Randen,
It looks as if you are having the same issue as I was. When I began with the TXV, I was running suction pressures in the 60 to 75 psi range. I just couldn't figure out why my high side wasn't getting hot enough. So I googled the following:
suction superheat r290
The first 2 hits, from bitzer and purdue, are definitely worth reading. That's where I read about the high superheat needs of R290. It has higher heat capacity than R22, so it doesn't superheat much when being compressed like R22 does. There are 2 main ways to add this superheat: lower your suction pressure or add a suction line hx. It goes against what I know about phase change systems: lower suction pressure and you reduce capacity. But guess what? The rule book lied! The scientists tested it out and it works. Who knows why? So I cranked my superheat WAAY up to see what would happen. Lo and behold, the unit poured out the heat immediately.
Now, my ASACHP suction runs in the 40-50 psi range on mild heating days. It has gotten down to 23 degrees f, at which time the suction pressure was closer to 35 psi. It used to peter out in heating mode around freezing temps. Now, I'm waiting for a cold snap to see how low it will go. If it doesn't pop another joint, that is.
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