About Space Heating

Virtually every residence in the United States has some form of space heating capability. Nationally, heating accounts for 58% of all residential energy usage. This percentage varies, of course, from household to household according to the local climate. For homes in Arizona, it may account for just 5%, whereas in northern plains states, it can be as much as 70%.

Several means of heating and distributing air throughout a house are in use today. Most means of heating involved combustion of fossil fuels or biomass. The most popular fuel of combustion for residential heating in the US is natural gas, accounting for 52.7% of all households in 2001. Fuel oil (9.3%), propane (4.5%), wood (2.2%) and kerosene (1.0%) make up the balance of combustion fuels. Of the remaining households, almost all, 29.2%, use electricity for heating.

Electricity itself is usually the product of a combustion process. More than half of the power generated in the US comes from the burning of coal and natural gas. It is significant to note here that electricity generated from fossil fuel converts only 30% of the fuel's total energy content into power. The bulk of the energy goes up the stack as waste heat.

At the residential level, electricity is converted into heat in two very different ways. Resistance heat converts the current directly into heat. Motor driven heat pumps extract heat from an outside source (air, ground, ground water) through a reverse-refrigeration process. Although the resistance heat is close to 100% efficient on its conversion, the heat pump can create much more heat for each kilowatt-hour consumed.

As shown in the table below, the use of different home heating methods varies widely by region. For example, 78% of all homes in the Eastern Midwest use natural gas, while only 27.4% of those in New England do. Note how the Northeast (Middle Atlantic and New England) account for vast majority of fuel oil consumption

U.S. Residential Heating Source by Region

As with the sources of heat, there are a number of different methods for distributing heat within a residence. Most homes (55.2%) distribute heat by blowing air through ductwork from 

As noted above, natural gas is the most common source of residential heat in the U.S. today. Gas offers the advantages of economy, efficiency, and cleanliness. It can also be used for other heating applications e.g. hot water, cooking, and clothes drying.

Natural gas has the environmental advantage of being a cleaner burning fuel than oil or the coal burned to produce electricity. With the proper mix of air, its only products of combustion are carbon dioxide (CO2) and water vapor. Although CO2 is a greenhouse gas, it is not deadly to humans as are carbon monoxide (CO) or sulphuric acid, which are by-products of oil burning. In addition, natural gas does not produce soot, which is common with both oil furnaces and coal-fired power plants.

Combustion efficiency is the percent a fuel's energy that is actually captured for heating. Although electric heating is theoretically 100% efficient, the energy conversion process used to generate that electricity is only about 30% efficient. By contrast, modern gas furnaces can operate in the mid-90% range. As evidence of their efficiency, the exhaust gas temperatures from the most efficient furnaces are only slightly above room temperature.

Annual Fuel Utilization Efficiency (AFUE) is the measurement of gas furnace efficiency that consumers can be used for comparison shopping. The US Department of Energy requires every gas furnace to be rated and labeled. AFUE should not be confused with combustion efficiency, which is the percent of usable heat that is extracted from the gas while the furnace is operating. AFUE takes into account all of the inefficiencies that result from the unit's on and off cycling during the heating season.

The only downside to gas is the volatility of its price, as anyone who paid heating bills through the '00-'01 winter can attest. But even at the highest recorded prices in the last thirty years, gas is still the cheapest heating fuel.

[Chart showing 30 years of average gas prices vs. oil and electricity]

The diagram below illustrates the operation of a typical gas furnace. Air from inside or outside the home is drawn into the combustion chamber by a fan. Inflow of gas into the chamber is regulated by a valve, which controls the ratio of air to gas to maintain the optimum combustion mix.

When the furnace is turned on, an electronic spark ignites the gas/air mixture to initiate combustion. The hot combustion gases flow through the primary and secondary heat exchangers before being exhausted to the outside. Concurrently, another fan draws air from inside the house through the ductwork that contains the heat exchangers. As the house air flows over the heat exchanger surfaces, the heat from the combustion gases contained inside the heat exchanger is transferred. The heated air is routed through ducts to supply registers located all around the house.

What has been described above is known as a forced air system, which is present in more than half of all U.S. households. It compares to a passive system, which does not have a fan. A century ago, passive systems were predominant. Ductwork would radiate from the furnace directly to each room in the house, giving rise to the term "the octopus in the basement". Passive was replaced by forced because it is more efficient way distributing conditioned air uniformly throughout the house.

Heating Oil

Since the energy crises of the 1970's, heating oil's share of U.S. households has diminished markedly, from a peak of XX % to under 10% in 2000.  Regardless, it still has a significant presence in the New England and Middle Atlantic states.

As shown below, oil furnaces operate in much the same way as a gas furnace. Oil is injected into the combustion chamber by a special nozzle. In general, the finer the nozzle, the finer the spray, the more efficient the burning will be.

In most U.S. residences, oil furnaces, as with gas furnaces, are mated with forced air systems.  

Propane

Liquified Propane Gas (LPG) is a condensed derivative of natural gas that is sold in rechargable containers. It is well suited to areas that do not have natural gas service or where portability is desired. As a derivative of natural gas, BTU for BTU, it is more expensive than natural gas.

Wood

Up until the twentieth century, wood was the predominant heating fuel for American residences, especially outside of urban areas. Although it has been largely replaced by fossil fuels, a small percentage of homes continue to burn wood for their primary heating needs. Over the last two decades, significant advances have been made in the energy performance of fireplaces and wood stoves.

An open hearth fireplace delivers less than 10% of wood energy to the room in which it is located. Furthermore, its consumption of room air for combustion draws unconditioned air into the house, causing drafts and cold zones.

Through the use of passive convection and sealed combustion, a modern zero-clearance fireplace delivers nearly 50% of wood energy into the room. Air for combustion is brought into the hearth from the outside, with the air pre-heated from the chimney's heat. Consequently, no draft-inducing vacuum is created in the house and the fire burns more efficiently.

Wood-burning stove have seen similar advances, but have gone a step further, adding catalytic converters to minimize air pollution. Freestanding wood stoves can achieve efficiencies close to 60%.

Despite the technological advances, wood burning does not do an especially good job of distributing heat in a larger home. Wood burning devices must also be tended and cleaned and cannot be turned on and off with the flick of a switch.

Electrical Resistance

Using electrical resistance for heat may make sense for a climate that sees very few heating days during the year. Although resistance heating is very efficient - all the heat stays in the house - generation of the electricity is not, as discussed above.

Heat Pumps

Heat pumps are the only heating systems that do not burn something directly or indirectly to create heat. Instead, they extract heat from the outside air via compressors, acting as an air conditioner in reverse. In the summer months, the heat pump functions as an air conditioner, drawing heat from the inside air and discharging it to the outside. The diagram below illustrates this process.

Air-Air heat pumps are suitable in climates where the outside temperature seldom drops below freezing. When the outside temperature is below freezing, there is not enough heat remaining in the air for the heat pump to extract to keep up with a home's heating needs. To supplement the heat pump's heat, electrical resistance is typically used. As discussed above, electrical heat is usually the most expensive option for residential heating.

Ground water and Geothermal heat pumps pick up where Air-Air heat pumps leave off. In most parts of the US, the ground temperature below the frost line is a constant 50-60F. By tapping into the ground water below, or running a closed cooling/heating loop underground, the heat pump has an ample source of heat. Lacking the wide temperature swings of the outside air, these heat pumps can operate at higher efficiencies than air-air units and do not require backup systems.

The only downside to Geothermal systems is their higher installation cost. To use the ground or ground water as a heat source/sink, trenches and/or wells must be cut into the ground. To determine if the expected savings will adequately offset the investment, talk with several geothermal contractors. Looking over your heating bills from past years, they should be able to provide a reasonable projection of your future energy savings.

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