The term "modulated" basically means variable speed. A few high-end HVAC systems have some form of variable speed fans and/or compressors, and even burners. Typically, these systems can run from about 40% to 100% of full rated output at any given time. This allows heating or cooling to run at a rate appropriate for the current need. Overshoot and undershoot are lessened, an efficiency is improved. Such systems typically cost around twice what a standard HVAC system would cost. Manufacturers argue that energy savings and extended hardware life due to less wear-and-tear of slower operation result in a payback less than the life of the equipment. The jury is still out, but they do seem to be getting more popular.

In our experience so far, all such systems have proprietary control systems, sensors, and user controls (thermostats). They are thus a "closed" system and cannot be controlled by whole-house automation systems or standard thermostats and are therefore cannot be interfaced with any of the equipment we sell.

Hopefully that will change someday. If anybody knows of any exceptions to this...please let me know at webmaster@hometech.com.