1. Repair or Replace

If you already have a mid-efficiency or high-efficiency furnace, it is probably less than 15 years old. You should try to have it repaired. Also, see if the furnace or the parts that are not working might still be under warranty. A good furnace can last 25 years or more; a lower quality furnace will last 10 or 15 years. If you are having a problem, hopefully, you have a good furnace that needs repair.
If you have an older, low-efficiency furnace you should think seriously about replacing it, even if it is currently functioning well. If you are planning in staying in your home for a number of years, it may save you money over time to install a new furnace and reduce your energy bills. The United States Environmental Protection Agency (EPA) and the Department of Energy (DOE) have created a Life Cycle Calculator for estimating the savings you will get over time by investing in a more energy efficient furnace.

If you decide to purchase the more efficient furnace, not only will you lower your bills, but you'll be reducing the amount of greenhouse gases you are generating while heating your home.

2. Brand

The most popular furnaces are brands that may be familiar to you from the air conditioning or water heater markets: Trane, Carrier, Goodman, Lennox and York. Furnaces have gone through design changes recently to improve efficiency. Furnaces should last 20 years or more. Some will last 25 years or more, while others will start causing problems after only 10 or 15 years. There is no way to know if a particular model of furnace is going to be reliable or not until many years after it is first sold and installed. Consequently, we recommend that buyers favor the leading brand manufacturers that have been in business for a number of decades. The manufacturers that have been in business for many decades are more likely to have the experience and knowledge to manufacture reliable furnaces. Trane, Carrier, Lennox, and York have each been in business for over 100 years.

3. Get Help or Do it Yourself?

Unless you are a licensed HVAC professional, we recommend you work with a professional to install your furnace. Most furnaces installed in the United States are natural gas furnaces, and the risks of making a mistake are very high.

We do recommend that you learn about furnaces so that you can be an informed and active participant in the process of buying a furnace and having it installed. When you are knowledgeable about furnaces, you'll be better able to:

• Interview and hire the service professional for the job.
• Provide input into selecting the furnace to be purchased.
• Make key decisions about the features that are important to you.
• More effectively monitor the professional's progress on the project.

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5. Power Sources 

If natural gas is not available to you or if you are interested in exploring the alternatives to natural gas, there are a number of other energy sources that you can use to power your furnace.  While these are subject to availability and lcoal codes and ordinances, they include:

Heating Oil: Heating oil powered furnaces have not achieved the fuel efficiencies achieved by the best natural gas furnaces, but they are coming close. The most efficient oil powered furnaces are 86% efficient, compared to 96.7% efficiency of the most efficient natural gas furnaces. So, if you are looking for a high-efficiency furnace, natural gas is your best choice. If you are looking for a mid-efficiency furnace, both heating oild and gas are options.

Propane: Propane is clean burning, like natural gas. Unlike natural gas, it is stored in a tank on the home owner's property. So, if a natural gas line cannot be run to your home, propane may be a good alternative. It is often used in rural areas, and many users are satisfied with this clean, efficient energy source.

Electricity: Resistance electrical heating tends to be more expensive than other energy sources. One exception is if you are using electric baseboard heaters and only heating the rooms you are occupying. You may make up for the inefficiency of electricity by heating your rooms in a targeted manner. Electric powered central heating is generally quite inefficient, and we do not recommend it.

Natural gas powered furnaces will continue to be a smart choice over the life of your furnace. However, nobody can know what the price of natural gas will be relative to other sources twenty years from now.

6. Sizing a Furnace

A properly sized furnace is necessary if you are to heat you home efficiently. A furnace that is too large for your home will turn off and on too frequently, and will waste energy warming up and cooling down. A furnace that is too small will not heat your home adequately on the coldest days.

 There are several, thorough ways to estimate the size of furnace that is optimal for a home. These methods are formulated and approved by standards committees or associations and are well documented. They include the Cooling and Heating Load Calculation Manual (1992) from the American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Heat Loss Calculation Guide (1989) from the Hydronics Institute, and several other methods. These calculations are quite detailed, and usually involve determining the number of ducts, the location of ducts, and the size of the ducts, the number of doors and windows and level of insulation, the depth of the walls, and other factors that impact heat retention. Some contractors will charge $150-$300 to perform this analysis and calculations for your home, and might deduct the cost if you select them to install your furnace.

There are also a number of less accurate methods of estimating the size of furnace. These include looking at your old furnace and assuming the replacement furnace should be of the same capacity, or asking you/measuring the square footage of the home, and using that as the basis for the estimate. These methods will not be as accurate.

It is worth the expense and effort to get your furnace sized properly, so you furnace works efficiently. When you contact professionals for estimates, ask them the method they use for calculating the size of the furnace. Work with professionals who use one of the industry approved and rigorous methods.

7. High-Efficiency or Mid-Efficiency?

A high-efficiency furnace will cost between $300 and $1,200 more than a mid-efficiency furnace, and is usually between 5% and 14% more efficient. Whether or not spending the extra money is a good investment depends on three factors: how long you plan to live in your home (and recoup the investment), the cost of energy to run your furnace, how much you use your furnace on a yearly basis. As the cost of natural gas and other energy sources rise, it becomes easier to justify investing in a high-efficiency furnace.

The other factor that can enter into this calculation is venting. If your home has no chimney or other limitations in its ability to vent waste gases, a high efficiency furnace may be a great choice. High-efficiency furnaces generate much less waste gas than a mid-efficiency furnace. High-efficiency furnaces can be vented using plastic venting systems and can be vented from the side of a home, so they can be installed in homes that do not have enough ventilation to support a mid-efficiency furnace.

8. Cost

The main variables that affect the cost of a furnace are energy source, capacity (size), and efficiency. All of these factors will have a tremendous impact on your energy bill. We have attempted to provide you the tools to put the initial investment in your furnace in perspective, by helping you to calculate how your new furnace will impact your energy costs. We encourage you to use the information provided here to understand the issues and factors relating to furnaces and to get started having your home evaluated for a new furnace; energy savings are probably available to you.