Knowing what to ask when it's time to replace or upgrade your furnace is critical to smart buying. Most homeowner's don't, since it's something they may only do once or twice in their lifetime.

Here are a few of the most important factors to consider if you're thinking about a new furnace.

• Size The size of a gas furnace refers to its heating capacity, not its physical dimensions. The capacity of a heating system is measured in BTUs (British Thermal Units) per hour. Generally, a unit that's too small will lack the ability to provide the comfort you desire. A unit that's too large will cost more to buy and run. A qualified heating contractor is the best person to recommend the correct size furnace for your heating needs.
• Venting The heating contractor can also determine if changes are needed in the venting for the new furnace. In most cases, the new furnace is placed in the location vacated by the old furnace. In some cases the same venting system can be used. However, many of today's high-efficiency gas furnaces require venting modifications such as venting out a side wall using PVC pipe or sleeving the existing vent.
• Efficiency The efficiency of a gas furnace is indicated by its Annual Fuel Utilization Efficiency (AFUE) rating. The minimum efficiency level required by the federal government on all new furnaces is 73 percent AFUE. The higher the AFUE, the more efficient the furnace. The more efficient the furnace, the lower your heating bill.
• Cost In general, the purchase price of higher efficiency units will be a bit higher than lower efficiency units. However, over the life of the unit, a high-efficiency gas unit will have lower operating costs. The lower operating costs help the furnace pay for itself. These operating savings could easily pay for the extra cost of increasing efficiency. Overall cost can also be affected by some installation factors, such as correcting venting or ductwork problems and the difficulty of removing the old furnace or unique challenges of installing the new furnace in the desired location.
• Contractor Perhaps the most important step in replacing or upgrading your home's heating system is choosing the right contractor to do the job. No matter how well a furnace is made, the comfort, reliability and energy efficiency are in the hands of the contractor installing it. Contractors affiliated with a nationally known manufacturer are usually a good choice because they've been factory trained in installation and service. Contractors who employ NATE certified technicians increase the likelihood of a good experience. They can also help you determine your comfort needs, make the right equipment choices and give you the best value.
• Be sure to choose an ACCA member.