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Are you considering buying a new furnace? Or, are you dissatisfied with the operation of your current furnace? Are you unsure whether to fix or replace it? Are you concerned about high winter utility bills? If you answered yes to any of these questions, this information can help. With it, you can learn about various types of heating systems and how to maintain your furnace, hire professional heating services, select a new furnace, and ensure that your new furnace is properly installed.

Understanding Furnaces

Many people buy or use furnaces without understanding their designs, components, and operating principles. Proper sizing, selection, installation, maintenance, and correct use are keys to cost-effective operation and lower overall costs.

How Heating Systems Work

Warm Air Furnaces

A warm air furnace provides heat by utilizing the heat energy of natural gas, propane, fuel oil or electricity. When a carbon based fuel is used (natural gas, propane or fuel oil), a heat exchanger directs the burned fuel to the outdoors via a chimney or plastic piping. It keeps the burned fuel seperate from the heat that is being distributed through the ductwork.  An electric furnace operates much like your toaster, elements get very warm when electricity is passed through them and the heat is distributed through the ductwork.  A blower in the furnace operates like a fan and provides the force behind the air movement in the ductwork.


A boiler also provides heat by utilizing the heat energy of natural gas or propane.  A heat exchanger directs the burned fuel to the outdoors via a chimney or plastic piping. The heat is transfered to water that is distributed through the home by piping.  A circulator or pump provides the force behind the water movement in the piping.

Heating Efficiencies

The U.S. Federal Trade Commission requires manufacturers to provide an annual fuel utilization efficiency (AFUE) rating on all heating equipment. This rating provides consumers with information about the operating costs of a particular model that is being considered.Boilers and warm air furnaces available today have an AFUE rating of  78% to 97% compared to heating appliances of the past that had ratings as low as 65%.

An illustration, but not actual data, of how the AFUE ratings compare. A heating appliance that has a 65% AFUE rating will utilize sixty five cents of every dollar of fuel that you purchase. The remainder of the heat 35% is directed to the outdoors via the chimney. A heating appliance that has a 97% AFUE rating will utilize ninety seven cents of every dollar of fuel that you purchase and only 3% is directed outdoors via a plastic pipe.

The initial purchase costs of a 97% AFUE heating appliance is greater than a lower AFUE rated appliance but in cold climates the increased costs can be offset in a few years.

Warm Air Furnaces and Boilers

Mid Efficiency Appliances

These heating units are the most basic heating appliances manufactured.  These units typically operate at an annual fuel efficiency rating of  78% to 85%.  They must be vented to a verticle chimney.  If the chimney is made of brick or masonary, a chimney liner must be installed along with the heating unit.

High Efficiency Appliances

These heating units typically operate at an annual fuel efficiency rating of 90% to 95%.  These units are vented to the outdoors with plastic pipe and must be connected to a plumbing drain.  Some operate in two stages to provide a better comfort level (more even temperature) in the home. For example, if the outdoor temperature is fifty degrees, the furnace operates in a low fire mode meaning it only runs at half of its capacity. When colder days occur, it automatically operates in high fire mode and runs at full capacity.

Very High Efficiency Appliances

These heating units typically operate at an annual fuel efficiency rating of 95% to 97%.  These units are vented to the outdoors with plastic pipe and must be connected to a plumbing drain.

The burners operate in two or more stages to provide better comfort levels (more even temperature) in the home. For example, if the outdoor temperature is fifty degrees, the furnace operates in a low fire mode meaning it only runs at half of its capacity. When colder days occur, it automatically operates in higher fire modes untill it reaches full capacity.

The blower of these heating units is controlled in conjunction with the burner operation to provide the optimum comfort level in the home. The blower runs faster when the burner is in high fire mode and slows down when the burner is operating in the low fire mode.

The blower is also utilized when the air conditioner is operating and can be controlled to increase dehumidification or increase efficency of the air conditioner.

An electronic thermostat that is always comunicating with the controls in the heating unit, decides the correct  stage that it should operate to provide the best comfort level in the home.

Maintaining Heating Systems

Older heating sytems may still be able to offer years of relatively efficient use. However, making your older heating system last requires you to perform proper operation and maintenance.

Heating Problems

One of the most common heating system problems is improper operation. If your heating system is on, be sure to close your home’s windows and outside doors.

Other common problems with existing heating sytems result from faulty installation, poor service procedures, and inadequate maintenance. Improper installation of your heating sytem can result in leaky ducts and low air flow. If a proper start up of a heating system is not performed after installation, the performance and efficiency of the unit is impaired. Service technicians that are not trained properly often overlook the cause of a problem and simply make a repair to get the unit to operate.

If your furnace fails, it is usually for one of the common reasons listed below:

*Inadequate Maintenance – If you allow filters and air conditioning coils to become dirty, the heating system will not work properly, and are likely to fail prematurely. The flame sensor requires pereodic cleaning to insure operation.

*Electric Control Failure – The heating system controls can wear out, especially when the heating system turns on and off frequently, as is common when a system is oversized. Because corrosion of wire and terminals is also a problem in many systems, electrical connections and contacts should be checked during a professional service call.

*Heat Exchanger Failure –  Frequent cycling of a heating system, improper gas pressure or improper air flow can cause a heat exchanger to develope a crack or void because of the continuous expansion and contraction of the heat exchanger metal.

Regular Maintenance

A heating system’s filters, coils, sensors and vent system require regular maintenance for the unit to function effectively and efficiently throughout its years of service. Neglecting necessary maintenance ensures a steady decline in heating performance while energy use steadily increases.

Heating System Filters

The most important maintenance task that will ensure the efficiency of your heating system is to routinely replace or clean its filters. Clogged, dirty filters block normal air flow and reduce a system’s efficiency significantly. With normal air flow obstructed, air that bypasses the filter may carry dirt directly into the evaporator coil and impair the air flow through the furnace. Filters are located somewhere along the return duct’s length.  Common filter locations are in walls, ceilings or in the furnace itself.

Some types of filters are reusable; others must be replaced. They are available in a variety of types and efficiencies. Clean or replace your heating system’s filter or filters every month or two during the heating season. Filters may need more frequent attention if the heating system is in constant use, is subjected to dusty conditions, or you have fur-bearing pets in the house.

Sealing and Insulating Air Ducts

An enormous waste of energy occurs when heated air escapes from supply ducts or when cold crawlspace air leaks into return ducts. Recent studies indicate that 10% to 30% of the heated air in an average duct system escapes from the ducts.For heating systems to be efficient, ducts must be airtight. Hiring a competent professional service technician to detect and correct duct leaks is a good investment, since leaky ducts may be difficult to find without experience and test equipment. Ducts must be sealed with duct “mastic.” The old standby of duct tape is ineffective for sealing ducts.

Obstructions can impair the efficiency of a duct system almost as much as leaks. You should be careful not to obstruct the flow of air from supply or return registers with furniture, drapes, or tightly fitted interior doors. Dirty filters and clogged evaporator coils can also be major obstructions to air flow.

The large temperature difference between attics, crawl spaces and ducts makes heat conduction through ducts almost as big a problem as air leakage and obstructions. Ducts in attics and crawl spaces should be insulated heavily in addition to being made airtight.

Buying a New Heating System

Today’s best furnaces use 30% to 50% less energy to produce the same amount of  heated air as heating systems made in the mid 1970s. Even if your heating system is only 10 years old, you may save 20% to 40% of your heating energy costs by replacing it with a newer, more efficient model.

Sizing Heating Systems

Heating Systems are rated by the number of British Thermal Units (Btu) of heat they can produce per hour

How big should your heating system be?  The size of a heating system depends on:

  • How large your home is and how many windows it has
  • How much insulation is in your home’s ceiling and walls
  • How much air leaks into your home from the outside
  • How much heat the occupants and appliances in your home generate.

A heating system’s efficiency, performance, durability, and initial cost depend on matching its size to the above factors.Make sure you buy the correct size of  heating system. Two groups, the Air Conditioning Contractors of America (ACCA) and the American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) publish calculation procedures for sizing heating systems. Reputable heating contractors will use one of these procedures, often performed with the aid of a computer, to size your new heating system.

Be aware that a large heating system will not provide the best heating. Buying an oversized heating system penalizes you in the following ways:

  • It costs more to buy a larger heating system than you need.
  • The larger-than-necessary heating system cycles on and off more frequently, reducing its efficiency. Frequent cycling makes indoor temperatures fluctuate more and results in a less comfortable environment. It also inhibits proper humidification levels because the humidifier has less time to humidify the air and the air filter less time to clean the air. This cycling also wears out the heating system more rapidly.
  • A larger than necessary heating system uses more electricity and fuel which creates added demands on electrical generation and delivery systems.

Hiring Professional Service

When your heating system needs more than the regular maintenance described previously, hire a professional service technician. A well-trained technician will find and fix problems in your heating system. However, not all service technicians are competent. Incompetent service technicians forsake proper diagnosis and perform only minimal stop-gap measures. Insist that the technician:

  • check for the correct amount of gas pressure
  • test for fuel leaks using a leak detector
  • check and test the vent/chimney system to make sure it is safe and working properly
  • check for and seal duct leakage in duct systems
  • measure air flow through the heating system
  • verify the correct electric control sequence and make sure that the heating system and cooling system cannot operate simultaneously
  • inspect electric terminals, clean and tighten connections, and apply a non-conductive coating if necessary
  • oil motors and check belts for tightness and wear
  • check the accuracy of the thermostat
  • test with an electronic combustion analyzer the performance efficiency of the heating system

Choosing a Contractor

Choosing a contractor may be the most important and difficult task in buying a new heating system. Ask prospective contractors for recent references. If you are replacing your heating system, tell your contractor what you liked and did not like about the old system. If the system failed, ask the contractor to find out why. The best time to fix existing problems is when a new system is being installed.When designing your new heating system, the contractor you choose should:

  • use a computer program or written calculation procedure to size the heating system
  • provide a written contract listing the main points of your installation that includes the results of the heating load calculation
  • give you a written warranty on equipment and workmanship
  • allow you to hold the final payment until you are satisfied with the new system

Avoid making your decision solely on the basis of price. The quality of the installation should be your highest priority, because quality will determine energy cost, comfort, and durability.

Installation and Location of Heating Systems

If your heating system is installed correctly, or if major installation problems are found and fixed, it will perform efficiently for years with only minor routine maintenance. However, many heating systems are not installed correctly. As an unfortunate result, modern energy-efficient heating systems can perform almost as poorly as older inefficient models.Be sure that your contractor performs the following procedures when installing a new heating system:

  • allows adequate indoor space for the installation, maintenance, and repair of the new system, and installs an access door in the furnace or duct to provide a way to clean the evaporator coil
  • ensures there are enough supply registers to deliver air and enough return air registers to carry house air back to the heating system.
  • installs duct work within the conditioned space, not in the attic or crawl space, wherever possible
  • seals all ducts with duct mastic and heavily insulates attic and crawl space ducts
  • when venting through the side wall of the home, make sure it is not annoying to your neighbors
  • verifies that the newly installed heating system has the exact gas pressure required and air flow rate specified by the manufacturer
  • locates the thermostat away from heat sources, such as windows, or supply registers

If you are replacing an older or failed split system, be sure that the evaporator coil is replaced with a new one that exactly matches the condenser coil in the new condensing unit. (The air conditioner’s efficiency will likely not improve if the existing evaporator coil is left in place; in fact, the old coil could cause the new compressor to fail prematurely.)