It is recommended that you emphasize most on the efficiency of the furnace. This is because if your furnace is more efficient you will be able to save on your energy bills. In fact, it is better to incur a little more initially as you purchase a new furnace because it will prove worthwhile in the long run as you save money on your utility bills for many years to come. The efficiency of a furnace is measured with AFUE rating. Higher the AFUE rating more is the efficiency.
The second important thing that you need to look into is the type of fuel that is used in the furnace. Furnaces are mostly gas-fueled or oil-fueled. Gas furnaces mostly use natural gas or propane while oil furnaces use oil as a combustion source. It has been seen that furnaces which use natural gas for heating is more efficient than the one which uses oil. Moreover, a furnace that uses oil is not much environment friendly hence the one powered with natural gases are preferred over the others.
Sizing of the furnace is another important aspect that you need to consider when you are buying a new furnace for your home. This is because unless you have a rightly sized furnace you will not be able to enjoy the ultimate comfort of a furnace. When you buy an over-sized furnace there will be frequent cycling on and off which in turn will cause unnecessary wastage of energy. On the other hand if you buy a smaller sized furnace it will not be able to heat the room adequately.