Looking at the trade offs, it seems that good old HDPE pipe will probably make the most economical sense for a large horizontal geo field.  This raises the concerns of Oxygen in the system even though some data indicates that even Oxygen as small as a few parts per million can cause some corrosion in cast iron.  Looking at the life of the Methanol or Glycol fluids, it just seems to make more sense to cut costs with HDPE and plan to change out the fluids every 3-5 yrs. 

Friend of mine that designs commercial systems told me they always spec an air separator to help combat these issues.  I am just curious if anyone here is familiar with them and their performance in residentials?  From the data I have found, the air that would naturally occur in the pipe during installation of a system is enough to cause problems, pretty much negating any real improvement with Oxy barrier tubings.  In short, you will always have O2 in the pipe, the better route might be to use a device to help trap and remove it.  That could substantially reduce the tubing costs. 

air separators cannot prevent corrosion. they remove air but cannot ensure a lack of oxygen. they should be used anyway since these systems are prone to air problems, but it doesn't stop your corrosion issue. you can stop the corrosion issue by not using any ferrous components on the HDPE side of the system, or by using anti-corrosion additives.

the air that is in the system during the initial install and even afterwards will cause corrosion. but once that corrosion is finished, you're all done. I don't know what kind of volume you are talking about or how much iron and even if I did I don't have the math to say what is safe and what isn't, but I can say the initial fill of oxygen is not typically a problem for hydronic space heat in residential homes that have only a few pumps and maybe some fittings in a boiler for "ferrosity" to take the corrosion. grrrrr Perhaps if you have a huge volume of initial fill water and a small amount of "target" for that oxygen it could be a bigger problem though.

of course, all hydronic systems worth their salt have air separators as well.

I agree - don't use iron/steel and then don't worry about it. But do purge air out of the system (for other reasons).

Large commercial systems have large (expensive) commercial pumps and bigger flush requirements. Resi geos have few ferrous parts and they are relatively inexpensive. Seperators are subject to failure as well (paticularly where antifreeze is in play). You might be trading one problem for another. If you put something in, make sure you can isolate it for repairs without having to re-flush the system.
j