Furnace and Air Conditioning Replacement Parts

Replacement Parts for Central Air Conditioners, Boilers, Furnaces and Heat Pumps

If your heating or cooling equipment is failing, troubleshooting the problem is only the beginning. You also need to figure out what each part does, how to find a replacement, and how to replace it. We have researched profiles of the most commonly replaced parts for residential air conditioners, boilers, furnaces and heat pumps. Each profile describes what the part does, how it works, which HVAC parts manufacturers make it, and how to buy a replacement.

Air Pressure Sensor Switch

Air pressure sensor switches are used as a safety control measure for heating, ventilating and air conditioning units. The switches act as sensors and will safely shut down furnaces if they detect improper drafting of flue gasses or low combustion.

Common Types of Air Pressure Sensor Switches

Depending on the system, one of the following sensor switches may be right for your HVAC appliance.

  • High pressure switches will disable a compressor when the pressure goes above a set point. The compressor will only resume when the pressure falls to an acceptable level.
  • Fan trigger switches will activate a cooling fan to lower pressure when a set point is reached.
  • Low pressure switches will disable the compressor when the pressure falls below a reset point.

How to Buy an Air Pressure Sensor Switch

When choosing an air pressure switch it is important to choose an adjustable range that is ideal for your purpose. Reliability is also important. A product that is able to withstand high shock and vibration conditions is likely to last longer. Lastly, the sensitivity to air pressure is also a factor to consider when making the purchase. Many replacement pressure switches cost around $100, though some can be as high as $400 when they are sold in combination with other parts.


Aquastats are used in hydronic heating systems to control water temperature. Hyrdonic systems use water as the transfer medium for both heating and cooling and are primarily used in residential applications. The aquastat has a sensing bulb that’s placed in a well in the side or front of the boiler. The bulb senses the temperature of the water inside the boiler and triggers the boiler to maintain the temperature between a low and high end set by the user. In a water heating system, your decision to turn your thermostat up–or it merely being triggered at your preset designation–makes hot water leave the boiler to circulate through the pipes. Naturally, some heat is lost in the process, so when the return water is sensed at its now-cooler temperature, the bulb on the aquastat calls the boiler into action to reheat the water for when the thermostat calls on it again.

Types Of Aquastats

An aquastat may be designed to control the temperature limit or to switch on the circulator. If used as a limiter, the aquastat closes at the low extreme and opens at the high end. But when used to operate the circulator, it closes on both temperature extremes. The former is referred to as direct action. For energy purposes, the high setting should be as low as possible to ensure proper heating. For a gravity hot-water system, a setting of 170 degrees is recommended; for a forced hot-water system, the recommendation is 200 degrees. Operating as the circulator, the aquastat is referred to as reverse action and should be mounted on the largest riser from the boiler. The temperature settings in either setup are used to prevent the boiler from firing too often.

Note also that the method for attaching aquastats can be different. Strap-on models are universal and can be used for circulators, hydro-air fan controlling or hot water makers. Clip-ons are normally used in hydro-air heating air handlers, while well aquastats are for boilers and, occasionally, hot-water makers.

Replacing Or Repairing An Aquastat

Before replacing, it’s important to test the aquastat. Remove the cover from the device, usually mounted on the outside of the furnace. Turn the main wall thermostat to its highest setting. This should trigger the furnace. Now drop the high temperature setting to less than 100 degrees. The furnace should go off if the aquastat is functioning properly. Return the aquastat to its original high point. If the burner doesn’t respond to the setting drop, or go back on within 10 minutes of returning the aquastat to its high point, then the device needs replacing.

If the aquastat needs to be replaced, before doing anything else the circuit breaker box and emergency cutoff should be used to cut the power. Next, disconnect the wires leading to the aquastat, being sure to label each wire with its respective terminal. Take out the screws holding the aquastat in place, as well as remove it from the furnace. Take the whole part with you to a dealer, and buy a matching replacement. Then simply remount the new aquastat, using your labeled wires to ensure proper connection.

Belt Drive Motor

On furnaces that have a blower, belts function in the same way as they do on a car, transferring the rotation of the motor to the fan.

Types of Belt Drive Motors

Belt drive motors function largely in the same way, though there are certain parameters that differ from model to model. The device itself looks like a large enclosed canister on its side with a rod sticking out of it. This is the case with virtually every model. However, when purchasing a belt drive motor, the following should be checked:

  • Horsepower: From as little as 1/6 HP to a full 3 HP
  • Electricity Distribution: Split-Phase or Triple Phase. These relate to the starting of the motor. A triple phase is started from direct connection (known as direct-on-line starting) while split-phase motors can be started using a starting circuit.
  • RPMs: 1725 is by far the most common, though some motors can run at variable RPMs, thereby using varying degrees of power.
  • Service Factor: This essentially states the overload capacity at which the motor can run without burning out. A higher service factor means the motor can handle more power without burning out. A 1 HP motor with a 1.25 service factor can operate at 1.25 HP without burning out. Service factors for these motors commonly run from 1.15 to 1.5.

Replacing A Belt Drive Motor

Before doing anything, users should check if the motor being replaced has a capacitor mounted on it. If so, the motor requires a separate type of motor known as a High Starting Torque Motor or Capacitor Start Motor. The actual replacement of the belt drive motor is simple, as it sits on the ground. The existing motor has mounting bolts on it, which will need to be loosened to remove the existing belts–should you decide to keep them. Switch out the motors, reattach the belts and tighten the mounting bolts to fix the tension, and the replacement is complete.

Boiler Relief Valves

boiler release valve

The primary function of a boiler relief valve is to prevent a boiler from exploding — a pretty important job! Relief valves moderate thermal expansion, which occurs when matter changes size in response to a change in temperature. In the case of a boiler, if the internal pressure exceeds what the boiler can manage, the relief valve opens and excess water or steam is released from the boiler into a relief pipe.

Pressure relief valves work by employing a spring-loaded diaphragm to hold the valve closed. In a dangerous pressure situation, the relief valve springs open and allows the water to pass into the discharge line. Bell and Gossett, manufacturers of A.S.M.E safety relief valves, advises to “treat relief valves as though they were as fragile as raw eggs. Never drop them on the floor or bang them with a tool.” Additionally, they say to never re-use an old relief valve in a new boiler.

Relief Valve Settings

Most household boilers have a pressure capacity of 30 psi. Manufactures determine the amount of pressure a boiler system can bare based on guidelines established by the American Society of Mechanical Engineers (ASME). The pressure relief valve should never have a pressure-release rating that exceeds the maximum working pressure of the boiler. More simply, the system for measuring the pressure of the boiler should align and not exceed the maximum allowable pressure of the boiler. Manufacturers provide a second rating for the full BTUH load of the boiler. This measures the amount that the boiler can unload in extreme circumstances.

If the relief valve on the boiler measures up to 30 psi, a setting of 19-20 psi falls within normal functioning range. Consult the manufacturer to ensure that the boiler, in fact, may manage pressure capacities up to 30 psi. If concerns persist, consider contracting a professional to test the blow-off settings of your system. It is not advisable to adjust the instruments on your boiler without professional guidance.


capacitor for air conditioner

Capacitors are electronic components that store electrical energy. The A/C capacitor provides voltage to enable the motor to spin and thus start the air conditioner. If a capacitor leaks oil or doesn’t hold a charge, it likely needs to be replaced. A compressor that hums but doesn’t start may also indicate a faulty capacitor. Modern A/C capacitors have a limited lifetime and often require replacement.

Common Types of Capacitors

Some A/C units use a single capacitor, but dual capacitors are more the norm. Dual capacitors serve both the fan and compressor motors. A/C capacitors are typically intended for use with motors that have specific voltage ranges and horsepower. Capacitors also vary by size. Starting capacitors—which can provide an extra voltage boost–may also be added to an A/C unit as a part of the repair process if the unit’s motor is having trouble starting up. This type of problem can occur in unbalanced cooling systems, where the refrigerant pressure on the sides of the unit are unequal, or for motors nearing the end of their usable lives.

Buying Replacement Capacitors

Replacing the A/C capacitor yourself can be hazardous. Even when not connected to a power source, capacitors can hold enough electrical charge to hurt, or even kill a person. Before proceeding, the terminals across the top of the capacitor should be shorted. When purchasing an A/C capacitor, the voltage and horsepower range of the A/C unit’s motor must match that of the capacitor. Most starter capacitors will match a range of different motors. Consult the documentation for the capacitor to ensure it will work with your A/C unit’s motor.

Blower Wheels

The blower wheel is a part of the blower assembly in an HVAC unit, and it helps air flow through the unit. Blower wheels that have become dirty can decrease the life of a furnace by reducing airflow, causing the compressor to fail, and can also increase the cost of running the unit. Dirty blower wheels can become unbalanced and put stress on the motor. This can sometimes be observed by increased sound when the unit is in operation.

Common Types of Blower Wheels

There are many variations of blower wheels used in heating and cooling units. Some types of blower wheels include squirrel cage, paddle wheel, radial wheel, tabbed, forward curve, backward curve, and many others. Blower wheels may be concave or convex, and double-inlet or single-inlet. Blower wheels may be made of steel, aluminum, plastic or other materials. Before purchasing a replacement blower wheel, make sure that it’s the correct size and type, and can handle the correct number of RPMs for your unit.

How To Buy A Replacement Blower Wheel

Dirty blower wheels can often be cleaned as a part of a servicing but are also inexpensive to replace if they become damaged. You should be comfortable working with your heating and cooling system before attempting to clean or replace the blower wheel. To remove or replace the blower wheel, turn the unit’s power off, short the capacitor (such as with an insulated screwdriver), then remove the wires from the blower assembly. On the underside of the blower assembly is a set screw, known as the “hub,” which holds the motor shaft and blower wheel together.

Circulator Pumps

A circulator pump is used to circulate gases, liquids or slurries in a circuit. Most often, these pumps are found circulating water in a hydronic heating or cooling system. The circulator’s job is to move hot water from the boiler to the radiators, and then return the cooled water for another injection of heat.

Types of Circulator Pumps

While the function of circulator pumps is generally the same, there are many different kinds. Among the designs are bronze sweat end pumps, stainless steel/bronze circulator pumps, cast iron pumps, pre-wired models and in-line pumps. Circulator pumps also vary based on horsepower, flow range (expressed in gallons per minute), head range (expressed in submersible feet of depth), motor type, and the maximum and minimum liquid temperatures they can be used in.

Replacing A Circulator Pump

The actual removal and reattachment of the pump is easy but is hardly the only task in adequately replacing the pump. To remove the old pump, the boiler should be turned off, as should the electrical supply to the boiler and pump. The circulating pump terminal cover should be removed, and the voltage at the terminals checked with a meter-reader to verify zero voltage. Note the wire colors and their respective terminal attachments, as well as the water flow direction, which is normally signified by a logo on the base of the pump. The electrical wires may then be removed as well as the vent plug. This step could lead to water spillage, which is why towels or a bowl should be on hand for cleanup/prevention. The pair of union nuts holding the pump in place can be removed, freeing the pump. The system should then be flushed and protected to ensure cleanliness and to guard against corrosion. The positioning of the pump should be checked, making sure it’s proper given the piping setup. For example, if installed in vertical pipe work, the pump installed should pump up. Only then should the new pump be installed.

Condensate Pumps

The combination of heating and cooling and water in a furnace or other HVAC product produces a certain amount of condensation. As such, these systems often need a device to eliminate the built-up water produced by the mechanism’s workings. Such is the role of the condensate pump, which is used to eliminate condensation produced by latent water vapor in an air conditioned building, steam from a radiator or heat exchanger, or, in the case of the highest-efficiency furnaces, the exhaust steam. These pumps normally run intermittently, draining condensate into a tank. There are variable settings for a pump that need to be considered, including power type and source, flow, shut off mechanisms and size.

Types Of Condensate Pumps

Commercial and industrial models are made using a broad array of materials, including iron, plastic, stainless steel, aluminum and brass. The two main types are sump pumps and boiler feed pumps. Sump pumps are used via HVAC equipment collection pans or tanks, preventing the buildup of water in the machinery by pumping out the amassed water. In a boiler feed system, the pump is a recycler of sorts, closing off a boiler system by condensing the emitted steam back into the water, thereby returning it to the boiler when it is once again heated into steam.

When seeking a pump, you must first be aware of the tank size the pump will be used in, as well as the number of pumps in the assembly. The pump will run on either AC or DC power, and can be operated using anything from gasoline to, in some outdoor models, solar power. A pump’s power is measured by maximum discharge flow, which can be expressed in gallons per minute, hour or day, depending on the model and expected usage for the product.

Repairing Or Replacing A Condensate Pump

Pumps usually aren’t repaired but rather replaced. While a pump motor can be repaired, it’s usually more cost-effective to simply replace the pump. That said, reports of motors burning out are rare. The problems most reported with pumps are not as much with the pumps themselves but issues related to the setup.

While there aren’t condensate pump inspectors knocking door to door, there are, in fact, regulations guiding pump installation. The International Association of Plumbing and Mechanical Officials develops the Uniform Mechanical Code, which dictates the following pertaining to condensate pump setup: “Condensate from air washers, air cooling coils, fuel-burning condensing appliances, the overflow from evaporative coolers and similar water supplied equipment or similar air conditioning equipment shall be collected and discharged to an approved plumbing fixture or disposal area. If discharged into the drainage system, equipment shall drain by means of an indirect waste pipe (a pipe upstream of a trap, meaning no drainage can be released downstream of the collection trap). The waste pipe shall have a slope of not less than one-eighth inch per foot or 1 percent slope, and shall be of approved corrosion-resistant material not smaller than the outlet size as required in either Section 310.3 or 310.4 (below) for air-cooling coils or condensing fuel-burning appliances, respectively. Condensate or waste water shall not drain over a public way.” The outlet, or pipe size, noted above ranges from three-quarters of an inch for up to 20 tons of refrigeration capacity to 2 inches for upward of 125 tons of capacity.

Condenser Fan

condenser fan motor

A condenser fan is part of a heat pump or central air conditioning system that circulates air across the system’s condenser coil to increase the transfer of heat. Though some condenser fan motors have sealed bearings, others need lubrication. In either case, though, fan functionality is vital, as the failure to maintain proper air flow can not only minimize the system’s efficiency but can cause the system’s compressor to fail. In an air conditioning system, the condenser fan’s role is really best explained by this simple fact: If the fan isn’t working, the device won’t cool anything.

Types Of Condenser Fans

Fan blades are essentially the same, so what’s really being discussed when talking about condenser fan replacement is the driving force of the assembly–the motor. Motors are gauged by varied horsepower, voltage and rotations per minute. Normally set up in a direct drive to the blade, condenser fans range from low-fractional horsepower in small residential units to more than 1 horsepower motors for large-scale commercial use. These fan motors can also come with an open-bearing or closed–also known as sealed-bearing–structure. The latter setup has no oil ports or oil port plugs. An additional consideration is whether the shaft is pointed up or down, which varies among models. Motor diameter is another variable that must be considered when buying or replacing the fan component.

Repairing Or Replacing A Condenser Fan

To minimize the need for repairs, it’s a good idea to check the fan motor to make sure the fan functions, preferably in late winter or early spring. To do so, shut down the entire condensing unit at the electrical disconnect box, set the thermostat to cool, and then restore power. Observe the unit to make sure the fan is turning and blowing a sufficient flow of air. As noted above, if the fan isn’t working, the device simply won’t cool the air.

First, check if the fan is somehow being impeded. Try to get some oil, such as WD-40, with a plastic extension to get the oil into the bearings and aid the mechanism. If that doesn’t work, the fan motor will likely need to be replaced. Earlier we mentioned the possibility of damage to the compressor if the fan fails. If it’s hot out and the fan motor fails, the unit will stop cooling and pressure within the condenser can rise to such a degree that it’s shut down by a compressor overload switch (though not all units are equipped with this feature, something work checking when buying a new unit). If there’s a problem, checking the fan will enable you to replace the fan cheaply as opposed to the costly expense of replacing the entire compressor because it blew when overloaded.

Now, should you decide to repair the fan, there are a few considerations. Most important will be the matching of the component traits mentioned earlier with your replacement. An error there could lead to your new motor–or worse your heating or cooling system–burning out. But also make sure the existing bracket will fit the new motor. If not, a new bracket will be needed, in which case the diameter of the existing bracket will need to be measured. For the actual replacement, follow the two wires coming out of the motor toward the main terminal block and remove them, ideally using a set of needle-nose pliers. Remove the mounting screws for the fan assembly and carefully withdraw the assembly from the motor comportment. A thin nut will hold the fan blade to the motor, which should be removed so the fan blade can be used with the replacement motor. Attach the mounting bracket to the new motor and reinstall the fan blade, which should have a rubber washer. Reconnect the wires from the main block, and reattach any protective covers for the assembly. Note that for any A/C or heating unit, at least 2 feet clearance should be maintained around the unit for proper air flow.

Electromechanical Actuator

Actuators are mechanical devices that convert energy into motion. They supply and transmit a measured amount of energy for the operation of a mechanism or system such as valves or dampers. They take energy introduced – a burst of air, electricity, or a liquid – and trigger a controlled reaction such as opening or closing a valve or damper. In addition to electromechanical actuators, other types include linear, pneumatic, rotary, and solenoid.

Electromechanical actuators use electricity to block, clamp or eject energy in order to move or control motors, pumps, switches and valves. Electromechanical actuators are popular as they are inexpensive to manufacture and, therefore, sell cheaply. Other advantages include: that operations can be automated, they are self-contained mechanisms, and display identical behavior whether extending or retracting. An important disadvantage is that they contain many moving parts that are prone to wear.

Electromechanical actuators are used in furnace blower and damper operating mechanisms. They open and close the vents as needed to blow hot air into the house and to expel gases to the outside. When the thermostat calls for heat, the electromechanical actuator is energized and drives the damper to its fully-opened position. Once this condition has been achieved, the actuator de-energizes, the pilot ignition system energizes, and the main burners ignite. After reaching the appropriate temperature, the operation is restarted to close the damper. The actuator then remains idle until the next heating cycle.


AC scroll compressor

The compressor is a pump that changes the air conditioner’s refrigerant or coolant (now commonly Puron, though in years past Freon was used) from a low-pressure gas to a high-pressure gas.

The high pressure created by the compressor induces condensation (the process of changing a gas into a liquid), and condensation produces heat. The refrigerant is pushed through a series of coils known as the condenser (or condenser coil) and the refrigerant changes from a gas to a liquid, releasing heat through the coils into the outside air.

The (now liquid) refrigerant leaves the condenser and passes through an expansion valve, which relieves the pressure on the refrigerant.

The refrigerant now passes into a second set of coils, which are typically located inside the space to be cooled. The new low-pressure environment induces evaporation, and the process of evaporation requires a transfer of heat from the surrounding area into the refrigerant. While the refrigerant is in the evaporation coil, it draws heat from the surrounding air and absorbs it within the refrigerant.

This process of raising and lowering the pressure on the refrigerant is how an air conditioner is able to cool a house.

Single-Stage vs Two-Stage Compressors

The main difference between a two-stage air conditioning unit and a standard, single-stage cooling system is the compressor. Because single-stage air conditioners run only on one speed, high, the unit provides a blast of cold air at one speed: high, whether you need that much cool air or not. This results in the potential for lost energy and money. Two-stage air conditioning units have a compressor unit that enables them to cycle from low to high, and can therefore adjust the amount of cool air that cycles through the house.

A two-stage air conditioner runs at its lowest speed about 80% of the time, and this low speed is usually sufficient to keep a home comfortable. As temperatures and humidity soar, the unit adjusts to its higher setting immediately. During the low speed, however, the unit runs nearly continuously and hardly ever shuts on and off. This allows it to dehumidify the air better; the cold blast of high speed is too quick to remove water vapor from the air. At low speed, water vapor has time to move into the coils and evaporate.

Continuous operation in the two-stage air conditioner also offers the benefit of consistent temperature. In single-stage air conditioners, the blast of chilly air leaves the room very cold, but when the system stops, the air becomes hot and stuffy. Two-stage air conditioners keep the air circulating and more consistently comfortable.

Finally, the compressor in a single-stage air conditioner is subject to more wear and tear as the unit is frequently turned on and off. The compressor in a two-stage compressor tends to have a longer life and save the homeowner repair and replacement.

Types of Compressors

Compressors come in many different types, but the two common to air conditioning units are the centrifugal compressor and the scroll compressor. Scroll compressing technology has been in use for a long time. In this setup, the compressor uses a pair of interleaved scrolls to compress, pressurize or pump liquids and gases. This design also boasts a superior capacity to handle liquids, enabling small amounts of dirt and liquid to pass through without damaging the compressor, while at the same time increasing reliability by eliminating stress on the motor. Scroll compressors are considered very efficient and used in examples such as Lennox’s HS-22, which includes a double-row condenser coil. Centrifugal compressors are considered best suited to continuous-duty applications, such as cooling units, ventilation fans and air movers because they give a high airflow at an efficient level but can’t achieve a high compression ratio, meaning they can’t handle large variations in pressure. Centrifugal force drives the compression of the refrigerant, rendering these compressors ideal for larger refrigerant volumes and low-pressure differentials.

Defrost Controls

A heat pump needs to defrost regularly when icing conditions occur. The trick is that the pump must run long enough to rid the unit of ice but not too long, so as to be an energy waster. As such, most heat pumps are equipped with a time-temperature defrost system. The time stops can be placed several intervals from 10 to 90 minutes, depending on the equipment manufacturer. The actual defrost cycle will run between 90 seconds and 3 minutes.

Types of Defrost Controls

A defrost control device uses a timer–either an electronic version or a clock motor–which includes mechanical stops or electronic jumpers implemented via a microprocessor on a small circuit board. These stops are placed at intervals generally ranging from 30 to 90 minutes. Larger systems that go beyond residential use can be equipped with a pressure switch to detect when the coils of the heat pump have become stopped up with ice. While such a setup can be used on residential units, it’s considered uncommon.

Replacing A Defrost Control

The following is taken from instructions to replace an ICM defrost control board with a UTEC defrost control board, part #917178A:

  • Disconnect power to the unit.
  • Remove the control panel cover, revealing the unit’s electrical components.
  • Remove all factory and thermostat wiring at the current defrost board.
  • Snip the plastic stand-offs holding the board in place and remove the board.
  • Install the new board in the same location, mounting its plastic stand-offs in the existing holes. Field terminals connecting to the thermostat should be located within the specified low-voltage barrier.
  • Replace all factory wiring to the new board.
  • Connect the wires from the new unit to the appropriate color-coded connect terminals.
  • Connect the stripped portion of the six wires to the thermostat wires and secure in place with wire nuts.
  • Check that all connections are secure.
  • Replace the cover board and return power to the unit.

Evaporative Cooler Motors

An evaporative cooler, also commonly called a swamp cooler, is used in dry, warm climates and functions by filling the air with water vapor. Each cooler requires an appropriate motor to keep it running efficiently.

Types of Motors

Evaporative cooler motors, like other electric motors, come as split phase motors and capacitor start motors. Split phase motors are ideal when the equipment stops and stars frequently. Capacitor start motors have superior efficiency and starting ability, and therefore, are used in equipment that is considered hard to start. Both motors can be used in air conditioners, blowers and fans.

How to Buy an Evaporative Cooler Motor

When choosing a replacement evaporative cooler motor, consider what you need and use the following features to help make a purchase decision:

  • Motor speed – most motors have 2 speeds
  • Maximum temperatures – motors can withstand temperatures as high as 300 degrees
  • Approval – is the unit electrically UL and mechanically approved for evaporative cooler applications?
  • Durability – choose a rust resistant model
  • Safety – choose features such as automatic reset thermal overload protection
  • Horse power
  • Make sure the voltage is right for your purposes.

Heat Exchanger

In the most general sense, a heat exchanger is simply a device that transfers heat from one substance to another. In the case of your gas forced-air furnace, this transference is done from the gas-powered flame to the warm air that you distribute around your home.

Today’s heat exchangers are typically made of stainless steel or aluminized steel in either a clamshell or tubular design. Clamshell heat exchangers are made up of two concave halves that, when joined either by welding or folding, form a passageway with an inlet and an outlet.

Tubular designs, which are comparatively newer, feature long U- or S-shaped chambers that create undulating series of tubes. Many manufacturers claim to use proprietary processes that strengthen the metal at the tubes’ curves, where normally the metal would stretch and thin during the bending process. In addition, tubular heat exchangers typically feature thicker walls and shorter seams than clamshell heat exchangers, reducing the likelihood of a failure.

How It Works

Once your thermostat directs your furnace that it’s time to produce heat, the furnace’s burner produces combustion gasses. These gasses enter the heat exchanger through one opening and transfer heat onto the heat exchanger’s walls. The combustion gasses are then either vented out of the home through another opening or, in the case of some high- efficiency furnaces, passed into a secondary heat exchanger, where more heat is extracted from the gasses before they are vented outside. As this is happening, the furnace’s blower motor moves intake air from the home over the heat exchanger’s now hot chambers where the air picks up heat before being pumped back out through your ducts and into the home.


It is important to maintain your furnace’s heat exchanger not only because it is a critical component of the furnace, but because a cracked heat exchanger can be dangerous. Should a heat exchanger develop a crack, its ability to vent combustion gas is impaired and, as a result, carbon monoxide could be circulated within the home.

Cracks can occur at seams or bends, as a result of corrosion, “metal fatigue” from working with a restricted air flow, weak joints or any other number of factors. Several things can indicate a problem:

  • Yellow or leaning flame (your flame should be blue)
  • Signs of corrosion, visible cracks or other instances of exterior wear and tear
  • A smell like formaldehyde
  • The presence of soot

In addition, many HVAC technicians recommend regular heat exchanger inspections for units more than 10 years old.

Linear Actuator

Actuators are designed to convert energy into motion in order to trigger a controlled reaction in a mechanical operation. The device takes the energy created using air, electricity, or liquid and transforms it into an action such as opening or closing valves, pumps, motors, or switches. Various types of actuators are available including electromechanical, linear, pneumatic, rotary, and solenoid.

A linear actuator applies the controlled force in a linear manner (as opposed to rotationally like an electric motor) by converting rotational motion into linear motion. Advantages to this type of actuator include: simple design and few moving parts, high speeds are possible, it is a self-contained mechanism, and displays the identical behavior whether extending or retracting. Unfortunately, these can only be used in low force applications.

Home heating furnaces use linear actuators to open and close the blower and the damper allowing hot air to blow into the house and gases to be expelled to the outside. The linear actuator is energized when it receives a signal from the thermostat for more heat. It converts this energy into action and it opens the damper. The actuator is de-energized once the damper is open and remains idle until the correct temperature has been reached. Once the desired temperature has been reached, the operation is restarted to close the damper. The actuator again remains idle until the next heating cycle.

Common Types of Linear Actuator

  • Belt-driven
  • Electromechanical
  • Moving coil
  • Spring return

How to Buy a Replacement Linear Actuator

Actuators are often sold as part of a replacement starter rather than individually. For instance, Honeywell sells damper parts for rigid round ducts that are triggered by a built-in actuator. Linear actuator prices vary greatly and can cost up to $600.

Pneumatic Actuators

Actuators are mechanisms that supply and transmit a controlled amount of energy for the operation of another mechanism or system. In other words, they convert energy – from a burst of air, electricity or a liquid – into motion. The action created may be to block, clamp, or eject and is used in mechanical applications such as motors, pumps, switches, and valves. Pneumatic actuators use the displacement of compressed air or gas to initiate the conversion into energy. Other types of actuators include electromechanical, linear, rotary, and solenoid.

Furnace blower and the damper mechanisms operate using actuators as part of the ignition system. Pneumatic actuators use a burst of air to open and close the vents as needed. When the thermostat calls for heat, the pneumatic actuator is energized and drives the damper to its fully-opened position. Once this condition has been achieved, the actuator de-energizes, the pilot ignition system energizes, and the main burners ignite. After reaching the appropriate temperatures, the operation is restarted to close the damper, after which the actuator remains idle until the next heating cycle.

How to Buy a Replacement Pneumatic Actuator

Replacement pneumatic actuators can be purchased through heating and cooling supply stores or contractors. Specifications needed to ensure the appropriate size include: maximum air pressure in PSI (pound per square inch), the nominal spring range in PSI, the stroke in inches, exerted force in PSI, the operating pressure range in PSI, and the size of the dampers the actuator will be opening and closing. Pneumatic actuators prices vary greatly depending, in part, on capacity: the range is from approximately $40 to $300.

Pressure Switch

A pressure switch is used to activate electrical current in a furnace or air conditioning unit once the set amount of pressure is either applied or released. In the case of a furnace or boiler, the switch is modeled as safety device to shut down the unit should the proper combustion air flow–which in this case is what provides the pressure–not be obtained.

Types Of Pressure Switches

Pressure switches in general can be either hydraulic or pneumatic. In the case of HVAC products, switches are normally pneumatic. Most recognizable manufacturers make their own pressure switches for their respective units. In many cases, there’s a single switch that works on most, if not all, of a company’s product line. Amana, for example, makes a three-wire, one-hose pressure switch containing an orange wire (common), yellow wire (normally open) and blue wire (normally closed). This device is considered a replacement for more than 200 different Amana furnace models. The three-wire, one-hose setup is common, though subtle differences make cross-brand substitutions inappropriate. Rheem makes a setup, for example, that fits more than 25 Rheem/Ruud, Coleman or York furnaces.

Replacing A Pressure Switch

It’s important to note that what often appears as a pressure switch problem may not actually be so. Remember that the switch’s function is all based on air flow. Accordingly, anything that disrupts air flow–such as plugged exhaust or intake vents, heat exchange surfaces coated with soot or a clogged switch hose (which can be caused by anything from dirt to spiders nesting)–can all cause fluctuations in pressure that alter the proper function of a pressure switch. As for the actual replacement, all electrical power to the furnace must be turned off. The burner compartment door should then be removed, and wires removed from the terminals on the pressure switch. The hose that attaches the pressure tap to the pressure switch itself should then be removed as well. Finally, the screws holding the switch to the front of the blower partition should be undone, allowing for the switch’s removal. Reassembly is a simple reversal of this process.

Rotary Actuator

Rotary actuators use electricity to block, clamp or eject energy in order to move or control motors, pumps, switches and valves. They use moving coils which allow the operator to control force, position, and speed – three qualities that are also repeatable. Rotary actuators are capable of high speeds and precise position, which make them desirable for certain applications.

Home furnaces use rotary actuators to trigger the opening of dampers and blowers. The action is triggered when the thermostat calls for more heat and sends a signal to the actuator to open a vent. Then, the open vent in the furnace sets off the series of mechanisms needed to raise the temperature in the house.

Once the damper is opened, the pilot ignition system energizes, and the main burners ignite. After reaching the appropriate temperatures, the operation is restarted to close the damper. The actuator remains idle until the next heating cycle.

How to Buy a Replacement Rotary Actuator

Rotary actuators using pneumatic energy (a controlled burst of air) can be purchased from online replacement parts vendors as well as plumbing and heating stores. The necessary specifications to buy the correct replacement include: the operating range in PSI (pounds per square inch), the maximum air pressure (in PSI), the nominal spring range (in PSI), the net force exerted (in PSI), and the net force exerted (in PSI). Pneumatic rotary actuators cost approximately $50.


Thermocouples provide a safety control in gas-fed appliances (such as furnaces), controlling the main gas valve to prevent the buildup of gas should the device’s pilot flame go out. Composed of two dissimilar metals that are joined on one end, the device reads the temperature and produces a small voltage that’s measured by a thermometer built into the thermocouple. If the temperature is too low, the gas valve closes.

Types of Thermocouples

Thermocouples are available in a wide variety of different combinations of metals or calibrations. The four most common calibrations are J, K, T and E. Each calibration has a different temperature range and environment, although the maximum temperature varies with the diameter of the wire used in the thermocouple. For example, a J component at its peak usable wire diameter–termed American Wire Gauge 8, or .125 inches –can withstand a temperature limit of 760 degrees Celsius. Different calibrations can withstand temperatures of more than 1,200 degrees Celsius. Thermocouple structures consist not only of the wiring itself but a wrapping of magnesium oxide insulation, which is then covered by a metal sheath. Junctions of thermocouple fittings can be either grounded, exposed or ungrounded.

Replacing or Repairing A Thermocouple

Because a thermocouple runs consistently at high temperatures, the component is one that wears out quickly. As such, the higher the operating temperature, the more quickly a thermocouple will lose its functionality. Generally, a pilot flame’s failure to stay lit is a defining symptom of a faulty thermocouple. Type K components will need to be replaced every year or two. A thermocouple running at a temperature beyond K level–uncommon in a gas furnace–may need replacing as often as every three months.

The actual replacement procedure is fairly simple. Inside the threaded connection to the gas line, unscrew both the copper lead and connection nut. Then, under the mounting bracket at the thermocouple tube, unscrew the bracket nut that holds the tube in place. Place the new thermocouple into the bracket hole, making sure the steel tube is pointed up while the copper lead is aimed down. Screw the bracket nut back over the tube and tighten the connection nut to the threaded connection, but only to a point slightly tighter than if twisted by hand.


The basic function of a thermostat is to turn a furnace or air conditioner on or off to regulate the temperature of your home. When the temperature falls below a pre-defined level (called a setpoint), the thermostat signals to your heating system to begin generating heat. Alternately, if the thermostat is connected to a cooling system, it signals the system to begin cooling your home when the temperature rises above a certain level.

Common Types of Thermostats

Thermostats are classified as either mechanical or digital, depending on their internal mechanisms. Mechanical thermostats (also called electromechanical or EM thermostats) typically have a thermometer coil and a switch (often made from mercury). Changes in temperature cause the coil to expand or contract, engaging the switch when setpoint temperatures are reached. Digital thermostats measure temperature with a device called a thermistor.

One increasingly popular type of digital thermostat is the programmable thermostat. (There are mechanical programmable thermostats called “clock” thermostats, but these are less popular for residential use.) These thermostats allow you to set different target temperatures at different times of the day. For example, you could set the temperature of your house to 68° F during the day, and 58° from 10:00 pm to 7:00 am. Many claim that such thermostats can save the average homeowner substantial amounts on their energy bills by automating such changes in temperature. (For example, the US Department of Energy claims that “You can save as much as 10% a year on your heating and cooling bills by simply turning your thermostat back 10% to 15% for 8 hours”.)

How to Buy a Replacement Thermostat

Installing a replacement thermostat can be a do-it-yourself task — although you need to be comfortable working with your home’s electrical system and your heating or cooling system. It’s also a relatively small job for a heating contractor.

If you own a heat pump, you need to do additional research before buying a thermostat. Many programmable thermostats don’t work efficiently with a heat pump in heating mode — so you need to buy a special thermostat.

Here are a few features you may want to consider when shopping for a new thermostat:

  • How important to you is simplicity? Some programmable thermostats can be quite complicated to program.
  • Do you want an LCD, LED or analog display?
  • How many different temperature setting do you need? More sophisticated programmable thermostats will allow you to define up to six different temperature settings.
  • Can you manually override the temperatures that you have programmed into your thermostat without having to reprogram your default settings?
  • Does the thermostat have a battery backup so that you won’t lose your settings if you lose power?

Who Makes Thermostats?

Honeywell claims that the T86 Round® mechanical thermostat is the most popular thermostat in the world. First built in 1953, the company has sold more than 85 million thermostats and was granted a trademark on round thermostats in 1990. Although Honeywell’s current models don’t include mercury, they have been criticized in the past for including mercury switches.

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