Types of Radiant Heat Systems - Open, Closed and Indirect
  A "closed" system is most often refers to a situation where your heat source is dedicated solely to the task of heating water for the radiant heating system. The term "closed" refers to the fact that the water in the radiant heat system re circulates continuously from the heat source through the tubing or radiators and back to the heater again.
  The "
open" system uses one heat source, your domestic water heater, to provide both floor heating and domestic hot water. The two systems are basically tied together. The same water that ends up in your hot shower or dishwasher, for example, has passed through the floor first. This is a very efficient system because one heat source is doing all the work. As long as the water heater is sized appropriately and matches your heating and domestic needs, the need for a "separate" heating system is eliminated. An open system using a tank type heater such as the "Phoenix" is the most efficient and simplest radiant system you can use and can be ordered with an internal solar coil.
  An "
open" system refers to a situation where the heat source - typically a water heater - provides hot water both for domestic use and to the radiant heating system.
  When there is a call for heat, water is drawn out of the water heat tank, it circulates through the heating system, and then returns back to the water heater. That same water mixes with the water in the tank which is also the direct source of the domestic hot water. So, when a faucet is turned on, water that has been through the heating system is furnished for showers, dishwashers, etc.
While the Universal Plumbing Code does allow for open systems, some local building inspectors do not.
  Before deciding on to a particular type of system, you should understand what is allowed in you area.
  An "
indirect " system allows you to separate your domestic/potable water from the water in your heating system and still use a single heat source. By pacing a heat exchanger between the water heater and the radiant heat system, separating the heating system water from the potable water.
  Rather than sending water directly into the heating system from the water heater, a pump pushes water from the tank through the heat exchanger which has series of plates inside. At the same time, the water in radiant system is being circulated on the other side of these same plates in the heat exchanger. Heat from the water heater is transferred through the plates to the heating system water. Hence, the heating system water is heated "indirectly" from the water heater. Indirect systems are "
closed" systems.

A tank type heater such as the Phoenix combined with a flatplate heat exchanger works best.

  I have been designing and installing radiant systems for over thirty years, although I resisted designing open system for many years. I no longer believe there are any health concerns with properly designed open systems.
  Any equipment we specify for an open system is approved by the manufacturer, their product liability insurance and all Universal National Plumbing Codes.
  We always include plumbing and equipment designs for both open and closed systems with every
radiant design we do, the choice is yours.