Like condensing warm-air furnaces, some condensing boilers have had problems dealing with the mildly corrosive nature of condensate and consequently heat exchangers are made from rather exotic and expensive materials. Again, like condensing furnaces, condensing boilers all have "forced" draft--a fan of some type supplies combustion air to the burner instead of plain ambient air in many conventional models.
Some forced draft equipment is quite loud--both inside and at their exhaust port outside. Condensing boilers MUST NOT be vented through a typical old house chimney!
Europeans (particularly Germans) are WAY ahead of the U.S. regarding condensing equipment. One of the main reasons for this is the fact that hydronics have a very limited market in the U.S.--thus less money available for research and development.
Even if a U.S. mfgr WOULD make their own condensing boiler the standard by which they are rated (the AFUE "number" you see from ASHRAE) DOES NOT ACCURATELY PORTRAY THEIR TRUE EFFICIENCY AS INSTALLED IN A SYSTEM!!!
While a few condensing boilers may "appear" American, they are really assemblages of "stock" parts from all over Europe. The condensing boiler with the longest and best track record assembled by an American company is most likely the Monitor MZ, www.mzboiler.com Most "simple" condensing boilers can be fueled with natural gas, propane or fuel oil.
Once your heat loss is complete, you have sized your radiators and designed a system appropriate to the temperature at which it will operate, you must consider how the system will be controlled.
The most common way (in the U.S.) is digital--the wall thermostat that everyone knows. The flow of heat is either "on" or "off" NEVER "in-between." Even if the system is divided into "zones" you can rest assured that the system IS digital if it is controlled by wall thermostats.
The other method to control (VASTLY superior by the way) is proportionally. The amount of heat delivered to a radiator depends on the temperature OUTSIDE and the desired temperature INSIDE--it is not "on" or "off" in most circumstance, but continually and automatically changing based on conditions--both IN AND OUT OF YOUR CONTROL.
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