While this may sound fantastic and hideously complex,
IT IS NOT! Fully self-contained devices called Thermostatic Radiator Valves (TRVs) have existed for DECADES and are known for high reliability, high efficiency and incredible comfort. These devices are REQUIRED in much of Europe. While there are many manufacturers (some even in the U.S.--even if they do actually make them in Mexico) the "best" are likely from a Danish company named Danfoss. As even "plain" radiator valves have become a specialty item in the U.S., the price of TRVs is not particularly high and WELL WORTH THE COST. You wind up with the ability to keep each and every space at whatever temperature you desire with amazing accuracy. While some systems using TRVs will still have a wall thermostat, the better way (in my mind) has NO indoor thermostat save the TRVs installed on EVERY radiator.
Until quite recently ALL boilers available for sale in the U.S. have been digital--i.e. full fire or no fire--nothing in between. In an attempt to reduce fuel use, many systems (both digital and proportional) use outdoor reset. Outdoor reset adjusts the temperature in the system based on the temperature outside--i.e. as it gets colder outside, the water gets hotter. Reset certainly works well and can certainly save energy, but those "problems" with low temperatures in conventional boilers can really bite you in the a## when teamed with reset.
Enter the condensing, modulating boiler. There are not many available in the U.S. at the current time. Of those few that are assembled in the U.S., problems (warranty-required INSIDE heat exchanger cleaning requirement in one and questionable exchanger material/longevity in another) preclude my recommendation. The "best" is likely the Vitodens by Viessmann (Germany) www.viessmann.us/
The Vitodens has an absolutely unique "burner" that does not produce flame in the conventional sense--it transfers the vast majority of its heat to the water via radiation. When installed in a system that has variable flow (TRVs), even the built-in circulating pump modulates! On such a system the burner will almost always be "on" at some level as long as the structure requires heat. Real-world "numbers" coming from this boiler are absolutely extraordinary regarding efficiency. Flue gas temperature is often only a few (less than five) degrees higher than the temperature of the supply water! Of course it is highly complex and very expensive but based on its use in Europe for much longer and stellar reputation for Viessmann reliability it really deserves the highest consideration. Remember that it is designed in Germany where TRVs are REQUIRED in any space larger than a closet and it will achieve its best efficiency when TRVs are used.
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site.
IT IS NOT! Fully self-contained devices called Thermostatic Radiator Valves (TRVs) have existed for DECADES and are known for high reliability, high efficiency and incredible comfort. These devices are REQUIRED in much of Europe. While there are many manufacturers (some even in the U.S.--even if they do actually make them in Mexico) the "best" are likely from a Danish company named Danfoss. As even "plain" radiator valves have become a specialty item in the U.S., the price of TRVs is not particularly high and WELL WORTH THE COST. You wind up with the ability to keep each and every space at whatever temperature you desire with amazing accuracy. While some systems using TRVs will still have a wall thermostat, the better way (in my mind) has NO indoor thermostat save the TRVs installed on EVERY radiator.
The Vitodens has an absolutely unique "burner" that does not produce flame in the conventional sense--it transfers the vast majority of its heat to the water via radiation. When installed in a system that has variable flow (TRVs), even the built-in circulating pump modulates! On such a system the burner will almost always be "on" at some level as long as the structure requires heat. Real-world "numbers" coming from this boiler are absolutely extraordinary regarding efficiency. Flue gas temperature is often only a few (less than five) degrees higher than the temperature of the supply water! Of course it is highly complex and very expensive but based on its use in Europe for much longer and stellar reputation for Viessmann reliability it really deserves the highest consideration. Remember that it is designed in Germany where TRVs are REQUIRED in any space larger than a closet and it will achieve its best efficiency when TRVs are used.