I am building a new house but with $ these days my upfront cash is less then originally planned so I have to cut the cost. I am putting in radiant heat and was originally going to install a Geo system as the heat source. Unfortunately I don't think I can swing the upfront costs so I want to put in something now that I can easily migrate to the GSHP in the future. I was thinking of using a On Demand Hot Water heater for both the Radiant and DHW. Then I would in the future add the GSHP and could use the On Demand heater for supplemental heat and maybe DHW. Am I on the right track?

if you are going to have instant hot water I will assume you have gas where you live...if you are going to be paying to be a customer of the gas company anyway then you would probobly be further ahead just using gas for heat anyway. If on the other hand you could cut out your gas bill all together you may save something from geo because the price of gas is cheap but at least around here there are like $50 in fees just to be their customer

It is Propane where I am building the house.

in that case I would ditch the instant hot water idea and just install geo with an electric tank and a buffer tank. In the long run you will save money, likely enough per year to pay the interest on the loan for geo. Its just wasteful to put something in only to plan to redo it later and alot of the savings from the always on how water will be negated by the geo install and the cost to install on demand hot water is likely far more than just putting in an electric tank so that is likely an area where you can save a bit of money to offset the cost

Just a thought - If you decided not to do the radiant heat, and go with a water to air geo system instead, could you do it now? Maybe you're set on radiant.

Already have the Radiant tubing in the slab - we are in the middle of construction and have run out of Cash for the GEO. I need advice on the best way to prep it to add the GEO later.

Size mech room to accommodate future geo unit.

Will you need ductwork for cooling? Put it in now

Place big conduits for future loop lines

Don't mix potable and radiant water

In the thread ~"straw poll" more than half the pros indicated they don't and wouldn't have on demand water heaters. It was a small sample and unscientific if that tells you anything.
Go with large Propane storage tank. It is generally cheaper if you don't go nuts on it and can be used as buffer or solar storage tank later.
I confess however I agree with the others not borrowing the money, so that you can pay extra and do it twice...?
Perhaps there is another feature you can wait to purchase that won't cost thousands each year in money burned.
i.e. linoleum instead of hard tile, pergo instead of wood floors........
good luck,
Joe

Joe, when you mentioned large Propane storage tank I thought you meant something like a 500 gallon LP tank, then I realized you meant a large propane water heater.