fellows I was talking with on a DX system (ECR) to water, also talked about oversizing their heat exchanger to get better transfer efficiency at lower temperature differentials. Is this a common tactic? Is it applicable to non-DX geo systems? Does it help?

It is not necessarily common, but we do it, too. When we do, we see much higher COPs heating water. It also translates into higher capacities.

I don't know if it's a good option for a water source system. I don't see why not.

This from Hydro-Temp which makes geothermal units. www.hydro-temp.com

"--Coil Size Options: For years, Hydro-Temp Corporation has manufactured extremely efficient heating and cooling systems by over-sizing the water and air coils by one size. For example, if a customer purchased a four ton unit, we installed five ton heat exchangers. We have since discovered that even greater efficiency can be produced by over-sizing the heat exchangers by two sizes--installing six ton heat exchangers with the four ton unit. With more surface area on the air coil, there is more heat transfer and dehumidification in the air conditioning mode. Our standard option provides the customer with heat exchangers that are one ton larger than the unit, while the over-sized option offers heat exchangers that are two tons larger. "

Farmboy,

Thanks for that piece of information.  This is something that I have believed for some time - actually ever since I drove a car that had an undersized heater core.

Ha! very cool, thanks for the feedback.

So now my follow up question: since it works, why isn't it standard practice? That is, why aren't the heat exchangers optimized for transfer in the first place?

I imagine it's cost, but how big is the cost differential between a 4 ton and a 5 ton heat exchanger, for example? rough order of magnitude is fine ($100, $1000, etc)