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Gazing at the innards of the newest residential heating and cooling equipment is a little like looking under the hood of a new car. Gone are the familiar basic components of yesteryear such as distributor caps and simple air filters. In their place is a jungle of electronics and m e c h a n i c a l c o m p o n e n t s intended to boost performance and to control polluting emissions. For cars, the technological maze under the hood has proved remarkably reliable as long as it is serviced by a well-trained mechanic. For residential hvac appliances the story is a bit more complex. Some appliances have enjoyed a relatively trouble-free evolution to higher efficiencies. Heat pumps and air conditioners, for example, were not radically modified to