Being familiar with commercial hydronic systems we always design systems with expansion tanks and air separators.

Looking for input on the benefits of a small inline air separator and expansion tank on a residential geothermal system.  I figure a small bladder type expansion tank could maintain pressure in the system during seasonal fluctuations/pipe expansion.

Shouldn't need them. HDPE loop pipe grows and shrinks a bit with temp changes accommodating loop fluid volume changes stemming from temperature changes, and there is no need to separate air if the system was properly purged of air at the get-go.

In days gone by expansion tanks were sometimes used, but to keep costs down they are seldom used today in residential. Micro bubbles also form larger bubbles that can be problematic in some loops. It's a cost thing.

Well, I put both on the ground loop and the load loop(s). In addition there is a high point vent valve on the top of the load supply tank. If you ever have to open the system(s) for a pump change, etc there will be some air to deal with.

Two Exp Tanks and two SpiroVents are about $250 + install. Probably cheaper than a service call to purge in case air is trapped somewhere.

If that's too steep by all means include high point vents. If your layout has the unit in the basement be sure the HP vent is piped to a spot for a bucket if it's inside the house.

Dick