[jeff5may]

^^ OBVIOUSLY EXPERIENCED (with R-290)^^

I have dropped bbq bottle propane into multiple (smallish capacity) cap tube systems without having to change any plumbing at all. They did great. The main thing to remember is that propane has a little less vapor pressure at the same temperature vs r-22. So at any given pressure, your saturation temp is slightly higher. Fwiw, With a cap tube I set the system charge in a hot tin shed on the hottest day of the year.

At low pressures, this difference is pretty trivial. For example, at 60 psig, r-22 has a saturation temp of 34 degF, and r-290 is at 37 degF. This difference widens as temperature increases. At 300 psig, this difference has grown to 10 degF.

In cooling mode, a stock cap tube will do just fine as long as you don't overcharge the unit with propane. For practical purposes, the gauge scale for R-22 can be used to set saturation point and superheat. I have found that if you charge by weight, about 1/3 of the nameplate value for R-22 charge is a good starting point. This will give you a little extra superheat vs R-22, but that's ok. We don't want to flood the compressor. Oil is more soluble in propane as well, so the extra superheat helps cut down on foaming effect. Fwiw, I set the system charge on the hottest day of the year in early afternoon. HOWEVER, I set the txv superheat during sub-zero outdoor temperatures. Warmer it gets outside equals more dT outdoors and more heat gathered per minute of runtime.

Heating mode is a different story. With any cap-tube metered system, capacity is always traded for low-temperature stability. If the unit is going to be operating in freezing (or sub-zero) outdoor temperatures, the cap-tube will have to be a lot longer than a cooling-only unit. Thus, the check valve and extra length of cap-tube is installed to limit suction saturation temperature in heating mode. This extra length of cap-tube also reduces mass flow and (maybe) heat flow from outdoors in. Depending on many factors, this safety cushion eats at heating efficiency.

For DIY ecorenovators, a choice has to be made between ultimate simplicity and ultimate efficiency. The old-school, cap-tube metered a/c and heat pump units are very reliable because they have very few moving parts. They just go and go like the energizer bunny, sometimes for a human generation or lfetime. If something fails, it is usually a capacitor or switching device. Much later in life, corrosion, friction, and fatigue take their toll as the unit approaches retirement age.

OTOH, the newer TXV or computer-controlled units cost less to operate because they push the efficiency envelope. They have many more moving and/or electronic parts, all of which increase the risk of failure. If something does fail, it is not as easy to identity what went wrong without gauges, thermometers, and other test equipment. While they operate, the energy savings is pretty substantial, but chances are the unit will not live as long. Sensors and transistors generally don't live as long as a piece of pipe.

With modern technology changing the way people use (and feel the need to be constantly upgrading) equipment, the manufacturers have followed this trend. Stuff just isn't built to live as log as it used to be. Whether or not you justify these changes makes a big difference in system design and operating parameters.