Many utility companies are offering rebates to customers who install dual fuel systems because such units help reduce demand during peak usage periods. Carrier Corporation goes one better with its 48HX Dual Fuel Heat Pump — an auto-switching system. In the summer, it operates as an efficient electric air conditioner (SEER = 12). When an outdoor thermostat senses a temperature drop to below what the company calls the "economic thermal balance point" of the region, the unit switches to gas for heating. The fuelswitch point is adjustable for different price conditions. Contact: ACLS, Dept. 848-101, 108 Metropolitan Dr., Liverpool, NY 13088; 800/422-7743. New Air Nailers Senco's newest tool release in this country is a pair of coil nailers — the SCN60 and