For years, I was a staunch radiant heating installer who picked up a few air-conditioning jobs to fill in the slow summer months in New Hampshire. But a growing demand from my customers for year-round indoor climate control led me to look for a system that would accomplish heating, ventilation, and air conditioning in one package. I wasn't willing to make the compromises imposed by conventional systems, in which heating and cooling needs are practically in conflict -- warm air rises and cool air settles, leaving no optimal place to locate the vents to satisfy both functions. I wanted a system that would work equally well in a chicken coop, a standard home, and a multimillion-dollar house. My search led me to the high-velocity system, a technology refined in the mid-1990s to its current reliable and efficient performance. Over the past six years, my crew and I have installed more than 300 high-velocity systems in both new and existing homes.

The most obvious difference between a high-velocity system and a conventional forced-air system concerns their basic operating cycles. In a conventional system, the air handler and boiler or condenser cycles on and off as the thermostat signals for heating or cooling, depending on the season. In a high-velocity system, on the other hand, the...

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