It is really dependant on the area of the country i have heard, here in Oregon we install ALL of our own piping and loops(except vertical) but even if we do vertical drilling($27 per foot) we install the headers, make all the connections and flush and fill the systems.

Personally I think its better when a company does their own piping. Lets say you have an issue and its short looped, the driller and geothermal installer can point fingers at each other and nothing gets done. If one person does it all, they can only blame themselves if the system doesn't work right.

as long as its right, it doesnt matter who does it. But....when I subcontract, I own the outcome.

Around here, the main contractor (I guess that's what you mean by "installer")
does the Manual J load analysis and designs the system, specifies HP tonnage,
specifies loop length & type, circ pumps, etc. and does the indoor plumbing,
ductwork, electric -- but subcontracts the outdoor work (drilling, loop pipe
installation, headers, grouting, trenching, etc.) to an IGSHPA-certified loop
installer.

It's simply a matter of who owns the machinery and who has the expertise
and trained techs for each part of the job. It also minimizes the need for
coordination. Indoor and outdoor tasks can be scheduled independently.

Indoor and Outdoor always run independent for us, since one is somewhat weather dependent, and the inside depends on the complexity of the job, not every crewman has the same skills. w-w or w-a? From our perspective, there is very little equipment needed for the header, 85% is horizontal anyway, which we all do in-house. I just don't want to own a drill rigg, and my skills do not include drilling.