R290 into 410A system
 

Lightning zapped 410A inverter split system at son's house, easiest fix was to buy a whole new unit as can later repair zapped unit and use for shop that now is just wood heat.
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Have used bbq propane for R22, how will that work for replacing 410A? Pressures will be a lot lower, maybe need to replace or 'finagle' txv?

Not the same at all. High molecular weight, high pressure. R410a is kind of a "high pressure extension" of R134a. Where R134a is good for Sub-Zero evaporator temperature and/or super high condensing temperature, R410a is best for HVAC system temperatures. The only advantage is that the system is physically smaller than the older refrigerant filled systems. It was pretty much invented to be different and not work well with any other gas.

One of the biggest problems with using a different gas is that of the lubricating oil compatibility.
If the oil is insoluble in the gas expect a very short life for the compressor

While R290 operates in the same neighborhood as R22, it doesn't even come close to r410s requirements. 410 runs at nearly twice the pressure with a different mass flow rate. If you installed r290, the btu output would drop 1/3 to 1/2 of rating, and since it's an inverter system, it would likely shut down on safety as low charge or erroneous SH readings. Sorry, this one's a no go.

Good luck with the fix though!

Why do you not wish to use r410a ?

Wyr
God bless

OP, If you are looking to use R290 because you think you can't buy the R410A, that's not true. You can buy it almost as easily as buying a can of R134A for a car, just from a different place than your auto parts store.