Hi,
I am an conventional hvac guy and have had my own business for a few years now.  I have recently started to educate myself on geothermal after running across a few interested customers and having to turn them elsewhere.  I want to make sure that I get this right the first time so here goes...
I understand the concept of head loss and how that applies to a loop field.  I just don't know exactly how to go through all the math and size the pumps accordingly.  If I do size the pumps incorrectly.  say I only put in one pump when I need two.  What will happen?   Will my gpm be really low.  Conversely if I put in too much pumping capacity, will my gpm then be too high.   What is the highest flow rate one would want on a three ton system.   Any input would be quite helpful.  I want to make sure I get this right before proceeding.
Thanks

Short answers:

One pump where two are needed will result in too low a flow across geo unit heat exchanger and possibly also subject loop field to laminar flow conditions when turbulent flow is needed for proper heat exchange.

An overpumped system will heat and cool OK but at reduced system efficiency. These pumps aren't particularly efficient, <50%, and their wattage can be a significant fraction of overall system power.

A 3 ton system should be OK on one pump, and flows should be 3 GPM / ton closed loop but as low as 1.5 GPM / ton open loop if open loop water temp exceeds 50 deg.

There are software packages out there to take care of the nuts and bolts of this, but the user needs to understand the underlying theory so as not to run afoul of the GIGO syndrome (Garbage In (to a computer or software package) results in Garbage Out)

You might benefit from IGSHPA membership / Heat Spring Institute classes, though they are a bit spendy.

Thanks for the info.  So essentially you are trying to match up the head loss of your pump(s) to the head loss in your loop field??

Also...  as far as I know pumps generally only come in one size.  Say I run across a loop field that requires 25% more than one pump can handle.  I then have to get two pumps and I am guessing that a great deal of efficiency is lost.  Should I be cognizant of this when designing my loop field. 

Geointerest,

Before you attempt your first geo install get some training!! IGSHPA training and certification is the best first step followed by factory training. Most manufactures conduct detailed classes. I've been to Climate Master and GeoComfort classes and learned from each. After you invest in your education take advantage of the factories design help. If properly done, the end result will be a great experiance for you and your client. If you try to take short cuts, the end result could be ugly. Geothermal requires different skills beyond HVAC and training is the best way to obtain them.

Circulators come in a wide variety of pumping capabilities. Your choice will be based on the pump curve(s) and the flow rate needed to obtain turbulent flow.

Bergy

Bingo - you've grasped the essentials.

Any given system will have a certain required (It'll actually be a fairly broad acceptable range) of GPM for adequate heat transfer and a total system (unit + loop field) head loss associated with achieving the range of GPM.  Pumps come in a huge range of sizes and all have published curves - graphs of flow vs head. Pick the one or combination that best fits the system need.

If you are a reasonably competent and thoughtful HVAC tech or designer you already know, whether by applied theory or empirical understanding, of these concepts as they apply to airflow. A given blower will (or won't) move the required CFM of air through the system's unique combination of trunks, runouts, elbows, filters, coils, grills, registers, etc.

The numbers and units look way different but the very same principals apply on the water side of a geo system.On the waterside you are dealing with long runs of tube, fittings , coaxial heat exchangers, etc.  It may well be you don't need expensive out-of-state training to apply the principals associated with ESP on the airside to total head on the water side.

It's all just fluid dynamics.