Many people find that gas pack heating cooling units are a really appealing solution to their home heating and AC problems. Obviously one of the most appealing elements is that heat and cooling are neatly contained in one simple unit that can be stored out of the way (most often on the roof).
These gas pack units use different fuels for the two functions that they are called upon to perform - gas takes care of the heating and then they go fully electric when all that is needed is air conditioning.
When it comes to the heat element, you should treat it like you would any residential gas heating. For a start check out the Annual Fuel Utilization Efficiency (AFUE) rating. You will be able to find units that are rated at 95 percent or more.
You also want to find a unit that offer a two stage heating system. These are newer technology that means the temperature swing problem experienced in older units is a thing of the past.
You should also take a look at the variable speed packs on offer - this is both a comfort and a cost saving measure. The most important thing is probably to go with a contractor and brand that you feel is trustworthy. You can get top quality guarantees on these units, and this is important since they spend their lives exposed to the elements.