Let's put flammability and legal issues aside. It's a common misconception that HCs are inherently more efficient. That maybe the case if you're doing theoretical comparison. Actual in-real equipment test results published in January 2003 ASHRAE Journal shows R-290 has a lower efficiency than R-22.

They found that R-290 is 3-6% lower in capacity, 5% lower EER and lower SEER than R-22 when used for air conditioning temperatures. Heat pump performance was also reported to decline similarly down to 17F. When you get under 17F outdoor temperature, they found the performance catching up. Reference: http://bookstore.ashrae.biz/journal/...=9439hwang.pdf

Saturation pressure of R-290 in low temperature applications is higher than R-22, so that's not a surprise. That's why they invented Freon 502 which was the gold standard of low temperature refrigeration refrigerant. For low temp (like ice cream freezers..) R 12 was another excellent refrigerant where R-134a does poorly.

There are plenty of marketing hypes and high mark-up sketchy "hydrocarbon second generation natural refrigerant" claiming their R-290 based refrigerant as more efficient than R-22. Finally, this brings objective view on this matter.

The researchers used a 2.5-ton R-22 heat-pump that was state of the art in 2003. I would guess a 12 or 13 SEER unit. Propane has a lower capacity than R-22 given the same compressor displacement, so they used a larger displacement type to compensate for this along with a modification to TXV to match capacity of R-22. They tested the system by the ASHRAE standards putting it through steady state and cycles to test the SEER value.



An Indian journal published the result of running R-290 in a window A/C designed for R-22.
R-290 caused a 11% loss of capacity, and a 3% efficiency gain. Wait a minute.. what's going on here? It's the same thing equipment manufacturer do to raise the SEER. You're making the coil bigger relative to BTU capacity. You're trading capacity for efficiency. I think the gain isn't because of supposed super power of hydrocarbon R290, but you're simply running the system at a lower capacity. Reference: http://www.doiserbia.nb.rs/img/doi/0...361000081C.pdf

I'd say the efficiency "gain" is more pronounced on older 7 SEER equipment that's running some 25-30 degree of rise at the condenser compared to a 13 SEER R22 equipment that's running less than 10 degrees of rise.