RV air conditioners remove heat from your interior air using a compressor-driven refrigerant cycle — they can typically cool the interior 18–22°F below the outside temperature, but not more.
Park in 100°F desert heat and your RV AC will get you to about 80°F, not 68°F. That temperature differential — roughly 20°F (delta T) below the return air temperature — is the physical ceiling for every rooftop unit. Understanding the cycle explains why that ceiling exists, what you can do about it, and when to call a technician.
The 7-Step Cooling Cycle
RV ACs don’t “make cold” — they move heat from inside to outside. Furrion and Dometic units follow the same basic refrigerant cycle. Step 1 — Intake: The evaporator blower draws warm interior air through the return grill and filters. Step 2 — Heat extraction: Refrigerant inside the evaporator coil absorbs heat and moisture, changing from liquid into a low-pressure gas. The cooled, dehumidified air returns to your cabin (Step 3). Step 4 — Compression: That gas moves to the compressor, which raises its pressure and temperature. Step 5 — Condensation: The hot, high-pressure gas enters the outdoor condenser coil; exterior fan air cools it back into a liquid. Step 6 — Metering: A metering device regulates liquid refrigerant flow back into the evaporator. Step 7 — Cycling: The compressor cycles on and off to maintain the thermostat setpoint; the fan keeps running.
Rooftop units require 120V AC power (for the compressor, condenser fan, and electric heat strips) and 12V DC power (for the thermostat, control board, relays, and reversing valve on heat-pump models). The 120V side comes from shore power (30 or 50-amp pedestal), an onboard generator, or a high-capacity inverter. The 12V side runs off house batteries, maintained by the converter.
Why Your RV AC Won’t Cool Below 20°F of Outside Air
The delta T limit — roughly 18–22°F — is the maximum heat-transfer efficiency the condenser and evaporator can achieve at standard compressor speeds. A typical 13,500 BTU unit draws about 15.5 amps at 115V AC (roughly 1,775 watts). On a 95°F day it produces an interior temperature around 75°F. Setting the thermostat to 68°F on a 100°F day does nothing — the unit reaches its delta ceiling and stays there.
Open windows swamp the system entirely because the unit recirculates interior air and cannot condition incoming outdoor air. Running an electric water heater and the AC simultaneously on a 30-amp system will trip the breaker.
Dirty filters and coils are the most common reliability killer. A clogged filter starves the evaporator of airflow, causing the coil to ice over and halting cooling. Roof debris blocking the condenser louvers prevents heat rejection. Clean both surfaces immediately when you notice weak airflow or ice. Most rooftop units prevent operation below 40°F to protect component seals — running an RV AC in cold weather can damage the compressor.
When to Troubleshoot and When to Call a Technician
Then check for blown fuses and tripped breakers on the roof unit’s power supply (both 120V and 12V circuits).
If the capacitor tests good and all fuses and breakers are fine but cooling is weak, inspect the evaporator coil and filters for dirt first — that fixes most field failures. If the air coming off the evaporator feels only slightly cooler than ambient, the refrigerant charge may be low. Freon leaks require a Certified or Master Certified RV Technician to repair. Never operate a unit with a known leak — running low on refrigerant destroys the compressor.
For heat-pump models, the reversing valve is controlled by the 12V DC system. A dead house battery or failed control board can lock the valve, so confirm 12V power to the thermostat before assuming a compressor fault.
If the AC cannot reach its rated temperature even after cleaning and with proper power, the unit is hitting its design delta-T limit on a hot day — covering windows with reflective shades, parking in shade, and insulating the roof above the AC can recover 2–5°F of performance.
Heating and cooling options are more nuanced than the cycle itself — our tested roundup of the best AC units for an RV compares rooftop and portable options for different RV sizes and power budgets.
FAQs
Does an RV air conditioner need a generator to run?
No — roof units run on 120V AC from shore power (30 or 50-amp pedestal), a generator, or a suitable inverter. Portable units can also run from a generator, typically requiring around 1,800 watts starting power for a 13,500 BTU model.
Can I run two RV air conditioners at the same time?
Only on a 50-amp electrical system. A 30-amp service cannot handle the combined draw of two 13,500 BTU units (roughly 30 amps total) plus a converter and other loads — the main breaker will trip. On 50-amp service, two units can run comfortably on separate legs.
Why does my RV AC freeze up?
The most common cause is restricted airflow — a clogged filter or blocked return grill. Cold liquid refrigerant cannot absorb heat when air isn’t moving across the coil, so moisture condenses and freezes. Fix the airflow source and run the fan only (no compressor) to thaw the coil before resuming normal cooling.
References & Sources
- Furrion. “How Does an RV Air Conditioner Work?” Explains the 7-step refrigerant cycle and power requirements.
- Dometic. “RV Air Conditioners” product page. Official specs for BTU ranges and operating limits.
- MyRVWorks. Carrier RV AC / Heat Pump technical manual. Technical repair procedures for rooftop units.