Main selection factors for refrigerant are:
- efficiency, amount of btu's exchanged during liquid to gas transition temperature and pressure for this transition required by compressor.
- Toxicity if system springs a leak.
- Flammability if system springs a leak. (propane is not good)
- Ozone depletion effect
- Globe Warming potential (compared to carbon dioxide emissions)
R-410A has an ozone depletion potential (ODP) of zero, meaning it does not deplete the ozone layer. R-410A does not contribute to ozone depletion because it only contains fluorine, unlike older refrigerants like R22 and R12 that contain chlorine or bromine. Fluorine is toxic when broken down, like when brazing copper tubing.
R410a is a mixture of 50% R32 and 50% R125 to get its flammability reduced.
Most refrigerant combo mixtures have a 'glide' slope which means the mixture creates a slight temp range for the transitioning between liquid and gas, which is undesirable as it reduces btu efficiency a bit.
R410a has a very slight glide of less that 1 deg F.
R454b has a glide of about 1.5 degs F. R-454B is a blend of R-32 and R-1234yf.
R32 has zero glide slope.
R-410A has a global warming potential (GWP) of 2,088, which means it is equivalent to 2,088 times the effect of equal weight of carbon dioxide.
R454b has a global warming potential (GWP) of 466 and zero ozone depletion effect. R454b is mildly flammable with an A2L rating
R32 has global warming potential (GWP) of 676 and zero ozone depletion effect. R32 is mildly flammable with an A2L rating. R32 high side compressor pressure is slightly greater than R410a but is net slightly higher efficiency than R410a.
R290 (propane) has global warming potential (GWP) of 3 and zero ozone depletion effect. R290 is highly flammable and has an A3 flammability rating. Too many lawyers in U.S. for manufacturers to risk using highly flammable R290 in U.S.
In U.S. A2L, slightly flammable rated refrigerant systems will have a gas sensor to detect refrigerant leaks that will shut down compressor and turn on air handler blower to disburse any leaking refrigerant, with a message on thermostat to immediately call for service.
For comparison, methane (natural gas and cattle farts) GWP is around 28 times that of carbon dioxide. New regulations for GWP will likely end up around <800, knocking out R410a but allowing R32 to squeak by.
In general, in U.S., conventional split, ducted systems will be dominantly R454b. Mini-splits R32
By manufacturer:
Trane (American Standard) - R454a
Lennox - R454a, R32 for contracted mini-split units.
Carrier (Bryant) - R454b
Rheem (Ruud) - likely R454b
Dalkin (Goodman, Amana) - R32
York - R454b
My personal preference is R32 which avoids mixture glide resulting is slightly greater efficiency.