Types of Radiant Heat Systems - Open, Closed and Indirect
  A "closed" radiant heating 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. See page seven for an example of a closed "radiant heating only" system equipment design
   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.
   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. See page nine for an example of an "open" radiant heating system using a tank type heater. The heater could be an instant water heater also. See page ten for an example of an "open" radiant heating system using an instant water heater.

While the Universal Plumbing Code does allow for open systems, if the manufacturer approves it.
                                             But 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 " radiant heating 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. See page eight for an example of an "open" radiant heating system using a tank type heater. See page eleven for an example of an "open" radiant heating system using an instant water heater.
  An indirect radiant heating system sends the 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.
   The heater you choose must have the Btu input capacity to heat both home and hot water at the same time. 

  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 heating design we do, the choice is yours.