[superlen]

Memphis,

Regarding staging vs variable speed, watch out for oil return issues when using multiple compressors. I have visited three different Geothermal manufacturers over the last 10 years and while visiting with their engineers, (electronics side, not thermodynamics side), two of the three had mentioned field reliability issues with some of their multiple compressor designs regarding oil return. All three were migrating towards variable speed designs and away from multiple compressors. Production designs always have to meet a certain cost constraint so they may have just been trying to stay too close to the edge, or maybe they didn't get enough test/validation time before they had to go to market. Any number of possibilities exist.


For larger systems, I think VS has been the norm for some time now. It's just now making it's way down into the 5T and smaller systems as found in residential. What's interesting is that it skipped around the 3-5T systems with the minisplits leading the way now.

My prefs if I wanted variable capacity would be in order:
1. VS VFD or brushless DC either one
2. Two stage scroll compressor
3. Multiple compressor

Being a hacker, I would do what you're doing, keep a lookout for the perfect choice, and play with what you can scrounge up easily for not too much money.

One question I would ask is unless you have all the components, evap/cond/both txv settings, charge, water flow, air flow, ect. tweaked in perfectly, how much more efficient will a true variable speed compressor be over say just a simple two-stage unit? I don't know the answer to that.

Len