Furnaces are rated by their AFUE (Annual Fuel Utilization Efficiency); an AFUE of 95% means that the furnace is able to deliver 95% of the heat in the fuel they burn; in other words, it will only waste 5% of the heat.
A 95% furnace is a high-performance furnace. Most furnaces have lower AFUEs - the best oil furnaces have their AFUEs a little above the 80%.
Energy Guide rating
When shopping, in case of high space heating requirements, consider a high energy-efficient furnace. Look for their Energy Guide label - an easy way of assessing their potential performance. The Energy Star programs provide lists for efficient gas and oil furnaces.
High rated furnaces have a better technology, especially sealed combustion (important for safety) and multi-stage burners and fans that can provide more comfort and quietness and more safety.
Be aware anyway. There are other factors to consider, especially the quality of the installation and the design of the ducts, or issues such as the airflow and regular maintenance. They are more important than small differences in the AFUE of the furnace.
To keep your furnace running efficiently, it should be professionally tuned-up every two to three years (gas furnaces) or every year (oil furnaces). Poor maintenance will cost you many hundreds or thousands over the lifetime of your furnace.
Replacing the filters is crucial, and a DIY task, but there are other important maintenance tasks (involving the burner and the heat exchanger, or the air and fuel flow...) that should be carried out professionally.
Ducted systems can lose a significant part of their heat they generate through ducts, namely when they are installed in a crawl space, garage or attic. Duct leaks and poor duct insulation are responsible for a large amount of heat loss.
Another problem with ducts is that they are often undersized, and can’t move enough air to heat the rooms they serve, which will decrease the efficiency of the heating system.
See, on these issues: Furnace Sizing & Chimney and Ducts
Installing a furnace is expensive and... critical for performance. An efficient furnace with a poor installation can be outperformed by a unit with a lower AFUE. The venting, the duct system, the sizing, the airflow and other installation details are of the greatest importance.
Hiring a knowledgeable contractor is money well spent. The furnace manufacturer may help you to find out a certified contractor, or you may look one at NATE (North American Technician Excellence), HVAC Excellence, Smacna or through the Resnet Qualified Contractor tool.
Our Video on Buying Criteria: