For some reason I find that I'm answering the same questions again and again.
1) How do I know the loops will be big enough?
2) How do I know the EWT will stay above freezing?
3) How do I know the auxiliary heater won't come on?
Here are the answers:
1) Because you did your homework and you know that your installer is either certified by the International Ground Source Heat Pump Association and has a string of satisfied customers behind him/her or it is a DX system where all loops are the same size.
We don't guess loop size we do what we know works with decades of our own or our trainer/instructor's experience to guide us.
2) If you are in a heating dominated climate it is anticipated that the water will get below freezing, that's why the antifreeze is there. There has been a trend lately to insist on extra loops (on this and other sites) because we have somehow led folks to believe that it is worth the expense. By and large it is not.
3) Again in a heating dominated climate the auxiliary coil is an inexpensive alternative to geo that is designed to accomodate 100% of your heating load.
A typical home in MI for example may find that a system designed to satisfy the thermostat 95 out of 100 heating days might be 2 tons (or more) smaller than a unit designed to handle all of the heating needs. The difference is thousands of dollars off the installation cost and often less than $100 added to the operating cost.
So, what does all this have to do with the title of this thread?
Ductwork is one of the single most common problems I see in geothermal installations.
Undersized duct work reduces the capacity of a heat pump (both air and ground source). It causes problems with distribution and it's really noisey.
The carpet baggers that will be offering geo soon, will hire an experienced geo excavator for the loops, but they will do the same lousy sheet metal work they did during the construction boom to satisfy code and bid minimums.
So IMHO if you really want to grill your bidding contractors, get off the loop questions. Instead ask them "how many square inches of return air is required by the International Mechanical Code for a heat pump?"
If they say 6 sq"/thousand BTU's, you've got a pro (unfortunately this same question might stump your local inspector so you'll have to enforce it).
A loop operating at 25* instead of 32 might cost $50 to $75/year but choked duct work could cost you hundreds + uneven temps and noise.
Luck to all,
Joe