I suggest you first investigate connecting into the house's current HVAC system. I would not be overly concerned that you will be on a concrete slab given the good insulation qualities of walls and windows. Most houses have over sized HVAC systems so it is likely your system could handle the additional load.

I have not heard of using a hot water heater as part of a radiant floor heating system. I would think you would have to set the hot water heater at its maximum temperature which would like shorten its life.

This post was edited by mike_home on Sun, Apr 27, 14 at 22:20

Agree with mike. I assume you are heating with propane.

IMO

You can definitely use a regular water heater for this small of a space. I use one now for almost 600 sqft. That being said, I assume you have AC with the existing forced air system, and as Mike said, your system is most likely oversized. This will be a far more economical choice. The only reason to go with in slab radiant is if you want the floor to be warm on your toes in bare feet. Just make sure you put 2-4" of insulation under the slab whichever direction you go.

Thanks for the responses everyone.
Here's the reasons why I was against using forced air vent:
1. I thought the concrete would be very cold, but maybe not according the above posts. I know my basement cement feels very cold.
2. There's no good way to get the ducts into the ceiling and that's not the best method for heating anyway.
3. According to local building codes, if I run the duct work in the dirt, then it either needs to be sealed, like blu duct which seems very expensive or else I need a drain tile due to condensation the inspector tells me.
4. I have no good way to connect the tile with the current house.
5. Then I have to do a sump pump, hole, that's radon proof.

So that's what lead me to this point. What do you think?

"For A/C, assuming it won't need much, I was thinking about simply a ductless mini split."

If you are going to get a mini split for cooling, then why not also use it for heating?

Throw a mini-split heat pump in there and be done with it. A good inverter mini-split can still crank out heat down to ~15F.

They're efficient, quiet, and you'll now gain a second source of heating/cooling if your main HVAC ever goes out.

Thanks for the input. I will do that.