This section focuses primarily on the many aspects of central heating, furnaces, and heaters for homes and businesses. We also provide and introduction to ductless heat pumps for rooms or smaller spaces. The central heating systems we focus on are fixed as opposed to being portable. To find resources on portable room heaters, we recommend you visit a site that focuses on those.
In some of the northern U.S. latitudes, especially those away from the west coast, the demand for heating (measured in number of heating degree days) can be up to 3.7 times that of the warmest parts of the U.S. This affects the heating infrastructure of an area including the type of fuels, heating equipment, and the services contractors offer. The climate also affects the design of homes, such as insulation and the presence of basements. Compared to a home on a concrete slab foundation, those with a basement provide more places to locate a furnace, ductwork, or fuel tanks. They can also take advantage of the natural tendency of heat to rise. Some homes with basements use the boiler type of central heating, which radiates warmth instead of using forced air. Due to the way the heat circulates, boilers tend to cause less dryness in the air.
The majority of this page describes the furnace and heating components within a “split” heating and air conditioning system with ductwork. The term “split” simply means that the some of the major components (on the A/C side, the evaporator coils and condenser coils and compressor) are divided into two cabinets connected by lines, one located inside and another outside. The other alternative is for the major components to be inside one cabinet, or “package”. Once the furnace and heat exchanger have heated the air, it must be moved to the rooms that need heating. In the U.S. about three fourths of the systems used a blower or fan in a “forced air” system to deliver the air. The other one fourth use some form of radiant heat transfer.
Heat pumps are a type of central heating system also. Due to their unique design and special considerations, we have given them their own detailed section on this website that includes heat pumps, both with ducts and ductless.
To help understand central heating systems, we will first sub-divide them by heat source. The majority of space heating around the country uses a natural gas (or propane, butane.) furnace for the heat source. The northeast region of the U.S. uses more heating oil. In areas close to coal mining, such as eastern Pennsylvania, coal can be used, either as the sole source of fuel or in units that can burn it or heating oil. Other than steam heating, which uses pipes to distribute the heat, most (about three-quarters) of central heating systems employ ductwork and a fan or blower to disperse the warmed air to where it is needed.
A bit of recent heating history
After liquid and gas fossil fuels became widely available, wood or coal burning heaters and furnaces went out of the mainstream for space heating in densely populated areas. The heavy weight and hassle of hauling wood or coal, plus deforestation and the smoke and air pollution episodes they produced in the early days of the industrial revolution played a large part in their decline as fuels.
Before forced air central heating became the norm, less advanced furnaces, boilers, and space heaters of various types provided most indoor heating for homes. Often the type of fuel available locally or the design of the house, in particular whether it included a basement or crawl space under the house, determined what type of heating system could be used. Where a furnace would not fit, or in warmer climates where space heating needs are secondary to cooling, gas or electric space heaters were the norm. The safer of the gas burning space heaters included a vent to the outside to help prevent low oxygen room levels and the buildup of carbon monoxide gas. In spite of this, many unvented gas burning space heaters were bought and used. These room space heaters, particularly the unvented gas ones, pose some safety concerns, including burns (people and pets), low oxygen/carbon monoxide gas situations, excessively dry air, fires, and more.
During the 1950s and ’60s in the U.S., there was a push for “all-electric” home label that promoted lower power bills and modern living through the use of electricity for heating. The all-electric home of that era used electric resistance heating elements as the heat source and the fan to force the warmed air around. When fuel prices first soared in the late 1970s, the all-electric home movement no longer passed the price test.
Still today, there are homes and buildings that linger as all electric due to the lack natural gas line infrastructure. The availability of natural gas can vary down to the local level–even within different neighborhoods or streets. In climates where the modern and efficient heat pump design has disadvantages, all-electric heating has proven to be quite expensive lately.
Furnace design and how they work
In a furnace, a fuel is burned and the heat produced goes through a heat exchanger into to the air distribution system for the home or building. The blower and fan and ductwork carry the warmed air to the rooms where it is needed. A vent pipe or flue transfers waste gases to the outside air. Depending on where they need to located within a structure, furnaces have different designs. These include conventional upflow, horizontal, and downflow designs.
Today’s central heating systems are more efficient
On the brighter side, today’s central heating systems with gas or heating oil furnaces are much more efficient than their predecessors. The heating industry uses a rating called the Annual Fuel Utilization Efficiency (AFUE) to measure the efficiency of converting gas into energy for heating. A high AFUE rating means the furnace can derive more heat from each unit of gas. This means both lower cost in utility bills and less environmental impact from emissions.
Only twenty-five years ago, the typical gas furnace had an AFUE of about 65 percent. Currently, regulations of gas furnaces require them to exceed 78 percent efficiency. The AFUE range commonly seen is 80 to 95 percent, and the most efficient furnaces have an AFUE of almost 97 percent.
Furnace and central heating maintenance and repair
Maintenance: plan ahead with regular maintenance, reduce the likelihood of emergency repairs, save energy, prevent buildup of dangerous carbon monoxide
A consensus of our research and interviews clearly encourages planning ahead through maintenance to prevent heater repairs. Why? If you wait until your heating system breaks, you will not only have to pay for the parts and labor. Most heater failures happen at the same time for everyone–during the first cold weather or times of intense use, such as the coldest day of the year. Since the heater repair companies are more likely to be most busy then, you will be more likely to pay for an after hours repair or emergency trip charge.
In addition to annual maintenance from your heating contractor, be sure to change all air filters in your heating system according to the manufacturer’s directions.
Make sure your ductwork is insulated if it runs through space that is not conditioned (i.e. in an attic instead of below the room ceilings.) Also, have your heating air ducts checked periodically for leaks. Sealing leaky ducts usually saves between 20 and 27% of all heated (and cooled) air, which would otherwise be lost through into un-conditioned space. Overall, skimping on regular maintenance and adjustment of your heating system can cause you to pay more each month on your utility bills due to wasted fuel and electricity.
In addition to financial penalties for inadequate maintenance, the combustion in a cracked, rusted, unvented, or improperly adjusted heater can produce dangerous Carbon Monoxide (CO) gas. Make sure the company you choose for heating maintenance offers this service by technicians who are properly certified. For more details on this topic, visit our page about carbon monoxide.
From a mechanical viewpoint, the main components in a central heating system can last up to 20 years. Due to improvements in fuel efficiency in furnaces, their economic life is much shorter. Twenty years also happens to be longer than normal mechanical and economic lifespan of most air conditioners, so this leads to decisions on replacing all or part of your central HVAC system. If you are “on the fence” regarding repair vs. replacing your heating or HVAC system, you will want to talk about this while setting the appointment with your heating and air conditioning contractor.
Ductless heating alternatives
Some situations call for heaters that do not require ductwork. Examples of suitable applications include one-room additions, offices, or garage apartments, and in a commercial setting, motel rooms. Although the equipment costs more than a portable space heater and needs professional installation, fixed ductless heat pumps offer some distinct advantages over portable space heaters.
Because of their design, ductless heat pumps:
- provide heating and cooling with one unit
- produce heat more safely than unvented gas portable pace heaters
- are usually more energy efficient that electric portable (“strip” heat source only) space heaters