Well since you asked...
I actually just had to deal with this issue a few weeks ago. I have a very old heat pump (central A/C unit) that works fine in the winter but does not generate very cold air in the summer because it leaks the refrigerant gas called R2 in my case. In the past, I would normally call an A/C technician to add more R2 gas once every two years or so. I have used this method to keep my old heat pump operational for the last six years despite the leak. I could buy a new heat pump, but I don't think this problem warrants replacing the whole system. The guy who refilled my unit last summer really pissed me off because he sold me this additional additive that he claimed would seal the leak, but instead my unit is back down to 60% to 50% this summer ahead of schedule. This means the air coming out of the heat pump is only slightly cool and it freezes itself trying to reach the target temperature set in the thermostat. Instead of calling any more A/C scam artists this year I decided to handle the problem myself.
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I initially bought two portable units (see first photo above) and installed them in my living room toward the back of my house (I don't spend much time in my living room so the additional noise is not an issue) and in my exercise room (where the noise from the portable unit doesn't bother me). Unfortunately, those portable units just create pockets of cold air and even with fans, I could not distribute the air evenly throughout my house. So I decided I needed to have heat pump working at least some of the time so that at least it could help distribute the cold air even if it is contributing very little cool air. After removing the thermostat cover and probing around, I figured out how to hack into the thermostat. I connected my own 8-bit microcontroller to it (ATMega328, same type that is used in Arduino Uno) and had the microcontroller use a timer to turn on the unit for 30 minutes and shut it off for 40 minutes in a loop. The microcontroller controls two transistors, which I soldered to the buttons on the thermostat. The transistors simulate pushing the buttons. The buttons normally allow you to raise or lower the target/desired temperature. The microcontroller sets the target temperature to 90 degrees to effectively turn off the A/C and sets the target to 45 degrees to turn it on. The timer prevents the heat pump from freezing. This worked out very well. The heat pump and the portable units work very well as a combo.
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For those whore are into electronics, yes that left-most red wire on the little green breadboard is not connected to anything in the second photo. It normally supplies power to the microcontroller, but I disconnected it when I took the photo. The green and blue breadboards and the wires coming out of them is what I added to the system. I made my own little cover out of some white poster boards to cover up the modified thermostat and make it look pretty.
I watched some Youtube videos and I am pretty sure I can add the R2 gas to my central A/C unit myself. I already ordered some equipment and the gas and now just waiting for them to arrive.
