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jeff5may · 11-26-12, 08:31 PM

Randen,
That TXV mod is sweet! I never would have thought of doing it. I have a similar TXV in my project. I have had to repair leaks in it a few times, and every time I adjusted the TXV for more superheat. It sure would be nice to be able to adjust that baby at will.

A word on R290 and superheat. If your TXV came set up for R22, the factory superheat setting is WAAAY too low! R22 likes 6-7K superheat in practice, whereas manufacturers are recommending 14-20K or more superheat for R290 systems. It seems that the propane dissolves in oil, making a kind of oil soda in the condensor side. This actually aids in heat transfer, since liquid propane soda has much better thermal conductivity than just oil. The problems happen on the other side of the TXV when this oil soda rapidly expands. Instead of fogging like R22 and oil, the propane soda fizzes and foams like warm beer! If your superheat setting is too low, the stuff spews right through the evap coil, transferring nearly no heat and killing your temperature split. If you listen to the evaporator, you can hear it! Sounds kind of like a car radiator that is about to spew from overheating.

You will see good high side pressure, yet low discharge temps. In my window AC unit, the SST was below freezing, but the bottom third of the evap hx never frosted up. After a few hours of operation, the suction line was heavily frosted all the way from 1/2 way up the hx to the compressor shell. When the unit defrosted, it behaved OK running through stock R22 cap tube circuit. Since then, I believe I have gone two turns up from the factory setting (it was set high anyway to deal with a distributor and its pressure drop). Once I got the superheat up to around 20 degrees F, I started to notice an improvement. I'm running at about 16-18K superheat now, and I just might turn it up again if i have to. Don't tell the HVAC guy for fear of being bludgeoned.

11-26-12, 08:31 PM	#1342
jeff5may Supreme EcoRenovator Join Date: Jan 2010 Location: elizabethtown, ky, USA Posts: 2,428 Thanks: 431 Thanked 619 Times in 517 Posts	Randen, That TXV mod is sweet! I never would have thought of doing it. I have a similar TXV in my project. I have had to repair leaks in it a few times, and every time I adjusted the TXV for more superheat. It sure would be nice to be able to adjust that baby at will. A word on R290 and superheat. If your TXV came set up for R22, the factory superheat setting is WAAAY too low! R22 likes 6-7K superheat in practice, whereas manufacturers are recommending 14-20K or more superheat for R290 systems. It seems that the propane dissolves in oil, making a kind of oil soda in the condensor side. This actually aids in heat transfer, since liquid propane soda has much better thermal conductivity than just oil. The problems happen on the other side of the TXV when this oil soda rapidly expands. Instead of fogging like R22 and oil, the propane soda fizzes and foams like warm beer! If your superheat setting is too low, the stuff spews right through the evap coil, transferring nearly no heat and killing your temperature split. If you listen to the evaporator, you can hear it! Sounds kind of like a car radiator that is about to spew from overheating. You will see good high side pressure, yet low discharge temps. In my window AC unit, the SST was below freezing, but the bottom third of the evap hx never frosted up. After a few hours of operation, the suction line was heavily frosted all the way from 1/2 way up the hx to the compressor shell. When the unit defrosted, it behaved OK running through stock R22 cap tube circuit. Since then, I believe I have gone two turns up from the factory setting (it was set high anyway to deal with a distributor and its pressure drop). Once I got the superheat up to around 20 degrees F, I started to notice an improvement. I'm running at about 16-18K superheat now, and I just might turn it up again if i have to. Don't tell the HVAC guy for fear of being bludgeoned. Last edited by jeff5may; 11-26-12 at 08:36 PM..