Should you replace a central air unit or go with a new air source heat pump or electric plenum heater in Buffalo Minnesota if the furnace lp is 15 years old?

A heat pump might be the best hedge against rising fuel costs, plus it offers the versatility of providing cooling as well as heating. However, heat pumps usually are not adequate to provide heating in really cold climates, so you would have to augment it with some other heat source. A 15 year old furnace should not need replacing, so using it with the heat pump might be the best solution.