If you see words and expressions that you are not familiar with, just consult this section.
Understanding the meaning of the following phrases will make it easier to understand your heating system options.
BOILER,  A boiler is a heating device that is designed to make very hot water (up to 250 degrees F, under pressure) for radiators and baseboards. Many boilers do not work well at lower temperatures and may need expensive controls to make the adjustment. Cast Iron or non-modulating, non-condensing boilers are not suitable for radiant heating systems. A tank type heater is a much better choice for radiant floor heating.
DOMESTIC HOT WATER HEATER,  A domestic water heater is engineered to make moderate temperature water. A storage tank is often provided, and additional precautions are taken to prevent corrosion. There is a potential for higher efficiency with a hot water heater because of lower operating temperature and lower exhaust gas temperature, but many low end water heaters do not take advantage of this potential. A few new tank type water heaters are now made for combined heating and hot water production. These tank type, 96% efficient, modulating and condensing heaters with advanced foam insulation are the most efficient radiant floor heat source you can use. They are also the most efficient domestic water heater you can use. They can also be purchased with an internal  coil to that can be connect to another heat source such as a wood burning boiler. They can be used as either an open or closed system to heat both house and domestic water. Some call this coil a solar coil, but solar should use a separate tank to be efficient.
COMBO HEATER, a water heater that is manufactured for combining hot water heating and space heating. These are approved by the manufacturer and meet national codes as combination heaters. This eliminates a second heater and the service associated with it. These provide better efficiency and insure a longer life to the heater because of the extra turbulence the heating system creates in the tank. This is the best system available and can be used as an closed or open system if approved by local code. Combo systems prevent short cycling and have a much higher actual system efficiency than boilers have.
  HEAT EXCHANGER,  A heat exchanger is a device that exchanges the heat from one fluid to another without a mixing of the fluids. A typical use would be to keep the potable domestic hot water separate from the radiant heating fluid. The stainless steel flatplate heat exchanger is the least expensive and the most efficient.
CLOSED SYSTEM,  A closed heating system is one which is closed off from the environment and its heating fluid is not interchanged. This can be a direct system circulating water through the heater directly, or a heating system that is separated from the heater through a heat exchanger. 
OPEN SYSTEM,  An open system is one that is connected to your incoming water supply system or one in which the heating fluid is replaced often. A domestic hot water heater is an open system because the heated fluid is constantly changed. Open systems require different mechanicals and materials. Open systems are not open to the atmosphere as some believe. All domestic water in your house is considered an open system. This is the same as an direct system.
POTABLE,  Potable means water that is safe to drink, often referred to as domestic water.
DIRECT SYSTEM,  A direct system is one in which the potable hot water in the water heater is used for space heating and domestic hot water for doing dishes, laundry, taking showers, etc. There is no separation between the two. This the same as an open system.
INDIRECT SYSTEM,  An indirect system is one in which the heat from the heating unit passes through a heat exchanger before it goes to the in floor tubes. This is the same as a closed  system.
TANKLESS HEATERS,  These are very small compact water heaters that try to heat the water instantly as the water passes through them.   See Page 9  We have had  good success using some brands of these as a closed or open radiant system for small homes and snowmelt systems.