Home Heating Basics

Space heating is the largest energy expense in most homes, accounting for 35% to 50% of annual energy bills. Upgrading your heating system could reduce your bills significantly. But how do you know what system is right for your home? Here’s an introductory look at the options available today.

Selecting a System

If you can’t seem to stay comfortable or keep your heating bills down, first hire a qualified home performance or heating contractor to help evaluate your home’s existing system and determine the best actions to take. It may be that a component of your heating system was improperly installed or needs a tune-up. In many cases, they may find that your heating system isn’t the problem, but rather that your home’s insulation, windows, weatherproofing, or ducts need help. 

Sizing the system. How much heat you need depends on the size of your house and how well it keeps heat in. Never figure out how big a system you need by basing it on the size of the old system. A heat-loss analysis is the only way to properly size a new heating system. (A possible exception exists when replacing steam or hydronic systems: The boiler needs to be sized to the existing radiators you have or plan to add.) A heat-loss analysis should include measurements of wall, ceiling, floor, and window areas and account for insulation levels and weatherization features, including any energy improvements that have been made. Online calculators and free software are available to make this task easier (see Access).

A new heating system should be sized no more than 25% over the peak heating demand. For example, if your home’s peak heating demand is calculated to be 60,000 Btu per hour, you should select a heating system with a heating output between 60,000 and 75,000 Btu per hour.

Efficiency recommendations. The efficiency levels you want to look for vary according to the type of system and fuel, as indicated in the “Selecting a Heating System” table on page 54. If you live in a cold climate and your house is well sealed and insulated, it usually makes financial sense to invest in the highest efficiency system available. Your heating contractor will help you determine the financial payback periods of the highest efficiency unit compared to lower efficiency ones.

If your home is still in the design phase, passive solar heating will offer the greatest up-front efficiency and long-term savings. Make sure to orient your home to utilize the energy the sun has to offer and specify appropriate amounts of thermal mass and south-facing glazing, and optimal insulation levels. This design strategy will generally allow you to install a smaller-than-typical backup heating system at a reduced cost and increased lifetime savings.

Common Systems

Forced-air furnace. The majority of North American households depend on a central furnace to provide heat. A furnace works by blowing heated air through ducts that deliver the warm air to rooms throughout the house via air registers. This type of heating system is called a ducted warm-air or forced-air distribution system. The air can be heated by natural gas, propane, fuel oil, electricity, or even biodiesel.

Furnaces and boilers (described below) are rated on their “annual fuel utilization efficiency” (AFUE), which includes start-up, cool-down, and other losses that occur in real operating conditions. The higher the AFUE, the more efficient the furnace or boiler. The AFUE rating for an all-electric furnace or boiler is between 95% and 100%. Units installed outdoors have a lower AFUE because they have greater jacket heat loss. A typical gas- or oil-fired furnace has a hard time keeping valuable heat from escaping up the flue, but “condensing” furnaces are designed to reclaim much of this escaping heat from exhaust gases. High-efficiency oil- and gas-fired systems (85% or greater AFUE) are typically condensing models.

Although it’s frequently overlooked, the electricity drawn by a furnace to power its motors and blow air through the house can be considerable—more than 1,200 kilowatt-hours (KWH) per year for some models, adding up to $100 or more to annual electricity costs. This power consumption is not factored into the AFUE ratings, so motor power and efficiency should also be considered when choosing a new furnace.