What Size Generator to Run an RV Furnace – Accelerate Net Zero

Keeping an RV warm while camping off the grid often depends on having a generator that can power the RV furnace’s 12V system and keep the house battery charged. Understanding the furnace’s power needs, the generator’s ability to supply 120V AC to the converter, and the typical startup surge helps travelers choose the right generator size. This guide breaks down typical current draws, provides sizing guidelines, and outlines practical scenarios to help you select a generator that reliably runs an RV furnace without overloading the unit or wasting fuel.

How RV Furnaces Work

Most RV furnaces are propane-fired heaters that distribute warm air through the coach. The furnace relies on three main parts: a gas valve and ignition system to light the burner, a 12V DC control circuit, and a blower motor that moves warm air through the ducts. The thermostat tells the system when to ignite and when to run the blower. The blower is a 12V device powered by the RV’s house battery or the battery charger when connected to shore power or a generator. Without a steady 12V supply, the furnace cannot operate, even if propane is available.

When the RV is plugged into shore power or running on a generator, the converter-charger supplies 12V DC to the house batteries, keeping the furnace’s 12V circuits powered. The generator’s 120V AC output feeds the converter, which in turn maintains battery voltage. This means that the furnace’s performance depends on both the 12V battery state of charge and the generator’s ability to support battery charging. If the battery is depleted, the furnace may run briefly on stored 12V energy but will stall as the battery voltage falls.

Different model families (for example Suburban, Atwood, or Dometic furnaces) may have slightly different current profiles, but the general behavior is similar: a 12V blower draws current continuously while the furnace is on, with short startup surges when the blower first starts or when the igniter engages. Understanding these power dynamics is essential for proper generator sizing.

Typical 12V Draw And Startup Surge

The furnace blower and control system consume most of the 12V power. Typical ranges reported by manufacturers and RV technicians are:

• Blower motor (continuous): roughly 6–8 amps at 12V
• Ignition/igniter during startup: typically 1–3 amps, energized briefly
• Gas valve during ignition: about 1–2 amps while energized
• Startup surge: can peak around 12–15 amps for a moment when the blower or igniter starts

In many installations, the total 12V load during operation sits in the 7–12 amp range, with brief surges above 15 amps at ignition. These values vary by furnace model, blower type, and whether the furnace runs with the thermostat in economy mode. A safe sizing approach accounts for both continuous draw and startup spikes.

Generator Sizing Basics For RV Furnaces

To determine an appropriate generator size, consider three factors: the furnace’s 12V draw, the 120V converter/charger load, and any other loads you plan to run simultaneously. The goal is to have enough continuous power to keep the battery charged and the furnace running without hitting the generator’s surge limit or causing overheating.

• Furnace-only operation (battery charged and converter active): you primarily need enough 12V supply from the battery. The generator’s role is optional, mainly to recharge batteries if you’re running the furnace for extended periods.
• Furnace with battery charging (converter charging at moderate level): you should plan for about 800–1000W of continuous 120V AC power to feed the converter at a mid-range charging rate (roughly 40–60 amps at 12V equivalent).
• Furnace with multiple essential loads (lights, water pump, fridge on propane, etc.): anticipate 1200–2000W of continuous 120V AC power, depending on which loads are active and how aggressively the converter charges the battery.

A practical guideline is to choose a generator with a continuous rating at least 25–30% higher than the peak expected load, to handle startup surges and provide a buffer for small, unexpected demands. For most RV setups focused on the furnace plus essential loads, a 1000–1500W generator is a common, comfortable starting point; for heavier use or larger battery banks, 2000–3000W offers more headroom. Pure sine wave inverters are often preferred if you want quiet operation and clean power for sensitive electronics.

Example Scenarios: Running Furnace With A Small Generator

These scenarios illustrate how generator size intersects with furnace operation. The figures assume a typical propane furnace with a 12V blower and a mid-range converter-charger. Always check your model’s exact current draw for precise sizing.

Key takeaway: If the furnace runs while the converter charges at a moderate rate, plan for roughly 800–1200W of continuous AC power. If additional RV loads are present, scale up to 1500–2000W or more, ensuring the generator can handle startup surges.

Choosing Between Portable Generators And Inverters

Many RVers choose between a portable generator and an onboard or portable inverter setup. Each option has trade-offs for running an RV furnace:

• Portable gas or diesel generators: Provide robust continuous power and are well-suited for charging the battery and running multiple loads. They require outdoor operation due to exhaust and noise, use fuel, and require regular maintenance.
• RV-specific inverter generators: Often offer cleaner power, quiet operation, and portability. They can power the converter and furnace while keeping noise levels lower than traditional generators.
• Inverters paired with a high-capacity battery bank: If the goal is silent operation, a sizeable lithium or AGM battery bank with a high-quality inverter can run the furnace for several hours, but charging capacity depends on the inverter’s input and any shore power or generator input.

When sizing, consider the combined load of the furnace and all other devices you plan to run. If you frequently camp with the furnace on while charging batteries and using lights, a 1000–1500W inverter generator is a practical balance of cost, weight, and reliability. For more demanding setups or larger battery banks, 2000–3000W may be warranted.

Safety And Operational Tips

Safety is essential when using a generator to power an RV furnace. Follow these guidelines to protect occupants and equipment:

• Ventilation: Run generators outside, away from doors and vents, to prevent carbon monoxide buildup.
• CO detectors: Ensure working CO alarms inside the RV when running any propane appliance or generator.
• Ventilation for propane furnace: Do not block furnace vents or intake/exhaust paths; keep area clear of flammable materials.
• Electrical safety: Use properly rated extension cords and power outlets; avoid daisy-chaining multiple adapters; use a transfer switch or RV inlet suitable for generator connection.
• Battery charging: The converter charger draws from the generator’s 120V output. Do not overload the charger; monitor battery voltage and temperature during operation.
• Starting with loads: Start the generator first, then turn on the converter, then the furnace. This reduces inrush stress on the generator.

Maintenance And Efficiency Tips

Keeping both the generator and RV furnace efficient reduces fuel use and extends equipment life. Consider these practices:

• Regular furnace filter and blower maintenance: Clean or replace filters; lubricate blower motors if applicable; a clean system runs more efficiently.
• Battery health: Maintain a healthy battery bank; damaged batteries reduce available 12V for the furnace and other systems.
• Converter charger settings: If possible, set the charger to an appropriate charging rate to balance battery health and generator load.
• Fuel management: Keep fuel for the generator topped up; run the generator long enough to burn off old fuel and prevent varnish buildup.
• Load management: Prioritize essential loads (furnace, refrigeration, water pump) during cold weather and reduce nonessential loads to keep power requirements manageable.

Understanding the interplay between the 12V furnace system, the RV’s converter, and the generator’s output makes it easier to choose a generator size that reliably runs an RV furnace. Always refer to the specific furnace model’s current ratings and the converter’s maximum charging capability to tailor the generator size to your exact setup.