RecPro 12V RV Air Conditioner: BTU Rating
You might be saying to yourself, “How can that small 12V AC be putting out 20,000 BTU?”
I’ve done some research and some number crunching and I’ve found that the ‘20,000’ number may not be too far off.
What are BTUs? Well, it’s sort of a tricky metric to understand and has several commonly used methods to determine, however each method can produce different results.
So what are BTUS? BTU stands for British Thermal Unit and is a measurement of the ability to heat or cool a given space. More BTUs means more heating/cooling power. The technical definition of BTU is the amount of heat needed to raise the temperature of 1lb of water by 1°F.
What are the testing methodologies and how are they different?
Different governing bodies have different approaches when measuring BTU capacity of air conditioners. Units are installed and run for a set time using different indoor and outdoor temperatures, as well as varying degrees of humidity. While testing conditions do differ, it’s worth noting that the harsher testing conditions typically produce the highest BTU calculations.
Example test conditions:
AHRI (Air Conditioning, Heating & Refrigeration Institute)
Indoor: 80°F @ 50% Relative Humidity
Outdoor: 95°F
American Trucking Associations TMC
Indoor: 73°F @ 50% Relative Humidity
Outdoor: 100°F
What were the testing conditions for the RecPro 12V Air Conditioner?
All testing and data collection were provided by the manufacturer and can be found in the original document here.
Testing environment:
Outdoor Temp of 125°F @ 45% Relative Humidity
Indoor Temp of ~120°F at start of test
Unit is installed and run at max for 75 minutes
The results don’t mention BTU. What gives?
We can determine cooling BTUs from the testing data using a simple formula:
Cooling BTUs = CFM x ∆h x 4.5
Where:
CFM: Cubic Feet per Minute of Airflow
∆h: The change in amount of heat, determined by factors such as humidity, pressure, and temperature, expressed as “Enthalpy”.
The 12V Air Conditioner fan is rated at 285 CFM giving us:
Cooling BTUS = 285 x ∆h x 4.5
How do we find the change in Enthalpy?
We can use the temperature and humidity results to find the equivalent Enthalpy within a psychrometric chart.
| 15 min | 30 min | 45 min | 75 min | |
| Inlet Temp* | 104.5 | 100.5 | 99 | 98 |
| Enthalpy Value (Inlet Temp) | 47 | 42.6 | 42 | 40.5 |
| Outlet Temp** | 88.5 | 82.5 | 79 | 76 |
| Enthalpy Value (Outlet Temp) | 34.1 | 30.1 | 28.2 | 26.3 |
| Change in Enthalpy | 12.9 | 12.5 | 13.8 | 14.2 |
*Inlet Temp based on Average Internal Temp Sensor Reading
**Inlet Temp minus Temperature Differential Air inlet-Outlet On Air Conditioner
Let’s plug in our enthalpy values and find our calculated BTUs.
Cooling BTUS = 285 x ∆h x 4.5
| 15 min | 30 min | 45 min | 75 min | |
| Inlet Temp | 104.5 | 100.5 | 99 | 98 |
| Outlet Temp | 88.5 | 82.5 | 79 | 76 |
| Change in Temp | 16 | 18 | 20 | 22 |
| Change in Enthalpy | 12.9 | 12.5 | 13.8 | 14.2 |
| BTUs | 16544.25* | 16031.25* | 17698.5* | 18211.5* |
As you can see here, values can fluctuate a little bit.
*Because enthalpy is dependent on humidity and other factors, it’s possible that with more data we could reach the 20k BTU number. Aside from cooling, air conditioners also pull some moisture from the air. We can infer that because the air conditioner lowers the humidity of the air, that our real enthalpy numbers may be slightly lower than estimated, resulting in a BTU increase. Additional data points such as relative humidity would help dial in the final BTU number.
While we don’t exactly see the 20-22k promised by the manufacturer, we are in the ballpark of 20k, while still running on ~1100W.
Conclusion:
BTU ratings are heavily determined by conditions and methods used in testing. These conditions can be more or less extreme than what you would normally experience in your RV.
Manufacturers will pick and choose, and even round numbers up in some cases in order to improve the salability of a product. Based on the information provided, we see approximately 18k BTUs at best; more data points could bring us closer to the promised 20k.
-RecPro Staff Writer
Premier Products 12V DC Air Conditioning Comparison Testing Results : Link