Sure, it could be made to work. You’ll have about 5 gallons of water in the lines under the floor plus an additional 2 gallons in the lines going to and from the floor. They have pumps that circulate hot water through large buildings where the heater is located far away from the point of use. In your application, the pump won’t have to be much more than 1/8 hp. One web site states that 1/2 inch tubing offers too much resistance to the flow of water, so it’s not quite as efficient as 5/8 or larger.

Now, as far as how much heat you’ll need to be comfortable and still have water hot enough for domestic use (laundry, shower, dishes) may need an expert in this area. If the water is too hot, it may be uncomfortable to walk on the floor. If just right, you may not have hot enough water for domestic use.

Just be sure you meet codes. There are systems that supply hot water for both heating and domestic use. You should get an expert in this area to design your system.

Some potable water heaters have space heating outlets. Check with the manufacturer of your heater to find out if it is ok. It shouldn’t need a huge pump, but if you are not sure how to do the work, leave it to the pros to make sure it is done properly.