When it snows, we buy shovels. When the mercury hits 100, we rush to Sears and drop $500 on an air conditioner. So it was when a mid-July heat wave wrapped the U.S. in sweat. Fedders, the leading U.S. maker, ramped up to 10,000 units a day, a rise of 25% over June's output. And its shares jumped 17% over the 5-day blast, cresting at 6.875 on July 17. Sales and profits are up 50% over '94, and the stock is up 68% since last October.