Can you mix R22 and R22a?
In the USA it is unlawful to mix R22 (chlorodifluoromethane)
with R22a aka R290 (propane, aka liquiefied petroleum gas)
(propane). While the two are not incompatible working fluids mixing
them is a bad idea even where it is not unlawful. R290 is a known
refrigerant with many excellent properties and one major drawback -
flammability. As a practical matter, R22 was created because early
air conditioners using ammonia, propane, and other toxic or
flammable materials were dangerous when they failed, and air
conditioning was not accepted as safe for the home. Propane has not
become any less flammable since that time. In the event of a rapid
loss of refrigerant (e.g. compressor terminal blowout) an
electrical spark (from the live wire on the terminal blown out) may
ignite it. In such an event any R22 also present would decompose
into highly toxic fumes greatly increasing the risk of accidental
death. As a further caution to tinkerers, commonly available LPG
propane (but not R22a or R290) has a sulfer compound "mercaptin"
added for leak detection that would have an undesirable effect on
the lubrication oil in a sealed system.