, and check voltage and airflow when troubleshooting.

How Do RV Heating Systems Work?

There are two primary types of RV heating systems: forced-air furnaces and hydronic heating systems.

In forced-air furnaces, outside air for combustion is forced through a sealed combustion chamber assembly and discharged to the exterior, while the interior air of the RV is moved across a heat exchanger. This design ensures that no products of combustion are mixed with the interior air.

Hydronic heating systems utilize a heated surface to warm water, which is then distributed to radiators throughout the RV. That surface can be heated by burning fuel (i.e., propane or diesel), an electric element, or using primary engine coolant to heat that surface. Some hydronic heating systems heat potable water and can be used for engine preheating.

Technician Tip: All RV heating systems should have a data plate that contains the manufacturer, model, serial number, and input and output (measured in BTUs/hour) of the appliance. This plate helps you identify your appliance for specific service and maintenance procedures. Your owner’s manual can also help you understand the type of heating system you’re working with. Do you need a general refresher on heating an RV?

How Does an RV Furnace Work?

If you’re working with a forced-air RV furnace, a blower fan powered by a small motor distributes heat throughout your RV. Most forced air furnaces burn propane to heat air, but some utilize diesel fuel (more common with hydronic heating systems).

Before we outline the process, let’s define a few key terms.

A sail switch is the first safety switch in an RV furnace. It prevents your furnace from attempting the ignition sequence if the blower motor isn’t running at a minimum of 75% of its rated speed.

A limit switch is the second safety switch. It opens at a preset temperature to prevent overheating. The limit switch is mounted on the heat exchanger or the burner assembly housing. It shuts off the burner but allows the combustion blower to continue running to lower the equipment temperature.

Safety Tip: The limit switch does NOT act as a thermostat. If your blower never shuts off, but your burner is cycling on and off at regular intervals, your RV should be inspected by a qualified RV technician immediately. This is a symptom of an overheating limit switch, which could result from blockages, faulty components, or LP gas adjustment issues that create a very dangerous situation. 

What Happens When You Turn on the Propane Heater in Your RV?

When you turn on your RV furnace, here’s what happens if it’s working properly:

1. The thermostat turns on and signals for heat.
2. An electrical current runs from the thermostat to the time delay relay built into the circuit board, which closes to energize the blower motor after a delay of 10-20 seconds.
3. When the motor comes up to speed, airflow activates the sail switch, closing a micro-switch and continuing the circuit. The micro-switch requires a minimum fan speed to close. Current can be interrupted if the motor isn’t running fast enough, there is a sticky switch, or an airflow blockage prevents the switch from closing. This circuit is independent of the thermostat and motor circuit.
4. The sail switch closes, sending power to the limit switch, which then sends power to the circuit board. Under normal operation, the limit switch closes and opens at a preset temperature to prevent overheating.
5. The circuit board delays ignition for 10-20 seconds, allowing the blower motor to purge any gas remaining in the combustion chamber.
6. The circuit board opens the gas valve and sends a high-voltage current to the electrode.
7. If ignition occurs, the electrode senses the flame and sends a current back to the control board, stopping the high-voltage spark and keeping the gas valve open
8. If ignition doesn’t occur, the board enters a lockout mode and must be reset before attempting again. Most circuit boards pause for 20 seconds, then try for 7 seconds. They’ll repeat this three times before going into a lockout.

Technician Tip: A lockout can result from a faulty circuit board, faulty electrode, faulty gas valve, or any combination of these issues. If locked out, conduct a brief inspection (LP gas on, no airflow obstructions, proper voltage, etc.) and then switch the thermostat off and back on. The circuit board should provide three more tries. 

This RV furnace parts diagram provides a visual representation of the Dometic DFMD30 Mojave propane RV furnace design. You’ll need to determine your furnace’s make and model and refer to its owner’s manual for more specific schematics.

Where Is My RV Furnace Located?

RV furnace location depends on the manufacturer and the floor plan. For example, the furnace in my Campsite Ultra travel trailer is set on the campside underneath the cooktop and microwave.

Regardless of your floor plan, understand that the furnace itself won’t be visible. You will see a return air vent (or vents) inside your RV, as well as the air intake/exhaust vent on the RV’s exterior. Here’s an example of what your return air and exhaust vents might look like:

Technician Tip: It is a good practice to locate the exhaust vent on the RV’s exterior and verify that the exhaust is present and clear. We’ve seen too many campers lean a lawn chair against the vent, which could present a fire danger.

If you’re shopping for an RV, it’s a good idea to look for a model with an exterior access panel to service the RV furnace. This panel isn’t included on all RVs, especially entry-level travel trailers.

How To Turn on an RV Furnace

Most RV furnaces are controlled by a wall-mounted thermostat. Depending on your make and model, you may have a digital control panel or an analog thermostat.

Most analog thermostats have a small On/Off toggle switch on top, and RVs with a digital control panel will have a display for turning your furnace on and off. In either case, your next step is to adjust the thermostat to a setting higher than the current ambient temperature inside your RV.

If your furnace is functioning properly, you should hear the blower motor turn on and feel air blowing out of your interior vents. Within 10-20 seconds, you should hear the pilot light ignite and feel hot air coming out of your vents shortly thereafter. If you leave your furnace on, it should continue to cycle on and off to maintain the desired temperature.

How Much Propane Does My RV Furnace Use?

To operate your RV furnace, you’ll need an adequate propane supply. If you’re not already familiar, take the time to learn about connecting and turning on propane in your RV.

The amount of propane your furnace uses depends largely on its BTU rating. BTU is short for British Thermal Units, which is a unit of heat measuring how much heat it takes to raise the temperature of one pound of water by 1℉ at sea level.

A furnace with a higher BTU rating burns propane more quickly than one with a lower rating. Your first step will be to determine your furnace’s BTU rating, measured in BTUs per hour.

Before we get caught up in the numbers below, remember that your RV furnace won’t run continuously once you turn it on and adjust the thermostat. It will cycle on and off to regulate the temperature inside your coach. This is crucial to remember when calculating the amount of propane your RV furnace consumes.

Let’s provide an example:

Example RV Furnace Specs

• Propane Capacity: 60 pounds
• Furnace BTU Rating: 30,000 BTUs/hr
• Electric Fireplace BTU Rating: 5,500 BTUs/hr

RV Furnace Propane Consumption Calculator

To start, we must remember that propane containers can only be filled to 80% of their total capacity. So while the Cougar’s advertised propane capacity is 60 pounds, its actual capacity is 48 pounds (60 x 0.8 = 48).

We can calculate the total BTUs of 48 pounds of propane from there.

1 pound of propane = 21,548 BTUs

48 pounds of propane = 1,034,304 BTUs

If our total propane supply accounts for 1,034,304 BTUs, we can divide that by our furnace’s BTU rating to estimate how many hours our furnace would run before that supply is exhausted.

1,034,304 BTUs / 30,000 BTUs/hr = ~34.48 hours

If we run our furnace and electric fireplace simultaneously, you can actually reduce your rate of propane consumption because you’re also relying on electric heat:

30,000 BTUs – 5,500 BTUs = 24,500 BTUs

1,034,304 BTUs / 24,500 BTUs/hr = ~42.22 hours

You can insert the appropriate figures from your RV into the example above to calculate how much propane your RV furnace uses. However, remember that these calculations are based on 100% run-time operation.

Other variables, such as cold weather and your desired temperature setting, impact your furnace’s potential operating hours on a full LP supply. For instance, you can roughly double the number of hours your furnace will operate if you estimate a 50% duty cycle (i.e., your furnace will run for half the time you’re camping).

Here are some tips to save propane in your RV.

Technician Tip: Remember there are other propane appliances in your RV. The calculations above represent only the available operating hours for your RV furnace, but they can be used if you know the BTU ratings of your RV’s other appliances, such as your water heater, refrigerator, stove, oven, and outdoor grill. 

How To Clean an RV Furnace

Pet hair can be a problem for certain RV furnaces. So here are a few RV furnace cleaning tips to follow:

• Thoroughly cleaning an RV after every trip helps keep it in optimal condition, and it also helps your RV furnace run more efficiently.
• Use a vacuum to minimize dust and dander. Remove the furnace’s return air vent to vacuum dust and pet hair around it, if possible. Just turn the stove off before doing so.
• Use a portable air compressor to remove dust, debris, pet hair, and insect nests from your furnace’s exhaust tube. Make this part of your seasonal RV maintenance checklist.

Technician Tip: Please note that forced air furnaces (not hydronic heating systems) are designed to operate without filters. If you were to acquire and find a way to install a filter on your forced air furnace, it would void any existing warranties and create a potentially dangerous situation.

Quick RV Furnace Troubleshooting Tips

Troubleshooting your RV furnace can be complex, but you can also schedule a service appointment for a professional inspection of your furnace. However, here are some basic steps to follow:

• Measure Line Voltage Using a Digital Multimeter. If less than 10.5 VDC, the blower motor won’t spin fast enough to activate the sail switch.
• Confirm Blower Motor Operation. If the motor isn’t reaching 75% of its rated speed, potential issues include low DC power, a faulty motor, a blocked return air inlet, or a blocked exhaust. 
• Evaluate Airflow Discharge. Open registers and verify air discharge by feel (or using an airspeed indicator). Ensure flow isn’t hindered by rugs or floor coverings. Verify that the discharge is warm and check for the presence of carbon monoxide using a carbon monoxide detector.
• Ensure Airflow Return. Verify the opening size for the furnace’s return airflow, ensuring it matches the specifications in your unit’s operating manual.

Use the links below for service and a more complete guide to furnace troubleshooting:

Learning how various parts of your camper function is imperative to healthy ownership and diligent maintenance. Here are a few additional resources:

• How Do RV Water Heaters Work?
• How Does an RV Refrigerator Work?
• How Do RV Air Conditioners Work?
• How Does an RV Slide-out Work?
• How Do RV Generators Work?

Do you have any other questions about furnaces and RV heating systems? Ask away in the comments below.

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