Atwood 8535-III Furnace Replacement: Best Direct & Alternative Options for RV Heating

Bottom Line Up Front

Your Atwood 8535-III DCLP furnace in your 1998 Holiday Rambler can be directly replaced with the Atwood 8535-IV Hydro Flame furnace, which is the current production model and a nearly exact fit. The key specifications match: 35,000 BTU output, direct vent design, and the same rough opening dimensions. You're absolutely right that parts for the older 8535-III are becoming scarce and expensive, making replacement the smart choice at this point.

The Atwood 8535-IV runs about $800-1200 depending on where you purchase it, and installation typically takes 4-6 hours for a competent DIY RV owner. The new unit uses the same 12V power connection, propane line size, and exhaust vent opening as your old furnace. The main differences you'll appreciate are improved efficiency, quieter operation, and readily available parts should you need service down the road. Given that your 1998 Holiday Rambler likely has the original furnace that's now 25+ years old, replacement is definitely the right call over continuing to hunt for increasingly rare repair parts.

Root Causes

Your frustration with finding parts for the Atwood 8535-III DCLP is completely understandable and represents a common issue with RV appliances from that era. Atwood discontinued the 8535-III model around 2007-2008, and by now most of the common wear items like circuit boards, blower motors, and heat exchangers are either completely unavailable or prohibitively expensive when you can find them.

The "DCLP" designation on your furnace stands for "Direct Vent Compact Low Profile," indicating it's a direct vent unit designed to fit in the standard RV furnace bay. The corrosion and rust you mentioned are typical problems with furnaces from the late 1990s, especially if your Holiday Rambler has seen exposure to salt air, road salt, or high humidity conditions. The heat exchanger and exterior housing on these older units weren't as well protected against corrosion as modern designs.

Beyond parts availability, your 25-year-old furnace is likely operating at reduced efficiency compared to when it was new. Heat exchangers develop hot spots and cracks over time, combustion chambers accumulate deposits that affect flame patterns, and blower motors lose efficiency. Even if you could find all the parts needed for a complete rebuild, you'd be looking at costs approaching or exceeding the price of a new furnace, without the warranty protection and improved efficiency of current models.

The repeated repairs you mentioned are also telling - once an RV furnace starts having regular problems, it's usually indicating that multiple systems are beginning to fail due to age and wear. Starting fresh with a new unit eliminates the cascade of problems that often plague older appliances and gives you the peace of mind of a full manufacturer warranty.

Before You Start

Before purchasing your replacement furnace, you'll need to verify the exact opening dimensions in your Holiday Rambler's furnace compartment. While the Atwood 8535-IV is designed to fit the same opening as your 8535-III, manufacturing variations in RVs from that era sometimes created slight differences. Measure the width, height, and depth of your current furnace bay carefully, and check that the exterior exhaust vent location will align properly with the new unit.

You'll also want to inspect the condition of your existing propane supply line, electrical connections, and ductwork before installation. The propane line should be 3/8" and in good condition without any signs of cracking or corrosion at the fittings. Check that your 12V power supply can handle the new furnace's amp draw - the 8535-IV typically draws about 7-8 amps during operation. If your RV's electrical system has been modified over the years, ensure you have a proper 12V supply with adequate wire gauge and fusing.

Plan for potential ductwork modifications as well. While the main supply and return air connections should match, you may find that the transition pieces or dampers need adjustment. Have some sheet metal screws, duct tape rated for HVAC use, and possibly some flexible ductwork on hand in case minor modifications are needed. The thermostat location and wiring should remain compatible, but verify that your existing thermostat is a simple on/off type rather than a programmable unit that might not work with the new furnace's control board.

Safety preparation is crucial for this project. You'll be working with propane connections, so have a good propane leak detector solution ready - soapy water works, but commercial leak detector is more reliable. Ensure your RV's propane system is completely shut off at the tank before beginning work, and have a fire extinguisher nearby. Plan to do this work during good weather when you can have windows and vents open for ventilation, and never attempt propane work in windy conditions where you might not detect a small leak.

The Repair Process

Start the replacement process by completely shutting off propane at your RV's main tank and carefully disconnect the propane line at the furnace to allow line pressure to be relieved safely. Disconnect the 12V power to your existing furnace at the main panel - don't just rely on turning off the furnace switch, as you want zero power to the unit during removal. Remove the interior return air grille and any screws holding the furnace in place from inside the RV.

From the exterior, remove the furnace exhaust vent cover and any screws or brackets securing the furnace from outside. Carefully disconnect the propane supply line using two wrenches - one to hold the furnace fitting steady and one to loosen the supply line nut. This prevents twisting stress on the furnace's internal gas valve. Disconnect the 12V power wires, making note of which wire goes where, and remove the thermostat wires if they connect directly to the furnace rather than through a junction box.

The old furnace should now slide out through the exterior opening. This is typically a two-person job as these units weigh 50-60 pounds and can be awkward to handle. Take photos of how the old unit was positioned and oriented before removal - this will help ensure correct installation of the new furnace. Clean the furnace bay thoroughly, removing any debris, old sealant, or corrosion that might interfere with proper seating of the new unit.

Before installing the new Atwood 8535-IV, inspect it carefully for any shipping damage and read through the installation manual completely. The new furnace should come with updated mounting hardware and possibly improved brackets compared to your original 1998 installation. Test-fit the new unit in the opening before making any permanent connections - it should slide in smoothly without forcing or binding.

Connect the propane line first, using new fittings and pipe dope or thread sealant rated for propane use. Hand-tighten the connection, then use wrenches to snug it properly - don't over-tighten as this can damage the threads. Connect the 12V power wires according to the wiring diagram, ensuring positive and negative are correct, and connect the thermostat wires. Many installers prefer to use wire nuts in a small junction box rather than direct connections, as this makes future service easier.

Secure the furnace in place using the provided mounting hardware, ensuring it's properly sealed against the exterior wall to prevent air leaks. The exhaust vent should align perfectly with the exterior vent opening - if it doesn't, you may need to adjust the furnace position slightly or modify the vent opening. Apply appropriate exterior sealant around the vent to prevent water intrusion.

Before testing operation, perform a thorough leak check on all propane connections using leak detector solution. Turn on the propane at the tank and check all connections for leaks before proceeding with furnace operation. Open the propane supply gradually and test furnace operation according to manufacturer instructions. and check each connection point - any bubbling indicates a leak that must be corrected before proceeding. Once you're confident there are no leaks, restore 12V power and test the furnace operation. The initial startup may take several minutes as air is purged from the propane lines.

Professional Help Needed

While this replacement can be accomplished by a skilled DIY RV owner, certain aspects of the job warrant professional assistance, particularly if you're uncomfortable working with propane systems. If you've never worked with propane connections before, consider having an RV service technician handle at least the gas line connection and initial leak testing. The consequences of a propane leak in an RV can be severe, making this an area where professional expertise provides valuable safety assurance.

Electrical work may also require professional attention if your RV's wiring system has been modified over the years or if you discover that the existing electrical supply isn't adequate for the new furnace. Some older Holiday Ramblers have wiring that doesn't meet current standards, and upgrading the electrical supply to the furnace location might require running new wire from the main panel. An RV electrical specialist can quickly assess whether your existing wiring is adequate and make any necessary upgrades safely.

If your furnace bay shows signs of structural damage, water intrusion, or significant corrosion, you may need professional body shop services to repair the mounting area before installing the new furnace. Don't attempt to install a new furnace in a compromised bay, as this can lead to improper operation, safety issues, and voiding of the manufacturer's warranty. Body shops that specialize in RV repair can assess the structural integrity of the furnace mounting area and make necessary repairs.

Consider professional installation if you discover during the removal process that your ductwork needs significant modification or repair. While minor adjustments to duct connections are typically straightforward, extensive ductwork modifications can affect the furnace's performance and efficiency. HVAC professionals familiar with RV systems can ensure that any ductwork changes maintain proper airflow and don't create restrictions that could cause the new furnace to cycle improperly or overheat.