alt Hacker NewsIf it is outside you can just use propane as refrigerant. Low warming value and it leaking to outside doesn't matter, not gonna catch on fire.
There are indoor devices using propane but they generally come with caveat "must be installed on room at least this m3", precisely so if it leaks it doesn't get to density that allows explosion
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I currently have R410a heat pumps with minisplits, when they're off warranty sometime in the future I might consider putting propane in them. The main question would be whether the oil used with R410a is compatible with propane as a refrigerant.
This is probably a little bit "dangerous" in the sense that I'll have high pressure propane in the line sets and minisplits inside the house, but it's a very small volume of propane and not really any more dangerous from a fire perspective than my propane fired condensing gas boiler.
[edit] a little internet sleuthing indicates that hydrocarbon refrigerants "might be" compatible with basically any oil, so I can probably just continue using whatever oil is in it.
I believe there are propane using split AC systems where propane is used only in the outside unit. The outside unit has a heat exchanger transferring the heat to some glycol-water solution which is what then circulates through the inside unit.