Quick review...
4 ton system (GeoMax 2)
4 vertical ground loops in 2 boreholes (2 per hole)
Rehau PEXa tubing
Manifold inside the house

The Rehau literature for the manifold refers to the manifold having adjustable valves for balancing the loops.  What exactly does balancing the loops mean and why is it done?  With only 4 loops, all connected to the same manifold, wouldn't the flow of coolant be balanced anyway, or does the first pipe get the most pressure, 2nd get a little less, and so on, leaving the 4th loop with significantly less pressure.  Does it really make a difference?

(Recall that the loops come in across the basement, travel across the ceiling, then down to the manifold.  The manifold is therefore upside down, which makes the individual gpm gauges not work...they always read zero.)  I know that screwing down any one of the manifold "caps/valves" shuts off the flow to that loop (the installer showed me that), but the adjustment seems to be simply on/off.

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Here's the Rehau blurb.  I have the 4 outlet model.

DISCOVER THE CONVENIENCE OF THE PRO-BALANCE™ MANIFOLD
Thank you for your inquiring about our PRO-BALANCE™ Manifold with Gauges.
Visual flow balancing is quickly accomplished with the PRO-BALANCE Manifold eliminating the use of tables, guesswork, and waiting for the system to respond to changes.
The manifold is delivered pre-assembled and ready to install, with the following features:
   Solid extruded brass headers with drilled and tapped connections
   Steel mounting brackets (assembled)
   Two 1” NPT isolation valves
   Two Combination air vent/fill ports
   Adjustable flow control valve for each circuit
   Flow gauge for each circuit, in easy-to-read graduated GPM increments
   Available with two to twelve outlets

http://na.rehau.com/construction/re...fold.shtml

if there truly is a flow gauge and adjustable flow control valve for each circuit, then balancing means to equalize the flow across each loop.

It is best to balance them by temperature.

Did you ever approach your installer regarding the gauges that don't function?

If heat pump performance is within spec, you will likely see little benefit to change.
It sounds as though the only change you could make would be to increase resistance to flow, so I would be inclined to leave well enough alone.
Joe

Yep.  He, too, was perplexed as to why Rehau doesn't design them for any angle installation, and I agree.  It seems that there are so many ways that piping could come into a house (up high, down low, across the basement, up from the floor) that designing a gauge that only works in one position is ridiculous.  He's been in touch with the company about it for them to redesign in the future.

Balancing by temp would mean making sure the return temp on each loop is about the same...in case one loop is particularly good/bad at absorbing/rejecting heat?
Balancing by flow would mean making sure the gpm in each pipe is equal?

Yes, exactly. If you are sure that all the loops are equal in performance and resistance to flow, then both methods yield the same result.