I got my first look at a “dry” unit the other day. A wholesale house had one on its showroom floor with a big sign “we have dry units!” I looked at the nameplate to see what it said about refrigerant. A refrigerant amount was actually listed. Instead of the familiar “factory charge” preceding the refrigerant quantity, it had “Field Charge” and then the amount. I asked about coil matching for older systems. They recommend 1/2 ton smaller condenser than the original for heat pumps and the same size condenser for air conditioning. Then I asked about warranty and they said the units came with a 5 year compressor warranty regardless of the coil connected to it. They pointed out that this was less than the warranty on new matched equipment. Many of the units are being sold to replace condensing units that have been stolen or chopped up in place, often in rental property. Another wholesaler requires you to take coil measurements and then uses software to try and figure out the coil SEER. If the calculated SEER is less than 13, they recommend against buying the dry unit. Companies that just a few years ago were cautioning against the perils of mismatches units are now selling units that will almost certainly be mismatched. They cannot provide any meaningful charging chart without knowing what coil is on the other end of the system.
If you do decide to install a “dry’ unit, make sure and follow good refrigeration system installation practices. The fact that you are creating your own special blend of equipment makes it all the more important to be careful, not careless. It would be wise to treat the job like a compressor burnout. Flush the lines and coil, install a filter drier, pull a good vacuum, and weigh in the charge using an accurate digital scale. You can see more about line flushing at Mainstream Engineering. They sell a flush designed for this type of use. You will need to adjust the data plate charge for line length and possibly coil mismatch, but the nameplate charge should be in the ballpark unless you have very long lines. I think it might be wise to start a little under, so I would not adjust for line length until after starting the unit. I found a good set of instructions for installing dry systems online provided by Century, a supplier that is offering dry units. No, these are NOT the folks who suggested the 1/2 under ton heat pump coil adjustment. The link to the guide is: